As regards the age of the channels (or more importantly, the age of their most recent activity), they have a fairly pristine appearance overall, with no sign of having been seriously degraded by the erosion that has been systematically decimating the Gale/Sharp stratigraphic column for a very long time, and they have also not experienced any significant burial by more recent sediments.
The channel complex also has a low density of impact craters, with few to none in the channels themselves. And although there are several craters in the “stage” area of the “amphitheater,” most of them are highly degraded or incomplete, and in the majority of cases are obvious paleo-craters that predate the higher layers of Mt. Sharp (as well as the more recent erosion associated with the channel formation, which has served to re-expose these “fossil” craters). Below is a clear example of such an exhumed paleo-crater, with a diameter of about 2.5 miles, at the head of the Northern Channel and in a high state of degradation (I have outlined its shape for clarity):
I previously cited another paleo-crater in the “stage” area, about 4 miles to the south of the above pictured one, located just to the SW of the mesa.
In the southern portion of the “stage” (specifically, the headwater area of the Southern Channel), there is one crater, around three-quarters of a mile in diameter, which is fairly pristine in appearance and obviously post-dates the collapse of the west side of the mountain (plus there are several small nearby craters, one of which is in the upper right of the photo, and which may be associated with the same impact, in that a single bolide may have fragmented prior to impact, something which is suggested by the tight clustering of these craters), but the central and northern portions of the “stage” are virtually free of any obviously recent (non-paleo) craters. And the aforementioned 3/4-mile-diameter crater has a sharp rim and largely intact ejecta blanket in a very highly wind-erosive environment (see below), so it must be fairly young, presumably no more than several tens of millions of years old, and beyond this, it provides no real information on the timeframe in which the western face of the mountain collapsed, and does not imply a more antique timeframe for that collapse than the past several million to tens of millions of years.
Also noteworthy is the fact that wind-carved “yardangs” (linear ridges oriented along the axis of the prevailing wind direction) are comparatively subdued in the “stage” area, whereas they are highly prevalent on most other surfaces of Mt. Sharp. There are some yardangs in the area of the headwater channel of the Grand Canyon GC, and in the “stands” above that point, but most areas of the “stage” are free of them, or only exhibit them in rudimentary form, which shows that the “stage” must be a comparatively young surface (which hasn’t had a chance to form many mature yardangs). This argument for youthfulness is especially compelling when we consider the fact that the wind erosion rate (and hence, presumably, the rate of yardang formation) on Mt. Sharp is VERY high (as we shall see below).
These properties of the channel complexes (fresh appearance, a comparative shortage of more recently-formed impact craters, and a comparative shortage of wind-carved yardangs) not only indicate that the channels (or at least their most recent fluvial activity) are geologically young, but also that they were carved into the mountain LONG after the vast majority of material filling Gale Crater had been removed by erosion, and in a time-frame where the mountain essentially looked as it does today. They are obviously not ancient “fossil” rivers or “paleorivers” that were covered up by sediments and that have now been uncovered by erosion, as shown by the lack of cross-cutting of the channels by erosion, and the lack of overlying (uneroded) sediments draped over major sections of these channels. And in any case, there is no plausible scenario that could have given rise to a “primordial” Mt. Sharp that featured these outflow channels, a mountain that exactly matched the dimensions of the current Mt. Sharp, which was subsequently covered by sediments, and which has now been implausibly re-exposed to perfection by erosion, including its drainage channels and depositional fans, effectively down to the under-one-meter-scale resolution of the photographs.
A related consideration that speaks to the recent origin (or activity) of the channels, is the fact that the impact cratering density on Mt. Sharp in general is not only low in relation to values commonly found on Mars, but is very dramatically so in comparison with the adjacent crater floor that surrounds Mt. Sharp (with small craters in particular, which are normally the most common size range by far, being comparatively absent on Mt. Sharp, which indicates a high erosion rate, since small craters fall prey to erosion much sooner than large ones). This can be readily seen in a topographic map or orbital imagery of the Gale/Sharp complex, and once again, most of the craters that are visible on Mt. Sharp can be shown to be paleo-craters created prior to more recent deposition (and subsequent erosion), which dictates that they must be excluded from any crater counts used to estimate the age of the currently-exposed surface (meaning that the “legitimate” crater count, for purposes of dating surface exposure, is much lower than it appears to be at first blush).
Below we see a cluster of craters south of the Grand Canyon GC, most or all of which are paleo-craters, as shown by their irregular shapes and on-lapping lobes of country rock. These craters formed during the time when the Lower Formation was being laid down, next they were covered by higher Lower Formation layers, and are now being uncovered by erosion. So they represent “fossil” craters entirely encased within a single formation, rather than defining erosional unconformities between formations separated in time. And perhaps a bit counter-intuitively, this instance of a terrain with a high cratering density, lacking as it does craters that are diagnostically of recent geologic origin (non-paleo), actually speaks to the youthfulness of the exposed surface of that terrain:
And some examples directly to the northeast of the Grand Canyon GC:
Collectively, these observations show that the visible surface of Mt. Sharp must be a recent erosion-created surface, and that if the more delicate features of the channel complex (such as the channel fill and delta at the end of the Northern Channel) were not of geologically recent origin, even startlingly recent, they would have been worn away long ago. But just how recent?
In the past, it was generally believed that the erosion rate on Mars is very low, and that the visible sand dunes (which on Earth would normally signify dynamic motion of sand grains through “saltation,” and active erosion of any exposed bedrock through sand-blasting) were mere “fossils” from an earlier time of Mars when the atmosphere was denser (it was also believed that the Martian atmosphere is currently too thin and the winds too weak to support saltation of sand grains, even with the comparatively weak Martian gravity). All of this has changed in recent years, however, as a result of several lines of evidence.
First of all, high-resolution photos of the surface (such as those by the Mars Reconnaissance Orbiter) have shown that many areas of the planet are almost totally lacking in small craters, which indicates highly active on-going erosion, as the impacts that produce such craters have been shown to be actively occurring, at the rate of approximately 200 per year globally , a figure derived from MRO observations of fresh craters that have appeared during several years of observation. In fact, on the basis of the relative shortfall in the number of small craters (which are created in far higher numbers than large craters, but are eroded away far faster) it has been estimated that parts of the planet are being effectively resurfaced, on the scale of even 2,000-meter-wide craters, on a timescale of under 9 million years .
Secondly, observations of sand dunes by the MRO have shown that they are actively moving [11,12], at a speed comparable to similarly-sized sand dunes on the Earth . This implies the active (and erosive) saltation of sand, likely to be basaltic sand in most cases (unlike the more typical quartz sand on Earth), and close parallels have been drawn between Martian conditions and those of Antarctic dry valleys on the Earth, where there are basaltic sand dunes migrating at a similar speed and causing erosion of basaltic bedrock at the rate of 30 to 50 micrometers (millionths of a meter) per year, and models of the saltation of sand grains under Martian conditions of atmosphere and gravity suggest erosion rates, where active dune migration has been observed (measuring approximately .5 meter of travel per Earth year), of 1 to 10 millionths of a meter per Earth year on a flat, level basaltic surface and 10 to 50 millionths of a meter per year on a vertical basaltic rock face , with much higher erosion rates to be expected with softer materials (such as most sedimentary rocks).
These observations, together with the “NASA Ames 3-D general circulation model for Mars” (GCM), have led to estimates of globally-averaged Martian wind erosion rates as high as 100 millionths of a meter per year . Which is one ten-thousandth of a meter per year, and when phrased that way may not sound like much, but these figures accrue to huge sums over geologic time: being very conservative and taking a value just one-twentieth (5%) of this global estimate (just 5 millionths of a meter per year), there would be an average of five meters (15 feet) of erosion per million years, and 15,000 feet per billion years! So while the erosion rate on Mars may be lower than the Earth’s, there is increasing evidence that it is not as far behind as has been generally believed, and in any case is almost infinitely higher than that of an airless body such as the Moon.
And the implications of this are extremely interesting when applied to the delta deposit of the Northern Channel: First off, let us (rather arbitrarily, for purposes of illustration) assume that the flank of Mt. Sharp is subject to erosion at the conservative global average I provisionally adopted above, of 15 feet per million years. Secondly, let us also assume that the typical Earthly ratio of the width to the depth of a channel also applies to Mars, namely around ten to one (which would seem to be a reasonable assumption, based on the general observation that Martian and Earthly river channels are similar in morphology). And since we observe that the width of the end of the Northern Channel (before it spreads into a fan) is around 2,000 feet, we then arrive at a depth of around 200 feet. And this channel is filled with old sediment and currently eroding, and if we assume for the sake of simplicity that 50% (on average) of the original thickness has been eroded away, then we get 100 feet of erosion….which, based on the assumed erosion rate of 15 feet per million years, means that the channel fill is only about seven million years old, since that amount of erosion constitutes just under seven of the 15-foot (per million year) increments! (It is possible that the channel has been widened by wind erosion since it formed, but if that is the case, our estimated channel fill thickness is less, and the estimated age even lower.)
But is there any reason to believe that the erosion rate in the foothills of Mt. Sharp even remotely approaches this value? After all, there are also wide areas of Mars that have a much LOWER than average erosion rate, especially the heavily cratered Southern Highlands (otherwise, all but the largest or most recent craters would be long-since worn away). However, it can be readily demonstrated that the flank of Mt. Sharp does indeed have a high erosion rate, based on the dearth of (non-paleo) craters, both in relation to the adjacent floor of Gale Crater and to global frequency distributions, and is in fact analogous to terrains elsewhere on the planet that have been categorized as very young on the basis of crater counts. And such a high erosion rate is also consistent with the observed dune migration rates on the adjacent floor of Gale Crater, amounting to as high a speed as .4 meter per Earth year [15,16].
To date, one of the most important results from the Curiosity rover has been the measured erosion rate on the floor of Gale Crater, at the “Yellowknife Bay” locale, of approximately .75 meter (2.5 feet) per million years, for a vertical mudstone rock face . Reducing this figure to one-sixth for a flat, level surface (based on the average ratios in the above-cited study  of basaltic sand migration), but compensating for the fact that the delta deposit is in fact on a sloping surface (a 7-degree grade), a reasonable approximation might be that the erosion rate for the delta deposit is .5 foot per million years, given the same wind regime as at Yellowknife Bay (and the same susceptibility of the local lithology to wind abrasion) . This would then result in an estimated age of about 200 million years for the channel fill (which would be young in relation to the generally-accepted chronology for Gale Crater, but far older than the figure I arrived at earlier). However, there is every reason to believe, on both theoretical and empirical grounds, that the wind erosion rate in the foothills of Mt. Sharp is FAR higher than on the floor of Gale. First of all, based on terrestrial analogues, wind speeds (and hence expected erosion potentials) should rise sharply with elevation, since friction with the ground (and resultant loss of velocity) is greatly reduced. And secondly, we empirically observe that there are very few recent (non-paleo) craters on the flank of Mt. Sharp at the elevation of the delta deposit, whereas they occur with variable but universally fairly high densities in all Gale Crater-floor locales…..and since craters are by comparison almost completely lacking on the flank of Mt. Sharp, a very rough (but plausible) visual estimate would that they are over an order of magnitude lower in frequency than on the crater floor where Curiosity measured the erosion rate (under one-tenth of Yellowknife Bay’s cratering density), which implies that the erosion rate on the side of Mt. Sharp should be over ten times higher than at Yellowknife Bay, perhaps on the order of 5 to 15 feet per million years for a flat, wind-facing surface with a 7-degree slope. Which brings us into the temporal range of perhaps 7 to 20 million years for the age of the delta deposit, which overlaps with my original channel-fill age estimate of seven million years (and this time, the figure is based on empirical data, not values arbitrarily selected for the purpose of example).
However, this method of estimating the age of the delta deposit, does suffer from a couple of weaknesses which impair its precision, namely that the total amount of erosion experienced by a given stratum is rather arbitrarily selected from a possible range, plus I am assuming that all sedimentary materials will erode at the same rate in a given wind regime. And later in this paper I will present an alternative (and more rigorous and precise) method for estimating the age of the delta deposit based on empirically-measured erosional deflations, and which also takes account of the variable rates of erosion of different lithologies.
Below we see a picture (courtesy Google Mars) of a one-mile-wide swath of land immediately to the west of the delta deposit. Note the almost complete lack of impact craters, with those present being highly degraded, almost to the point of being unrecognizable as such (and in most or all cases likely to be exhumed paleo-craters):
Now we see a one-mile-wide swath of ground centered on the location where Curiosity measured the erosion rate as being 2.5 feet per million years (just inside the edge of Yellowknife Bay, which lies to the upper right of center). Note that all nearby terrains are densely peppered with impact craters, in various states of erosional degradation, for which a rough visual estimate indicates a frequency in excess of one order of magnitude higher than for the delta deposit region, even in the more rapidly eroding (hence lower-crater-density) Yellowknife Bay strata:
Here is a closeup of Yellowknife Bay, which features a conspicuous 130-foot-diameter crater in its center (arrowed, and fairly fresh in that it retains a rim and significant bowl depth), as well as several smaller craters in various stages of erosional degradation. This demonstrates that Yellowknife bay, even though it has a relatively high rate of erosion and a lower crater count per unit area than its immediate surroundings, has a much longer crater retention age than the area of the Northern Channel’s delta deposit, hence the delta deposit’s environment must be experiencing an erosion rate that is far higher still.
As a cross-check of the above results, we can also compare the terrain encompassing the delta deposit to one which, on the basis of its low crater count, has been independently dated as being between 2 and 30 million years old: the Cerberus Fossae region, about 1,000 miles northeast of Gale, where eruptions of both lava and liquid water from fissures (such as the one shown here) completely resurfaced the terrain. Below we see a typical one-mile-wide swath, and it can be seen that there are very few craters, perhaps on a par with the area adjacent to the delta deposit, but with those craters looking considerably crisper than those near the delta deposit (and obviously being non-paleo craters). So while we are dealing with a different mechanism here, lack of craters due to deposition of new surface material vs. lack of craters due to ongoing surface erosion of old materials, the similar dearth of craters is suggestive that the effective ages are similar, for the purpose of dating events that have left their mark on these respective surfaces (and below, I will suggest that there may be a direct causal linkage between the Cerberus Fossae eruptions and the geologically recent fluvial activity in the Gale/Sharp complex).
So the startling conclusion from these observations is that the channel complex of Mt. Sharp (or more precisely, its most recent activity and the Northern Channel’s delta deposit) may date from under 30 million years ago, and Gale Crater was largely filled with a 2,800-foot-deep lake within that timeframe! Which will require a sea change in how we view Martian evolution and chronology, since such features (and environmental conditions that would accompany them and support them) are commonly assumed to be a hundred times older than they actually are, and it turns out that Mars is still a very dynamic planetary environment, both geologically and climactically, and able to offer more than a few surprises! And the implications for what Curiosity may find when it reaches the delta deposit are truly huge, and I will have more to say on this matter later in this paper.
Furthermore, this youthful estimate of the age of the Mt. Sharp channels provides robust support for the artesian hydrant hypothesis of Mt. Sharp, as there is no other conceivable mechanism that could produce such a flow of water in the geologically recent history of Mars.
Some researchers, such as Le Deit et al. , assume that the channels/canyons on Mt. Sharp date from the early history of Gale, perhaps 3.5 billion years ago, but this is utterly inconceivable in light of the erosiveness of the local environment, which is demonstrated by the low crater count. And based on the erosion rate measured in-situ by Curiosity (and the local and global erosion rates were likely much higher in the distant past, when the atmosphere of Mars was thicker), any shallow “drape” feature such as the delta deposit would have been completely worn away ten or even a hundred times over, if it were of such an antiquity.
Fairen et al.  allow a somewhat more recent timeframe (Hesparian and Early Amazonian) for the occurrence of fluvial activity within Gale, but they dismiss the possibility of Late Amazonian activity, apparently without taking account of the evidence posed by the Northern Channel’s delta deposit (which they don’t discuss, although I consider it to be the single most important feature in the entire Gale/Mt. Sharp complex, for the purpose of understanding that complex’s history). And fortunately, the delta deposit is precisely where Curiosity is headed, and should arrive there around the middle of 2014!
RECENT AND CURRENT LIQUID WATER OF MARS
I would offer the following interpretation: the Mt. Sharp artesian hydrant has been inactive most of the time during recent geologic history (mostly dormant for the past three billion years), but due to a rare confluence between a warmer surface climate and an increase in the subsurface temperature gradient arising from deep magmatic activity, the ice plugging the springs of Mt. Sharp melted and surface access for the aquifer was re-opened, at the same time that the aquifer itself was revitalized and pressurized by regional volcanic activity. And these factors synergistically combined to cause liquid water to begin working its way out of the mountain, presumably in a weak area between layers or through fractures, with what was initially minor seepage creating ever-larger openings, and this water began the process of carving the outflow channels, until the western face of Mt. Sharp’s upper mound was undermined and collapsed, opening an even more direct route for the aquifer to discharge its contents. And it was likely that the Gale Crater was largely filled by a deep and massive lake at that time. But eventually the aquifer drained sufficiently to greatly reduce its head, and what was originally a torrent had become a trickle, and even that was subsequently shut off when movement toward colder conditions froze solid the openings of the springs.
It should be noted that below-surface magmatic activity (and associated surface volcanic activity) could very well coincide with a warmer planetary climactic regime, because increased volcanic activity leads to outgassing of greenhouse gases and higher temperatures and atmospheric pressures, greatly increasing the ability of the surface to support liquid water, in both channels and lakes. So the collapse of the west face of Mt. Sharp’s upper mound, the creation of the channels, and the filling of a Second Generation Lake, may not only have occurred at the same time, but coincided with a peak in the volcanic activity of Mars, thanks to a one-two-three punch that could have been delivered by igneous activity: 1) increased subsurface heat that would have melted large areas of deep subsurface ice and revitalized and pressurized the regional aquifer, 2) outgassing an atmosphere that increased surface temperatures and helped to melt through shallow subsurface ice that may have been sealing off the artesian hydrant, and 3) creating a (temporary) temperature and pressure regime that once again allowed liquid water to be stable on the surface. And below I will have more to say about the timing of geologically recent Martian volcanic activity.
However, the history of the channel complex is apparently not a simple one involving a single event or relatively short period of activity, and at least two discrete periods of activity, separated by a large gap of time, were evidently involved. This is demonstrated by the apparently young age of the Northern Channel’s delta deposit, whereas there is no obvious colluvium or lag deposits still extant from the collapse of the west face of the upper mound. In addition, the amount of material in the delta of the Grand Canyon GC is just a small fraction of the huge volume of sediments that were excavated in the formation of that canyon. So the collapse event, and the formation of at least the ancestral Grand Canyon GC, obviously occurred long ago, probably in early to mid Amazonian time, long enough ago for wind erosion to have degraded and dispersed the aforementioned debris. And yet if all the channel activity had occurred that long ago, and a corresponding amount of erosion had occurred since the last outflow, we may expect that the Northern Channel’s delta deposit would have been entirely worn away, and the channels in general would look far less fresh than they do today, even to the point of being mostly eroded away. So my conclusion is that the complex is old, and has been generally dormant but not extinct, and that there was a recent burst of activity that freshened up the channels and deposited new sediments, as well as filling Gale with a 2,800-foot-deep lake. And it is possible that additional outflows occurred within the interval bracketed by the first and last outflows, whenever conditions were optimal for a revitalization of the Mt. Sharp artesian hydrant.
In addition, the Northern Channel was likely a fairly late feature to develop, and may have formed during the most recent period of activity. This is suggested by the observation that a delta begins at around minus 12,500 feet in the case of the Northern Channel and at approximately minus 12,700 feet in the case of the Grand Canyon GC, which I interpret as meaning that the Grand Canyon GC largely dried up prior to the high point of the lake, and that the Northern Channel took over as the primary conduit for flow, topping off the lake. And this is consistent with the fact that the head of the Northern Channel is at a lower elevation than the Grand Canyon GC’s (the floor of the “stage,” approximately representing the unconformity between the Upper and Lower Formations, is tilted towards the NW), which would have favored the flow of water into the Northern Channel, once whatever remaining barrier to the northward flow of water had been eliminated by erosion or collapse (or else a separate subsurface-to-surface conduit opened that supplied the water that formed the Northern Channel….in either case, this was long after the primary collapse of the west face of Mt. Sharp’s upper mound, but before the “stage” area had been fully eroded). This interpretation is also consistent with the funnel-shaped erosion pattern on the north end of the “stage”, graphically showing how the water was being directed into the Northern Channel from a broad swath of the surface area of the “stage,” once such a pathway had been made available. It is also consistent with the fact that the Northern Channel did NOT involve the excavation of a huge amount of material (unlike the Grand Canyon GC, which has hundreds of times the volume of the Northern Channel), which not only suggests that the Northern Channel is far younger, but also explains how there can be a lack of colluvium or other debris (aside from the delta deposit) downhill from the Northern Channel’s source, even if it is a young feature, whereas there must have been an enormous pile of material dumped from the end of the Grand Canyon GC by the time that its primary erosion was completed (close to one cubic mile), which would have required a very long span of time to erode away through wind action, likely hundreds of millions of years at the minimum (perhaps the large dune field beyond the end of the Grand Canyon GC is largely the end product of the breakdown of the debris that was excised in forming the canyon?). And so I would also interpret the visible channel fill and delta of the Grand Canyon GC as simply the product of the most recent discharge from the artesian hydrant, a far less voluminous flow than the one that carved (at least the lion’s share of) the Grand Canyon GC (although this later-stage flow was still powerful enough to move 30-foot-wide boulders).
In any case, the apparent youth of the delta deposit and of the current surfaces of the channels, have broad implications for the quest for water (and thus current microbial life) on Mars: if the hydrant of Mt. Sharp has been recently active, this implies that a stable liquid aquifer still exists at depth (probably with the benefit of dissolved salts lowering its melting point). Which would greatly increase the chances of life on Mars, as a stable long-lived aqueous environment is infinitely more favorable than the scarce near-surface moisture on Mars that may remain frozen for thousands to millions of years between melt cycles.
An ongoing debate has involved whether Mars still has liquid groundwater, at any depth. For unless it has enough water to virtually saturate the crust (i.e. to mostly fill the available pore spaces through most of the vertical column), whatever water is there will, by now, have been locked away in the “cold trap” of the upper crust (the “cryosphere”), leaving nothing at depth to form an active aquifer, even if the temperature there is above the melting point of water (as it must be at some depth, even if that point is several miles down). But this evidence from Mt. Sharp, suggesting geologically recent outflows, weighs in on the side of there being an active aquifer (the recent discovery of “recurrent slope lineae” has no bearing on this issue and also cannot explain the Mt. Sharp channels, as the lineae are believed to be shallow surface phemonena, overlying permafrost, and resulting from “deliquescent” salt deposits absorbing moisture which melts at warmer times of the year, creating brines that seep downhill, but only at a low flow rate, and with little erosive effect).
Further evidence supporting a positive “water budget” for Mars (and hence a present-day active aquifer) is the recent analysis of igneous Martian meteorites, evidently derived from material in the upper mantle of Mars, which show that Mars has (or had) at least as much water in its mantle as the Earth does, which greatly increases the odds that the planet has had plenty of water to play with .
Additionally, striking evidence for a currently-active aquifer in Mars (and one still capable of producing artesian springs) can be had from a location about 1,000 miles to the northeast of Gale, namely Athabasca Valles, a water-carved outflow channel complex which, together with associated volcanic flows, are believed to be among the most recent volcanic and large-scale aqueous features on Mars, estimated to have an age of just 2 to 30 million years, which is a blink of an eye in Martian geologic time, and should provide us with a good snapshot of what the current aquifer is like (the uncertainty in the age of the outflows is a result of the small number of available craters to count in calculating its age, uncertainties in the Martian cratering rate over time, and suspicions that more than one outflow event occurred). The water that carved Athabasca Valles emerged from cracks in the Martian crust (Cerberus Fossae) from which lava was also extruded, and the prevailing interpretation is that a rising plume of magma cracked the crust and the associated “cryosphere” of Mars (the vertical thickness of the crust in which any pore and fracture-residing water is frozen), allowing water to escape from a pressurized aquifer below the cryosphere. And it is estimated that, to create the necessary flow rate to carve the observed channels, the aquifer had an effective head at the surface corresponding to a vertical column of water nearly 3 miles high , mainly as a result of the nearby Elysium rise and volcanic complex (where the tallest point, Elysium Mons, towers 11 miles above the outlet for the flood). But the fissure from which the water erupted is only 2,000 feet lower than the headwaters of the channels on Mt. Sharp, PLUS the Gale/Sharp complex is in an even better position to generate a head from a pressurized aquifer, thanks to the adjacent Southern Highlands. So there should be a copious liquid water source that could still be tapped today by the Mt. Sharp artesian hydrant, the only limitation being the plugging of the conduit(s) with ice due to the prevailing low surface temperatures at this time.
In fact, the approximate confluence between the estimated age of the Cerberus Fossae eruptions on the one hand, and my estimate of the age of the most recent Mt. Sharp channel activity and the formation of the Second Generation Lake on the other (all occurring perhaps 30 million years ago or even more recently), strongly suggests that there may have been a relationship between the two events. First off, the fissure eruptions of magma and water, by providing carbon dioxide and water vapor, would have temporarily thickened the Martian atmosphere and thus raised the surface temperature. And secondly, it is entirely possible that the two locations share a common aquifer across the relatively flat, one-thousand-mile sub-surface of Elysium Planitia, in which case the additional pressurizing of the aquifer by the heat of the magma plume could transfer to increased pressure at Mt. Sharp. And thirdly, the thermal pulse involved with a rising magma plume may have had a regional effect on the subsurface geothermal temperature gradient, melting portions of the cryosphere from below and increasing its permeability beneath Gale Crater (while warmth at the surface simultaneously acted on the top of the cryosphere). And if these events happened to correspond with a warmer climactic cycle on Mars as a function of orbital cycles, the cumulative effect could be very significant, with the atmospheric effects of the “orbital forcing” synergistically combining with the atmospheric effects of the volcanism.
Indeed, there is strong evidence of a warmer, wetter climatic regime during and after the time of the Cerberus Fossae aqueous and volcanic outflows (whether such a climate was due to the outflows or simply coincident with them, or a combination thereof): there are a number of features in the Athabasca Valles area which suggest a “periglacial” landscape (much like the arctic regions of Earth), in that there are extensive polygonal structures in the surface suggesting there was ice at shallow depth (permafrost) experiencing repeated (presumably seasonal) freeze-thaw cycles, plus thermokarst-like depressions and pingo-like mounds, extensive channel networks that cross-cut pre-existing terrains and which suggest melting of ice and drainage of liquid water, and extensive cirque and spur complexes apparently produced by intense sapping and mass-wasting, and together these features suggest that the water released by the fossae did not simply evaporate or soak into the deep crust, but resided long enough at the surface (presumably for at least centuries) to produce these landforms, implying a warmer, wetter, and higher-surface-pressure regime during that time . But because the lava flows are very closely associated with the aqueous flood features, both spatially and temporally, and the lavas apparently overlaped and covered the flood features in many cases, a great deal of controversy has been generated over the interpretation of specific features, as to whether they are water-caused or lava-caused, given that both types of flow can sometimes produce similar-looking features. However, given the fact that (even under a predominately volcanic interpretation of the landforms) there are numerous “rootless cones” caused by lava flowing over permafrost and generating explosive eruptions of steam, it is demonstrated that surface or near-surface ice was stable at that latitude at that time, which is not true today, so regardless of the interpretation of specific Athabasca Valles features, it appears that there was a more Earthlike climactic and atmospheric regime several million to several tens of millions of years ago on Mars, and one in which we were far more likely to encounter equatorial surface ice, and even liquid water.
The aqueous flows associated with Athabasca Valles also yielded a topographic feature that cannot be produced by lava flows and is unequivocal proof of a periglacial landscape with far higher temperatures than today, namely “sorted stone circles” which are produced by repeated freeze/thaw cycles in polygonially cracked ground, and are identical in morphology to terrestrial examples in Arctic regions. Furthermore, it has been estimated that, in order to support such processes, temperatures at this locale would have had to be 40 to 60 degree Celsius warmer than at present . And given that this location was only 1,000 miles from Gale, and Gale is at a lower elevation and latitude and hence apt to be warmer, the implications are that temperatures at Gale could have been well above the melting point of even pure water, averaging between day and night and seasonally, and with daytime highs approaching 100 Fahrenheit! And if such conditions had persisted for a significant period, they would have served to melt at least the upper portions of the ice blocking the artesian hydrant of Mt. Sharp (see below for a further explanation of this process).
Not only in Elysium Planitia, but in the plains of Utopia Planitia to the northwest of Gale Crater, there are landforms highly suggestive of very recent (very late Amazonian) fluvial and periglacial activity , including thermokarst-like depressions apparently left behind by ponded surface water that was stable for long enough periods to form terraces, signifying surface temperatures and pressures very substantially higher than those of today.
There are also unequivocal, recent aqueous features at Lyot crater, where there are no associated volcanics. Lyot is a 147-mile-diameter impact crater located in the Northern Lowlands but at almost the opposite side of the planet from Gale, and at the fairly high latitude of 50 degrees, where surface ice is currently stable over appreciable periods. And at Lyot there are what appear to be currently active glaciers, as well as gullies and river channels and depositional fans produced by glacial meltwater that has apparently flowed as recently as several million years ago . And while the mechanism for this water generation is different from that of Athabasca Valles, in that it is not from a deep aquifer but rather surface or near-surface melt on top of a uncompromised cryosphere (much like what occurs in spring and summer in Alaska, on top of permafrost), as such it serves to argue even more strongly for the occurrence of globally warmer, wetter, and higher-atmospheric-pressure episodes in the geologically recent history of Mars.
There are a number of additional locations that have experienced glacial, periglacial, and fluvial activity in the last few million years, all believed to be powered by Martian obliquity cycles, whereby a given terrain will periodically serve as a cold trap that accumulates snow and ice, and is subsequently subjected to a ”thermal excursion” that melts this ice (see below). Examples in the southern hemisphere are Gregg crater , and the Argyre impact basin , as well as several recently-formed, mid-sized, mid-latitude craters showing fluvial-related activity as recently as several hundred thousand years ago, including debris lobes, channels with fans, braiding, terraces, and natural levies .
The Cerberus Fossae floods also provide support to my hypothesis that the ancient sediments in Gale Crater were soaked (and subsequently lithified) by artesian flows from the central peak of Mt. Sharp, as the estimated hydraulic head in the Cerberus Fossae event(s) is several thousand feet higher than the highest surviving layer of the Upper Formation, and thus it could not be argued that there would have been insufficient hydrostatic pressure in the regional aquifer to emplace water where required by my hypothesis. And we may expect that the aquifer over 3 billion years ago would have been at least as active as it has been in the recent geologic past, plus the upper crustal milieu just over 3 billion years ago would have been sufficiently evolved to have featured a cryosphere under which the water would have been trapped and subject to pressurization, and able to exit the surface in only a few select locations (only where open fractures commuted from the surface to the bottom of the cryosphere).
AN EMERGING PICTURE
It is generally accepted that, due to variations in the axial tilt (obliquity) and orbital eccentricity of Mars, the planet experiences periodic warming and cooling cycles, at least on a regional basis (the Martian equivalent of the Earth’s “Milankovitch cycles”, which influence the glacial and inter-glacial cycles of the Earth, only much more extreme in the case of Mars, due to the lack of a large stabilizing moon), and this has been empirically supported by the observation that fresh permafrost has apparently been emplaced in the recent geologic past (on a scale of several hundred thousand years) over wide areas of Mars, as shown by the near-pure water ice excavated by meteor impacts that have occurred during the time in which the planet has been under orbital observation. Also providing evidence is the ”fresh” ice directly excavated by NASA’s Phoenix lander, ice that had seemingly been emplaced during a warmer and wetter period well under one million years ago, and which must be of such a “recent” origin as shown by the fact that it lies only a few inches below the surface (if it were older than a few hundred thousand years, it would have evaporated/sublimed down to a greater depth). And it is conceivable that, being near the equator, Gale Crater warms up enough during some climatic cycles (but probably requiring the random confluence of regional subsurface magmatic activity) to melt through the ice that might otherwise be plugging the conduits of Mt. Sharp, allowing a temporary outflowing of those artesian springs, until the climate chills enough to re-freeze the plumbing.
The diagram below illustrates how this could work, where dry crust is brown, ice-containing crust (cryosphere) is light blue, and liquid aquifer is dark blue:
Due to the good thermal insulating properties afforded by the mass of Mt. Sharp (not just due to its thickness but also to porosity, as it consists of sediments of lower density and thermal conductivity than either the typical crustal basalt or of water-saturated sediments), beneath the mountain we may expect an uptick in the level of the underground contact between the liquid and frozen aquifers. Simultaneous with that, we likely have a relative depression of the top of the frozen aquifer within the mountain in relation to the surface directly above (i.e. a cryosphere upper surface having a slight upward bulge below the mountain, but lying at a greater distance below the ground surface than for the surrounding, lower-elevation terrain), since we may expect that the mountain would have run relatively “dry” (due to gravity-driven outflow) the last time its internal temperature was high enough for water to be liquid (similar to how we have to drill down deeper to reach the aquifer when we are on top of a mountain on the Earth, even though there is an upward bulge in the aquifer under that mountain). So the net result is that the cryosphere should be unusually thin inside/under the mountain, and if we have an extended period of surface temperatures above freezing (quite likely coinciding with an increased subsurface thermal gradient), we may expect the cryosphere to be exceptionally subject to melting at this location (assuming the temperatures held long enough for significant warmth to soak into the core of the mountain, perhaps hundreds or thousands of years). In fact, a warmer regime that would not even begin the melting of the broader cryosphere (due to the cryosphere’s high thermal inertia resulting in turn from its thickness/mass AND from the high latent heat required for the phase change of such a thick column of ice), could well melt through the thin portion of the cryosphere inside Mt. Sharp, a process which would also be favored by the equatorial and below-datum location of Gale Crater, which raises the average temperature in relation to most portions of the Martian surface, and which would result in an unusually-thin cryosphere to begin with in the Gale area.
Brine (that is, water that is saturated with salt) has a freezing point far lower than that of pure water, specifically minus 21 Celsius in the case of sodium chloride brine, but minus 50 Celsius in the case of calcium perchlorate, which in fact is a temperature below the current day-to-night average recorded by Curiosity (which is around minus 40 Celsius), so even a perchlorate concentration below saturation would permit liquid water under the current surface temperature regime (and with depth below the surface, the temperature is going to do nothing but rise). And calcium perchlorate salt is known to be a very abundant compound in the Martian soil, making up .5 to 1 % by weight, so we may expect this highly water-soluble compound to be a significant “contaminant” in any liquid water we find on or within Mars (or any frozen water on or below the surface that was once in a liquid state, but NOT atmospherically-precipitated snow and frost on the surface, or glacial flows resulting from those, as such compacted snow would consist primarily of distilled water). But wherever such contamination occurs, it will serve to greatly lower the melting point, depending on the concentration (indeed, it appears that droplets of perchlorate brine were seen on the legs of the Phoenix polar lander).
It has recently been determined that the globally-averaged thermal gradient within the crust and mantle of Mars is lower than previously believed, meaning that the interior of Mars is colder than previously modeled, and that (all else being equal) the cryosphere may reach twice the depth previously believed . However, the recent relevation concerning the abundance of perchlorate “antifreeze” on Mars and its presumed “contamination” of surface and sub-surface water has largely negated this projection of a deeper cryosphere, and it may be that at low-elevation equatorial locations (such as Gale), the top of the liquid aquifer is only a mile or two below the surface.
And if the central peak of Mt. Sharp and the subsurface of the crater floor were a nexus of hydrothermal activity early in its history (as is believed to happen in all large impact craters), then we may well have an unusually high salt concentration in subsurface fractures which could also be conduits for water, and so it could be that the portion of the cryosphere below Mt. Sharp is currently composed of slushy, semi-frozen ”salt water” that is almost warm enough, under current conditions, to permit upward migration, and any substantial and prolonged period of warmth could send it above that threshold. In fact, conditions could normally be at a “tipping point” such that any increase in subsurface temperature, as little as a degree or two, from either top-down (surface climatic) or bottom-up (deep magmatic) sources, or better yet from a combination of the two, could send conditions over the threshold that would result in a reactivation of the artesian hydrant.
Because of the thermal inertia of a planetary crust and mantle, any subsurface thermal excursion caused by a rising magma plume will take millions of years to develop and dissipate, and it may be that during any time of activity in the Elysium or Cerberus Fossae complexes, conditions at Mt. Sharp are for an extended time ”primed” for an outflow, but the final ingredient that is needed to allow a new aqueous discharge is a radical thermal excursion in the surface climatic regime, so as to melt the near-surface ice blocking the plumbing in Mt. Sharp, and this is something that has happened only rarely in the Amazonian era (possibly as few as two times).
Increased subsurface warmth by itself might be inadequate to re-activate the Mt. Sharp artesian hydrant, because the top of the cryosphere would still be more-or-less exposed to the frigid surface conditions. And conversely, surface warmth (by itself) would likely not last long enough to melt through even a thin cryosphere, hence my suggestion that it is only during a rare confluence between regional subsurface geothermal activity and a strong surface thermal excursion, that we experience a reactivation of the artesian hydrant.
But what would happen to such water from such a source if it reached the surface, would it simply boil away as is generally expected for water exposed to the surface of Mars? The “triple point” for pure water occurs at a pressure of 6 millibars (the conditions of pressure and temperature where frozen, liquid, and gaseous water all exist in equilibrium, and representing the lowest pressure at which a liquid phase is supportable), and since Gale Crater is significantly below the Martian “datum”, the atmospheric pressure there is typically about 8 or 9 millibars, which would permit a liquid range (for pure water) of several degrees. But having the water in the form of a perchlorate solution could result in a liquid range of at least 50 degrees under the current pressure regime of Gale Crater. And in a high-obliquity stage of Mars’ orbit (where the planet is almost on its side, and each polar cap would point almost directly at the sun for much of the year) we would expect to vaporize all the dry ice at the poles, which would nearly double the atmospheric pressure , increasing the liquid range still further (and also providing an increased greenhouse effect).
Indeed, surveys of the current north polar cap have shown it to be only 4 million years old, and to consist of periodic layers of ice and dust laid down from the start of that timeframe , permitting age-dating much like the rings of a tree. So just prior to that timeframe (and probably on a number of separate occasions in Amazonian times), the volatiles of Mars were elsewhere on the planet, likely doing more interesting things, and presumably constituting carbon dioxide and water vapor in the atmosphere, and water ice (and even limited liquid water) on the surface. And not coincidentally, that 4 or 5-million-year-ago range is also believed to be when Mars experienced its most extreme cycle of obliquity in recent geologic history, with an axial tilt of approximately 40 to 50 degrees, which would have caused the “melting” of the polar caps and prevented their redevelopment until the obliquity had greatly lessened [30,31]. And it is further believed that such extreme cycles happen every several million years on average, and in some cases have involved an axial tilt as high as 80 degrees .
So during portions of such a climatic cycle, we can expect conditions at Gale to permit the melting of (at least salty) water and its flow over the surface, and even pooling for a limited time (and although the evaporation rate would be fairly high, that is likely to be mitigated by the formation of an ice cover, which would be encouraged by the fact that evaporation is a cooling process). And if we had a coincidence with major volcanic activity (or an asteroid or cometary impact, all of which could increase the surface pressure and thus temperature), then all the better for generating conditions which could not only support stable surface water, but also bring about substantial melting of the thin cryospheric layer inside Mt. Sharp, which if assisted by increased warmth from below, could once again permit direct outflow from the pressurized aquifer. And so I am suggesting that one such orbitally-forced warmer and wetter cycle coincided with the Cerberus Fossae surface eruptions and associated deep magmatic activity, all of which combined synergistically to allow a revitalization of the Mt. Sharp artesian hydrant, as well as conditions that permitted the existence of a relatively short-lived but fairly deep lake in Gale Crater (in other words, during the radical thermal excursion and presumed higher atmospheric pressure that is recorded in the periglacially-derived landscape of the nearby Athabasca Valles).
And so it may be that we have the misfortune of visiting Mars during one of its less-interesting phases, and there have been comparatively recent times when the planet was warmer and wetter and hosted a thicker atmosphere, and even featured flowing rivers and playa lakes at some spots on the planet (especially low, near-equatorial locales such as Gale Crater), and in so doing boasted more than a passing resemblance to Earthly conditions. And while such phases will likely occur again, sadly it is also the case that they are becoming more and more infrequent (and less intense) over geologic time, as the internal heat of the planet gradually winds down.
In any case, the (antiquated) notion of Mars currently being a “dead” planet no doubt had its genesis in the disappointing photos that were returned from the first Martian flybys, which only captured ancient heavily-cratered terrain, giving the impression that Mars was “Moon-like” and that it shared a similar history. And even though a more dynamic geology was discovered by Mariner 9, the impression (or bias) persisted that such features (such as fluvial channels) were simply flukes or exceptions to the general rule, and that all the visible surface features of Mars, including all signs of active liquid water, were to be dated in the billions of years ago, and that (like the Moon) the planet had been (mostly) frozen in time since shortly after its formation, a position that has been disproven by recent observations, and a viewpoint that I have argued against on the basis of the features of the Gale/Sharp complex. And although there were a number of fascinating locations on Mars vying to be the landing spot for Curiosity, it is now obvious that NASA could not have picked a better spot!
THE ATMOSPHERE OF MARS & BIOLOGICAL POSSIBILITIES
Mars is continually losing atmosphere to space, and it needs to be periodically replenished by volcanism, and we can assume that the atmospheric pressure fluctuates wildly and, all else being equal, is more-or-less in proportion to the level of recent volcanic activity. And during extended lulls in volcanic activity, the planet may reach a point of having even less atmosphere than what we are seeing today (also, it is suspected that when the tilt of the axis is near zero, as it is every few hundred thousand years, that the polar caps are even more efficient cold traps for carbon dioxide and water than they are now, and the atmosphere ”collapses” to a surface pressure only a fraction of its already-meager value).
It is commonly stated that Mars has suffered a high rate of atmospheric loss over time because of not having a magnetic field, which (it is stated) results in the top of the atmosphere being directly exposed to the solar wind, which continually strips it away. But while this may be a significant factor, it is not the primary one….if lack of a magnetic field destroys atmospheres, how then does Venus (which lacks a magnetic field) have an atmosphere almost 100 times as dense as the Earth’s? And why has not the Earth lost its atmosphere over geologic time due to the periodic reversals in its magnetic field, where the field strength drops to zero? And in fact, the magnetic field is a double-edged sword, because it serves to funnel charged particles into the atmosphere at the poles (hence the aurorae), putting that part of the atmosphere under even greater stress than it would otherwise be.
The primary reason that Mars has had difficulty maintaining an atmosphere, is the small mass of the planet (1/10 that of Earth) and the resultant lower escape velocity (5.0 km/sec vs. Earth’s 11.2), causing Mars to have a smaller separation between the average speed of upper atmospheric atoms and the planet’s escape velocity. And in a gas (in accordance with the Maxwell–Boltzmann distribution), plotting a graph of the number of atoms vs. the speed produces a bell-shaped curve, where at a given temperature, atoms cluster around one speed (which in fact IS their temperature, which is a measure of average atomic/molecular speed), but this also means that there is a small fraction of atoms (due to random collisions and absorption of photons) that greatly exceed the average speed, and will in fact exceed the escape velocity of the planetary body, and so are lost to space (see graph below). This is referred to as “Jeans Escape,” and Earth also loses some atmosphere by this means, but at such a negligible rate that its atmosphere has a half-life of many billions of years, whereas the half-life of the Martian atmosphere would likely be measured in the hundreds of millions of years (here’s another way of looking at it: kinetic energy increases with the square of the velocity, and Earth’s escape velocity is over twice that of Mars, so it is over four times harder for an atom to escape from the Earth than Mars, and so it rarely happens on Earth, whereas it’s happening all the time on Mars). And so if Mars had not experienced periodic volcanic activity over geologic time, which serves to (at least partially) replenish the atmosphere, by now it would be almost as airless as the Earth’s Moon.
There have been previous estimates that Mars is currently losing atmosphere to space at the rate of approximately 100 tons per day, but the figure should be refined by the current MAVEN mission.
Another recently popular idea is that Mars lost most of its atmosphere because of the huge impact (from a Pluto-sized body) that created the Northern Lowlands, since such an impact would have blasted most of the primordial atmosphere into space. But this is irrelevant for current conditions, because even if that collision had not taken place, most of the original atmosphere would have been gradually lost to space anyway, and the planet would have still needed to depend on volcanic activity to maintain what atmosphere it has. In fact, it has been previously estimated that, given the amount of volcanism that has left a visible record on Mars, a total of 1.5 bars of carbon dioxide has been outgassed over time (1.5 times Earth atmospheric pressure equivalent), assuming that the gas composition of Martian lavas is similar to that of Hawaiian basaltic lavas….but this is an estimate that is likely to only rise in the future, now that a number of ancient craters that were assumed to be impact craters have recently been reinterpreted as supervolcano calderas .
Mars may very well have acquired microbial life early in its history, either originating in situ, or else seeded from space, inside meteorites blasted from the Earth or other bodies. But because of the harsh surface conditions, principally due to the thin atmosphere, and with relatively habitable episodes being increasingly fleeting over time, it is unlikely that life has maintained a foothold on the surface of Mars, and if there is life on the planet, it resides deep underground in the safe haven of the aquifer, where conditions could be stable for billions of years. And in fact, I would be surprised if there is NOT currently live bacteria in the Martian aquifer, as there was presumably an exchange of meteoric material in the early solar system that would be capable of transplanting viable microbes, and some of these would no doubt be the anaerobic, “rock eating” type that derive energy and nutrients from the inorganic minerals in the deep subsurface, and are found living at depths of up to 3 miles on the Earth, under conditions likely very similar to the deep subsurface of Mars, and thus would be a “natural” species to transplant between planets.
Although it awaits further analysis and confirmation, the recent announcement of a possible discovery of biologically-related carbon-rich spheres and microtunnels in the 1.3-billion-year-old Martian meteorite Yamato 000593 is consistent with this scenario, in that the microscopic features observed could conceivably be the product of ”rock-eating” bacteria. As stated by the authors, “… textural and compositional similarities to features in terrestrial samples, which have been interpreted as biogenic, imply the intriguing possibility that the Martian features were formed by biotic activity” .
Below, from White et al. , we see a scanning electron microscope image of a layer in the meteorite, composed primarily of the water-generated clay mineral iddingsite, with the spheres in the red circle showing twice the carbon abundance as the background level (circled in blue).
Next we see a thin polished section of the meteorite, showing the microtunnels:
In fact, it has been cogently argued that life on Earth may very well have originated on Mars by means of such a vector, as the smaller planet would have cooled off faster and been able to support liquid water on its surface many millions of years sooner than the Earth, and with the high flux of asteroidal debris throughout the early solar system, causing much material to be rapidly exchanged between at least the inner planets, such life would have no doubt been seeded to Earth just as soon as conditions were cool enough to support it, giving such life a jump-start over any of indigenous Earthly origin. And taking this line of thinking a step further, it has even been argued that our lineage of life may have first appeared on Ceres, a water-rich “dwarf planet” which would have cooled off to the point of supporting liquid water even before Mars, and which would have been a perfect candidate, both in terms of its orbital position and low escape velocity, to have viable biological material blasted from it that would subsequently seed the planets of the inner solar system!
Even given the current minimal Martian atmosphere (or less if the planet were to go for a long period without volcanism), the cold trap of the upper crust prevents most of the planet’s water from being lost to space, at least for a very long time span (much as Jupiter’s moon Europa operates, with a liquid water ocean protected from the cold vacuum of space by the icy crust above), a fact that demonstrates how important it is that we obtain a sample from the deep subsurface if we are to assess the biological status of the Mars. And since heavy drilling equipment is out for the foreseeable future (equipment that could very well need to drill down several miles), our only option is to take advantage of a natural shortcut, which I am proposing Mt. Sharp may very well be.
And if we do obtain samples of intact biology from Mars, either live or very well preserved, it will be of the utmost interest to decipher its chemistry and map its genetics, to see if we can answer the question of whether it and Earthly life have a common origin!
AN ALTERNATIVE PROPOSAL FOR MT. SHARP’S GENESIS
It has been proposed by a team of researchers from Princeton University and the California Institute of Technology  that Mt. Sharp is basically a big dune, having been created by wind rather than water (and with the outer portion of the crater volume NEVER having been filled with sediments). Their “SWEET” hypothesis (Slope Wind Enhanced Erosion and Transport hypothesis), which has recently received considerable press coverage, states that a convection cell develops in response to the daily warming cycle experienced by the crater floor, which causes katabatic winds to sweep down the inner crater rim and pick up material from the outlying parts of the crater, whereupon they rise as they move towards the center of the crater, depositing their load in the stagnant-air zone in the center, and over a span of several billion years this process gradually built up the mountain we see today. And this hypothesis may indeed take account of some of the anomalous features of the Gale/Mt. Sharp complex (namely the existence of Mt. Sharp and the fact that the outlying parts of the crater floor are well below the grade of the surrounding landscape), but I will argue that there are clearly-visible features of the complex that are incompatible with their model, and that there are alternative explanations for what they present as evidence in support of their model.
First off, there is at least a prima facie implausibility to what they are proposing: looking at Mt. Sharp, there are no signs of ongoing deposition, in fact there are no signs of deposition (save for channel fill and some relatively small aeolian bedforms) in any time-frame but the very distant past, and instead there is a landscape basically carved out by erosion (looking at first blush much like the landscape of the America’s desert Southwest). So where is the hand of that process today, which they envision? Why is not deposition continuing to occur?
The authors of the study state that their model predicts that, once the mountain reaches a certain critical size and shape, the mountain modifies the winds such that they become a net erosive force, and what we are seeing today is the late end stage of that process. But would we not expect that, if the erosive winds reduce the mountain to a considerably smaller size, that the depositional process would resume at some point, mimicking its pattern of growth when the mountain was initially of that smaller size? In natural processes of the sort proposed, what we normally see when they “max out” is a movement towards a steady-state equilibrium, either asymptotical in nature, or else oscillating about a point of equilibrium, where forces (eventually) balance each other out (in this case, the forces causing erosion and deposition). But what we see instead in the Gale/Sharp complex, is a landscape that has been deeply and systematically ravashed by erosion for a VERY long time, with graphically exposed layers that have been very deeply cut, carving out buttes and hummocks and yardangs and ridges and scarps and canyons, with no indication that this process has slowed down or ever will slow down (let alone reverse itself), as the sparse impact cratering on the flanks of Mt. Sharp show that it is a surface experiencing ongoing intense erosion. And considering the depth of this relentless erosion, and its asymmetrical nature as can be seen in a topographic map, with massive chunks of the mountain missing, it would come as no surprise if the total volume of Mt. Sharp has been reduced to a fraction of what it once was….and yet there is no sign of any recent wind-generated deposition except for a thin dust cover on the upper slopes. So I would argue that the burden of proof must be on the authors, to explain how such a “counterintuitive” scheme could work in the real world, which is at odds with what we normally observe across a broad range of natural physical and geological processes.
Below we see two fine examples of the graphic erosion-carved landscapes of Mt. Sharp. The first is from the Mars Reconnaissance Orbiter, showing a spur defined by dramatic cliff-bench scarps on the east side of Mt. Sharp, and the second is an extreme zoom shot by Curiosity, of the heavily-dissected, butte and knoll-riddled foothills of Mt. Sharp in the area of the Northern Channel, taken while the rover was still several miles away (note the shadowed canyon of the Northern Channel in the upper left portion of the landscape):
As far as any direct evidence that the mountain was once far larger, there is indeed an apparent ”outlier” of the Lower Formation, an eroded low-relief layered exposure, in association with a medium-sized crater (quite possibly an exhumed paleo-crater), and largely covered by sand dunes, which extends to a point about 3 miles from the western margin or Mt. Sharp. Which not only shows that the mountain was far wider at one point in time, but that, by extension, the strata making up Mt. Sharp likely extended clear to the crater wall (the furthest westward extent of this outcrop is only about 6 miles from the first foothills of the crater rim in this portion of Gale Crater). Below is a pair of figures from Anderson and Bell :
There are also stratigraphic features of Mt. Sharp which are difficult, if not impossible, to reconcile with the SWEET hypothesis. The Upper Formation is of a totally different character than the Lower, both in terms of composition (it has spectral characteristics similar to globally-distributed Martian dust, not showing the hydrated clay and sulfate signatures of the Lower Formation), and also in terms of its depositional history: the entire Upper Formation was evidently laid down in a very short time, probably only a few million years for the entire 6,000-foot-thick column, on the basis of the almost complete lack of exhumed paleocraters (in other words, sedimentation was occurring so fast that there wasn’t a chance for any significant number of impacts to occur, while a comparable thickness of Lower Formation strata has many paleocraters, and evidently the older formation took at least a hundred million years or so to be laid down). Whereas, based on the SWEET model, we would expect a more “uniformitarian” pattern of deposition at all levels of Mt. Sharp.
The visible bedding morphology is also very different between the two formations. The Upper Formation beds exhibit strikingly obvious cross-bedding when exposed by erosion, as we see below in high-resolution photos from MRO of the terrace-like or cliff-bench layers of what I call the ”stands.” So the conclusion is that the Upper Formation consists of a stack of lithified dune fields, which is consistent with the fact that they were deposited above the minus-6,000 foot mark, which is believed to represent the high-water mark of the former Northern Ocean (and thus the maximum water level in Gale Crater):
Whereas the beds of the Lower Formation are more-or-less horizontal, are laterally continuous over an extended distance, and lack obvious cross-bedding in most facies (and where cross-bedding does occur, I would interpret it as the product of subaqueous turbidity currents, as I will discuss later), and these are properties consistent with being lacustrine in origin. In fact, such a lacustrine origin is strongly suggested by the placid, orderly character they exhibit when exposed in cross-section by erosion, as we see below in another shot of the area near the Northern Channel. And yet the SWEET authors are claiming that these two radically different morphologies formed through the same mechanism (aeolian deposition) in the same setting.
It is also unclear how the angular unconformity between the Lower and Upper Formations can fit within the SWEET model, which again proposes a relatively steady-state mound-growth process controlled mostly by the structure and dynamics of the Gale Crater complex itself, and not so much by regional and global climactic changes and disruptions that could radically alter, and in fact temporarily reverse, the Gale depositional milieu.
Additionally, the apparent cross-bedding of the Upper Formation beds suggests active sand dunes, and it is difficult to picture how a dune field could occupy the top of a mountain (whereas, in all previous known examples, dune fields occupy horizontal and sub-horizontal terrains), much less how sand would make it up to that elevation…..Martian winds are capable of transporting sand-sized grains, as we see in the active dune fields, but this is horizontal transport, and even on Earth, we don’t normally see such large and heavy particles being transported upslope over extended distances (only fine dust, which does not characteristically form dune-shaped bedforms). Le Deit et al.  have incorporated SWEET within their models, and state that at one stage of the development of Mt. Sharp, ”Remobilized deposits such as sands accumulate at the top of Aeolis Mons forming the (Upper Formation).” However, simply stating that this happened does not make it so, and provides no mechanism to explain how such granules could have been transported en masse to such a location.
Furthermore, cross-bedded dunes imply the presence of fairly strong horizontal winds, which are not only incompatible with the basic model of SWEET (which postulates that the area occupied by Mt. Sharp is a stagnant air zone in which particles fall out of the air and accumulate), but such horizontal winds would also serve to drive any such loose sediment off the top and down the side of the mountain, preventing any such accumulation. But the presence of cross-bedded dunes is perfectly compatible with the model of Gale Crater being totally filled with sediments, where during the deposition of the Upper Formation, the upper exposed surface of the crater fill was a 90-mile-wide “sand sea” consisting of air-fall material (such as volcanic ash) brought in from outside of Gale, as well as material eroded from the crater rim and central peak.
But the single biggest problem with the SWEET hypothesis, is that it is incapable of accounting for the existence of the large channels/canyons on the west side of Mt. Sharp, as there is no conceivable mechanism in their model for generating a flood of water inside, or on top of, what is basically a big sand dune! Heavy rain over an extended period might do the trick, but as we saw earlier, there is not believed to have been rain on Mars during the timeframe in question (we’re more than a billion years too late), and even if there were such rain, it apparently didn’t fall on Mt. Sharp, as the surface of the mountain is sufficiently indurated to have, of necessity, developed tell-tale surface runoff gullies (which it lacks). And as we saw earlier, similar arguments hold against ice/snow-melt being the water source.
A related problem with the SWEET hypothesis is that the authors state that Mt. Sharp is currently in a late-stage erosive regime, but if this is so (and for the sake of argument, we grant the plausibility of the switch from wind-generated deposition to wind erosion that they hypothesize), how then do we account for the delicate, easily-erodible channel features, such as the delta at the end of the Northern Channel? Such features, in their model, should have been worn away long ago…..being shallow surface features and “drape” formations, they are by default the most recent features of Mt. Sharp, and if the mountain were to erode, they would be the first to go…..and in their model they certainly are not positing recent conditions that could have created them (which don’t merely involve small-scale flows of water, but ones sufficient to carve a “Grand Canyon,” transport boulders several meters wide, and form a 2,800-foot-deep lake!).
I won’t delve too deeply into the ongoing debate as to whether the Northern Lowlands were occupied by an ocean at the time that Gale Crater formed, except to state that I find the evidence of paleo-shorelines and of paleo-deltas that debouched into that ocean to be fairly convincing, much as the similar evidence of deltas and topographic benches within Gale demonstrate the existence of the Second Generation Lake (plus, an intriguing line of evidence has recently been brought to light , the apparent discovery of scour marks from oceanic currents in the Northern Lowlands). And if the prior existence of such an ocean is proven beyond any doubt, it would strongly count against the SWEET hypothesis, as Gale Crater would be expected to share a common water level with such an ocean, and certainly not be dry (or nearly dry) as SWEET proposes, since the shoreline of that ocean would either overlap with Gale or else be so close that groundwater movement would certainly fill Gale (even if my proposed artesian hydrant didn’t), as the lowest point of Gale is about 9,000 feet below the putative water level of that ocean.
Even neglecting the oceanic hypothesis, how plausible is it that Gale Crater, with a floor up to 15,300 feet below Mars datum and one of the lowest points on the planet, and with a surrounding topography highly favorable for the formation of a powerful hydraulic head, would be dry or nearly dry (as SWEET hypothesizes), when massive floods on the planet have been sourced from aquifers located at much higher levels, as late as mid to late-Amazonian times (at least 2 billion years after Gale formed)? Surely seepage (even if there was no direct inflow, which seems improbable) would have filled Gale in a timeframe (late Noachian) that has left very extensive records of surface aqueous activity on Mars, when the planet had far more internal heat, and a very thin cryosphere coupled with a near-surface aquifer that surely would have intersected the walls of a crater such as Gale.
Above is the theoretical argument for the existence of a large and deep lake in the early history of Gale Crater, but in addition we have the argument from historical precident: namely, the fact that there is compelling evidence for at least one massive lake in Gale, in the relatively recent geologic past (as shown by the Northern Channel’s delta deposit and other features in the Gale/Mt. Sharp complex), and it is difficult to imagine how, if the hydrological system of the Gale/Mt. Sharp complex were that powerful, at that late a date, that it would have been incapable of generating at least as massive a lake in the distant past, when conditions for generating (and sustaining) such a lake were far more favorable.
The authors of the SWEET study argue that the fact that layers at the outskirts of Mt. Sharp appear to be inclined by several degrees, is evidence in support of their hypothesis, in that their model of wind-generated deposition predicts such an inclination (and they also claim that this inclination is incompatible with any alternative depositional model). See below picture, from Kite et al. :
However, I do not believe that the inclination of the beds supports their SWEET hypothesis, for two reasons. First of all, the measured beds are precisely the ones that are the poorest choice for fitting within their model or lending support to their model, because they are the ones that appear to be (on the basis of their mineralological makeup) water-deposited in a lacustrine environment, not wind-deposited (as SWEET’s proposed mechanism for the creation of all of Mt. Sharp’s layers involves). If the authors are measuring the tilt of beds, they should instead sample ones that are unambiguously wind-deposited (as are the layers above around 9,000 feet from the base of the stratigraphic column). But given the vagaries of slope inherent in wind deposition and cross-bedded dune formation, we are unlikely to have any consistent pattern of bed inclination in the case of aeolian processes, either within the context of SWEET or more traditional models. This is unlike deposition in standing water, where gravity is the primary determinant of bedding-plane formation, and unless other factors are coming into play, results in horizontally-oriented beds….whereas aeolian bedding is almost never horizontal, and can lie at any angle between that and the maximum angle of repose of 30 to 40 degrees. And indeed we see this at work in the bedding planes of the Upper Formation layers, which are tilted by 5 to 10 degrees to the north-northeast, which is inconsistent with the predictions of SWEET (which predicts an approximate 3-degree radially-outward tilting of the mound layers), an observation which has no apparent explanation within the context of their hypothesis, and in fact constitutes an additional stratigraphic anomaly with respect to SWEET. So it would appear that, in principle, Mt. Sharp is incapable of providing any beds that have the potential of lending support to their hypothesis.
The second reason I do not believe the measured inclination of the beds supports the SWEET hypothesis, is that the authors have overlooked an important factor that likely explains the tilting of the Lower Formation layers: namely, the presence of a central peak “island” in the middle of the lake, a geographic feature that appears to have been far larger than conventional models of central peaks (based primarily on Lunar examples) would predict. Our indication that this is the case, is the fact that what appears to be the central peak, is jutting out the top of Mt. Sharp to a height of 3.4 miles from the crater floor, and exceeding the height of most portions of the crater rim by several thousand feet:
There is an obvious anomaly here, because this is several times higher than we would expect on the basis of other craters of similar size , and so there is definitely something unusual about the central structure of Gale Crater. Gale may be a variation of “complex crater” that features not only a central peak, but associated uplift features in the central area of the crater floor, and in fact the central peak may be just the tip of the iceberg, representing the high point of a surprisingly broad and massive central uplift, or possibly a central peak surrounded by a raised ring or rings, and which in turn serve as the core of Mt. Sharp, with the sedimentary deposits constituting a relatively thin veneer on this basement (at least near the outer edges of Mt. Sharp). And I would suggest that the structure of Gale may resemble that of Utah’s Upheaval Dome, which features concentric uplift rings around the (highly eroded) central peak. Uplift Dome was previously believed to be a salt dome, but it has now been established as an impact feature, thanks to the discovery of shocked quartz and impact melt (photo courtesy of Earth Impact Database, University of New Brunswick):
I would suggest that the difference between Upheaval Dome and Gale Crater, on the one hand, and “classic” Lunar craters on the other, may be that the former two involved impacts in a planetary crust with an active and saturated aquifer, rather than a “dry” crust like the Moon’s, and the large amount of water resulted in extensive hydraulic fracturing, and uplift due to steam explosions. And this is supported by the fact that Upheaval Dome is only 3 miles across, a size that would be insufficient to generate a multi-ringed “complex” crater in a Lunar setting.
In addition, at least in the case of Gale, the large amount of thermal energy absorbed by the vaporizing water may have allowed more of the rock surrounding the central peak area to remain solid rather than melting to form a magma lake, which could contribute to a high and broad central uplift. Obviously, further study of the geophysics of crater formation is in order, to define how radical differences in crustal compositions and water content affect the resulting impact structures, but of course there is a relative scarcity of potential examples to study on the Earth due to the high erosion rate.
In any case, if there is a major uplift in the center of Gale Crater (as suggested by the height of the central peak), we may expect that the sedimentary beds would have been laid down on a surface that was sloping towards the outer portion of the crater, and so it is natural that we would see the bed inclinations that we do. Deposition in a shallow playa lake will of necessity result in horizontal beds, but if deposition occurs subaqueously in a deep lake that has a gently inclined floor, the resultant sedimentary beds will share that same inclination.
Also, if the central peak and uplift constituted a large (both high and broad) island rising out of the Gale Crater lake, we may expect it to have provided considerable eroded material to add to the sediments, and we may expect this to result in a thicker section of such sediments near the shoreline of the island, with the thickness of the beds gradually pinching out towards the center of the lake’s waterway, generating the illusion of greater bed inclination than is actually present (but which in itself may be considerable, due to a radially-directed outward tilt of the uplifted basement).
Plus, the presence of the central peak and island could have served as a catch point for wind-blown sediments, whereby the physical barrier to airflow would have caused a local drop in windspeed, in turn causing the winds to dump much of their load, and thus providing additional sediment to the central area of the crater lake.
In addition, if the waterway between the island and the crater rim was sufficiently wide, and the wind speed sufficiently high, we may have had wind-generated waves, which would have served to push sediment up against the shoreline, tending to produce a wedge-shaped depositional cross-section for near-shore beds. And an interesting feature of waves when encountering an island, is that they tend to refract around both sides, so we may expect a consistent pattern of onshore waves on all sides of such an island, regardless of the initial wind and wave direction (but of course, in such a mileau we may also expect appreciable sediment movement in the reverse direction, due to turbidites carrying material downslope from the island’s shoreline).
So as with any shoreline, we may anticipate an appreciable slope (both upwards from the lake’s surface onto the dry land of the island, and downward below the water’s surface, much like a continental slope in miniature), with both basement and overlying depositional factors playing their parts, and we may naturally expect these factors to impart an angled slope to the bedding as we progress away from the island, towards the center of the waterway.
It is also possible that the initial central peak may have augmented itself by serving as a hydrothermal vent, since the residual underground heat of impact is believed to persist for hundreds of thousands of years in the case of large impacts, and this heat could yield mineral-rich brines which would be discharged from the surface and deposit their load, building up “spring mounds.” This process is believed to have occurred on a more limited scale in Mars’ Vernal Crater, and occurs on Earth in volcanic settings, and if it occurred in Gale, would also be expected to have added to the slope of the bedding, both directly and indirectly (Rossi et al.  proposed that the entire mound in Gale crater is a spring mound, but this is unlikely based on the lateral extent and horizontal continuity of the beds making up Mt. Sharp, which instead suggest deposition in a crater-spanning lake, although a spring mound may lie underneath the visible bedding of Mt. Sharp, and of course my hypothesis also involves springs, but ones with a sufficiently large discharge volume to fill the crater with water, and not simply provide evaporate deposits).
Simply extrapolating the downward slope of the visible central peak (and ignoring the possibility of a broader uplift and/or concentric rings) results in a projected island with a width of at least 20 miles (over a quarter of the bowl width of the crater), whereas in the model presented by the SWEET authors, this feature is totally ignored, and their diagrammatic cross-sections of Gale and the formation of Mt. Sharp show the central peak as a tiny, short, insignificant feature, a small fraction of the height of the crater rim and of the actual central peak height and width (and so the SWEET model actually misrepresents the structure of Gale Crater). Whereas I would argue that this feature needs to be factored into any model, and that it is highly unlikely to be a coincidence that this anomalous central peak size and structure is paired with a unusual central mound of sediments. The linkage, I am postulating, being that the enhanced central structure of the crater is the result of an impact in an active aquifer, and the central structure that was so produced, became an outlet for that very same aquifer.
In the catalog of Martian craters, Gale is of a size that is transitional between craters typically hosting only a “simple” central peak, and ones with a outer peak ring approximately half the diameter of the crater, and if it does host such a ring, it may well define the curved northern edge of the Mt. Sharp sedimentary mound, and account for the radially-outward tilt of the beds at that edge. And as pointed out by Allen, Dapremont, and Oehler , projecting such a hypothetical ring around the entire circumference of gale (see figure below), shows an alignment with an arcuate line of hills in the southeast quadrant of the crater (and, I might point out, in the SW quadrant as well, to a lesser extent), which may be outcrops of just such a ring:
In fact, there is another piece of evidence that supports a model of there being a central uplift in Gale almost as wide as Mt. Sharp itself: to the NW and SE of the central peak, there are chaotic zones of large fractured blocks (please refer to the earlier plan photo of the central peak), somewhat of the character of shorter secondary peaks flanking the main central peak, and which have been interpreted as “megabreccia” (breccia consisting of kilometer-scale blocks) that are a remnant from the original formation of Gale Crater , when a fractured basaltic basement upwelled to form the central peak complex (but did not melt, although it was surrounded by a lake of impact melt which solidified to form the crater floor). And if this is the case, then here we see a sampling of an uplifted surface that is considerably larger than even the central peak. In the case of the better-exposed elevated region, the one to the SE of the central peak, the outer visible face experiences an elevation drop of about 3,000 feet over a linear distance of 18,000 feet (with a starting point 9,000 feet above the northern floor of Gale Crater, located 45,000 feet east-southeast from the summit of the central peak), and this surface slopes radially outward from the central peak, at an angle of about 9 degrees. And if this slope is characteristic of the broader basement beneath Mt. Sharp, projecting this angle yields an uplifted basement approximately 35 miles wide, the best part of the width of Mt. Sharp itself (which measures about 55 miles wide). And since, in most cases, mountains and other uplifts decrease in their slope towards their outer margins, it is quite possible that this uplift is nearly as wide as Mt. Sharp, tapering down to a slope of just several degrees in its outer portions, in which case it could be adequate in and of itself to explain the several-degree radially-outward inclination of the beds near the edge of Mt. Sharp.
The pattern of convective, katabatic winds in Gale Crater that the SWEET authors hypothesize may indeed occur, and be responsible for the crater floor being scoured and eroded over time, rather than steadily accumulating sediment (especially in the “Yellowknife Bay” area and in the broader basin or “moat” directly to the north and NW of Mt. Sharp), and it may also account for the layer of dust that covers the upper reaches of Mt. Sharp (whereby fine sediments picked up from the crater floor are carried upslope and deposited on the mountain), but it cannot account for the formation of the mountain’s stratigraphic sequence, and the empirical observation it claims as evidence in support of itself (the inclination of the lower beds) could just as readily be explained in the context of a competing model.
The greatest usefulness of the SWEET hypothesis lies (ironically enough) in the fact that it has provided a wind-generated mechanism for explaining how the outlying portions of an original crater-filling mass of sediments could have been eroded away, which heretofore was a major difficulty for the older (and competing) model that Gale was originally filled with a crater-spanning lake that deposited crater-spanning and crater-filling lacustrine sediments. Specifically, it provides a mechanism for eroding a basin to an ever-deeper level (in this case, a large paleo-crater to a point near its original floor), rather than the processes of erosion (and subsequent deposition) filling in a basin (which is the more typical outcome).
In itself, my proposed explanation for the erosion of the outer portions of the Upper Formation (the fact that they were never lithified, due to being physically removed from the central peak’s source of lithifying water), does not fully account for the deep erosion experienced by the outer portions of the Lower Formation. Although being exposed to the wind after losing their protective cap would have caused the lower formation layers to be incised, there is prima facie no accounting for the depth of their ultimate deflation (to a point one mile below the grade of the adjacent landscape outside the crater rim).
However, once the outer portions of the Upper Formation had been removed, we can expect an erosive convection cell to be set up, basically in line with the one described in the SWEET hypothesis, as we would have a ring-shaped depression surrounding a central mountain, and we may expect daytime heating to cause katabatic winds to rise from the lower-elevation region, carrying entrained sediment, and if upper-level winds were to shear off the top of such a convection cell, there would be a net removal of material from the floor of the crater (and from the Gale crater complex more generally), which would likely produce very substantial deflation over geologic time intervals.
Below we have a figure from NASA showing the wind patterns they predict for Gale Crater, and they are similar to the ones proposed by the SWEET hypothesis. Although they are not identical to SWEET, in that they show daytime katabatic winds RISING against the inner crater wall (rather than descending), plus they show lateral movement of wind over the surface of the crater floor, which would present a highly enhanced erosive potential for the crater floor. But in common with the SWEET hypothesis, they show winds rising over the slope of Mt. Sharp, and if these winds hold entrained sediment that has been eroded from the floor of the crater (which is empirically supported by the observation that the flank of Mt. Sharp has been subject to intense ongoing wind erosion, as demonstrated by the dearth of recent small impact craters and the high prevalence of yardangs, which implies in turn that the upslope winds are strong and have a high level of entrained erosive particulate matter), then there would be a net removal of material from Gale Crater over time, if these upslope winds typically encounter a horizontal wind shear after cresting Mt. Sharp.
And such a mechanism, once it was initiated, would begin a positive feedback loop that would increase the strength of the erosive wind regime over time, as the floor of the crater would be progressively deflated and the differential elevation increased (and thus the differential diurnal heating, and the convective wind strength). Plus, these phenomena were primarily occurring in a time when the Martian atmosphere was much denser than today, and thus potentially far more erosive.
So I conclude that we have a readily-available mechanism to explain the erosion of sediments (originally filling Gale Crater) to a point well below grade, a mechanism (inadvertently) proposed in large part by the competing SWEET hypothesis, where SWEET was originally fielded in an attempt to provide an alternative explanation for Mt. Sharp’s genesis, in the face of what originally seemed to be intractable difficulties with the more traditional crater-filling lacustrine model of sedimentation. Which is a rather interesting example of the old philosophical notion of “delectical reasoning,” whereby the original idea (the “thesis”) leads to its “antithesis,” which are subsequently combined into a new “synthesis.”
Besides the tilted beds (for which we have alternative explanations) and the erosion-below-grade problem (also apparently explained), the other seemingly intractable problem with the traditional model of crater-filling sedimentation, was the survival of the center of that sedimentary mass (in the form of Mt. Sharp), which I have explained by means of my artesian hydrant model (and which, as a side benefit, also neatly accounts for the geologically-recent canyons eroded in the flank of Mt. Sharp, as well as the existence of a Second-Generation Lake).
There are also some examples on Mars of filled craters which are at an initial to intermediate stage of exhumation, and which demonstrate that the developmental sequence that I am claiming for Gale does in fact occur on Mars, and by implication there is no need for invoking an ad hoc process to explain Mt. Sharp, such as SWEET’s formation of a giant sand dune in the center of a crater. The prime example is Asimov, a 52-mile-diameter crater located at 47 degrees south latitude and 355 degrees west longitude, which was obviously filled to at least the crater rim, but which is now being exhumed, mostly along the outer edge of the bowl, and if it follows a path like I believe Gale followed, it should eventually be left with an isolated mound in the center (image courtesy NASA/USGS):
Gale likely looked similar to this in the mid Hesparian era, shortly after the “Great Erosive Event” began, forming pits along the inside of the crater rim in the less-lithified outlying sediments, with those pits progressively enlarging and deepening until they formed a continuous ring, and with the convective wind pattern set up by that topography serving to feed the further development of erosive winds, in an ever-accelerating process. And in fact Gale’s ring is still expanding to this day, with Mt. Sharp growing ever smaller, although the rate of erosion has greatly diminished due to the thinning of the Martian atmosphere over billions of years.