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geology of Gale Crater and Mount Sharp
serpens
post Jul 2 2014, 11:17 PM
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A slight aside. I wonder at the validity of using high obliquity ice events to explain the channels and other features. Taking an extreme case of say 80% obliquity, Gale being equatorial will still have the sun directly overhead twice a year, and each pole will receive maximum insolation once a year when the other pole is dark as the tropic are positioned close to the poles. There would be transfer of significant amounts of CO2 and some entrained water vapour between poles and some increase in air pressure during transfer as is evidenced in the current environment. But how great a rise at the equator and how much ice could reasonably be expected to deposit, albeit for a short term is uncertain although there would probably be increased Aeolian activity. I expect that there are probably a number of models relating to this, but models of Earths climate are somewhat unreliable despite the wealth of empirical data available so perhaps models of Martian climate should be treated as indicative, not absolute. The point I am trying to make is that I really do not think that glacial activity, or even significant deposition of ice would feature at Gale, regardless of changing obliquity although early on when Mars had a denser atmosphere, high obliquitys would have been extremely energetic periods.
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Gerald
post Jul 3 2014, 12:20 AM
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That's why I would prefer to wait for more reliable data. There are hypotheses ranging from a mild wet early Mars to Kite's snowball. May be both is true in hundreds of millions of years. A one degree change of obliquity on Earth is thought to cause ice ages. What would happen on a 10 degree change? A larger ice shield at the poles may lead to a run-away cooling due to higher albedo. We don't know whether there have been oceans on Mars, and if so, when, and to what extend? What's the consequence for the humidity of the atmosphere, and the amount of snowfall? That's simply too complicated to estimate, at least for me. I'd think, that climate changes on Mars have been much more severe, and more frequent than on Earth, and Mars should be cooler than Earth on average due to the larger distance from the Sun. But if there had been much CO2 or even CH4 in the Martian atmosphere, there might have been a strong greenhouse effect. On the other hand, SO2 resp. elementary sulfur in the atmosphere (due to volcanism) would probably lead to additional cooling.
We also don't know the vertical temperature profile of early Mars. High mountains, like e.g. Kibo (Kilimanjaro) on Earth (still) have an ice cap despite their proximity to the equator. Without additional information, I'd apply this observation also to early Mars, and consider the fainter Sun.
With all these uncertainties I can't at least rule out temporary ice caps on Mt Sharp as a valid hypothesis, on the current thin basis of knowledge.
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David Palmer
post Jul 3 2014, 02:48 AM
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Reply to Gerald:

No, I did mean what I said, that groundwater would be enriched in heavy hydrogen and oxygen isotopes, relative to atmospheric precipitation (and thus any residual bound water in minerals and sediments that interacted with this ground water, such as ones eroded and transported by spring water, would show such an enrichment).

Any atmospheric precipitation will be biased towards light isotopes, because lighter molecules are slightly more apt to evaporate. This effect becomes significant in some environments on Earth....in the polar regions, deuterium concentrations in snow can be 40% lower than in sea water. And the same is true for groundwater vs. atmospheric moisture....groundwater will have a relative over-abundance of heavy isotopes, compared with water that has evaporated and subsequently precipitated.

For example, see http://www.whoi.edu/cms/files/kcasciotti/2...t1996_14124.pdf

You are quite right in saying that a lacustrine origin for the Lower Formation would not prove my hypothesis....but it is a necessary condition for that hypothesis to be valid (a necessary but not sufficient condition, to use terminology from logic).

The first condition lays the groundwork for the possibility of my hypothesis being right, but the later two conditions are more directly a test of it (low cosmic ray exposure age, and chemistry and isotopic ratios characteristic of groundwater involvement).

You state that a low cosmic ray exposure age could also be the result of rapid erosion (of very ancient materials), and not signify recent deposition. However, my point is that IF rapid erosion of a relatively thin formation is occurring (several hundred feet thick, in the case of the channel fill), that observation suggests that the strata in question is fairly young (I am claiming less that 100 million years old), because given such a rapid erosion rate, any such thin (surface) deposit would otherwise be long gone. The only alternative would be that a (now vanished) overlying stratum was protecting the channel fill until recent geologic times, but this would seem an unlikely coincidence, especially considering that (per the law of superposition) the channel fill is one of the most recent deposits in the Gale/Mt. Sharp complex (it overlies the mound-skirting unit, which in turn embays the Peace Vallis fan deposits).

So it can be seen that my "artesian hydrant" hypothesis is a scientific hypothesis in the truest sense of the word, with very specific, quantitative, field-testable predictions (another of which, unfortunately, may have to wait till the distant future to be tested....namely my prediction that there will be underground conduits in Mt. Sharp, to be mapped out by high-definition ground-penetrating radar or seismic studies).

Dave
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David Palmer
post Jul 3 2014, 03:36 AM
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Reply to dialogue between Serpens and Gerald:

I don't agree with Kite's model for the origin of the Mt. Sharp sedimentary column, however I do recognize his contribution to climatic modeling for Mars, such as in: ”Seasonal melting and the formation of sedimentary rocks on Mars, with predictions for the Gale Crater mound.” ¯ Icarus 223 (2013) 181-210.
http://gps.caltech.edu/~kite/doc/seasonal_...ntary_rocks.pdf

In that article, he presents the results of models that suggest that Gale would be one of the most likely places on Mars to experience liquid water flow, as a result of obliquity-driven precipitation and subsequent ice/snow melt. And I figure that the proof is in the pudding: the entire Gale/Mt. Sharp complex (the mountain, the crater floor, and the crater rim) shows unmistakable signs of repeated aqueous events (as well as glacial and periglacial activity) over a very long time, and the natural interpretation is that these events were obliquity-driven, although volcanism also would be expected to play a major role (during periods of intense volcanism, much carbon dioxide and water vapor would be injected into the atmosphere, and high obliquities correlated with volcanic activity should result in an especially favorable mileau for Gale Crater).

I also agree with much of Kite's aeolian model for Gale Crater (I attribute the deflation of the outer portions of the crater fill to such a wind regime).....where I break with him, is in the temporal assignation of the climactic regimes he defines: his description is probably a good one for the latter half of the Hesparian and for the entirety of the Amazonian, but the early history of Gale (and of Mars in general) was FAR wetter than his model suggests (and that's the environment in which the majority of the Mt. Sharp sedimentary column was laid down).

Dave
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serpens
post Jul 3 2014, 05:09 AM
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Apply Kite's model to the massive, upper stage of Mount Sharps formation and it fits very well indeed. It would probably be best if you couched your statements as beliefs as opposed to facts. For example you may believe that "the entire Gale/Mt. Sharp complex (the mountain, the crater floor, and the crater rim) shows unmistakable signs of repeated aqueous events (as well as glacial and periglacial activity) over a very long time". There is no wide agreement on that, nor definitive evidence, so the statement is your interpretation, not a fact. You have a deep belief in your high pressure aquifer release hypothesis as the cause of the channels but again, that is a personal belief rather than a fact. If you want to explain Gale topography as the product of a wide area, high pressure aquifer then you need to marshal some defensible arguments in the context of the region. Try considering the possibility that the volcanic activity /uplift involved an ambulatory process and think about the flooding effect and ocean rise if the volcanic activity was coincident with an existing ocean. Pointing to Cerberus Fossae and saying here be the aquifer is drawing an exceedingly long bow. Explain why the release occurred in the central mountain and not the equally fractured crater walls or the low terrain to the North.
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David Palmer
post Jul 3 2014, 12:11 PM
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Reply to Serpens:

There is quite a lot of ground to cover here.

I disagree with your interpretation of the headwater of the Grand Canyon....you state that it doesn't narrow prior to disappearing at the bottom edge of the Upper Formation (the implication being that if it were not in unconformable relation with the Upper Formation and buried by that formation, it should pinch out to a narrow gully prior to disappearing)....but I believe that the reason for this is that the visible channel never was the conduit for water that proceeded to flow down the Grand Canyon. Such a conclusion is suggested by the fact that this "headwater channel" is too small a channel (it has too small a cross-sectional area) to have contained the huge flow required to cut the Grand Canyon, move ten-meter-wide boulders, and deposit the delta at the terminus of that canyon (both its width and depth are a small fraction of the dimensions of the lower canyon). We observe that the channel suddenly changes to a canyon at approximately the 7-mile mark, and this canyon proceeds to widen and deepen as we move downstream, but WITHOUT tributaries that could explain the corresponding flow increase and erosive capability.

I would suggest the following as an explanation for this apparent paradox (that the channel had a high flow rate without a headwater or tributaries to support it): although there may have been some flow in the visible headwater channel, what we are interpreting as a channel is primarily a subsidence feature on top of a largely hollowed-out subsurface (which is where most of the flow took place). This is a pattern we see fairly often in Earthly settings, where there is a sudden transition from shallow to deep runoff channel, involving flow through fissures or porosity of the rock beneath what we see as the visible channel, until the bulk of that flow transitions to being surface flow, from springs and general seepage. And I would suggest that we are seeing the first of these (dormant) springs where the channel suddenly deepens (at the 7-mile mark), and that there were additional springs in the canyon floor and walls, augmenting the total flow, as we moved downhill.

Support for this model is provided by the occurrence of numerous box canyons in the western lobe of Mt. Sharp, apparently brought about by groundwater sapping from springs, with stubby heads that have been eroded backwards into the mountain (but with no signs of being fed by surface flow), and with termini featuring debris lobes. The "Grand Canyon" simply happens to be the largest of these, and it appears that Mt. Sharp is highly fractured, featuring dormant springs that can become active again under the proper conditions.

The fact that the headwater channel doesn't narrow prior to disappearing into the hill, is consistent with the channel being a collapse zone, wherein the rise in the lay of the land as we move further east, eventually creates enough of a roof arch above the subsurface flow to prevent subsidence.

As I have previously stated, if the Grand Canyon were the product of overland flow from a substantial distance (across a hypothetical tableland in the eastern portion of Gale), then it would be a deep, wide, mature channel by the time that reached the spot where we see the "headwater" channel, and the Grand Canyon would not be expected to begin with such morphology.

Also I find the concept of a tableland in the eastern portion of Gale to be suspect, as there is no obvious way to account for its subsequent erosion, while at the same time the western face of Mt. Sharp remained comparatively unmolested (such that channels and thin delta deposits survived subsequent geologic history).

I do not believe that SWEET is able to account for the Upper Formation, the most serious flaw being the extensive cross-bedding (see photo), which could not be the result of the predicted winds, but only of horizontal winds, and in any case, a dune field could not be expected to form on top of a mountain, and if it managed to, said horizontal winds would quickly send it packing downslope.

Attached Image


I do not consider Cerberus Fossae irrelevant for understanding Gale. Those outflows show that Mars still has an active aquifer that is under pressure (and one which is capable of reaching the surface), and there is no reason to believe there is any relevant subsurface barrier across the Elysium plains between the two locales, so they should in fact share the same aquifer.

To see how water has repeatedly left its mark on the Gale/Mt. Sharp complex, all that is needed is to look at photos of the crater floor (showing multiple generations of fans and channels stacked on top of each other) and photos of the crater rim, showing numerous stream-carved valleys and canyons that were obviously not created by some huge catastrophic Noah-type flood, but over an extended time from innumerable individual events, likely in association with obliquity changes and also volcanic and possibly asteroid-collision events, with the net result being a landscape that looks very much like the Southwest desert terrain of the US (which has also been subject to periodic fluvial events). This is not just an opinion on my part.

I consider it highly unlikely that the foothills of Mt. Sharp (and the channels) have had a protective aeolian cover on them that has recently been stripped away, because of the immense damage that such a stripping action would also have delivered to the underlying surface, given that an extension of the Upper Formation would likely require violent stripping action to remove, considering how well-lithified the Upper Formation appears to be.....likely to a comparable degree as the Lower Formation (the implication being that the surface on the lower flank of Mt. Sharp is young, with the channels most recently active several tens of millions of years ago). You stated that we can't really tell the relative state of lithification until Curiosity reaches the deposits, but unfortunately Curiosity will never be able to drive up to the Upper Formation, so we have to make do with HiRISE imagery, which indicates that the Upper Formation is fairly erosion-resistant. And if indeed it is an outlier of the Medusa Fossae formation, as seems likely, the upper mound must be well-lithified indeed, to have survived an erosional environment that has removed all other local exposures of that formation, including at lower levels....and that fact fits very well within my artesian hydrant model, in that water would have been available to provide a degree of lithification that was highly atypical for the Medusa Fossae.

You stated that it is implausible that aquifer outflows would occur in Mt. Sharp but not in the "equally fractured" crater wall, or in the low terrain to the north. I would suggest that aquicludes play the same role in the Northern Plains as they do in the floor of Gale. And as regards the crater rim/wall, I don't see any reason to think that fractures would be deep enough at those locations to reach the aquifer, which would be separated from the surface by at least a mile of basalt and a very solid cryosphere (whereas the fractures would be expected to reach very deeply indeed under the center of the crater). Although I can't vouch for its detailed accuracy, the included figure (courtesy of NASA) shows how there would be deep fractures beneath Gale, but not necessarily in the crater rim/wall.

Attached Image


The reason that I robustly support my Artesian Hydrant hypothesis, is that it seems to account for all empirical observations of the Gale/Mt. Sharp complex, and is also internally coherent logically, whereas there are serious difficulties with all other models I have seen, either in terms of their basic theory or in terms of meshing with the data.

Dave
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Astro0
post Jul 3 2014, 01:31 PM
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ADMIN NOTE:
Sorry everyone, I believe that this topic may have run its course.
There is no discussion here, only contribution and refusal to hear those contributions.

The Admin/Mod team have been more than patient and we will now discuss whether this topic gets re-opened.

Contributors to this thread and other members are welcome to send me a direct message and add comment on the course of this discussion or provide valid reasons as to why this topic should continue.


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