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This topic has been discussed here over a spread of time and in various threads but not recently, I think. This article might be a good way to kick off some more discussion from a present day perspective if anyone's interested:
Agree the article concerning a variety of construct mechanisms for Martian river valleys might apply to 'river' valleys on other bodies with liquid on their surface at some point in their evolution including Earth and Titan. Might suggest the author next apply their comparative analysis to Titan's variety of 'river' valleys.
That is another interesting hypothesis on the formation of drainage networks on Mars but as with all such it should be considered hypothetical rather than definitive. In fact the abstract in Nature states that the study modelling revealed that valley formation involved fluvial, groundwater sapping, glacial and sub glacial erosion with fluvial and subglacial the predominant mechanisms. This is a different picture to that cast by the Science Daily article.

I would make a few observations. In the main the distribution of valley networks does not match the distribution of evidence of glacial activity. The valley networks are distributed in the equatorial and mid latitude range. In the Southern highlands the glacial remnants are generally south of the valley networks and above the equator are to the further north, primarily along the dichotomy (Deuteronius, Prontonilus and Nilosyrtis Mensae). Interestingly, to the South of the Tharsis rise the valley networks are closer to the pole with glacial remnants to their North, closer to the equator, supporting the idea that the development of the Tharsis rise caused tectonic dislocation.

Another point is that the dating of the drainage networks indicates that they were formed over a span of some 200 million years at the boundary between the Noachian and Hesperian periods, pretty much when Curiosity's ground truth has confirmed a warmer wet environment. As with Nick Hoffman's white Mars hypothesis and dburt's impact gardening hypothesis, concepts that look compelling from analysis of orbital data are revealed erroneous with analysis on the ground.
Where we see modern evidence of glacial activity doesn't tell us much about where glaciers were located in ancient Mars. It's worth pointing out that the climate models (depending on how much stock you put into them), consistently show the highlands to be a cold trap, and that the largest cold traps are associated with the highest densities of valley networks. Even in a relatively warm and wet environment, scattered ice caps or ice sheets probably existed scattered across the highlands. We wouldn't necessarily see evidence of them, since most unambiguous glacial landforms involve either the direct presence of ice or sediments deposited by a glacier. Bear in mind that we see substantial erosion of sedimentary rocks and deposits all over the Martian surface, so unless there is unusually high-quality preservation of these landforms we're unlikely to see direct evidence of a Noachian-Hesperian ice sheet from modern topography. If those ice sheets existed, the clearest remaining evidence of them would likely be cirques and glacial valleys cut into the high-standing hardrock massifs of the highlands. Even those wouldn't be unambiguous evidence, since they'd likely be modified by fluvial and aeolian erosion in warmer/dryer epoch.

I'd also caution about extrapolating global conditions from Gale Crater. The modern rock surface is ~4 km below datum, while most valley networks are located between 1 and 3 km above datum. So the Gale Crater basin would probably be recording a much warmer environment than the valley networks. Additionally, the Gale Crater lake (estimated at 3.5 - 3.2 Ga) appears to substantially post-date the Noachian-Hesperian boundary (~3.8 Ga), and so records a slightly different slice of Martian history. So while there's ground truth for a warm, wet environment at Gale Crater, that doesn't necessarily mean that the environment the valley networks formed in was as well.

EDIT: My main research area so far has been to look at intercrater basins in the Noachian highlands, which appear to host sedimentary deposits. I have a couple of HiWish requests awaiting fulfillment targeting where the valley networks open into these basins. There is one region with a weird, speckled rock at CTX resolution that I suspect might be a tillite. This was recently picked up and I'll share it once it's in the PDS (probably next month's release).
QUOTE (serpens @ Aug 6 2020, 06:48 PM) *
...As with Nick Hoffman's white Mars hypothesis and dburt's impact gardening hypothesis, concepts that look compelling from analysis of orbital data are revealed erroneous with analysis on the ground...

I'd be curious about where you came up with the unusual term "impact gardening hypothesis," insmuch as I've never heard it used before. Nick Hoffman (on his web site about 20 years ago), based on available orbital data, and my coworkers and I about 15 years ago, based entirely on later rover "analysis on the ground," to use your term, independently came up with the hypothesis that many of the layered rocks seen by the orbiters and later by the three rovers on Mars could have been deposited by impactoclastic density currents (could be IDC deposits, in other words), analogous to the well-understood PDC deposits deposited by pyroclastic density currents on Earth (and also "base surge" deposits deposited around nuclear blast sites on Earth). In other words, they could be distal layered blast beds, resulting from large blasts on a planet with an atmosphere and abundant subsurface volatiles (unlike the Moon). Therefore, to what compelling "analyses on the ground" are you specifically referring? BTW, I gave a talk on this subject earlier today at a scientific meeting on impact cratering where "cold weather" (mainly on Mars) vs. "warm weather" (on Earth) glaciation was also discussed.

I don't recall if you ever participated, but the subject of impact sedimentation was excessively discussed on this web site about 15 years ago, when my coworkers and I first proposed it, and it was never refuted, despite your possible recollections. I see no need to repeat this discussion now, despite the considerable additional supporting data available, and I'm sure the site moderators would agree. Best wishes.
I was wondering if that would demand your druthers... dburt, to say nothing of the contentious issues, I will say it heartens me to see that you are still reading this site on occasion. Not for the contentiousness, but I enjoy the food for thought.
Thanks to all for the insightful comments. I agree that the history and geography of Mars are both extensive enough to accommodate many different climatic conditions and a wide variety of valley-forming processes. The Science Daily article says as much despite its icy headline. I would make just two general observations that this work would seem to support:
1/ Subglacial water flow could have persisted through a wider range of planetary conditions than surface rivers. This would include cold periods with low atmospheric pressure.
2/ There is likely to have been much more ice on Mars in the past than there is now since it is inexorably depleted by evaporation/sublimation and atmospheric leakage. It is therefore reasonable to imagine that it was a major feature of the landscape in ancient times.
dburt, I've seen 'impact gardening' used to describe your hypotheses when reading over some of the past contributions on this and other forums, but I am not sure where. For some reason it struck a chord and no disrespect was intended. I don't think anyone would argue that impact effects, both base surge and ejecta did not contribute significantly to stratification, as did volcanism, aeolian, fluvial and lacustrine influences. As far as I am aware impact effects have been considered in analysis of rover investigations since the beginning. For example Frakik, Grotzinger and Edgar "Potential Recognition of Accretionary Lapilli in Distal Impact Deposits on Mars: A Facies Analog Provided by the 1.85 Ga Sudbury Impact Deposit". If I may quote from the abstract to the paper "While no impact-induced base surge deposits have been confirmed on Mars, it is likely that they will one day be discovered, and it is important to establish criteria for their recognition in the rock record". Personally I find the evidence for fluvial, deltaic and lacustrine deposition in Gale Crater compelling. The forthcoming contribution from Perseverance will no doubt provide another piece of the jigsaw puzzle.
Impact gardening is a well-understood and well-defined process of erosion by micrometeorites on airless worlds like the Moon. It is not a hypothesis, has no relevance to planets with an atmosphere, like early Mars, and certainly has no relevance to density currents. I was therefore astounded to see you use it so out of context, and would be equally astounded if your recollection of others using it was correct.

Regarding the article you cited, a glance at the author list told me basically all I need to know about it. It is an "apples to oranges" comparisons of impacts on Mars to the Sudbury impact which happened in shallow sea water (as did Chicxulub, that ended the age of dinosaurs). Land deposits by Sudbury have long since been eroded, as have virtually all marine deposits, save for a few widely scattered basal breccia deposits (possible tsunami deposits). A slightly less biased assessment is Calvin, W. M. et al., 2008, Hematite spherules at Meridiani: Results from MI, Mini-TES, and Pancam: Journal of Geophysical Research, v. 113, p. E12S37 which concludes (in its abstract): "no single terrestrial analog provides a model that can account for all attributes of the spherules on Mars". More recently, actual Mars concretions are described in detail by Sun, V.Z. et al., 2018, Late-stage diagenetic concretions in the Murray Formation, Gale crater, Mars: Icarus, 321:866-890 (you didn't recognize these on 06/09/17 because they weren't spherical enough). I noted that the article made no mention of or comparison with Meridiani spherules (or the very similar spherules at Gusev). Also it didn't mention the unusual detrital blueberries (actual spherules) in Gale discovered by members of this forum.

Personally I don't find the much abused Mars phrase "consistent with" very compelling, but that is my own preference as a practicing scientist. And I also am looking forward to observations by Perseverence, especially after I observed the large, young impact crater Hargraves not far away (I once co-authored a paper on paleomagnetism with Bob Hargraves). Apologies to the moderators for being so didactic; that is the professor in me, I guess.
While I have well and truly achieved my three score and ten my memory has not been impaired so far and I assure you that my recollection is correct. I admit that my use was wrong. I had never actually encountered the phrase in its correct use.

".... a glance at the author list told me basically all I need to know about it". That statement however left me a little perplexed.
Gonna ask everyone to go read rules 2.1, 2.6, and 2.7 before commenting further, and if needed go back and edit their posts as applicable.

Right now I'm just asking.
In case it wasn't obvious, I meant that it would be as improbable for that particular set of coauthors to decide that the Meridiani spherules were not concretions as it would be for me and my coauthors to suddenly decide that they were. Just our human nature, with no disrespect. To the moderators, neither of us appear to be new posters, but I apologize if a wayward, ill-informed comment by one of us seems to have led the discussion astray. Please remove or move those irrelevant, uninformative portions of the discussion if appropriate (rule 2.5). Incidentally, Nick Hoffman, for whom I have the greatest respect, used to post here too, but I doubt that he still follows the site.
Thanks for posting that link. It's pleasing to find that the full PDF is freely available.
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