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ngunn
Mars discussion is a bit thin at the moment due to the conjunction but I could not let this pass without thanking Emily for yet another superb article elucidating a highly complex topic.

http://www.planetary.org/blogs/emily-lakda...hypothesis.html

The idea that the last liquid water on an increasingly airless Mars would have worked its geological magic under a covering of snow makes a lot of sense.
Phil Stooke
wrong thread? But a great blog post by Emily. Note that there have been other suggestions about water activity under snow blankets for several years. As far as I can remember they started with Christiansen and colleagues suggesting the crater wall gullies formed under melting snow.

Phil

mhoward
Moved to the Mars forum.
mcaplinger
QUOTE (ngunn @ Apr 19 2013, 01:05 AM) *
The idea that the last liquid water on an increasingly airless Mars would have worked its geological magic under a covering of snow makes a lot of sense.

The paper is well worth reading, but as it says in section 6.2, it really doesn't address the valley networks and alluvial fans. (It tries, but doesn't completely succeed to my taste.)

As a general rule, explaining Earthlike morphology with non-Earthlike processes needs a high level of motivation. ("If it looks like a duck", etc.)
IMHO, section 1 oversells that motivation in the case of early Mars, though I need to read Haberle 1998, which I hadn't recalled was so definitive. But as a jumping-off point for discussion, the paper is very good.
ngunn
QUOTE (mcaplinger @ Apr 20 2013, 05:05 PM) *
As a general rule, explaining Earthlike morphology with non-Earthlike processes needs a high level of motivation.


True, but Mars morphology is not always Earthlike. One reason I like their idea is that it provides a way to build structures like the Gale crater mound by making windblown sediment stick preferentially in particular places favourable for snow melt. From a purely terrestrial perspective Aeolis Mons is more like a surrealist painting than a geological construct.

mcaplinger
QUOTE (ngunn @ Apr 20 2013, 01:10 PM) *
True, but Mars morphology is not always Earthlike... From a purely terrestrial perspective Aeolis Mons is more like a surrealist painting than a geological construct.

I'm not convinced that a lot of geomorphologists would agree with this statement. If the mound is an erosional remnant instead of being formed in place, I'm not sure it's that remarkable on its own.
serpens
QUOTE (mcaplinger @ Apr 20 2013, 04:05 PM) *
As a general rule, explaining Earthlike morphology with non-Earthlike processes needs a high level of motivation. ("If it looks like a duck", etc.)


So true. The explanation for Mt Sharp could well apply to the apparent aeolian buildup in the upper regions and this was discussed at some length in the geomorphology of Gale crater thread, including Kite's previous hypothesis for the creation of Mt Sharp based on katabatic driven deposition. But with respect to the lower hydrated sulphate and smectite layers I would tend to fall back on the basics. Steno's principle of original horizontality implies that these layers were once more extensive and in fact should have covered the area of the crater, meaning that the moat is an erosional end state and the fan and channels are an early artifact.

As Emily points out in her article, at high obliquity the poles receive high insolation but Im not so comfortable with her statement that this translates to snow being comfortable (happier) at the equator? Relative to the equator the sun is overhead twice a year and atmosphere and entrained water vapor will transfer between the mid to high latitudes. The high obliquity means that the transfer is complete (no residual cap) and the area of deposition would extend through the mid latitudes. Souness et al 'An inventory and population-scale analysis of martian glacier-like forms' seems to confirm that the mid latitudes are the area of unstable deposition.
elakdawalla
QUOTE (mcaplinger @ Apr 20 2013, 08:05 AM) *
The paper is well worth reading, but as it says in section 6.2, it really doesn't address the valley networks and alluvial fans. (It tries, but doesn't completely succeed to my taste.)

Mike, one question that I had about this paper was that it takes as fact the notion that most of the observed sedimentary rocks postdate the valley networks, so that it makes sense to talk about different climates prevailing during valley network formation time and the time during which the sed rocks were getting deposited. (I guess the sediments deposited by valley network formation are either obscured or destroyed, in this scenario.) But now I'm reading the Brian Hynek stuff, which states that although valley networks formed on the oldest terrain, that if you age-date the largest networks you find they all seemed to form in late Noachian or early Hesperian. So I guess I'm not sure of the chronology. If valley networks formed only at the end of the Noachian and into the Hesperian, are they coeval with or are they older than the layered clays and sulfates that Kite was trying to explain?
mcaplinger
QUOTE (elakdawalla @ Apr 21 2013, 07:09 PM) *
Mike, one question that I had about this paper was that it takes as fact the notion that most of the observed sedimentary rocks postdate the valley networks...

I am neither an expert nor a big believer in age dating by crater counting, especially on Mars where we have burial and exhumation and complex erosional processes to worry about. Certainly Malin and Edgett assign most of the sedimentary rocks to the Noachian; see the whole discussion of age relations starting on page 1929 of "Sedimentary Rocks of Early Mars" Science 290, December 2000. But I'm not convinced anyone really knows in detail.

I reread Haberle ("Early Mars Climate Models", JGR 103 E12, November 1998) and it has plenty of "escape clauses" for how early Mars could have been warm enough to have large amounts of liquid water, so I think the motivation for a more exotic mechanism is not absolute. That said, the last few paragraphs of Malin and Edgett they advance a somewhat "Kite-like" hypothesis (before Kite did, obviously):

QUOTE
The second scenario is substantially more
exotic and attempts to conceive a plausible but
uniquely martian explanation for what is ob-
served. In this model, modulation of atmospher-
ic pressure by astronomical perturbations, com-
bined with catastrophic modulation of sediment
sources, gives rise to conditions recorded by the
layered, massive, and thin mesa units.

serpens
Wordsworth et al recently published the results of a 3 dimensional model in Icarus 'Global modelling of the early martian climate under a denser CO2 atmosphere: Water cycle and ice evolution' . Well worth the read although I wonder why they treated the solar flux as a constant (441.1Wm2 representing a faint young sun) rather than treating it as a variable. Empirical evidence suggests that both Earth and Mars were warmer than can be catered for under standard solar evolution / climate models.
mcaplinger
QUOTE (serpens @ Apr 22 2013, 04:31 PM) *
I wonder why they treated the solar flux as a constant (441.1Wm2 representing a faint young sun) rather than treating it as a variable.

Climatologists love the faint young sun paradox because it gives them an interesting problem to work on. They're not astrophysicists so they can't work on the possibility that the Standard Solar Model is wrong or incomplete. smile.gif
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