CRISM Corner Options

[Guest_AlexBlackwell_*]

Nili Fossae in Natural Color and Across the Spectrum
MRO CRISM Release
September 11, 2007

[djellison]

They got it - CRISM of the MGS gullies.

http://crism.jhuapl.edu/gallery/featuredIm...amp;image_id=95

It's looking like dust slides.

Doug

[ugordan]

It's looking like dust slides.

Well, it was fun while it lasted.

[AndyG]

Well, it was fun while it lasted.

Is it over? I don't see how "Dust slides" per se answer questions relating to the face of slopes, which I believe tend to show a preference in terms of their bearing.

And if these slides were caused by liquid water, then that water would evaporate quickly from the scene. So the surface materials brought down by undermining and slip wouldn't have the chance to be modified by salts in the water, and might still chemically resemble dry surface materials from around the area...

Andy, needing more evidence one way or another.

[djellison]

modified by salts in the water

So where are the salts?

Doug

[marsbug]

Well, it was fun while it lasted.

While this may be a blow to hopes of water flowing down gullies right now, the morphological evidence for formation by liquid water, in the last few million years, maybe the last few tens of thousands, is stronger than ever. Dry slides may be running down the gullies today but the evidence from HIRISE suggests very strongly that dry slides did not form them. So the party doesn't have to be over, gullies were still likely formed by liquid water on mars in the (geologically) recent past, and as Andyg points outthere are still questions about the gullies that need answering.

[djellison]

I think this new data takes it from 'possibly water' to 'don't know'

Doug

[AndyG]

I don't feel happy with the apparent "slope preferentiallity" based on dust alone. It seems to be lacking a missing (volatile?) ingredient.

This pdf suggests any post-slide remnant hygroscopic salts on the surface are more likely to attract dust over that of surrounding less-salty surfaces - so perhaps these are buried quickly.

Burial must also occur just after a release, when surface materials above the release point are undercut and fall onto an otherwise salt-surfaced gully.

Admittedly the other volatile options - especially the explosive CO2 one - would leave no trace as per these CRISM findings.

Andy

[dburt]

So where are the salts?

Great question. Although I'm not a spectroscopist, my incomplete understanding is that IR spectra can pick out only sulfate salts. They are utterly incapable of detecting chloride salts, in which the relatively long, weak chemical bonds do not vibrate at the right frequencies. Inasmuch as chloride salts are more soluble in water, and exhibit far greater freezing point depression than sulfates (more than 50 degrees C vs. less than 5 degrees), one would expect that subsurface brines would be enriched in chlorides, not sulfates, and that sulfates would instead remain at and very near the surface (Knauth and Burt, 2002, Icarus; Burt and Knauth, 2003, JGR). General sulfate enrichment at the surface was detected by both MER rovers, as well as by earlier Viking and Pathfinder missions, and by CRISM itself. If the light-colored deposits at the foot of gullies consist of an emphemeral thin coating of chloride salts, deposited by evaporation of subsurface chloride-rich brines, they should be "invisble" to CRISM. Of course, the deposits might consist of dust, as hypothesized, or of dust coating "sticky" (hygroscopic or deliquescent) salts too. The important point is that only sulfate salts (which wouldn't be expected in extremely cold subsurface brines anyway) seem to be eliminated by the CRISM data.

-- HDP Don

[elakdawalla]

Huh. Interesting point, Don.

Wish I could just put on my hiking boots and go check out that gully!

Emily

[jamescanvin]

Quite a good article from the BBC about this today.

[SteveM]

Is it over? I don't see how "Dust slides" per se answer questions relating to the face of slopes, which I believe tend to show a preference in terms of their bearing....

But the sample of gullies studied recently in Science by McEwen et al. show:

There is no favored slope aspect: The bright deposits are on east-, west-, equator-, and pole-facing slopes.

Clearly a systematic sampling of the orientation of a large number of gullies is needed to resolve this, but the slope aspect does not seem to be a convincing argument for melting water (or CO2).

Steve M

[marsbug]

Gullies have been observed on slopes too shallow to support a dry flow, which argues for some kind of volatile. But I dont know much about the mechanics of dust slides, perhaps a violent event (an impact?) could trigger a dust slide on a slope otherwise to gentle. Also; would it be possible for anyone to confirm or deny that CRISM would not pick up chloride salts? I agree that the CRISM data puts a dent in the gullies=water argument but I'm a long way from being convinced its dead. And although I dont give it much credence personally we've yet to see anything that kills the 'exploding CO2' theory.( Minor rant incoming) Press releases on mars water seem to fall into two camps: 'mars is bone dry' or 'mars is soaking wet'. I'd like to see one that falls into the 'mars is definately icy and may be slightly damp' catagory!

[ElkGroveDan]

The "dust slide" notion may explain some of the gullies, but there are a whole bunch more that obviously aren't or can't be dust slides.

When it was hypothesized to be water, skeptics (rightfully) demanded to know the source of the water. Well now I want to know the "source" of all the "dust" that might be above each gully in question, especially in this image that MSSS called the "Weeping 100." MGS MOC Release No. MOC2-235, 22 June 2000

Attached thumbnail(s)

 

[dburt]

Quite a good article from the BBC about this today.

Well, Richard Kerr has a "damp, but not wet" Mars quote at the end of that BBC article anyway. A bit of a change from his "giant vanished lake or sea" remarks for Meridiani made when Science chose a super-wet Mars as its 2004 "Discovery of the Year". So maybe the pendulum is indeed swinging back towards dryness. Still, I find it curious that the boulders covering the northern plains (and earlier found in abundance at every landing site save Meridiani) have not yet been attributed to impacts, but rather to some sort of floods or glaciers or volcanism. Nevertheless, I see no conceptual problem with northern ocean deposits that later was covered with lava flows and/or impact debris (including boulders), any more than with the putative lake beds in Gusev covered with the possible impact debris we now see there (breccias, plus fines at Home Plate). Who knows, there may even be lake beds deep under Meridiani (although they'd have to be deeper than the bottom of Victoria Crater, apparently). Finally, as Emily noted out last June, there seem to be boulder beds interspersed with some finely layered Meridiani-like beds in MRO images, at least consistent with an impact origin for everything. Orbiters image only the surface, and landers and rovers visit only the surface, so speculation will continue about the subsurface, despite new radar data showing a heavily cratered basement to the smooth Northern Plains.

Regarding the mysterious gullies, the apparent lack of restriction to particular insolation angles indeed suggests they may not be caused by simple melting of surficial snow or ice, but it probably doesn't rule out escape of deep chloride brines, which carry their own antifreeze (also antivapor). The newly reported high slope angle of gullies with light-toned deposits seems difficult to reconcile with a brine flow origin though, or at least the control is not obvious. In contrast, the strict absence of gullies from near the equator suggests some sort of strong climate and ice/melting effect (even if ice is only the rock cement) - it doesn't seem consistent with simple debris flow. At the very least, I imagine it will turn out that not all gullies form by the same mechanism, any more than they do on Earth. The upcoming Martian Gullies workshop being organized by LPI's Allan Treiman (the first person foolish or courageous enough to suggest a dry debris flow or landslide origin for the Mars young gullies, athough he didn't get a citation in the latest MRO Science article) should be of interest to devotees.
http://www.lpi.usra.edu/meetings/gullies20...s2008.1st.shtml

I'll probably end up going, although it's only a month before LPSC.

--HDP Don