Hi all,

Maybe this has already been discussed before, but I havn't found anything about it on Google or forum archive search ...

The HiRISE image PSP_005550_1440 caught my attention because the RGB color composite shows what looks like a very prominent view of possibly ice/frost covered gully channels ...

The white/bluish coloring (given the red/brown rock color) seems plausible enough for ice/frost but could this color also be
a result of some kind of salt/minearl deposits only ... ?

I can not remember any previous images that show such prominent light toned features directly within the gully channels.
So I'm wondering why this image has not been discussed before, especially in light of the ongoing debate on Gully formation processes
(i.e. the "Liquid/H2O related" versus "dry avalanche" theories)







What seems intersting to me is that due to the relatively equator-near location of 35 degrees latitude this possibly ice/frost deposits do not look like the "ordinary" frost covers often seen in shadowy places on higher latitudes terrain ... which suggests a direct relation between the deposits and the Gully channels...

However as I'm no geologist/expert myself I would like to ask what others in the forum think about this.

Nirgal

Is that a false color image? If so, I wouldn't be quick in implying that's ice only because it looks blue. There were other color composites released earlier that, for example, showed dust at the bottom of channels as deep blue, almost ocean-blue.

EDIT: I see it's a RED + BLUE/GREEN composite. Even so, contrast stretch might have been applied to the two channels, enhancing the blue look of the material in the channels. I'd actually expect real ice to be white in these kind of composites, they'd be bright in the red channel as well (maybe even brighter due to dust contamination).

False color, yes.... but with the brown rock as calibration reference, the coloring of the light toned material seems at least plausible.

I'm not *implying* that this is indeed ice/frost .... just looks like it could be... (The topic title should be changed from
"Probable ice/frost ..." in "Possible ice/frost ... " however)

BTW: I for myself am rather drawn to the "dry-avalanche, non-(present)liquid" related gully formation process theories ... all the more I was surprised
by the gullies in this image

You're right: contrast-stretch induced color distortions should be the greatest caveat when visually interpreting colors ...
(Should take a look at the individual channels from the raw PDS data ...)

In the mean time, a quick look with the IAS viewer at the greyscale version (which is, if I remember correctly, directly derived from the RED channel)
shows that the light toned areas are indeed very bright in that channel as well (so bright, that many pixels are completely saturated...

the only other places where one can usually see saturated white pixels in RED channel images are either very bright sun-lit rock edges or
the higher latitude ice/frost deposits (or, possibly salty deposits as also seen within MER rover tracks)

It looks to me as though the "white" bits are on sun-facing slopes. I'd guess they're bright because of that, and not compositional differences.

The title of this thread definitely perked me right up, I'll tell you that...!

Looking at the detail image



It seems that the bright/bluish color seems to coat all the "branches" of the Gully channels, regardless of the individual branch's relative orientation to the sun ... and that none of the many rock facets outside of the channels do show
similar light colors despite being oriented dircectly to the sun ....
Of course this does not prove anything ... just an interesting observation possibly worth further investigations



I have to admit that the wording "probably ice coverd" in the topic title is pre-mature (as already noted above)
and should be changed to "possibly ice covered ..." or "very light toned ..."
because it's not my intention to spark any wild speculations about present H2O or something (to the contrary: I for myself
do not believe in present-day liquid water on Mars at all... )

However, after looking at literally thousands of HiRISE images, this one really caught my attention for the very
pronounced light toned channels that do neither look like just sun-lit bright slopes nor like the usual
frost covers found on higher latitude terrain.

One possible explanation (other than ice/frost) could be that the color differences are due to
some mineral/salt (recalling the very bright salts sometimes seen in the MER rover tracks)

What do the experts think ?

The orientation on this image is interesting to me.
Take a look at it up the other way.
High points look like low points now and viceversa.
Click to view attachment
Has anyone got any ideas what's high and what's low here?
It might tell a different story.
Astro0

Assuming that poofy dust settles down into the valleys and that this frame has normal valley fill....

Looking at the image, I would guess that the sharpest features are at the top and that the broadest features are at the bottom.

The sharp lines are the ridge boundaries at the top, and the broad (bluish) channel-like features are lower downslope and thus are the bases of the valleys.

(It is possilbe that the broad bluish features could be at the top of the ridge, making a gentle ridge cap, but then you'd have to explain why the valley bottoms are so sharp and fill-free.)

-Mike

After searching the HIRISE archives for similar images I found the following interesting entry PSP_004469_2195

where the headline at the HiRISE site says: "Gullies Produced by Possible Snow/Ice Melting"

with center latitude, denoted as 39 degrees north, i.e. also not a polar type of terrain ...

Unfortunately, the corresponding JPG-images seem to be corrupted (?) ...

Has anyone managed to obtain the uncorrupted versions of PSP_004469_2195 ?

Would be interesting to compare the multi-band (raw R/GB/IR) spectra of the (supposed) ice/frost areas to the light toned areas in PSP_005550_1440 ....

That image probably won't be able to show much, 10 July 2007 was in the heart of last years' dust storm.

Another interesting image found on the HiRISE site

PSP_003252_1425

The caption there says:

"The bright gully deposit has a very fluid-like appearance, and has not been covered by other gullies or debris flows, indicating a young age. The brightness is a mystery; it could be due to minerals formed from water or ice. Alternatively, the flow that made the gully may have removed a thin coating of relatively darker dust and soil, revealing a brighter substrate. In any case, this feature is probably indicative of recent flow of water or water-rich material on Mars."

Fascinating stuff

Fascinating images Nirgal, Mars just continues to throw delights at us, doesn't it?

By the way, I have finally - don't laugh! - got broadband access on my computer, and this morning I have finally managed to install and use the HiRISE IAS viewer, too. It's a revelation, truly it is. I had a passion for Mars before, and thought I knew its surface pretty well, but this makes it seem almost like I have a whole new world to explore. It's been an everyday routine for many of you here for ages now, I know, but to be able to zoom in with IAS on individual rocks, and ledges, and see such fine detail, well, it's staggering, and very humbling too.

So, to all the people here - and there are many, you know who you are - who helped me with crops and close-ups of areas in the pre-broadband days, to use in my Outreach talks, a huge thank you. And to anyone out there who is not sure if it's worth installing and using IAS, I urge you to do it. It's a wonder, and will transform the HiRISE images for you.

I am going to get sooooooooo behind writing my new book now...!!

Sort of like what Spirit did here but on a much larger scale?