Journey to Mt Sharp - Part 1: Site 7 to Waypoint 1, Sol324 [Jul4,'13] to Sol391 [Sep12,'13] Options

[Ant103]

And my take on corrected the tilt of the MaHLI frame

[James Sorenson]

The ground in front of us on Sol-327

[ngunn]

I want to ask about a feature that I have for some time privately been calling the 'blue hill'. It apears at the extreme right of this recent frame peeking over the nearer reddish terrain but in front of the dune-and-canyon lands beyond. I can't find any other extended patch of ground that matches it in colour. It's not dark enough for dune, not red enough for dust and too smooth (one would think) to be naked bedrock. Any thoughts?
http://mars.jpl.nasa.gov/msl-raw-images/ms...6000E1_DXXX.jpg

[Phil Stooke]

This may help with your question. This is a small part of the big Mastcam pan, a perspective distortion, compared with a CTX mosaic to identify features. The CTX mosaic is south-up to match the roughly south-facing Mastcam view. Lines on the CTX show the edges of the section of the pan shown above. Your blue hill is really part of the dune field. There is quite a bit of variability in the dunes, maybe related to when they were last active.



EDIT - just noticed my bottom line in the CTX image should be a bit lower to include a bit of outcrop in the dunes.

[ngunn]

Thanks Phil, that's very helpful, and a very interesting possible explanation potentially providing a handle on dune chronology.
So we may think of recently stabilised dunes turning a paler bluish grey first and only developing a reddish crust over much longer timescales.

[fredk]

We could see that hill (and other similarly coloured patches) more clearly when we landed - check out these MC views:
http://mars.jpl.nasa.gov/msl-raw-images/ms...3000E1_DXXX.jpg
http://mars.jpl.nasa.gov/msl-raw-images/ms...0000E1_DXXX.jpg

Phil, I agree with most of your IDs there, but I think the hill in question is closer:

[Phil Stooke]

Very good! Yes, that is a better view. So the 'blue' hill actually is an exposure of a different rock within the dunes.

Phil

[ngunn]

Or the exposure of a fossil dune among the active ones. It is after all both dune-shaped and quite smooth. (I recall that fossil dunes underlying active ones have been observed on Titan. Maybe there are terrestrial examples too?)

[dvandorn]

It is obvious from the HiRISE imagery that the dune field is multi-toned, and we see the exact same thing from a distance from Curiosity. However, I'm a little confused as to why a "fossilized" dune would appear more bluish than the darker, assumedly more actively recent dune structures.

If a fossilized dune is of a lighter color than more active dunes because of accumulation of the ubiquitous red surface dust, wouldn't that tend to make the fossilized dunes more reddish than bluish?

I'm wondering whether or not the apparent color difference may indicate a different mineral composition in the different structures within the dune field. Of course, we'll need to wait and see what we find when we get there -- MSL certainly has the tools and sensors required to make that determination once we get there.

-the other Doug

[ngunn]

If a fossilized dune is of a lighter color than more active dunes because of accumulation of the ubiquitous red surface dust, wouldn't that tend to make the fossilized dunes more reddish than bluish?

That's what I thought at first, but maybe there are two processes at work. When the dune first stabilizes maybe it 'bleaches' relatively quickly due to some process other than dust accumulation.

Curiosity is unlikely to visit that feature so unless there is a similar one close to the place where our traverse threads through the dunes it may not give us a definitive answer. It's probably not a priority question, but fun to think about all the same.

EDIT: It could just be a different kind of bedrock as Phil and Doug have suggested, but then you still have to explain why it remains free of the ubiquitous red dust.

[fredk]

Nearby is a prominent two-toned hill - I don't recall it being identified before on orbital imagery. Here it is, with the orbital view rotated so the MC view is approximately from the bottom:

Edit: replaced with higher resolution orbital view.

[dvandorn]

The dune fields are multi-toned, it shows up in the HiRISE images and shows up better in the Curiosity images (at least as far as the fine color delineations are concerned).

I also think we need to remember that the human eye tends to apply a contrasting color level to what is essentially gray if it is in contrast with a reddish color. It's been especially true on Mars that what may appear greenish or bluish is actually a pretty neutral gray, the eye seeing a bluish or greenish tint in the absence of the surrounding red.

My best guess is that the dune fields are primarily varying shades of gray, and when we're looking at them from a close-up vantage point we'll see a range of tones from light gray to a dark, almost black gray. (Of course, there is going to be at least a little of the ubiquitous red dust mixed in there, but not a whole lot, from the way it looks from a distance.)

-the other Doug

[Gerald]

EDIT: It could just be a different kind of bedrock as Phil and Doug have suggested, but then you still have to explain why it remains free of the ubiquitous red dust.

Many more factors will influence color and specularity, e.g. grain size, local wind (dust removal below saltation threshold for sand), surface texture, adherence properties of the surface, abrasion by saltating sand grains during storms.
If we can fix all other parameters, assume low abrasion but high local dust removal, and the dune is made of basalt sand, it might be worth to check, whether color can change from grey to reddish-brown like weathering basalt on Earth. That's at least plausible in an oxidizing environment.

Paper on reflectance/specularity
Abstract on abrasion by saltating particles on a crust.
In-depth paper about dust adherence.
Wikipedia about basalt (including oxidation)

[Guest_Actionman_*]

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If we can fix all other parameters, assume low abrasion but high local dust removal, and the dune is made of basalt sand, ...

"If" being the key word. "IF" the dark dune is segregated basalt sand, where is the basalt the sand coming from? directly under the dune?

[Guest_Actionman_*]

Nice match fredk.
A whole lot of textures going on.