[X] My Assistant

[Bill Harris]

Here are a series of false-color L257 Pancams of the current Oppy traverse stop. I'm still in the "arch the eyebrows and muttering 'fascinating' " mode so I don't have any words of wisdom right away. We can discuss over the next few day/sols before she moves on to the next miracle...

Cabo Frio: (order is #3, #1, #2 from left):

This post has been edited by Bill Harris: Nov 12 2006, 03:36 PM

[CosmicRocker]

I am so blown away by the long baseline anaglyphs of the opposite rim that I am having a difficult time thinking of anything else.

Here is my quick-n-dirty interpretation of the stratal geometry of Cabo Verde on a photo posted yesterday by Nix. ...

tdemko: Thanks. That looks good. Although they were not apparent on some of the images we saw at first, those dune foresets are there if we look for them, even though the upper part seems massive at first. I really was not expecting the ejecta to be so thick here. That, and the apparent, later collapse features are giving me fits. I think the only way I can deal with this is to retire from my day job.

Another thing that continues to confuse me is the way the term "evaporite" is so frequently used here. I thought that all of the rocks we have seen are really sandstones that have been later cemented by evaporites. What is the "evaporite" layer many keep talking about? Is it the upper part of the Burns formation seen at Endurance?

Not necessarily relevant, but do caves form in evaporite (salt, on earth) in the same way that they do in limestone? In other words, is there an evaporite version of karst terrain to look for on Mars?

Algorimacer: Such things exist, but they are very uncommon compared to the karst terrains created in carbonate rocks, afaik. There are a lot of reasons for that, but imho the most important reasons are that salts are simply quite soluble in water, and such rocks have little mechanical strength. Carbonate rocks are only slightly soluble in plain water, but more so in commonly available, acidified water, and carbonate rocks have significant mechanical strength...so cavities formed in them are more durable.

O'Doug: These are probably quite soft rocks, but I would think not quite as friable as you suggest. Regardless of that, the important thing to consider in erosive environments is the relative durability of the layers. These cliffs exist, regardless of how incompetent they may be.

[dvandorn]

Another thing that continues to confuse me is the way the term "evaporite" is so frequently used here. I thought that all of the rocks we have seen are really sandstones that have been later cemented by evaporites. What is the "evaporite" layer many keep talking about? Is it the upper part of the Burns formation seen at Endurance?

Actually, I believe I was the first one here on UMSF to refer to the light-toned, layered bedrock first seen in Eagle and later seen, well, everywhere at Meridiani as evaporite. It was about a week after I first used the term that I can first remember Squyres using it during a press conference or in one of his updates. I started using it because of the Anatolia-like crack features I could make out in the enhanced DIMES images, which suggested strongly to me the polygonality of dried sea floor beds. It just made sense to me that the surface we were seeing was the result of the dessication and shrinkage of a wet sand or mud unit; when the first-look at the rocks revealed them to be composed largely of sulfur salts, I laid the label of evaporite on them.

Technically, I suppose only the sulfur-salt matrix of the light-toned rocks are actually formed from evaporation of acidic, high-sulfur groundwater, and the resulting rocks are more accurately described as sandstones with evaporitic cementation. But since the high-sulfur-salt-content rocks were formed by multiple epochs of evaporation (and possibly sublimation?) of sulfurous, acidic water, it's just been a good shorthand to refer to them as evaporite rocks. And it differentiates them from the more pure sandstones that seem to underly the evaporite layers (the lower portions of the Burns formation).

I just keep wondering -- if there's a Burns formation, oughtn't there be a Smithers formation?

-the other Doug