So, there's a significant feldspar content in the ubiquitous Martian dust, eh? It will be interesting to see if we find source rock that is anorthositic, or source rock that is granitic.

Or both.



-the other Doug

Steve Squyres' term "dog's breakfast" seems appropriate, here.

-the other Doug

Well -- if it is a dust coating, the dust is preferentially coating just the upper unit above the visible contact, which is definitely eroding more quickly than the dark-toned basaltic rock below the contact. This dark-toned rock is dust coated in some places, but even where dust-coated the dark-toned unit looks darker than this light-toned upper unit.

The upper unit also shows a fairly consistent coloring, with slight hints of a light gray showing through, indicating a dust coating over all surfaces, while the darker-toned unit shows dust coatings only in those places where wind erosion has not stripped it off. In particular, the dark-toned surfaces just below the contact have generally not been dust coated, while the light-toned unit above the contact is either dust coated or actually is composed of material of a light reddish color.

There are also very distinct textural differences between the units above and below the contact, including but not limited to the greater wind erosion seen in the upper unit as compared to the lower unit. This definitely speaks to this being an actual contact between rocks of different composition.

-the other Doug

Yep -- lots of interesting contacts and inclusions in some of these rocks. The one that really stood out to my eye, though, is the rock in the upper left of the imaged grouping:



It's a touch difficult to see because the upper unit is the same color as the soil behind it, but there is definitely a very bright light-toned, reddish unit forming a layer at the top of this rock. It doesn't look like a preferential dust coating, the contact is quite regular, even giving the appearance of the light-toned unit filling voids in the top of the dark basaltic-looking unit, as if it was emplaced in a fluid manner. Also, the light-toned unit is eroding more quickly than the lower dark-toned unit.

If I had to guess, this could be the result of a flow of mud or sediment that covered over a layer of basalt, and the mud or sediment has since lithified. You have to wonder, though, what exhumed this particular chunk to leave this contact above-ground? Are we looking at evidence of the deflation of fill material from the Gale floor?

-the other Doug

Looks exactly like dried mud to me, too. I've often seen dried mud curl up at the edges of cracks like that.

I guess the question is whether or not this formed in place, here, or whether the entire rock was transported from elsewhere. Looks like an extensive enough general distribution to think it formed in place, but I imagine we'll need quite a few more sols and ChemCam shots to figure that out.

I can't wait to hear what the elemental composition of this stuff is. I bet it's some water-altered form of basalt, but we'll see.

BTW -- seeing what looks like potentially lacustrine (or at least alluvial) materials popping up, does anyone else get the feeling that this is the kind of thing we were hoping to see on the Gusev floor, and didn't?

-the other Doug

You think so? Haven't noticed much of anything yet that appears feldspathic, and IIRC earlier landers found feldspathic materials primary as very fine grains in the ubiquitous dust layer. Same thing with the carbonates -- they're there, but mostly as very fine grains well-mixed into the primarily basaltic dust.

Heck, even the fine-grained Martian basalts we've seen at the various landing sites seem absent of the plagioclase laths often seen in terrestrial and lunar basalts.


This is the exact point I've been making about the Gale floor from the beginning, and it's a lesson we seem not to have learned real well from earlier landings. Yes, it's crazy-interesting when we find anomalous rocks strewn about, but at the V1 and Pathfinder sites especially we found that the rocks themselves were a crazy-quilt scattering of materials washed there by ancient floods. Here in Gale, a lot of the surficial rocks seem to have been washed there by running water, as well -- be they floods, encroaching seas or even just small streams running down from the central mound and from the rimwalls.

Yes, it's interesting to look at the depositional process in terms of how the rocks got here and the kinds of transport that could have been involved. But the rocks themselves are scattered, there is no real stratigraphic information to be gained from them, and since they (for the most part) did not form in place they contribute very little to our understanding of the landform(s) on which we happen to be standing at the moment.

Now, the lithification of the muds that slid in when these larger rocks were emplaced from their origin points to where they sit now, yeah -- that's rather interesting. But except for those places where the lithified muds are exposed (as, perhaps, at Glenelg), and especially where they haven't been torn up by post-emplacement events like impacts, these beds seem to be primarily buried under detritus that's been transported in from elsewhere. I sure hope there are enough strata exposed here at Glenelg for us to get a good handle on the alluvial deposition sequence.

-the other Doug

The one depositional process that has continued at a pretty steady state through to the present day is impact. Perhaps, as the atmospheric density declined and the armoring process proceeded, a relatively higher impact flux distributed larger-grained impact debris on top of dunes and ripples composed of finer grains, which were already in the midst of developing indurated surfaces.

-the other Doug

I would still like to see if there is any sign of Surveyor IV in Sinus Medii. The best co-ordinates for the intended landing site I can find are 0.4N and 1.33W.

Yeah, I know that if the solid retro exploded as it neared exhaustion the craft would have been too far up for anything truly recognizable to be left after the shrapnel hit the surface. But there is always that tiny chance that Surveyor IV actually completed its landing safely and the failure was simply with the comm system.

I wonder if anyone has ever looked for either a landed Surveyor within the targeted footprint, or a set of craters where the pieces could have fallen...?

-the other Doug

Also, remember that the ripples have a cemented layer at top. Perhaps there were multiple cemented layers produced by multiple episodes of cementation, and the uppermost layers are breaking down into the larger-grain fines?

-the other Doug

This is not to say that we haven't seen quite a large number of mini-impacts (especially in the Meridiani ripple fields). I'd guess those kinds of tiny craters are primarily secondary impacts where small gravel is spit out of nearby impacts.

However, I agree with the majority, here. The image in question looks like ChemCam laser hits on unconsolidated material to me.

-the other Doug

Only if the target returns fire...

-the other Doug

OK -- well, the arcs are far bigger than the outside arc of the RAT's circular footprint. I guess the RAT was turned on and dragged across the surface, eh? First time I've ever seen that.

Another first for Opportunity!

-the other Doug

Did Oppy drive over some portion of the later MI image coverage area? There are two arcs of what appear to be scratches in the less-durable covering layer on the right side of the image linked below; one large arc and another much narrower arc tangent to the larger one:

http://qt.exploratorium.edu/mars/opportuni...29P2956M2M1.JPG

These "scratch arcs" look decidedly artificial to me.

-the other Doug

Indeed. That's why I put "varnish" in quotation marks the first time I used the word. It's some kind of hardened layer, but the look of flow along its top really makes me think of something that splashed into a sheet then "froze" in place.

-the other Doug

I'm more fascinated with what appears to be a flow pattern in the "varnish," resulting in mini-lobes that appear to flow from right to left across the image.

I've never seen desert varnish freezing in any kind of flow pattern before... at least, not unless it comes from transient moving water.

-the other Doug

Is there any way to tell if Little Shiny was present before they started vibrating the scooped sample? I can imagine little bits of kapton tape or similar things breaking off during the vibration cycle, but if this was there prior to the scoop vibration, maybe it's just a native bleb of melted metal or glass? There's no reason to believe you couldn't get such little blebs in the Martian soil -- there are likely a lot of sources of source material, and certainly enough impacts to generate little melted blebs every once in a while.

-the other Doug

Actually, I've discussed this with Eric Jones who compiled the ALSJ, and I'm comfortable with his interpretation that this sentence was "The ground mass is, uh, very fine." This was one of the items on Neil's observation checklist pre-flight, to describe the ground mass, and there is even a recording of Neil during a simulation saying of the sand that had been set up around the ladder on the training unit that "the ground mass is sandy, like beach sand," which of course it was.

Yes, Mars is not Luna, and you get very different erosion processes on a planet that has even a thin atmosphere vs. a moon that has none. In general, there is much greater variation in grain sizes in Martian soils as opposed to lunar soils. On the Moon, you either have very fine dust or rocks from cobbles up to boulders. Because grains in the Martian soil were transported by water (a long time ago) and since by wind, you get much more well-sorted soil fines and a lot of very fine grains deposited by the global dust storms. Add to this the continual impact mixing, and, well Martian soils are far more complex than anything you'd ever find on Luna.

-the other Doug

Likely very similar all around Mars, with some hard-to-predict local admixtures. Remember that Mars gets global dust storms and a lot of the dust gets distributed globally.

-the other Doug

The tightly layered rocks at Home Plate in Gusev were laid down by a hydrothermal system, according to the best theories I've read. (Think: Yellowstone Park hot springs and pools.) And the rocks here at the approach to Glenelg look, at least at first glance, like the Home Plate rocks.

Could there have been hydrothermal activity here?

-the other Doug

For me, Ikeya-Seki was the first contemporary comet of which I was aware (I was 9 years old at the time), and for us in the northern hemisphere, I recall it to be an unviewable bust. I never did see it with my own eyes.

The very first comet I recall seeing was Comet Bennet in 1970. By that time I had a small reflector telescope, and I recall setting it up in the back yard on a cold March or April morning at like 3am and looking at the coma and streaming tail through my little 'scope. Didn't look like much, just a fuzzy patch with no definition near the core, but with my naked eye I could see the tail covering about 30 degrees of the sky. Faint, but rather impressive.

Kohoutek was a bust as well, never even spotted a fuzzy patch in the sky. Hyakutake was the next comet I saw, and I never saw a lot of a tail from it, just an elongated fuzzy patch in the sky. Really not that impressive.

Hale-Bopp was very impressive to me, the near-in tail was very bright and the comet was very clearly visible in the daylight sky. I flew to England while Hale-Bopp was in the northern sky, and I recall out my window seeing the tails (H-B had that cool spiked double tail) rising out of the pale green glow of the aurora borealis as we sped along from the U.S. to England along the great circle route.

I never saw anything but pictures of McNaught, though I understand it was impressive to those in the southern hemisphere.

I'm really hoping that ISON will out-perform Hale-Bopp. All we can do, I suppose is wait. And watch the skies!



-the other Doug

I used to think along those lines, too. But the orbiters (especially Odyssey) have seen almost no indication of hydrogen in Hellas -- i.e., no indication of subsurface ice or even strongly hydrated materials.

Instead of harboring water in the past, these results tend to indicate that Hellas has never seen much water at all. I think that's likely why it has never been considered as an attractive landing site, even though it is such a low spot that the atmospheric pressure there is higher at the surface than just about anywhere else on Mars.

Hellas would be a wonderful landing site if you're looking to examine Martian mantle materials, because it is certainly deep enough to have exhumed mantle rocks. Geologically speaking, it's very attractive. But since the main interest in Mars is (and, I think, always has been) the investigation of water, habitability and life, the geologic questions that drove the exploration and analysis of the Moon are taking a back seat to the water- and life-seekers when it comes to Mars.

-the other Doug

Viking 1 was also targeted to an outflow channel area (where it was thought at the time that there may be a remnant water table near enough to the surface to support microbial life). Unfortunately, these locations have been dry for megayears, and the rocks in the area were washed in from a great variety of locations, making it nearly impossible to judge their geologic context.

-the other Doug

One rim of Gale Crater is quite a bit higher above mean than the other, right? Even though it appears to be a regular ringwall kind of structure, not breached nor significantly out of circular. It could be that much of Mt. Sharp was deposited in horizontal layers and the overall ground mass below the entire crater could have tilted before the deflation that exposed the central mound and revealed the horizons we now see as the floor. The entire subsurface table tilting would account for the different heights of the rim between north and south.

As to what could have caused the entire subsurface below Gale to tilt -- well, the Tharsis bulge was responsible for enormous deformations of the crust. Also, if this area of Mars ever went through extensive glaciations, the entire subsurface could have been pushed down by the weight of the glaciers during the deposition of Mt. Sharp's layers, and has since recovered its original elevation and orientation via isostatic rebound.

Finally, if the material that supposedly infilled the entire crater (and has since been deflated) was emplaced by a rapidly moving force, such as the rush of waters or repeated pyroclastic flows from the same vent area, well, that material could have piled up on the far wall and filled back from there. If the force emplacing the materials was consistently from the same vector, you would get layers that are tilted in a sort of compromise between the gravity vector and the emplacement vector.

In other words, there are a lot of ways on Mars that you can get tilted and discontinuous rock beds, you don't have to assume tectonic processes.

-the other Doug

I took a quick look at the open front page of your favorite comet forum, Gladstoner, and seemed to see several people congratulating an Anatoli Nevski for the discovery. Will this be Comet Nevski, then?

Got to be a good connection to be made to Alexander Nevski, somehow, if so. "Oh, noble yeoman, thy tail shineth so bright..."

-the other Doug

Well... I know that Oppy has seen an awful lot of trace elements in the Meridiani soils and rocks, including sulfur, chlorine, arsenic and even carbon (though Phoenix was much more well-suited to identify carbon). As I recall, Phoenix found more carbonates in the soil than it found perchlorates.

I know it's always dangerous to judge Martian processes based on Terrestrial processes. But the various exotic admixtures we see in the Martian soils are roughly what I would expect if bodies of very salty water evaporated, leaving the easily soluble elements and minerals as evaporites which were then well-mixed by impact, volcanic and aeolian processes into the rocks and soils over megayears.

-the other Doug