On sol 886 Oppy's microscopic imager takes shots of a new type of stone previously unseen in Meridiani planum. This place was already shot on sol 885 by the Panoramic Camera, showing a patch of soil looking like gravel.


These stones are clear, chalky looking, with angulous fractures, but slighty rounded as if transported for a while in a river bed. They also show some hollows or veins in some places. In the background there are small more or less rounded stones evoking the smaller blueberries already seen in the beginning.


So I wonder what these stones could be. The most conservative explanation would be that they are freshly broken jarosite, submitted to a bit of eolian erosion. But other explanations are possible. We shall remain in the expectative until some analysis is done.

Can you point out specific rock in the MI images? I see some pieces of broken evaporite, but not what you describe.

--Bill

This rock (the big one)


Yes, it could be evaporites, but on other images they appear dark, which evaporites never do. (This is a context view of the MI images, see the same image in a similar wavelenght than the MI images)


even if it is evaporites, it is rather unusual to find them under the form of freshly brocken small blocks. Beagle crater must be very young, even more than Fram crater.

With my opinion there is a kind or erosion of the evaporites. They would lose their chemical binding water, and this lefts only some powdery stuff (try to heat some plain gypsum crystal, it turns to powdery plaster). So any kind of block seems to "evaporate" (which is usualy interpreted as a wind erosion). So on young craters, we find all sizes of blocks, on medium craters only big bloks, and in old craters the king of flat "pavement" seen on Endurance rim.

Are we talking of this strange, blue/gray rock pictured on Sol887 by PanCam (L456 filters)?

Ah, I see what you mean. During the UMSF outage this morning I played with some of the recent Pancam images and found some interesting spectral (or color) characteristics if you blink between images of the same spot in L2 and L7. In L2 (infrared) they do appear dark. Hopefully we'll have a full filter sequence planned (I've not looked at the tracking site, yet).

This is a new rock type, and this is a unique area.

--Bill

Anyone remembers Wopmay, the brain-like rock in Endurance? It also had some rounded surfaces.

Isn't this the stuff which Emily in her blog referred to as "like a mound of small gravel that's been dropped atop the sand dunes and Meridiani bedrock"?

If so then it seems more like ejecta from Victoria. (In fact given the position of the mound it would not surprise me if we're looking at the first example of material from the annulus which seems to ring Victoria.)

If it is Victoria's ejecta then to be lying on top of the dunes is I guess the first indication of just how recent that crater is.

I notice there are some much bigger blocks, presumably of much the same material, visible in this Pancam pic from Sol 887, so I guess we'll be seeing a lot more of this stuff (and even bigger rocks) as Opportunity moves out onto the annulus and closer to the crater.

======
Stephen

Dilo, I was not speaking of this bluish rock. But it sounds very new too, thank you to point at it. Hope they will have a closer look to it. It looks like some iron or pyrite, but it is difficult to know from only its color. If it is iron, it is interesting to note that it is not oxydized, so that it is recent.


Yes bill and stephen, it is what I was speaking about. Emily's blog show it too.

If there was not the dark infrared image, I would think that it is just broken evaporite. That would be already special. But these infrared dark blocs don't look like evaporite, or at least not the common model.


This, together with Dilo's block, show us what we expected from recent images: there are new rocks near Beagle (Stephen I don't think it is Victoria ejecta, it is too fresh. Remember that fresh jarosite shows many whitish blocks of all sizes, that we don't see near Victoria)

Where these rocks come from? The impactor which created Beagle? But we have now TWO types of rocks to explain.

These are the same as the ones I earlier mentioned as very fine grained and almost glassy and conchoidal. The MIs have shown that they are not quite conchoidal, and the dust coating makes it difficult to say if they are glassy. Some of the pancams do seem to display very bright portions at all wave lengths, but I hesitate to call them specular reflections.

It gets a bit confusing when looking at the MIs, which appear uniformly bright. But the filters we have in the visible band do indicate the cobbles are dark to humans.

Dilo: Good job spotting one in the calibration target's background.

I don't know what they are, but they seem to be different from the dark cobbles seen earlier, though I must admit to not having really gone to compare them in detail. I am not certain they are a new rock, but they might be. I thought I'd have a go at making a stereo-pair from the sol 886 MIs. This is not perfect, and I am not 100% certain why this seemed to work, but attached is an *experimental* anaglyph made from two of the MIs. If my dialup allows me the time, I'll also try to attach a crosseyed pair for those who prefer those, but you'll need to resize it for comfort, or force it into StereoPhotoMaker.Click to view attachmentClick to view attachment

Noticing that there seems to be some confusion as to which pebbles Richard is discussing (at least in my mind), it would be helpful if someone would post an annotated image.

CosmicRocker, I was speaking of the same rocks you show, there is no mistake.

Dilo spotted another one which seems different... but his image is (clearly) a human color, while your MI images are infra red. In human colors both rocks appear dark, so that they could be the same.

Dilo's is more interesting, because it is larger and easier to analyse. I don't know if they can put the instruments on my rocks, because the largest is only about 3cm.

Hello back again from one day without UMSC, with bad withdrawal symptoms and trembling fingers I join in only to find the discussion over these new found rocks in full swing.
I agree with Richard Trigaux that this rock look like the kind of mineral to me that might be one 'ore' perhaps iron and yes it looks recent so my thought is that it isnt from Victoria either. If we find many it might be Beagle but if theres only one or a handful the source might be somewhere else also. To early to tell, lets keeps our eyes peeled.

Time for a Meridiani lithology refresher. Drop by Aldo's MarsGeo.com and most particularly the Meridiani Lithology pages. We are seeing a lot of new rock types, but we also need to remember what we have seen.

--Bill

Friends,

As an old flint knapper, I'm not going to say that these rocks are chippable, or even that they have been chipped, but they certainly look different from what we've seen before. . . .

It's hard to tell anything about the surface nature of many of these rocks. They were fragmented by a megaton explosion and have lain around for thousands of years being gently sandblasted.

--Bill

The rock in question, in the MI images, looks like it has microclasts. It seems to have more than one type of material in it, and yet it all seems to be about equally resistant to erosion.

I think, quite possibly, what we have here is a pebble ground out of a piece of impact melt. Such melt often contains microclasts of material swept up with the melt sheet.

Now, whether it's melt from the Victoria impact, or the Beagle impact, or even a farther-reaching impact, this is the kind of thing I'd expect to see in impact melts on Mars.

-the other Doug

Inpact melt?

I don't think that jarosite can melt, it would rather change of chemical composition, from a sulphate to an oxyd (a common industry process). Which could, after, melt...

I'm not sure what the sulfate evaporites do when heated, but remember that underlying the evaporite is a supposed (hopeful?) basaltic unit, which likely comprises most of what remains as the surface of the ejecta apron. This is the importance of getting to Victoria, we'll get to see further down into the section than we have so far. And this is also the importance of continuing to travel further downhill once we compete the survey of Victoria, it will take us even further down in the geologic column.

--Bill

Oxide? But where would the oxygen atoms come from? Hmmm, I suppose if it was an aqueous environment...

It's an interesting question though - what, chemically speaking, would happen to evaporite subjected to a brief blip of intense heat and pressure?

No problem, the oxygen is already in there. Sulphates are compositions of the sulphate ion SO4-- with one or two metallic ions (2x M+ or M++) together with water molecules which are required to make the sulphate crystal as we find it.

When heated, (about 100°C??) usualy a sulphate first lose its crystalisation water. It is what happens when industry does plaster from gypsum.

When heated more, (about 200°C??) the sulphate ion breaks, releasing SO2. The remaining oxygen is now available to combine with the metal, it cannot do otherwise anyway because it is already reduced so it cannot escape the combinaison with the metal ion.


If we heat still more, the oxyde melts, a its own temperature, whic can vary from hundred to thousands degrees.




What happens to shocket sulphates? probably what happen to other crystals. I found an interesting link to this about heating asteroids (alternate PDF file). They explain that crystals submitted to intense shock waves undergo a variety of transformations: irregular fractures and melt pockets, together with a heating increasing with the intensity of the shock wave. For low intensities, the effect would not be very different with jarosite, but when we come to higher temperatures, the effect must be different, as jarosite cannot melt without undergoing a chemical change, involving emission of gasses.

Thanks Richard for the excellent explanation; I see that I remember even less chemistry than I thought I did

The place where I got lost in this discussion was where jarosite was assumed. Referring back to the original post, why would freshly broken jarosite be the most conservative assumption?

There has been a lot of work done on this subject in connection with the Chicxulub impact which occurred in carbonate/sulphate rock and it is well established that very large amounts of SO2/SO3 and CO2 were liberated. There is little doubt that the CO2 remained in the atmosphere for a fairly long time (millenia), while there is more doubt about the sulfur. Sulfuric acid from volcanboes remains in the stratosphere for a few years, but some people think that the sulfuric acid from Chicxulub would quickly have reacted with other material (dust) in the ejecta and fallen back to Earth. Of course things might work differently in the very thin and dry Martian atmosphere.

tty

Because it is the most common rock here. Most, if not all, jarosite blocks seen until today were old, with rounded shapes and apparent layering. These one would be fresh, with no erosion, so that they have angular shapes and the layering was not yet outlined by differential erosion. But please note that when I wrote this I did not noticed yet that these blocks are dark in visible light, so that Jarosite becomes unlikely.

As I recall, the majority of the rocks here are magnesium sulfate, keyserite. Jarosite is an iron sulfide that was discovered by the Moessbauer here at Meridiani. We'll have to see the results of the recent MB, but I don't think that these new rock types are jarosite.

--Bill

Errr? what I heard until recently was that Chicxulub impact punched throught two cristaline (granite-like) layers (which were dated with uranium clock) and that the limestones layers were deposed on it afterward, giving a flat surface obliterating the crater. So that there would be no carbonates involved.

On the other hand, the Ries crater in Germany made into a granite layer, covered by some tens of metres of limestone and shale deposits. There is a work about the granite melt, which gave a layer of about 10m of lava spread all over the crater rim, and nice glass-like light green tectites known as moldavites (because they were found in Moldavia). I know the limestone-shale layers formed large blocks (several hundred metres) which were lifted and then fell back on the crater bottom, sometimes turned upside down, but still with the layering recognizable (shattered I imagine). But I don't know how they responded to shock and heat. We can imagine that shale and clay can form a lava too, we can even try to melt some. Some carbonates too can exist in a liquid form. But sulphates, I don't know.

The light-toned outcrops vary a little in composition, but on average, the sulfate composition is somewhere around 2 parts Mg-sulfate, 1 part Ca-sulfate, and 1 part jarosite. There's also a non-trivial amount of silicates mixed in.

tglotch: Thanks for that. Those are similar to the ratios I've seen for the light-toned rocks in the Eagle and Endurance outcrops, but I didn't realize they are still about the same. As for the silicates, I think I've heard they are about 30-40%.

Is there any idea of what the silicates are? I don't think phyllosilicates (clays) or it would have been front-page news and I suspect that they are generic silicates, as in "left over from weathering processes".

--Bill

The Mini-TES data indicate that the majority of the silicate component is amorphous silica--that is a product of rather than a leftover residue of the weathering process. The rest looks like Na-rich plagioclase feldspar. Phyllosilicates are modeled, but with low confidence, and probably aren't really there. There is no evidence in the Mini-TES data for pyroxenes or olivines. Moessbauer data show a component that could be pyroxene or glass.

Tim

Tim,
Welcome to the forum! We will all look forward to posts from your unique perspective.

Thanks, Tim. By terrestrial experience, it would seem that the Na-plagioclase suggests alkali basalts which in turn suggests a magma chamber at a spreading center. This does not necessarily mean tectonics, but a very "primeval" or source of the basalt. Assuming, of course, that the source of most rocks on Mars is basalt. "Amorphous silica" suggests to me formation as a hydrothermal mineral near hot springs, which may be a stretch here in Mars. Eventually we'll get enough data to understand the rocks on Mars, but for the most part, we know just enough to leave this soft-rock type befuddled...

New Pancams are up at Exploratorium today. Of interest are a couple of close-in Pancams showing the soil surface at Oppy's feet. And it shows that there is pretty much the same thing that we saw over at Jesse Chisholm: many angular-subangular fragments of light and dark toned rocks, with some of the "new type" of rock.

Pancam view at Beagle

--Bill

Bill,

The feldspar used in the Mini-TES deconvolution models is oligoclase, which is about Ab90. Its still a plagioclase, but less intermediate than we're used to seeing on Mars. There's no indication of Kspars or other alkali components. There's also no pyroxene or olivine in the rock which is interesting. A working hypothesis of mine is that at some time before the jarosite was deposited, the weathering environment at Meridiani was less acidic. Over a broad range of neutral pH's, the weathering rates of olivine, pyroxene, and Ca-rich plagioclase are faster than Na-rich plagioclase. So, if you start with a basalt with an intermediate feldspar (perhaps with Na-Ca zoning) and weather it at near-neutral pH, you might get something like the silicate component you see today. On the other hand, if all you have is very acidic water weathering the system the whole time, its harder to square the weathering rates of the silicates with what you see today.

Finally, the presence of the hematite spherules within the outcrop is very suggestive of a diagenetic process, which may also account for the silica we see in the outcrop. I don't think it necessarily has to be a high temperature process.

Tim

Nix, the impact was on the Yucatan carbonate platform which already existed at that time, though the crater extended well into the Early Palaeozoic/Precambrian basement. It is true that the crater quickly filled in with carbonate deposits (the impact area actually was in a shallow sea). The ejecta blanket (which is locally preserved in Belize and Cuba) is essentially all carbonate blocks, some of them huge.

tty

Thanks, Tim. Eventually we'll be able to reconcile an ultimate igneous-volcanic source for the rocks we see today at Meridiani, and will be able to propose a series of environmental conditions that have existed in the past. Mars is a wonderfully dynamic planet and answers are here waiting to be unlocked.

One reason I've been wanting closer looks at "the dark basaltic" rocks we've encountered is that the "holy grail" of the basal unit underneath all this, which is a likely source for the sands, sulfates and Blueberries we see today, will allow us to better understand the weathering processes that have occurred. Eagle, Endurance, Erebus and Beagle were too shallow to show anything, but Victoria might be. At least there are plenty of dark cobbles scattered about, so at least it will expose deeper units of the evaporite formation. Ultimately, it would be great to continue the traverse to the south-southeast, downhill and presumably down-section. We probably won't make it, but we can dream...

--Bill

That was the wheel scuff at Jesse Chisholm. That set didn't come down until last night.

Oh fiddle.