[X] My Assistant

[Stu]

You can almost feel your boots skidding on these beautiful rocks as you clamber and step over them, can't you..?

[Gladstoner]

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[CosmicRocker]

... I'm not yet sure whether we are looking at a coherent stack of everted rim bedrock or a rubble-pile of ejecta...

Me, too.
It seemed to me that we should be able to make some guesses about the ages of these rocks from their dips and strikes, so I took a look around.

... If these are lapilli ...?

Lapilli are a very specific type of spherule. Should we expect a certain kind of internal structure in them as compared to glassy spherules?

... unless Endeavour's current "rim" is not actually the original rim but instead an erosional remnant of the ejecta deposited outside...

I'm not sure, but since the apparent rim of Endeavour as seen topographically appears to be discontinuous and significantly eroded (and, since we are all making wild guesses here lately), I'll guess we are somewhat outside of the original rim.

[serpens]

Lapilli are a very specific type of spherule. Should we expect a certain kind of internal structure in them as compared to glassy spherules

From the split examples in the MI they look more like a product of accretion than condensation - maybe perhaps.

[ngunn]

Lapilli are a very specific type of spherule. Should we expect a certain kind of internal structure in them as compared to glassy spherules?

As a non-geologist I'm here to learn from those who are, and I'd appreciate some help with definitions. Here: http://www.google.co.uk/url?sa=t&rct=j...KDA&cad=rja I read that lapilli are small round stones produced by volcanic eruptions or meteorite impacts and that they can be formed either by accretion from a vapour cloud or by solidification of drops of melt. Is that how you're using the term, and if so what is the distinction between the latter case and the glassy spherules you refer to?

(I do apologise for descending into semantics but I want to make sure that my posts do not muddy this fascinating discussion through loose use of terminology on my part.)

[xflare]

Does anyone know of a cross section of a tektite? This is the only image I could find online:
http://www.panoramio.com/photo/3894593

The latest MI shot shows the cross section structure a bit better http://qt.exploratorium.edu/mars/opportuni...M5P2935M2M3.JPG

They do bear a superficial similarity to the very last image on this page http://www.meteorite-times.com/Back_Links/...te_of_Month.htm

[Gladstoner]

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[dburt]

Tektites are glassy bits of impact melt that hardened in the air; their shape need not be spherical, and they are probably irrelevant to the subject of this discussion. Lapilli are gravel-sized fallen melt stones, without reference to specific shape or origin (only size), and the term alone therefore is ambiguous.

As is being discussed, there are many types of spherules and other intrinsically rounded objects, of various origins. Spherules formed by some combination of chemical and physical accretion, such as concretions (formed by pure chemical accretion owing to abrupt changes in chemical properties of groundwater that reduce mineral solubility) and accretionary lapilli (formed by a combination of condensation and physical accretion - possibly involving electrostatic charge - of particles in a turbulent, cooling and mixing gas cloud, and thus not unlike hailstones in concept) are NOT HOLLOW on the inside, unless perhaps internally weathered or altered (as most are on Earth).

The spherules under discussion, or at least many of them, ARE distinctly hollow, and some appear to have objects in their center. This feature (hollow, possibly with junk on the inside) is characteristic of spherules formed by devitrificationof a hot hydrous glass, especially of the variety called lithophysae. As microcrystallites of anhydrous silicate minerals grow radially outwards, in many cases around a central object or megacrystal that served as a nucleus, they release steam that can inflate the spherule as it grows, leading to a hollow inside (except perhaps for the remant central object). The images haven't shown any yet, but some lithophysae even develop an onion-like or layered outer structure (but with open space between the layers), as a rind of crystallites develops, then steam escapes past it, inflating the still soft hot glass, then another rind of crystallites grows, and so on.

Oppy may have been imaging impact glass cementing breccia ever since it arrived at Cape York. Martian impact glasses and melts, compared to those found elsewhere, should be especially hydrous and full of salts, both of which characteristics would favor devitrification (crystallization), so finding spherules formed by devitrification of hot glass or melt shouldn't be surprising. I repeat, this is just one suggestion among many, and need not be correct, but it appears to be the only one so far that accounts for the intrinsically hollow structure with junk inside and an apparent rind on the outside that has just been imaged here. Greater magnification revealing a concentric outer rind of microcrystallites (as mentioned by Bill Harris above), or new images revealing a separated outer onion-skin structure, both would support the hypothesis. I hope this background information helps the discussion.

[ngunn]

I hope this background information helps the discussion.

It certainly helps me make sense of it. Much appreciated.

[serpens]

After years of sulphate sandstone and a winter looking at a chunk of suevite this is ultra cool. There does not seem to be much deformation in the spherules and if they are devitrified glass wouldn't this indicate a lack of compaction? ie. If they were formed pre-Endeavour then they would have been deposited pretty close to the old surface . A thin section of unweathered rock would come in handy about now. I wonder if the APXS will be able to provide an indication of the nature of the matrix? The next Opportunity related LPSC papers should be real interesting.

Edit: Just to put CosmicRocker's look around in context, is this a view of the outcrop from the Sol 2751 position?

http://marsrover.nasa.gov/gallery/all/1/n/...J7P1907R0M1.JPG

[Bill Harris]

This is going to be a very interesting stop on the traverse. I want to say "the most", but it keeps getting better. This is one reason why I've been jumping up&down waving my arms about doing a thorough leg of the traverse across CY, from the Meridiani side, across the rim remnant and into the Endeavour bowl. The depositional, weathering and erosional history of this area is so incredibly complex that we need to understand this history to understand the details.

Good points, dburt. I'm glad that you weighed in. The "hollow" appearance of the spherules doesn't bother me, but it provides another clue (or puzzle-piece). What it tells me is that the interior has a different "hardness" tenacity" "gooeyness" (whatever) than the outer rind. These spherules have been here a LONG time and have been through a lot of secondary, tertiary and quad-ternary mineralization. And that is beyond what a fresh spherule was like. We are seeing broken spherules that have been gently "sandblasted" by aeolian erosion for aeons. We need to see more MI examples, under several different lighting conditions. And the snaggle-toothed RAT needs to section a couple of fresh spherules.

I'm waiting for tdemko to weigh in. I see several "sedimentary" structures that are taunting me. Something has happened there.

And I see that "they" are calling it "outcrop" without choosing a name, yet. I'm still pushing for the Gracie Allen Formation...

--Bill

Edit: Just to put CosmicRocker's look around in context, is this a view of the outcrop from the Sol 2751 position?

May well be. Just after Oppy hitthe high point of Shoemaker Ridge, she started heading downhill to the West side of CY, so we never got a good, close view of the current stop. But IMO, your image is close (and if not the same spot, it's the same outcrop line on the hillside).

[dvandorn]

And I see that "they" are calling it "outcrop" without choosing a name, yet. I'm still pushing for the Gracie Allen Formation...

I thought they were calling the whole area Fin Ridge or something like that... though I guess they need some kind of separate name for just the outcrop within the ridge.

BTW, Gracie Allen Formation? I *like* it!

-the other Doug

[dvandorn]

I'm on tenterhooks waiting for some imagery and APXS work that would indicate whether or not these spherules are deformed hematitic concretions or some form of lapilli. If lapilli, the big question would be whether they were formed by volcanic or impact processes. There is plenty of evidence on Mars for both processes.

Of course, this rock bed could have been formed by a combination of the two processes -- volcanic lapilli could have been spread around the fringes of the impact target sites for any one of the very large craters in the area, and been caught up in a moving tide of impact melt to become clasts in a melt matrix. The same thing could have happened with concretions being embedded in impact melt, and impact-generated lapilli could always have been caught up in a pyroclastic flow of some kind that left them embedded in tuff.

Isn't it fun how impacts are both a blessing and a curse? A single impact into an intact pile of rock beds shows you wonderful evidence of the emplacement and composition of the beds over time, and exhumes rocks from deep within the strata. On the other hand, throw impact after impact onto a set of rock beds (as happened during the LHB) and you get a ground-up, brecciated set of jumbled strata, greatly enriched by impact melt (which can have profoundly altered composition from the target rocks). And with each large impact you're creating new impact melt that is an amalgam of the elements not just in the surviving rock beds but also in the impact melt already emplaced by prior impacts, and in ejecta from other impacts that could have been transported there from hundreds of kilometers away (and exhumed from very different depths of strata). It seems to me it would be almost impossible to straighten the whole mess out into its original emplacement and compositional stratigraphy.

-the other Doug

[Bill Harris]

calling the whole area Fin Ridge or something like that.

They called it "half Fin" at one time last week, but several of the pancam sequences have been called simply "outcrop".

Just a thought. Eight years of trundling over the Burns Fm can do that to you...

--Bill

[ElkGroveDan]

I'm on tenterhooks waiting for some imagery and APXS work that would indicate whether or not these spherules are deformed hematitic concretions or some form of lapilli.

I've heard a lot of well thought out theories on these, but six bucks and my right ... spherule says these are the same hematite concretions we've seen across the rest of Meridiani.