[X] My Assistant

[dburt]

Impact melt can also form a lining within the crater. In smaller craters, a glassy lining is made up of impact melt. (Since we have never landed in or taken samples of the floor of a large lunar impact crater, we have almost no data on how abundant impact melts might be in the floor units of large impact craters.)

If this *is* local, I would imagine that the suevite was emplaced in the ejecta and has since eroded out of one of the prominent capes. Being harder than the surrounding rock, it would simply fall out of the cape structure and tumble down the inner slope of the crater.

Other Doug - All good, but let's remember that the piece of glassy-appearing brecciated rock in question (found along the rim of Victoria), let alone the piece of breccia that started this discussion, are not yet proven to be suevite (impact melt breccia) and that we have no idea where they actually came from (another suggestion was, as I recall, that the rock on the rim might be a fresh meteorite). Nevertheless, some useful generalizations have been made - that the furthest part of the ballistic ejecta blanket generally represents the near-surface material, and that as you approach the crater, more deeply excavated material is represented (it's ballistic path went almost straight up, and so it came almost straight down). This ballistic ejecta pattern, of course, ignores the fines that interacted turbulently with any vapor or atmosphere present and potentially went out extremely far as radial-flowing density currents (impact surge clouds). Also, the larger the impactor (and the softer the target), the more excavation energy is wasted in frictional heating, meaning that the crater formed is proportionately shallower, and that the quantity of impact melt formed is greater (that is, big craters are not as deep as and probably contain more impact melt than you might otherwise expect, especially in soft targets). A water-rich target might also favor the formation of impact melt (hydrous melts can form at lower temperatures than dry melts, in general), so martian impact cratering could have formed more melt than lunar impact cratering (or perhaps just bigger vapor explosions). Again, I'm not an expert, but have been trying to learn. So corrections would be welcome.

That said (and related to your earlier post stating a longstanding desire to see impact melt), where's all the impact glass that might be expected on Mars? Hydrous melts crystallize more readily than dry melts, water being a catalyst. That gets rid of glass. Quenched glasses also break down (weather) far more rapidly if minor water is present, especially it's acid and salty. So, perhaps that's where the glasses went on Mars as opposed to the Moon. On the other hand, glasses, being amorphous (noncrystalline), are difficult to detect spectroscopically, I'm told, and the spectra of "surface type 2" (Northern Plains, in general) are not a bad match for material rich in glass (or amorphous clays and/or silica formed from glass or another source). So my short answer is I don't know, but if the impact glass is indeed gone, water is probably to blame, one way or another. In this regard, geologically ancient glasses are generally lacking on Earth, owing to the abundance of water. Crystalline rocks last a lot longer.

First Doug, feel free to move this if it's again getting too general (and it probably is). I can't resist rambling - just ask any of my students.

-- HDP Don

[CosmicRocker]

Here's the part that I'm curious about. The Victoria impactor apparently slammed into an 800 meter thick pile of hydrated, sulfate-cemented, silicate sediment. Wouldn't the sulfate salts act as a flux, lowering the melting point and dissolving the silicates, then mechanically dispersing them as fine particles, as the impact cloud expands and cools?

[dvandorn]

Good discussion, wherever it eventually ends up "belonging"...

Yeah, I know there's no proof that we've actually seen a piece of suevite yet. I do sincerely believe that there must be a fair amount of impact melt on Mars, if only because, as HDP has pointed out, impact processes have likely predominated surface change on Mars for a long, long time. And Mars has been very dry (over much of the globe), to a pretty decent depth, for a long time, too. You'd expect to see less and less hydrous effects on the impact melts as Mars has become drier and drier, would you not?

Then again, no one really knows how much water is locked up in rocks and ices beneath Mars' surface. I guess it's possible that Mars retains enough subsurface volatiles to continue to preferentially degrade impact melts over other impact products, leaving much of the planet covered with ejected chunks of rock (some of it not even very shocked), and very little melt.

I don't suppose it's possible for Oppy to go visit this little chunk of what could possibly be suevite and take a real long sniff...?

-the other Doug

[jvandriel]

A mosaic of Opportunity's arm taken on Sol 1407

with the R0 Navcam.

jvandriel

[dburt]

Here's the part that I'm curious about. The Victoria impactor apparently slammed into an 800 meter thick pile of hydrated, sulfate-cemented, silicate sediment. Wouldn't the sulfate salts act as a flux, lowering the melting point and dissolving the silicates, then mechanically dispersing them as fine particles, as the impact cloud expands and cools?

CR - Vicky is a relatively small crater (smaller than Meteor Crater, AZ), the cratering didn't go all that deep, and explosive vaporization of hydrated salts may have greatly exceeded melting, if there was any. Impossible to know, and no close terrestrial or lunar analogs are available. Now it's all buried under material that caved in off the friable former walls of the crater, as the crater enlarged itself by slope retreat, and even most of that material is buried under dune sand (which may also cover ejecta or dusty surge deposits derived from craters younger than Victoria). I guess we are lucky that the upper cliff exposures are as good as they are.

-- HDP Don

[CosmicRocker]

Thanks. I guess I didn't say it very clearly, but the possibility of significant vaporization is exactly what I was wondering about. I understand that that this is a rather small crater, and that if any local impact melt existed, it would be buried.

[gallen_53]

Yesterday I was chatting with a colleague about MER and he claimed that MER-B (Opportunity) is dragging a wheel. I replied that he was mistaken and that only MER-A (Spirit) is dragging a wheel. Are all six wheels of MER-B still working?

[djellison]

Opportunity has a front right wheel STEERING actuator that is broken...i.e. it's stuck facing just about forwards. BUT - it still turns to drive the vehicle.

Doug

[dburt]

...the possibility of significant vaporization is exactly what I was wondering about...

CR - That would probably depend on how "wet" (including ice) the area was at the time Victoria formed. Presumably, given its equatorial location and seeming high permeability, any ice (cryosphere) or brine horizon was buried more deeply than Victoria cratering ever reached, leaving salt dehydration as a source of vapor. Then we get into how to interpret the word "significant" and I won't hazard a guess. All I'll say is that the target rocks were quite weak and at least somewhat hydrous, and so probably were heavily disaggregated by the impact. That is, lots of sand blasting to flay your skin and relatively few big pieces of ejecta to hit you on the head. And that's just a guess.

-- HDP Don

[Tesheiner]

Checked the PCDT web right now and Opportunity should be moving to another site/target on sol 1416.

[fredk]

Oppy appears to be doing a cloud imaging campaign, and there are many frames where you can see clouds, even through all the dust on the lens! Here are a few of the best frames, from 1413:
http://marsrovers.jpl.nasa.gov/gallery/all...26F0006L0M1.JPG
http://marsrovers.jpl.nasa.gov/gallery/all...26F0006L0M1.JPG
http://marsrovers.jpl.nasa.gov/gallery/all...26F0006L0M1.JPG
http://marsrovers.jpl.nasa.gov/gallery/all...26F0006L0M1.JPG
I wish I could flat-field those things!

(Sorry for the distraction from the Mercury party! )

[ngunn]

Oppy appears to be doing a cloud imaging campaign

Excellent. I've always wanted to see more of the clouds, the forms they take and the ways they change.

[Doc]

By the looks of these images, these are the typical water ice cirrus clouds which migrate south for the winter. These ones are apparently moving from north east to the southwest
(following the local winds?)

[Tesheiner]

Opportunity moved about 2m downslope during sol 1416.
Take a look to this rear hazcam picture. That's the rat hole made on the Lyell layer, which is now way behind the rover.

(Edited to point to a picture at the correct side of the planet.)

[fredk]

Thanks for the heads up, Tesheiner, but it sure is strange how much Victoria suddenly looks like Gusev!