In that image, the rock looks to be a breccia, which might suggest ejecta over meteorite.

Gray:

I see slightly water-altered vesicular basalt... ...so: Ejecta!

Bob Shaw

Given the rock types we've seen here, that is a reasonable conclusion. But the little voice in the back of my head says "not basalt", so I'll reserve judgement til we *thwack a couple more samples.

--Bill

Yes. It is interesting. Fine grained, but clearly not homgenous. I'm not sure I'd call it a breccia, but there are chunks. I suspect they will investigate them further, especially considering Steve Squyres' "surprise" 8/17 update.

" Oh, yeah, and the cobble we looked at with Opportunity isn't a meteorite, it's a martian rock... and one that's very different from anything we've ever seen before. Busy times..."
http://athena1.cornell.edu/news/mubss/

Bill: I think I hear that little voice, too. Let's hope they thwack them some more.

Stitch of Sol556 MI pictures

Geo-trivia...

I wonder why they are calling these "cobbles" when they would normally be called gravel or pebbles? Then again, one might wonder why we refer to the aeolian structures as dunes when they might actually be ripples.

These dark angular rocks of interest that we are initially looking at are below cobble-size (64mm-256mm) but I think they have been looking off the planned route and seeing larger rocks of interest, so we may be looking at actual cobbles soon. We "caught the first two minnows" to determine whether or not closer examination was warranted.

It looks like she is doing an eastward jog to the evaporite bedrock of the Erebus Highway so there are going to be plenty of interesting things to look at.

--Bill

No new info on the cobbles. Doug, could you ask Steve about these?

I have a new theory about the appearance of the rock in this RAT hole.

We're approaching Erebus, right? Erebus is a bigger crater than Victoria, although a lot older. In fact, it seems likely that Erebus was formed before the waters receded.

That would mean that Erebus ejecta would have been scattered on top of an existing evaporite layer, only to become embedded in later evaporite layers.

I think the dark cobbles we've been seeing, increasing in population density as we approach Erebus, are in fact the remnants of Erebus ejecta that have eroded out of the evaporite. I also think that the dark clasts visible in these MIs are small-grained ejecta particles that became embedded in the evaporite as it formed around it, during evaporite deposition eras that post-dated the Erebus impact.

And I think these materials may represent the sub-floor material that lies below the evaporite layer.

What do y'all think?

-the other Doug

I tend to agree, Doug^2. And remember that we are seeing a dark unit in the walls of Erebus which is clearly the source of the dark angular chunklets. I also see the dark unit in a smaller crater along the way, so we don't need to risk detouring across Erebus. I am especially interested in seeing the contact between the evaporite and this dark rock.

Not only do we have the excavation of Erebus, but also of "Terra Nova", a larger crater predating Erebus.

Have we heard anything about the minerology of the dark rock she Mossburged?

--Bill

The dunes are stratigraphically higher than the evaporite, but we dont' know if the dunes are "original" in this area or are sand that's windblown hundreds or thousands of kilometers across Meridiani.

We dont' know what's missing from on top of the evaporite, there may/probably-were other layers that weren't consolidated enough to survive.. the cobbles may be from within or on top of that/those layers.

The evaporite is stratigraphically higher than "XXXX", where XXXX is a softer layer than the evaporite, maybe unconsolidated, that refuses to form outcrops in craters like Endurance or Victoria.

The "XXXX" unit is on top of what appears to be another evaporite layer, judging from a crater some few tens of km to the southwest <I think> with a bench of what looks like more evaporite halfway down, then more unconsolidated layer below the bench down to the bottom, which is occupied by a gorgeous dune-network that puts the one in Endurance to shame.

Below that, only Mars knows, for now.

The cobbles well could be ejecta from Victoria and/or Erebus, deposited on the dunes or now missing higher layers and "lowered" down onto the evaporite over time. I was *really* frustrated Steve declined to mention them during the briefing and didn't get Q&A'd afterwards.

The dunes are the recent/current part of the section; I barely consider them "rocks", but a soil or regolith cover. If this were a terrestrial geo-map, the dunes unit would be colored yellow and labeled "Qal"...

But they are recent, ephemeral and unconsolidated. I imgine that the source area is to the NW, upwind of the prevailing winds. The prevailing wind direction likely shifts with polar drift/precession/orbital changes, which change the climate over many kiloyears. Remember, on the large scale, weathering erosion and deposition on Mars work over timeframes of 10's of thousands of years.

I dunno what the stratigraphy of the area is; we've only seen a couple of small windows and need to look at more roadcuts. I've been meaning to search the MOC/Msss archives and piece together a regional map of the area, but haven't, yet.

What image was the "crater some few tens of km to the southwest" on?

--Bill

"What image was the "crater some few tens of km to the southwest" on?"

I've got that @#$@ image on a hard drive on a computer that's not currently running (NT format), and on any of some 50 CD's of minimally sorted and undocumented images and files, and my netting computer is Winbloat 98se.. so It can't read that drive. I gotta find the time to get the other puter running again and files consolidated so I can find things....

Just think of me under a pile of CD's and books and LP's and VHS and.... with one mummified hand sticking out into the air....

I'll try to find that image.. it's not a huge crater.. 5 km maybe.. but very neat.

OMG, I see that others are experiencing bouts of Image Inundation, too. I've got many, many images that "I remember..." or that I come across and have no idea specifically which image it is...

Early on, a year and a half ago when this was to be a 90-day mission, I started renaming "nifty" Rover images in the "oppy0001.jpg" format and now I have hundreds of images with a non-standard filename.

If you can't fine it easily, don't sweat it. It'll turn up...

--Bill

I have a pdf presentation entitled "Mars Express OMEGA and Opportunity Coordination"
by R. E. Arvidson that includes several maps/images of the Opportunity general area, along with interesting figures about geological composiiton.

I have no idea where I downloaded it from

If anyone thinks it can be helpful and can't find it, I can email it (it is too big for posting here, 19 Mb)

Edit: Ok, I have cannibalized the pdf for this MOLA map, I hope it is useful