Maybe we can find tehre a connection to the brightness of the beacon...

What a pleasant, chatty leisurely thread this is, away from the frenetic passions of false democracy evident elsewhere….

I agree that “Corner/Bullseye/Whatever-JPL-Will-Call-It” is post-Victoria, relatively fresh, and it really does look fascinating already. I don’t think we’ve yet seen a crater rim so clean of the Meridiani drifting dune material. Speculating on what is perhaps a rim of pale broken evaporite ejecta blocks piled too high to be inundated with dark drifts, I have been thinking of the different types of crater morphologies we’ve seen on this voyage.

Eagle had no blocks on its sandy rim, while Endurance did have a bedrock exterior rim in places, but lacked scattered ejecta. I keep coming back to little Fram around sol 88, which is fresh enough to have exterior and interior broken blocks, inter-laced with the ubiquitous dark drift. This is in contrast to a significant little crater we passed on sol 818 (new pancams just posted, but I don’t have the skills to make a proper pan). This is perhaps a bit larger than Fram size and has a clear circular bedrock rim but not much blocky ejecta evident. The rim of this little crater reminds me of the Payson edge of ancient Erebus. Is it just millennia of slow erosion that turns a jumbled Fram into a flatter crater 818?

Maybe not that simple. What happens when a big impact like Victoria occurs near to an older Erebus or little crater 818? Doesn’t a powerful supersonic wind scour the landscape for miles around, blasting away the loose ejecta of older craters and flattening them down to look like rimless Erebus and 818. I don’t know whether this is a recognised phenomenon in this field of study, but it does seem to me that big impacts in an atmosphere, like Mars has, are very different from say the moon. And proof if needed that Corner with its raised rim post-dates Victoria.

Kenny

....gasp...whew....'scuse me while I catch my breath in here...
Yes indeed, Kenny, the shock waves, hypersonic blast of superheated air, and gravity flow of proximal ejecta are all parts of the latest hydrocode modelling for impact cratering. I've seen no studies of the effects of these on a neighboring crater, because the research focuses on the larger, more scattered craters on Earth. I have little doubt though that Victoria impact vastly accelerated the erosion of Erebus and any others nearby at the time. The thing about Mars that confuses the issue is that the vast stretches of time surrounding the current topography, combined with a currently slow rate of erosion, allow for multiple cycles of deposition, cratering, burial, exumation, and reburial. If you follow the MOC images at msss.com you've seen copious evidence for these cycles. When a crater, like Corner, has the classic features of freshness, we can assume that it has escaped these cycles. As for the rest, without raised rims, blocky ejecta, etc., who can say?

...right...back into the fray...

Thanks for the X-stereogram, but... boulders? I see no boulders along the way, only drifts and outcrop flats. Are you sure you have your eyes crossed properly?

Otherwise it seems clear that CC is sitting on a ridge - the ground behind it is not visible for a long way off.

Before the left half of the crater about 2/3 to 3/4 I see rocks larger than usual. Boulders may be an exaggeration, but they're bigger than the usual cobbles...

--Bill

I'll agree on a boulder aligned with the east (left) edge of CC, most of the way there. The other white flecks I'm not sure about yet, we'll see!

The dark feature on the horizon, has this been visible/discussed yet?

Would that be the edge of the crater ?
Nico

I wondered about that too, Tesheiner straightened me out that its the little craterlet on Victoria's blanket in this thread.

Interesting how close it looks from here, and if we can see its profile, it really shows that Victoria has no substantial slope up it, were looking pretty flat and there is no slope hiding Victoria's lower flanks. its somewhat of a sunken crater compared to Endurance. Makes me wonder if it compacted the underlying landmass by melting the permafrost or rock, sinking the whoel assembly minimizing its profile on the horizon. Which might account for the small rise separating Victoria and Erebus.

This is what I think we're looking at. These images are adapted from Nix's anaglyph (de-anaglyphed) and Tesheiner's Route Map (South is up). The edge of the Victoria rim on the left may actually be the nearer west rim, but you get the idea.

--Bill


NOTE: I agree with Joffan, and re-did the image to point at the proper rim-spot on Victoria.

I agree that the rough patch on the horizon is the small crater you identify in the diagram Bill, maybe that should be "Garter Crater". Your green arrow should perhaps point to the same place as Tesh's Infallible Route finishes.

I think I was wrong before about Corner being on a ridge; it's just the uniform dark ejecta behind it that gives the illusion of distance behind it.

It's just another beacon. My gut tells me it's on the far rim.

In re the discussion of crater morphologies, specifically as seen in the Meridiani area...

First, if Mars' cratering history is anything akin to the Moon's, then a vast majority of the visible craters were made at the end of the Late Heavy Bombardment. Yes, there has been a continuing cratering process, but the rate has been much slower.

Second, the LHB occurred back in a period when Mars may have had a much thicker atmosphere and a lot more volatiles in and on its crust.

I truly think that one factor in the cratering morphologies is the relative volatiles content of the impact target. I think the old, very subdued craters may have been made by impact into soggy ground (or perhaps even water-covered ground). Sharper-looking craters were made after most of the volatiles disappeared. The former feature subdued rims and ancient crater fill almost level to the rims, while the latter feature much more lunar-like morphologies, arguing against the presence of volatiles in the targets at creation.

So, I would argue that Eagle, Endurance, Fram, Victoria and Corner are all examples of impacts into a dried-out ground target, while Erebus, Terra Nova and other ancient craters in the etched terrain are examples of older impacts into a ground that held an active water table, close to the surface -- if not actually into a body of water.

-the other Doug

This all sounds logical, DV, but won't a more parsimonious explanation suffice: Eagle...Corner formed in the last 500 Ma, and Erebus... formed in the 500Ma before that. In other words, do we really need the LHB and the "wet" period on Mars to account for what we see on Meridiani today? How can we distinguish without absolute dates on the craters?

We've not really had the chance to examine smaller craters on the ground before the MER missions, but there's a clear set of crater morphologies visible from orbit which certainly *do* demand to be interpreted as impacts into volatile rich materials: the uniquely Martian 'rampart' craters. These are characterised by an ejecta blanket complete with flows, and ending abruptly in lobate scarps. Such craters often resist later water erosion, too, and can form islands in the midst of some of the catastrophic floods.

How tiny craters work in volatile rich periods and/or materials is interesting - but we must remember that secondaries will predominate, with much lower impact speeds and thus lower overall energies.

Bob Shaw

Good points, Bob. And Shaka, as for the importance of knowing how volatile-rich the targets were when given craters were formed -- I would think that this would cause more than just morphological differences. We ou ght to be able to see differrences in the rock types making up the ejecta.

I would think that ejecta from a volatile-rich target would be mineralogically quite different from that from a dried-out target. Without plugging that potential difference into our analysis of the minerology around each type/age of crater, I don't think we're going to be able to come up with analyses that make sense.

-the other Doug