Another abyss covered by dunes. Coincidence? I think not.

Brian

Ice (or at an least ice/soil mixture) is a soluble karst-forming rock even in some areas here on Earth.

As far as I know karst here on Earth occurs in the following materials:

limestone
dolomite
gypsum
anhydrite
rock salt
ice (particularly ice/loess mixtures)

Note that gypsum and anhydrite are sulfate rocks.

tty

What if there were a more uniform, fairly thin "ice cap" over deep, somewhat uneven permafrost? Could an impact create enough heat to create this feature in a single event of localized thawing and flooding, followed by longer term slow melting or sublimation of the remainder of the overlying ice?

--tc

Ed:

I'm not trying to be particularly contrarian by pursuing the karst topographic aspects of Meridiani, but instead I'm trying to extract some interesting ways of looking at and understanding what we see - it's a thought experiment for me as much as anything!

But, it's an attractive one!

As for the terrain, who's to say that - apart from regional tectonic changes - the landscape would have been quite the same shape when/if a water/ice substrate with/without a liquid water topping was extant? We might be seeing a tilted landscape, shrunken down over previously water-filled voids and ice lenses. There's good reason to think there may currently be ice lenses in Martian polar dunes - imagine if they sublimed...

As for the 'tell-tale marks' - well, we certainly see a strange pavement over/under the area we've traversed, including the slopes of Endurance and Victoria. There *are* troughs and holes, too. And there's the ripples, and a few indisputable impact craters. I think the jury could vote either way!

The notion of a karst landscape isn't at odds with any of the theories put about by the MER PIs - they've concentrated on the small-scale processes as evidenced by the cross-bedding etc, but I can't see any way that a karst interpretation precludes any of that (and, indeed, it fits well with the Anatolias out there).


Bob Shaw

Ted:

I think you're perhaps touching upon the circumstances around the formation of rampart craters - good HiRISE images of which may be very illuminating (once they've finished with Northern Plains sandpaper).

I'm also intrigued by the famous Mars Express image of the 35km-wide crater with an ice-cap inside it, and the way that would interact with the local geology (although in that case the ice vastly postdates the formation of the crater).


Bob Shaw

A very small rampart crater, perhaps. There would need to be water flowing back into the hole, and having it flow over a layer of ice might address the issues of irregular topology of the existing surface but fairly regular scalloping of the crater itself.

--tc

What we need are predictions from each theory than can be tested with observations. It seems to me that there's at least one problem with the standard theory, ie. that it doesn't account for the capes and bays...

Nprev,

The cochoidally fractured pebble was discussed on page 5 of the thread "Victoria Annulus; Discussions about Victoria's Apron", Sept. 3, 2006. Doing a search on keyword "chert" will help you zero in on this page.

Glenn

I still haven't seen any explanation for vertical cliffs that does not involve three things: undermining, collapse, AND removal of the detritus.

The "prow-like" form of the capes is interesting. The cliffs are vertical or near-vertical everywhere where they are deeper than... a small distance, 0.5m or so, at a guess. That would imply (if you are correct that there are no other explanations of vertical cliffs) that the undermining process occurred (or is occurring) all the way around the edge of each cape, but lower and lower, the further away from the rim the cape gets. Or, that the very tip has been exposed to the process for longer than anywhere else, and the further back from the tip you look, the shorter the exposure. (Just thinking out loud...

Earlier up the thread I had a wild idea about the bays being due to the surface sagging down as a distinct unit as material was removed below it. Now I come to look back (eg. Capo Verde ) it's interesting that the bedding plane of the surface of the bay appears to be parallel with the present-day surface, ie sloping inwards at 25-30 degrees. However the planes exposed in the sides of the capes are roughly parallel with the surface of the annulus.

Shaka, you're right... I've got a bad case of Marsaholism here. It's gone midnight, I have a big pile of real work half-completed on the dining table to be completed by tomorrow evening, and here I am, busy not doing it... :>

Er... ...perhaps we're thinking backwards.

The cliffs may not be the key - perhaps it's the formation of the bays which are strange...


Bob Shaw

Thank you very much, Glenn. Ashamed to say that I haven't had the time to chase this down as promised...been buried under homework & reserve military duty to boot. I'll give it a look.

I'm thinking there were bays in the original vertical cliffs due to an irregular collapse pattern. If you start with bayed cliffs it's easy to see the bays acting as wind tunnels where the cliff edges would be systematically planed off at an angle until the crater fill piled up level with the erosion surface, yielding a smooth bay profile while leaving the top part of the cliffs exposed on the promontories.

Regarding the escalloped bays, whats to prevent the following?

The plains were saturated due to the then-current water table level (or permafrost) at the time of impact, so any crater rim irregularity would be amplified by water erosion as the seepage carried the soft soils down into it, since the amount of seepage would be proportional to the surface area progressively exposed, bays should tend to elongate while capes should stay the same. Since the soils were likely to be much looser back at the time of impact, the bay material was more easily carried downward to fill and flatten the bowl floor, which perhaps didnt have time to develop much of a bathtub ring shoreline with the water that seeped into it before it sublimed away, unless its buried.

This seems simpler a scenario to me than the underground river drainage network scenario. Although Endurance should have similar bays in miniature unless it was created in a period when the soils were signifficantly more dried out, which may not necessarily mean its newer. We'd also expect analogs of this type of process on Earth, any geologists care to set an amateur straight?

The capes have not 'stayed the same' as they were immediately after impact. There are no raised or everted strata even on the capes so they too have expanded outward by collapse, erosion or both (either quickly or slowly), though not as far as the bays.

There is no need for the volatile rich material to have been present right up to the surface at the time of impact. If there was a dry layer on top not fully penetrated by the Endurance impact then Endurance would not be expected to the exhibit the scalloped rim and pronounced vertical cliffs that might plausibly be associated with mass wastage by sublimation of buried volatiles.