[X] My Assistant

[dvandorn]

This event deserves its own thread.

To quote the illustrious Dr. King -- "Free at last, free at last, thank God Almighty, we're free at last!

So, how much y'all want to bet we'll stay here for a few sols, looking very carefully at the tracks and waiting for the MER Team to figure out how to proceed from here?

-the other Doug

[dvandorn]

All of this is my best guess -- after all, I'm not an Areologist. But it seems to me that the east-west trending "ridge" is indeed the remnants of the northern rim of an ancient impact crater that has been filled in, its rim almost entirely obscured. The question is not so much whether the "dark patch" north of Erebus is an ancient crater (which I t hink it almost definitely is), the question is how and when was it filled.

If it was just filled by windblown dust, it could be a huge pit of soft powder that will act as a rover trap. But I don't think that's how it got filled.

I think this crater was made *before* Mars dried out and froze up, and that the ancient sea filled it. That would mean it's filled with sediments from the ancient sea, which means that it's filled with more evaporite and sandstone. Which ought to provide good, solid footing under the drifts.

I also don't think there is anything like duricrust on the drift surface that was "broken through" when Oppy got stuck. I think that Oppy got stuck because the drifts are more and more powdery in the white-ish fish-scale drift arcs and it managed to get itself in a position where it had three wheels on one side of the drift crest and three wheels on the other side, causing each side to try and slide in opposite ways. Since the rover can't slide to both sides at once, instead, it dug into the powder rather than riding on top of it. Yes, there is some kind of cementation process that occurs to the soils over a lot of Mars' surface, and yes, there are layers in these drifts that demonstrate varying compositions of the dust that makes them up. But these drifts formed a *long* time after there was rain on Mars, I think -- I don't think that groundwater or precipitation has caused the drift surfaces to become cemented.

In the long run, what I *don't* want to see is the MER Team lose its courage and fret about rover traps over every hill and beneath every drift. The whole purpose of the MERs is to see what's over the next ridge, and as long as we have a fair degree of confidence that we can dig out of soft sands when necessary, I say we should try and visit as many of the landforms here as we can. The darker "fill" inside the ancient crater north of Erebus looks like a slightly different landform, and without any ground-truth that arguies for that fill being unsafe, I'd say go for it...

-the other Doug

[Guest_Richard Trigaux_*]

Trader,

I also noticed that the volume of disturbed soil seems lesser that before it was disturbed. How to explain this?


This would imply for instance that the dunes were formed in an unique episode, with more air and snow powder. After, the snow evaporated, lefting a low density material which remained in place, forming like a bread with a crust and hollowed inside.

But of course this is very prospective.

I noted on photos of Mars that often the surface facies changes drastically in some hundred of metres, without apparent reason. And as a matter of fact, it changed, from the hard flat layers with very small dunes around Endurance, to larger dunes with less or no hard layer under.

[dvandorn]

I also noticed that the volume of disturbed soil seems lesser that before it was disturbed. How to explain this?

Compaction of soft, fluffy powder. Not hard to explain at all. And the material lower in the drift kicks out quite nicely, so it's already more compacted.

This would imply for instance that the dunes were formed in an unique episode, with more air and snow powder. After, the snow evaporated, lefting a low density material which remained in place, forming like a bread with a crust and hollowed inside.

There is similar dust and sand drifting all over the planet. I seriously doubt it was all created in one big global snowstorm three billion years ago, back when snow was possible on Mars. You have to remember, it has been eons since Mars' atmosphere was thick enough to support precipitation like that. These are classic windblown drifts -- there's no reason to postulate precipitation when none has occurred for at *least* millions of years. Besides, we've seen minor changes in the drifts (outside of Endurance) in the time Oppy has been at Meridiani. No, these drifts are still in the process of creation/deflation, and I feel pretty confident in saying they're all simple aeolian features.

I noted on photos of Mars that often the surface facies changes drastically in some hundred of metres, without apparent reason. And as a matter of fact, it changed, from the hard flat layers with very small dunes around Endurance, to larger dunes with less or no hard layer under.

First, I bet there's a hard blueberry-paved surface under these drifts. Second, the facies don't change without reason -- we just don't see the reasons at first glance. In this case, the difference is probably as simple as wind erosion shifting from eroding the blueberry paving to eroding a greater percentage of evaporite outcrop, resulting in different grain size and mass. Which creates drifts of different composition and structure. Add to that the windbreaks caused by the low rims of Erebus and its fellow ancient craters (making up the ancient crater cluster that lies to the west of Victoria), and voila! you have all the reasons needed for the change in the facies.

-the other Doug