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Fraser
2005-May-09, 04:57 PM
SUMMARY: The European Space Agency's Mars Express took this image of 140-km (87 mile) Crater Holden on the surface of Mars. This crater is very old, with numerous smaller impact craters inside it, which formed later. It's also heavily eroded, with the characteristic central mount nearly completely covered by sediments. The rim of the crater has been cut in several places by gullies, which seem to form small valley networks.

View full article (http://www.universetoday.com/am/publish/crater_holden_uzboi-vallis.html)

What do you think about this story? Post your comments below.

wstevenbrown
2005-May-09, 05:57 PM
Exquisite photograpy!

I'd be willing to bet that the content of the main crater was permafrost at the time(s) of the three lesser impacts. Water + CO2 + silt. After the minor impacts, the surface melted and slopped over into the crater in the eight-o'clock position and plated it with rapidly-cooling melt. Note the sharp cutoff at the 'high-water' mark. Most of the inflow paths were subsurface (did not pass over the crater rim). Subsequent impacts with crustal flexion collapsed the roofs of the flow channels, forming the rilles.

Speaking of water, the presence of a haze there now suggests to me that the crater floor is still permafrost, with a few leftover, uncollapsed ice caves. Nice place for a garden.

Best regards-- Steve

eburacum45
2005-May-10, 12:19 AM
If there is a lot of water just below the surface it might be possible to get at it fairly easily, the planet might support a small population of colonists after all.

The thing that seems to be very different about Martian geology is the absence of large folded mountain regions like the ones we see on Earth- many craters seem to penetrate a fairly flat, unfolded series of layers, which develop canyon-like regions but not raised mountains (other than volcanoes).

Guest
2005-May-10, 01:30 AM
the pictures and info from this craft has been great !

lswinford
2005-May-10, 01:58 PM
wstevenbrown, but the large channel to the southwest (between 4 and 5 o'clock on the crater circle as presented) shows flow towards the crater. That appears to be a distinct surface flow streambed.

I bet it is an old volcanic caldera. My first thought, with the little peaks in the middle, were something like a broader Crater Lake (I keep forgetting whether that was in Oregon or Washington state), but has filled in with sediment. Olympus Mons shows that magma flows are very fluid, so it could have built a cinder cone inside the crater and later flooded the caldera to some equilibrium point of pressure. Sand storms then later iced this cake, and obviously punctuated a few times with impact craters.

Whichever, the sharpness of the peaks in the middle has me puzzled. For something so apparently old, how do sharp peaks like those get sustained over such a time with a comparatively undisturbed flat fill around them? The crater walls are sloughing off, but there seems comparatively scant debris around those very sharp peaks. That must be pretty hard rock, if you ask me.

wstevenbrown
2005-May-10, 04:16 PM
Go back to the source document and look at the overview panel, which covers the entire area of the cropped display photo. It is color-coded to reflect approximate vertical relief. Using the display photo as a directional reference, primary inflow was, as you suggest, at 4:30, with small supplementary spill-ins up to 2 o'clock. Primary outflow was at 12:30-1:00. Most of the time, the flow was underground. With seasonal melt-volume changes, tho, the entire structure would have swirled like a flushing toilet. This by itself may have accounted for the slopover into the 8:00 crater, but I don't think so. My intuition shrieks that the three vertically-aligned secondary craters were a chain event by a (originally) 1/4 km object, as the thin atmosphere has no time in which to cause more extensive breakup. See if the perceived standing-wave patterns in the refrozen permafrost are a figment of my diseased imagination.

I suspect the walls of the crater act as a spoiler to keep the notorious winds from scouring the central peak-- possibly, above a critical size, all central peaks are preserved? Looking at that again, if the crater were really large, the central peak would not be protected.

There's plenty here to think about, and conclusion-jumping is aerobically very sound. ;) Best regards-- Steve