[X] My Assistant

[MichaelT]

Martian terminal velocity. An iron ball 1 cm. in diameter travelling at almost 800 km/sec would seem to have a

It's 800 km/h not 800 km/sec. Otherwise we'd be talking about a cloud of iron plasma not an iron ball
Michael

[Bob Shaw]

I think we need to review and classify the range of features that have entered this discussion, in order to be sure we are all talking about the same things. Many of the features that have been discussed are quite different from the original mini-craters mentioned by JPL so long ago.

In all honesty, they jumped the gun. The range of features is indeed large, and some look very much like sapping while others don't. I think the whole gamut of features could do with a really vigorous shaking to see what falls out, and even calling the things 'mini-craters' may be leading us all astray.

Bob Shaw

[Bill Harris]

Cataloging the minicraters is one of my round-tuit projects. There is indeed a great deal of variability of these features; the only common link is a) they are cup-shaped and b ) they are not large.

--Bill

[CosmicRocker]

It's 800 km/h not 800 km/sec. Otherwise we'd be talking about a cloud of iron plasma not an iron ball
Michael

Whoops! That was indeed my error. I'm so accustomed to thinking of high meteor speeds that I guess I automatically used the wrong units. I did understand helvick's argument that small meteors on Mars might be expected to arrive at Martian terminal velocity. For us Americans, 800 km/hr is equal to about 730 ft/sec or 500 mph, almost as fast as a low velocity rifle bullet or a passenger jet. I was picturing a 22 caliber bullet hitting the sand on earth. In my experience, such impacts cause more disruption than we see in all but the larger mini-craters we've seen. With Mars' lower gravity, I'd expect larger craters.

It is a bit of a judgement call, and now (with your prodding) that I've thought about it further than I did last night, I realize that a primary impactor of smaller mass could be expected to create the craters we've seen. Thanks for correcting me.

[ngunn]

Doug:

I don't think so, no - or at least, not always. Impacts can result is some quite non-intuitive effects (for example, it's been suggested that on Earth it's possible that tektites 'surf' shock waves and end up with very non-linear distributions). And meteorites are often cold in their interiors - one with a nice ablative coating of ice should survive really well...

Bob Shaw

Following up your posts I found some interesting things by googling 'Icy meteorites'. Not only are they expected to survive passage through the Martian atmosphere quite well but they have been postulated to exist even here on Earth. Foeldi, Berczi and Lukacs proposed searching for ammonia-water meteorites on Antarctica in 1995. I don't think they found any! As far as I know the Tagish Lake meteorite remains our best example so far of a terrestrial meteorite containing ice.

But it seems we have a plausible mechanism here for the formation of Martian craters, both small and large, new and old, from both primary and secondary impacts. If there was once an extensive frozen ocean on Mars we might expect to find a rash of impactor-less dimples formed by secondary ice projectiles in highland palaeosurfaces of appropriate age.

[Guest_Bobby_*]

From Today's nav cam images. Looks like a mini crater has formed over one of the rovers tracks?

http://qt.exploratorium.edu/mars/opportuni...CUP1988L0M1.JPG

[kenny]

From Today's nav cam images. Looks like a mini crater has formed over one of the rovers tracks?

You mean the little feature apparently over-lapping the near track a few metres back? I think I see a discontinuity between the track alignment before and after this feature, and something similar at the corresponding point on the opposite wheel track, if you look carfeully. Seems to me like a little sideways "jiggle" in the Rover's motion.

Kenny

[kenny]

Bobby

This Navcam mosaic from Tesheiner in "Back to Work" thread shows a better view of one of the discontinuities I mention above, at the right end of the trail.

Tesheiner pan

[CosmicRocker]

Hehe, there was a recent pancam image showing one of these elusive mini-craters in the process of formation.