Bill:

There *was* an earlier discussion here on that very point - only in reverse. The matter of what size of object could make it to the surface got some serious consideration. I'll try and find it...

Bob Shaw

I remember previous discussions, now that you mention it. I just did a quick browse through _this_ thread and there was discussion on this on Page Three, specifically Post #32. Whew, there has been a great deal of thought put in on these minicraters!

--Bill

The discussion of "dimples" in this article made me think of some of the microcraters we have seen. Atacama

I seem to recall that we went round in circles quite a bit in that discussion. The reality on earth is that objects of all sizes make it to the surface although the vast majority burn up very high in the atmosphere, and the stuff that does make it to the surface may be very different in size from the object(s) that initially hit the upper atmosphere.
The most relevant item for these mini craters will be martian terminal velocity for various densities and shapes given that it is extremely unlikely that anything small will hit the ground at anything higher than its terminal velocity. For small fairly compact objects Martian terminal velocities are around 5-6x faster than the equivalent on earth. i've tried calculating this in the past and came up with a number ~790km/h for the terminal velocity of a 1cm diameter iron ball on mars vs 127km/h on earth using the same formula. The Martian number is probably close to supersonic in the martian atmosphere so the reality is probably a bit slower, at any rate the martian number will be 5x or more higher than the earth case in general.

Thanks. That is an excellent summary, helvick. For the "mini-crater" features that might be created by impacts, it seems clear that such small craters would need to be the result of pretty small objects travelling at Martian terminal velocity. An iron ball 1 cm. in diameter travelling at almost 800 km/sec would seem to have a lot of energy, and result in more significant craters. Secondary or tertiary impacts might impact at much lower velocities. Some of the things we have seen seem likely to be of impact origin, but some, somewhat differently appearing features really seem to be the result of a kind of sapping/subisdence process or even an aeolian process.

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.

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

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

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

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.

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.

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

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

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

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