[X] My Assistant

[jaredGalen]

Looks like the streak spotted back in March is most likely a bit of a comet called Wiseman–Skiff. According to this article

Fantastic little addition to Spirits 'research portfolio.'

[dvandorn]

This discussion points out why I have a certain distrust of mathematical models that predict real-world behaviors -- they often don't account for the occurrences that lie on the shallow edges of the bell curve.

It's all well and good to talk about the number of joules generated when a body enters an atmosphere, and describe on paper what happens to such a body when that many joules of energy are transferred to it. But such a simple calculation doesn't take into account such facts as abalone describes -- golfball-sized objects that survive entry into Earth's atmosphere and whose deeper structures aren't even significantly heated.

Models are good ways to understand how the real world works -- as long as you constrain your expectations as to exactly what you'll let those models predict. The real world is "fuzzy," it includes chaotic phenomenah that cannot be predicted by models. And models are always more simplistic than the behaviors they attempt to predict -- any model complex enough to predict all of the observed phenomenah in the universe will be no less complex than the universe itself.

-the other Doug

[Bob Shaw]

So: Some 'minor' (10KPS!) velocity differences between Earth and Mars impacting objects, no effective oxidation, some intense heating but also rather *little* heating in some cases. Presumably, strange shock effects here and there, too.

But: (and this is a big one) There *is* a huge difference between the dynamics of an object zooming through the upper atmosphere of the Earth and one heading for Mars. And it's this: the Martian atmosphere is thinner than ours, and goes out a bit further, and so on (but in many ways would behave like our own at a very high altitude) but it *doesn't* have a 1Bar surface pressure. Objects won't hurtle in then shatter on the 'hard' lower atmosphere, then drift to a halt on the surface. Instead, they'll keep going and I'd wager that their terminal velocities are *not* as on Earth! Volume (and mass) rise as the cube of the diameter, while surface area goes up only by the square, so small objects should be far less effectively slowed on Mars. We're talking about small objects hitting HARD.

I suppose the buckshot effect on some rocks *might* be impact created, then...

...(eating words previously uttered!).


Of course, there's the Heat Shield meteorite to consider...

[helvick]

So:  Some 'minor' (10KPS!) velocity differences between Earth and Mars impacting objects, no effective oxidation, some intense heating but also rather *little* heating in some cases. Presumably, strange shock effects here and there, too.

It's a significant difference but 50kps is an outrageously fast speed nonetheless. At atmospheric entry speeds the surface of the object and the molecules in the atmosphere are atomized to plasma. Then there is all of the complex shock wave hypersonic stuff.

I don't think the differences are as significant as you think. At the altitude that most meteors vaporize on earth (90-100km) the martian atmosphere is thicker due to the lower gravity and on Mars there is more atmosphere above that altitude than on Earth. The shattering\disintegration effect you describe does not happen because the objects hit a 1 bar (or a 0.1, or 0.01 bar) plateau it starts to happen right at the edge of the atmosphere.

You are spot on about terminal velocities - they are much higher on Mars - the lower gravity slows things down a bit but the 0.01 bar ground level atmospheric density is what counts. A 10cm iron rough sphere has a Vt of ~700m/sec on Mars, ~100m/sec on Earth and for fun ~30m/sec on Titan.

Of course, there's the Heat Shield meteorite to consider...

While I still maintain that the "average" meteoride will vaporize unless it's very massive on Mars or Earth there clearly are objects that aren't average.

The points made by dvandorn and abalone about meteorites found on earth are true - these things regularly hit the surface even on earth. The outer atmosphere gets pummelled by the perseids, leonids, taurids and countless other showers of debris but to the best of my knowledge none of those actually makes it to the ground. The stuff that does has to be significantly different from the average in some way, certainly by having an orbit that results in an impact well below the 50-60km/sec that results from the "normal" impact from a retrograde object.

This Arecibo Meteor Velocity Study shows that around 5% come in at around <10km/sec. Those could survive and many probably do.

So if there is a small but not insignificant percentage that should survive then if the Martian surface is extremely old shouldn't the entire surface be blanketed with meteoric debris or dust\debris that has a significant component that is of meteoric origin?

[Bob Shaw]

All very true - but I still think that the population of 'landers' will differ from the Earth. The fast chaps are one thing, but the other guys...?

I've said to as many people as I could for as long as I could that the moon should be an interesting place to look for meteorites, but with the Mars meteorites on Earth and the Antarctic experience in particular I'm beginning to wonder whether Mars - with it's putative long-term mobile ice features and aggressive sublimation - might actually be a perfect environment to look for the remnants of early planetary (plural) surfaces? Consider: we might have analogous situations to Antarctica (Allen Hills etc, blue ice) on Mars which might preseve not only proto-Mars but also rocks from earth, or Venus!

This whole area deserves a l-o-t of thought - imagine a sample return mission which could obtain material from several planets at once...