Mars Meteorite origins |
Mars Meteorite origins |
Oct 8 2007, 04:57 AM
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Senior Member Group: Members Posts: 2530 Joined: 20-April 05 Member No.: 321 |
Walking the line between wacky and sure to be unoriginal:
The martian meteorites are quirky in the ages, in that about half of them have ages of roughly 175 million years, whereas most of the martian surface is clearly older than that. There must be a selection effect biasing the samples we find. Is it possible that altitude (on Mars) is a major selection factor? The top of Olympus Mons is above 90% of Mars's atmosphere. Of course, that represents a very small fraction of the surface area of Mars, but when you add in the heights of the five biggest volcanos on Mars, you get a still small but nonzero area of the surface of Mars at high altitude AND likely to be the last places on Mars to get paved over with lava. When an impactor strikes these areas, it faces much less air resistance (and spalling) on the way in, and then any ejecta flying skyward also faces much less air resistance on the way out. Additionally, sheets of lava should make for more elastic collisions than the dusty regolith at lower altitudes. Could it be that these selection effects favor the highest volcanos so greatly that ejecta from these places outnumber ejecta from the rest of the surface combined? |
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Oct 8 2007, 01:57 PM
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#2
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Member Group: Members Posts: 593 Joined: 20-April 05 Member No.: 279 |
It's an interesting idea, and thank you for making me think about it - but it's not one I think is correct.
Firstly, from an altitude and velocity point-of-view, at the top of Olympus Mons the energy needed for rocks to escape Mars is still over 99% of that required by materials thrown out from the bottom of Hellas, around 30km below. Not much to bias things here. Looking at the atmosphere issue: Martian escape velocity is close to 5km/s. This feels fast, but with Mars' reduced (mean surface) pressure, that's a drag equivalent to a speed of around 500m/s in Earth's atmosphere. I suggest that's warming but not hugely damaging to any rocks that survive being accelerated to escape velocity in a fraction of a second - and at these speeds therefore free of the atmosphere in just a few seconds. That's not very long for drag to have an effect. So it seems there's little statistical bias against materials surviving being cast from lower altitudes, nearer the mean surface. A quick google later - this old space.com article gives an insight into why younger rocks might be preferentially selected. (The suggestion is that common-sized impactors have to hit younger, less disrupted surfaces in order to create escape velocity debris). Andy |
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