QUOTE (AndyG @ Oct 8 2007, 06:57 AM)
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
Yes, I definitely didn't posit that there was any significance to the altitude getting us higher in the gravity well. Even Olympus Mons is pretty small on that scale. </sacrilege>
Per the drag issue, I'd raise another couple of points: In any impact where some ejecta is capable of escaping the atmosphere, there must be other ejecta that are going not quite fast enough to escape, and even a little drag could make the difference.
I don't have any expertise in the area, but if it turns out that a typical one-in-five-million-years impact to strike Mars can only eject a tiny fraction of its mass, then even a slender margin could have a major effect on how much debris escapes. By analogy, how many humans can run the 100 meters in less than 10 seconds vs. how many can run it in less than 9 seconds (about five people and absolutely zero, respectively).
Additionally, the drag would only be minimal for the stuff that's popping straight up, which should also be a tiny fraction of the mass (zero, even?). There should be much more mass traveling more tangential to the surface, so it would have to cover a ground track of many tens or even over 100 km before rising out of the atmosphere. Essentially, the same sort of horizontal path through the atmosphere that makes the Sun look so much redder and dimmer before sunset. I believe on Earth, the light path to the Sun at sunset involves about 15-30 times as much air mass as takes place when the Sun is at zenith. Now contrast the situation in the martian lowlands with what you'd have atop Arsia Mons. The horizontal path off the top of Arsia would almost be in the vacuum of space already, whereas the stuff flying at shallow angles off the lowlands would have to spend around half a minute in the atmosphere of Mars.
Finally, per the space.com article, the favorability of lava sheets is what I was referring to with my comments on inelasticity of the surface. Another factor favoring these same locations.
I'd like to see a model of the spallation that gives the full 3-dimensional account of how much stuff sprays out at all angles, and apply the atmospheric drag as a factor for lowlands and the loftiest heights. It seems to me that there are some possible effects making the multiplier a very large one, but the devil is in the details. If I change my 100 meters example to 47 seconds vs. 46 seconds, then 1 second isn't such a big deal anymore. The question would be if the maximum velocity of ejecta would be very close to the important threshold here (escape velocity). The closer it is, the more a tiny factor like air pressure would make on the amount of ejecta escaping.