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?
Hey! I like it. Wacky or no, it deserves some comment at least. Hope you get some well-informed replies.
I think it's arguable that while there are 34 known SNCs, they come from only three main groups; all of the meteorites from one group may represent only one meteorite-launching impact. If there were, in fact, only three impacts, you're bound to have selection effects on the ages.
--Emily
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 - http://www.space.com/scienceastronomy/solarsystem/mars_knocks_021107.html 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
Mass ejection from major impacts should not be much affected by the local atmospheric pressure since they literally punch a hole through the whole atmosphere. It might have some effect on smaller impacts.
However I think it likely that the martian meteors we have all come from just a few relatively recent impacts. Most of the stuff in Earth-crossing orbits would be swept up pretty quickly. It could be pure coincidence.
I don't quite buy the argument that an impactor has to hit hard rock to accelerate fragments to 5 kms-1. The Chicxulub impactor landed in limestone/gypsum which is not particularily hard and it still ejected enough material for the re-entering stuff to virtually fry the whole planet, including the antipodes (antipodal fragments must reach more or less orbital speed i. e. 8-11 kms-1 ).
An observation: we have never found any terran meteorites suggesting that most of the stuff ejected by Chicxulub and the group of major impacts at the Eocene/Oligocene border has already been swept up (admittedly a limestone meteorite might not be recognized as such unless it landed in Antarctica).
By the way this might be a good question Emily: just why don't we ever find terran meteorites, one should think that they should be at least as common as martian ones?
I'm trying to figure out who to ask about the dynamics of Earth ejecta vs lunar ejecta and so forth. Renu Malhotra is a name that's popped up after some Google searching; does anyone else interested in this question see any other researchers who might know?
--Emily
Perhaps Jay Melosh at the University of Arizona?
http://www.lpl.arizona.edu/faculty/melosh.html
Yeah, that's a good one. Thanks for the suggestion.
--Emily
To put some figures to this intriguing idea, I've used http://httphttp://www.grc.nasa.gov/WWW/K-12/airplane/atmosmrm.html of the Martian atmosphere and run density/quantity measurements for straightline paths at a variety of starting altitudes and exit angles. The results are tabulated in a jpg http://personal.strath.ac.uk/andrew.goddard/atmosphericquantity.jpg. (I think straight lines are acceptable in this initial instance, where hypersonic material which isn't going to fall below escape velocity, is being slowed for just a few seconds).
In the table, alt is in kilometres above or below the reference radius, the angle of ejection goes from 0 degrees (parallel to the surface!) to 90 degrees (straight up). Results are in % of the vertical amount of atmosphere above the 0km altitude.
The higher the percentage figure, the greater the effects of drag. As you point out, higher altitudes allow for "wider exit cones" depending on whichever drag figure marks a cut off for any particular launch velocity - but with regards to material "popping straight up" it's interesting to see that material released at lower angles than 90 degrees doesn't get badly affected by drag until the angle really is quite low: drag effects are only 10% worse at 65 degrees, 40% worse at 45 degrees: finally doubling at 30 degrees. (In the real - i.e. non straightline, and slower particle - world, exit cones would be, I accept, narrower than these.)
The Martian atmosphere is not great at retarding rocks. I've seen figures suggesting that the freefall terminal velocity for Earth meteorites is around the 180m/s mark. Under lower Martian gravity, but with far thinner air, it would seem likely that (spent) meteorites which hit Mars come in around the 1km/s figure - a fifth of escape velocity.
You mention spallation models, and I agree that this is the key to a greater understanding - though I wonder if a 3d model is necessary, since planetary rotation effects (+/- 250m/s at the equator) could be handled along with drag and exit angle in a 2d case just as easily. What is rather intriguing is the effect of drag, given that it rises with the square of the velocity. There's a chance that - for a bell-curve of initial material speeds and mass distributions (and is that right?), a wider cross section of material launched at different speeds and angles could reach minimal escape velocities...though, as you suggest, are we just seeing the tip of the Olympus-berg?
Andy
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