Mars Meteorite origins Options

[JRehling]

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?

[AndyG]

To put some figures to this intriguing idea, I've used a basic model 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 here. (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