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geokstr
2004-Nov-23, 11:32 PM
It just struck me - we have identified meteorites from Mars here on Earth. Yet given the distances involved, it would seem unlikely that very many actually landed here. But the moon is VERY close by, and obviously must have taken quite a pummeling for a very long time. Shouldn't this planet of ours be literally covered in moon rocks?

eburacum45
2004-Nov-24, 12:42 AM
Lunar Meteorites (http://www.lpl.arizona.edu/SIC/moon/lunar_meteorites/)

ngc3314
2004-Nov-24, 02:54 PM
It just struck me - we have identified meteorites from Mars here on Earth. Yet given the distances involved, it would seem unlikely that very many actually landed here. But the moon is VERY close by, and obviously must have taken quite a pummeling for a very long time. Shouldn't this planet of ours be literally covered in moon rocks?

There's already a pointer to a list. One thing which did not seem intuitively obvious, and only emerged with detailed simulations in the 1980s, is that the number of lunar and Martian meteorites is comparable. Many bits blasted away from the Moon escape the Earth-Moon system entirely, and once that happens they are not much more likely to re-encounter the Earth than are Martian ejecta. Ahh - there it is. I'll quote from something I posted to the dinosaur e-mail list a few years back:

Martian meteorites are not a new discovery, but getting the idea established took some work on the plausibility - is it likely that a rock could be accelerated from an impact to Martian escape velocity (0.43 of Earth's or 4.8 km/s after leaving the atmosphere) without being (a) melted, (b) pulverized, or (c) metamorphosed beyond chemical recognition? The clinchers were numerical modelling which showed that, at the right distance from the impact, near-surface material was indeed accelerated gradually enough to survive almost unaltered (see H.J. Melosh 1995 Meteoritics 30, 545), and bits of solar-system dynamics showing that indeed there existed orbits reachable from Martian ejection that cross the Earth's in a reasonable time (since the cosmic-ray exposure times for the meteorites are only millions of years, much less than the age of last melting for the rocks). This is discussed by Wetherill 1984 (Meteoritics 19, 1), who finds that the typical time in between planets is of order 10 million years, and that ejection as small fragments is more efficient than as large fragments which must then be fragmented by collisions with other (inner asteroid-belt?) pre-existing objects.

It has been a bit of a puzzle that meteorites of lunar and Martian attribution are about equally numerous. Martian meteorites are more massive, seem to come from deeper in the interior, and are geologically unusually young compared to the Martian surface as a whole. Gladman and Burns have proposed an explanation (Lunar Plan Sci 27, 421, 1996) - but I can't tell what it is since the abstract doesn't say and we don't have the journal. It is also interesting that the possibly aquaeous origin of carbonates and the presence of PAHs in ALH84001 has been reported in other papers by overlapping sets of authors - apparently a few people saw this coming.

The issue of crossing between Terrestrial planets was examined by Melosh and Tonks (1993, Meteoritics 28, 398), who explicitly note the biological implications. Their abstract states, in part, "The results show very little dependence on velocity of ejection. Mercury ejecta is nearly all reaccreted by Mercury or eroded in space--very little ever evolves to cross the orbits of the other planets (a few percent impact Venus). The median time between ejection and reimpact is about 30 m.y. for all erosion models. Venus ejecta is mostly reaccreted by Venus, but a significant fraction (about 30%) falls on the Earth with a median transit time of 12 m.y. Of the remainder, a few percent strike Mars and a larger fraction (about 20%) are ejected from the solar system by Jupiter. Earth ejecta is also mainly reaccreted by the Earth, but about 30% strike Venus within 15 m.y. and 5% strike Mars within 150 m.y. Again, about 20% of Earth ejecta is thrown out of the solar system by Jupiter. Mars ejecta is more equitably distributed: Nearly equal fractions fall on Earth and Venus, slightly more are accreted to Mars, and a few percent strike Mercury. About 20% of Mars ejecta is thrown out of the solar system by Jupiter.

The larger terrestrial planets, Venus and Earth, thus readily exchange ejecta. Mars ejecta largely falls on Venus and Earth, but Mars only receives a small fraction of their ejecta. A substantial fraction of ejecta from all the terrestrial planets (except Mercury) is thrown out of the solar system by Jupiter, a fact that may have some implications for the panspermia mechanism of spreading life through the galaxy. From the standpoint of collecting meteorites on Earth, in addition to martian and lunar meteorites, we should expect someday to find meteorites from Earth itself (Earth rocks that have spent a median time of 5 m.y. in space before falling again on the Earth) and from Venus."

The atmospheric barriers from Venus and Earth also seem formidable, so it seems easier for Mars to seed inward than the innermost planets to send rocks to Mars (on top of the current larger density of potential impactors near Mars-crossing orbits).

kucharek
2004-Nov-24, 03:31 PM
It just struck me - we have identified meteorites from Mars here on Earth. Yet given the distances involved, it would seem unlikely that very many actually landed here. But the moon is VERY close by, and obviously must have taken quite a pummeling for a very long time. Shouldn't this planet of ours be literally covered in moon rocks?
If we would have no geological or other activities on the surface, we may. But the earth's surface is changing so quickly, that much is lost.
There is this idea that going to the moon would enable us to find specimen of earth's crust much older than we can find here. Maybe nearly the whole geological history of earth may be preserved on the moon in form of meteorites from earth that hit the moon.

geokstr
2004-Nov-25, 01:13 AM
ngc3314 wrote:

Venus ejecta is mostly reaccreted by Venus, but a significant fraction (about 30%) falls on the Earth
30% of the TOTAL ejecta from Venus lands on the Earth? How can that be? It would seem the only way that would be even remotely likely is if all the ejecta thrown towards the Earth's orbit would somehow stay in the plane of our orbit, then come all the way out here and stop directly in our path until the Earth came around to sweep it up.

The circumference of Earth's orbit is 940 MILLION kilometers and the diameter of the Earth is only 13,000 kilometers. How could the ejecta from Venus be aimed anywhere near well enough for 30% of it to fall here? Doesn't make any sense; Melosh & Tonk's models must be assuming a lot of really good marksmanship by somebody is involved here.

kucharek said:

If we would have no geological or other activities on the surface, we may. But the earth's surface is changing so quickly, that much is lost.
The models cited don't take the moon into account. Since we are so close, much of the ejecta would have been aimed right at us, and doesn't need to go far to be caught in our larger gravity.

The only thing that makes some sense is something along the line of kucharek's comment about the relatively high geological activity eroding and subducting the ejecta from the moon. And then only if we can assume that the moon stopped getting pummeled almost immediately after it formed, and all the Martian rocks are very young (which they are).

Am I understanding this correctly? (I do not have a science backgound, but have always been interested in it, but do read a lot, so be gentle, OK?)

ngc3314
2004-Nov-25, 02:55 AM
ngc3314 wrote:

Venus ejecta is mostly reaccreted by Venus, but a significant fraction (about 30%) falls on the Earth
30% of the TOTAL ejecta from Venus lands on the Earth? How can that be? It would seem the only way that would be even remotely likely is if all the ejecta thrown towards the Earth's orbit would somehow stay in the plane of our orbit, then come all the way out here and stop directly in our path until the Earth came around to sweep it up.

The circumference of Earth's orbit is 940 MILLION kilometers and the diameter of the Earth is only 13,000 kilometers. How could the ejecta from Venus be aimed anywhere near well enough for 30% of it to fall here? Doesn't make any sense; Melosh & Tonk's models must be assuming a lot of really good marksmanship by somebody is involved here.


I haven't looked at the papers in several years, but their numbers may result from a basic law of Solar-system dynamics - objects in solar orbit remain in solar orbit until they hit something (or have a close enough planetary encounter to eject them from the system). That is, the eventual fate of Venus ejecta wil be collisions with one or another of the inner planets, and that will mostly be Venus again with a significant contribution from Earth (since very little Venus ejecta makes it out far enough for Jupiter to be a big factor). The Venus favoritism follows from the fact that the ejecta will have orbits that go very close to the point at which they departed the planet. And Earth collisions must be vastly helped by planetary perturbations changing the eccentricity and incliinations of the rocks' orbits on million-year timescales; otherwise they could be stuck in out-of-ecliptic orbits that don't go remotely near other planets.

In the simple in-plane case, the chance of hitting Earth per crossing of our orbit would be 13,000/940,000,000 or about one in 70,000 (so the typical time to collision is about 70,000 years, depending on the debris' orbital period; short that I would imagine). In the out-of-plane case, perturbations mean that there are only windows in time when the orbit can cross, so the clock stops between these windows.

geokstr
2004-Nov-25, 08:04 PM
OK, let's see if I understand this:

You can't just look at the size of the Earth itself, but also at the region of space surrounding the entire circumference of Earth's orbit, where the Earth's gravity is stronger than any other body's. That would be what? - (approx) halfway between Earth and Venus and 3/4 of the distance to Mars, and even a much greater distance perpendicular to the plane of the solar system, because there wouldn't be any other body's gravity competing with Earth's in those directions. That IS a pretty big place, probably a doughnut shaped region flattened in the plane of the solar system.

If the ejected material takes up orbit anywhere in that entire region, it will eventually, over millions of years, fall, or be slowly tugged, into the path of the Earth and land here? If it took an orbit anywhere outside that region, it would eventually be pulled into the orbit of whatever body had the strongest gravity in THAT region of space.

Did that make sense?

Perhaps that is at least part of the hundred tons of meteors and micrometeors the earth accretes every day - just continually sweeping up the mess.

Thanks for all the enlightenment.

Evan
2004-Nov-25, 10:01 PM
It's not quite like that. If something is ejected from Venus with velocity above escape velocity it will assume a solar orbit that will come back to the point where it left Venus (same as launching a satellite). The orbit will certainly be very eliptical and would impact Venus again after one orbit if it weren't for the fact that by then Venus has moved on.

If the orbit has an aphelion (furthest point from sun) that extends to the orbit of Earth then it has a chance of impacting upon Earth, regardless of the orbital plane of the ejecta. If it even comes close to Earths orbit it will eventually be perturbed by Earth/Moon (enough to miss Venus at perihelion) eventually not to mention Jupiter which affects everything in our system. Some of the Apollo asteroids fit this description. The object need only cross the ecliptic near Earths orbital distance. The inclination of the objects orbit doesn't matter.