According to New Scientist, a Russian meteorite expert believes that he has found a chunk of Phobos. A meteorite that fell in a Soviet military compound in Yemen in 1980 is apparently carbonaceous and meteoritic looking (though in a rather weird way) but also has some volcanic minerals embedded in it. Volcanoes are not something one expects in the asteroid belt, where most meteorites come from; asteroids aren't big enough to have hot volcano-driving innards. So Andrei Ivanov of the Vernadsky Institute of Geochemistry and Analytical Chemistry in Moscow thinks the best way to explain the volcanic fragments is by appealing to Phobos, or possibly Deimos, as a source. Phobos looks like an errant carbonaceous asteroid that has been captured by Mars's gravity. The same impacts that throw rocks from Mars to earth will also throw far more rock into orbit around Mars itself, and Phobos will sweep up some of that debris. So Ivanov reckons Phobos offers the most plausible source for his carbonaceous-with-hints- of-volcanism meteorite.
If this is right (and according to New Scientist, Ivanov's American colleague Michael Zolensky of Johnson Space Center isn't entirely convinced), then this is the most wonderfully well-traveled mongrel rock imaginable. Some of it started off in the belt before finding itself in orbit around Mars; other parts were forged in Martian volcanoes and later blasted into orbit. (I say "later" because I assume that even a vast Martian volcano can't muster the oomph to throw fire-fountain fragments straight into orbit. I'd love to be wrong about that though. A moon brushed by the feathered edges of a volcanic plume rising from the planet below would be a wondrous thing.)
And then the whole mixed-up concretion was thrown to earth by another impact.
The idea of stuff moving from planets to moons without the benefit of rocketry is not in itself an untoward one -- it's the idea of stuff moving from planet to planet via an intermediary moon that's wild. Rocks from the earth must be thrown to our moon just as rocks from Mars must be thrown to Phobos and Deimos. Indeed, this is one of the better scientific reasons for going to the moon. There are hardly any samples of rock from the first half billion years or so of earth history on the earth; erosion and plate tectonics have destroyed almost every trace of our planet's early childhood. However, rocks that were blasted off the early earth (and in those days of heavy bombardment there would have been a lot of them) and landed on the moon might be preserved. John Armstrong of the Weber Sate University sums up the idea well when he calls the moon "earth's attic" -- the place where our house's ancient dust accumulates undisturbed.
And the transit system also works the other way round. Dozens of meteorites on the earth have been identified as coming from the moon. And there's every reason to think that fragments of Phobos are falling to Mars. As Sho Sasaki of the University of Tokyo argued in this abstract, even small impacts on Phobos will kick up dust that will then go into orbit around Mars. Most of this dust will fall to the Martian surface; some of it will hit Phobos again, dislodging yet more dust in its turn. If this secondary dust production is efficient enough, there could be a self-sustaining process going on in which Phobos is gently eroded away as it orbits through an exfoliating dust ring of its own making; the ill-fated Japanese Nozomi mission was going to look for a dust ring generated this way. Sasaki thinks that if this ring-erosion were going on a metre of Phobos's outer surface might be lost every million years or so, and most of it would end up on the surface of Mars. (In this it is merely anticipating the whole moon's fate; tidal drag is bringing Phobos ever closer to Mars, and at some time in the future the moon will come crashing down, probably having broken into fragments first. It's possible that some of the craters on the martian surface mark the graves of ancient moons that fell to Mars.)
As it happens, evidence for something rather like what Sasaki is describing going on elsewhere in the solar system is published in today's Science (there's a brief account avaiable to non-subscribers in The Economist). Saturn's moon Iapetus has long been known to be dramatically darker on one side than the other. A team of radar researchers who've been bouncing signals from the Arecibo dish off Iapetus argues that this darkness may be best explained as an accumulation of dark dust given off by other moons on the "leading edge" of Iapetus. This is probably a more plausible, if less thrilling, explanation for the asymmetry than Arthur C. Clarke's suggestion in "2001" that it might be a way of drawing our attention to a large alien monolith. (In the movie the monolith was moved to Jupiter; but Doug Trumbull liked the effects shots he'd developed for Saturn's rings enough that he used their descendants in a film of his own, "Silent Running", a few years later. Saturn is not a very good place to put a spaceship carrying lots of plants that want to photosynthesise. But it definitely looks pretty.)
So fragments of Mars may be embedded in Phobos, and fragments of Phobos are probably drizzling down on Mars. In fact, this process could conceivably have some relevance to the methane story. When I looked at the question of whether the martian methane might be generated from meteoritic material (back here) I used a figure for the influx of material on to the martian surface which ignores the contribution from Phobos. If Sasaki is right, and if I'm reading him right, you might get a few hundred tonnes of carbonaceous material every year from Phobos on top of the main flux. My suspicion is that it isn't enough to make a big difference. But it's an intriguing wrinkle.
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