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The Planetary Society Weblog
By Emily Lakdawalla
Mixing up the early inner solar system
Apr. 5, 2006 | 10:46 PDT | 17:46 UTC
There was a press release in my inbox this morning from the Royal Astronomical Society about new simulations by Jonti Horner and coworkers of the formation of Mercury. Mercury -- as the MESSENGER team likes to tell you -- has by far the densest bulk composition of any planet, something that has been tough to explain. With these simulations, Horner has shown that a popular hypothesis, involving a giant impact, could make a dense Mercury from a protoplanet that looked more like Venus and Earth. Here's some snapshots from the simulation: Simulation of the formation of Mercury from a giant impactMercury's very high density has been difficult to explain. Computer simulations of a giant impact event early in Mercury's history have shown how the original rocky material surrounding the proto-Mercury (blue) could have been blasted off into space by the oblique impact of a smaller protoplanet, leaving Mercury with a disproportionately large amount of metal-rich core material (red). The three images show Mercury at 2, 8, and 194 minutes following the simulated impact. The third view is zoomed out by a factor of 6 to show the scattering of the proto-Mercury's rocky mantle particles. Credit: Horner et al. 2006 |
Taken this far, it's just another planetary formation simulation. But Horner and his team took another step: they took the endpoint of this simulation -- the positions and velocities of lots of big ex-Mercury chunks -- and ran another simulation forward in time millions of years to see what would happen to all those chunks. A goodly number (50%) crashed back on to Mercury within 4 million years, but that leaves a lot of Mercury sprayed out in the inner solar system, being scattered by interactions with the Sun, Venus, and Earth; some of it eventually hits Earth. The release closes: "Given the amount of material that would have been ejected in such a catastrophe, it is likely that there is a reasonable amount (possibly as much as 16 million billion tonnes [1.65x10^19 kg]) of proto-Mercury in the Earth." For context, the entire mass of Earth is 5.9736x10^24 kg, so the ex-Mercury stuff accounts for less than a thousandth of a percent of Earth's mass, not very much. Still, it's interesting to think about all the different places in the solar system whose rocks, metal, ice, and dust could have contributed to our own little rockball.
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