Earth may have once looked a little more like Mars: a planet with two moons. It's not the first time the theory has been trotted out by Erik Asphaug, but it's getting new life thanks to an upcoming conference about our natural satellite. The University of California at Santa Cruz professor will share the theory at the September conference, and the Telegraph has a preview. Asphaug says the second moon would have been much tinier—about 1/30th the size of our moon. But after orbiting Earth for a few million years, it would have had quite the impact, literally, on its larger twin.
Asphaug believes that "it would have collided with the moon to leave the one large body we see today." And what PhysOrg terms the "strange terrain" on the moon's far side could very well be the remnants of that little moon, which could have left a layer "tens of kilometers deep" upon impact. A neat factoid from PhysOrg: Mars may one day look more like ... Earth. The bigger of its two moons, Phobos, is expected to crash into the planet sometime in the next 10 million years. (In other moon news, astronomers have named two Pluto moons—and ignored the people's No. 1 choice in the process.)