In a post on 26 August the blogger known as "Metageologist," Simon Wellings, pointed out an odd little paper that appeared a few months ago in American Mineralogist, in which a group of Japanese researchers studying an African meteorite found three tiny pieces of what looks for all the world like eclogite.
Until now, eclogite has been a quintessential Earth rock. Typically, it's what happens to basalt when it's carried deep into the mantle during subduction, subjected to high pressure, and somehow spat up again to the surface too quickly to revert to the minerals it would rather be. And yet here it is reported, three millimeter-sized fragments of it anyway, in a chondrite meteorite from out in the asteroid belt.
Eclogite, as we understand it, can only form in an active planet that can (1) differentiate enough to create basalt and (2) subject that basalt to sustained high pressure. Its presenceeven in a few crumbs in a meteoritedemands a parent body as large as the Moon that was shattered into tiny pieces. We don't happen to have a candidate parent, except possibly Theia, the "Mars-sized object" that is thought to have collided with Earth, way back in the Hadean Eon, to create the mess that formed the Moon.