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A case of arrested development

The asteroid’s density would make sense if it were completely solid, free of voids or cracks. However, Rosetta researchers measured the asteroid’s surface craters and identified huge fractures throughout, suggesting the asteroid is relatively porous, a finding that didn’t quite square with the team’s density measurements — after all, the more porous an object, the less dense it should be.

Weiss and his colleagues, including MIT professor Richard Binzel and former MIT professor Linda T. Elkins-Tanton, now head of the Carnegie Institution for Science’s Department of Terrestrial Magnetism, offered a likely explanation for the discrepancy: Perhaps the space rock contains a dense metallic core, with a once melted interior underneath its fractured crust.

The direct observations from Lutetia may provide evidence for a theory developed last year by Weiss, Elkins-Tanton and MIT’s Maria Zuber. The team studied samples of chondrites, meteorites on Earth that have remained unchanged since their early formation. They found samples from the meteorite Allende that were strongly magnetized, and theorized that such magnetization most likely occurred in an asteroid with a melted, metallic core. The theory was seen as a big shift from the traditional picture of most asteroids as primordial, unmelted objects.

If a metallic core does indeed exist, Lutetia would be the first asteroid known to be partially differentiated: having a melted interior overlain by progressively cooler layers. The asteroid would also represent a snapshot of early planetary development. As the solar system began to take form 4.5 billion years ago, planets formed from collisions first of dust, then of larger chunks of rock. Numerous chunks remained relatively small, cooling quickly to form asteroids, while others grew with each collision, eventually reaching the size of planets. These large bodies generated an immense amount of heat – but as a new planet melted from the inside, it cooled from the outside, forming a crust around a molten core.