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ESA probe finds halfway house asteroid

Lutetia_closest_approach_(Rosetta).jpgPosted on behalf of Rick Lovett.

An asteroid known as 21 Lutetia may be a missing link between Earth-like worlds and more primitive space rocks known as chondrites, an international team of planetary scientists announce in a trio of articles in the latest issue of Science.

The asteroid, first observed in 1852, was the first large asteroid to be visited by a spacecraft, when the European Space Agency’s Rosetta probe flew past it on 10 July 2010.

Rosetta’s primary mission is to visit comet 67P/Churyumov–Gerasimenko in 2014, but on the flyby of Lutetia it was able to measure the asteroid’s gravity and photograph it from many angles, allowing scientists to make accurate maps of its three-dimensional shape.

What was revealed was an object that looked like a knobby, banged up potato, 121 kilometers long by 101 kilometers wide and 75 kilometers thick. And from precise measures of its shape and gravity field, the scientists found Lutetia to be surprisingly dense, averaging about 3.4 grams per cubic centimeter.

That’s one of the highest densities ever measured for an asteroid, says Benjamin Weiss, a planetary scientist from Massachusetts Institute of Technology who was part of the research team. And it’s not possible, he says, if the asteroid is simply a “rubble pile” of broken fragments, as is the case for many other asteroids.

At the same time, however, Rosetta’s instruments showed Lutetia to have a rocky surface, covered in fractures – also inconsistent with its high density.

What this means, Weiss says, is that the interior of Lutetia must once have melted, causing void spaces to collapse into a solid core.

But the melting, produced by short-lived radioisotopes such as aluminum-26 (plentiful in the early Solar System), didn’t spread all the way to the surface. Rather, it halted, leaving a partially melted body with a differentiated interior, but a primordial surface.

That’s a surprise. “Traditionally,” Weiss says, “it was not thought possible that asteroids could melt on the inside only, while retaining an unmelted crust. It has been assumed that planetary evolution only produced two kinds of bodies: fully differentiated, like Earth and Mars, or totally unmelted like many asteroids.”

The new find, he suggests, may mean there is a “whole spectrum” of “strange planetary beasts” that lie between these extremes.

Other scientists are intrigued. Francis Nimmo, a planetary scientist from the University of California, Santa Cruz, calls the find “interesting,” noting that it means there may not be a one-to-one correspondence between asteroid classes and the types of meteorites produced when impacts knock chips off of larger asteroids. “A single body could produce many different classes of meteorites,” he says.

Planetary astronomer Mike Brown of California Institute of Technology adds that the find might lend support to theories that the earliest protoplanets didn’t form by slow, stepwise accretion from ever-smaller ancestors.

“An approximately 100-kilometer, partially differentiated planetesimal sounds a lot like support for ideas that have been put forth in the past few years that asteroids are born large – and thus hot,” he says – though he notes that this is still highly speculative.



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