Cold asteroids may have a soft heart

A new analysis of one of the most well-known meteorites on Earth provides strong evidence that the prevailing view of many asteroids is wrong. Rather than randomly mixed blobs of rock and dust stuck together, it appears that the asteroid that was the source of the Allende meteorite was large enough to have had a molten core, even though its surface remained cold and solid. The new view also suggests that astronomers’ view of how planets like the Earth formed may need revision.

The Allende meteorite fell in Mexico in 1969, shattering into thousands of fragments as it slammed into the Earth's atmosphere and strewing them across dozens of miles of desert. More than two tons of scattered pieces have been found, and it has become perhaps the best-studied meteorite ever.

When the solar system formed, planets built up through the slow accumulation of smaller objects that collided and stuck together. When these growing collections of rubble reached a certain size, radioactive elements within them heated up enough so that the rock melted, and heavier elements tended to sink toward their cores. This separating process (known as differentiation) ended up producing concentric layers of different composition, structured like the layers of an onion. In the metallic cores at the centers of these bodies, swirling eddies of molten metal would produce a magnetic field. Planetary scientists have long thought that asteroids that formed cores must have completely differentiated and melted throughout their interiors. Now, new findings by planetary scientists at MIT and other institutions suggest that may not be the case: that many asteroids with cores might be only partially differentiated, with their outer regions largely unmelted. ...

The new analysis shows that while newly formed asteroids melted from the inside out because of their radioactive elements, their surfaces, exposed to the cold of space and continuing to accumulate layers of new, cold fragments, remained cold. Computer modeling of the cooling process by Elkins-Tanton clearly shows this disparity of a molten interior and cold, unmelted crust, she says.

The decisive new evidence came from studies of the way mineral grains within the meteorite are magnetized: the magnetic orientations of all the grains line up, showing that they became magnetized after the material had all become stuck together, rather than being a remnant of earlier magnetic fields in the swirling cloud of dust from which the object formed. In addition, using a form of radiometric dating involving isotopes of xenon, they could determine that the magnetization took place over a period of millions of years. That rules out an alternative theory that the grains could have become magnetized as a result of a brief pulse of magnetism in the cloud of dust itself.

The finding has implications far beyond the specific asteroid that was the source of this meteorite ...

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