[meteorite-list] Magma Oceans Sloshed Across Early Asteroids

[Ron Baalke]

http://www.newscientist.com/article.ns?id=dn7522

Magma oceans sloshed across early asteroids
Jeff Hecht
New Scientist
June 15, 2005

Oceans of molten rock, or magma, covered some asteroids in the early
solar system, reveals a new study of meteorites. But researchers are
still puzzled over why other asteroids apparently did not melt at all.

In the solar system's first few tens of millions of years, collisions
between rocky objects and the decay of radioactive isotopes melted the
interiors of large objects. Magma oceans - perhaps hundreds of
kilometres deep - lapped over the Moon, the Earth, and other planets,
allowing dense material to settle towards their centres in a process
called differentiation. But the extent of asteroid melting had remained
unclear.

Now, Richard Greenwood at the Open University in Milton Keynes, UK, and
colleagues have analysed groups of meteorites thought to have come from
the 530-kilometre-wide asteroid Vesta and from a second, still-unknown,
asteroid.
          
Short half-life

They found all of the meteorites from each source shared the same ratios
of oxygen isotopes, suggesting both asteroids must have melted almost
completely. "It's an exquisite piece of work," says Michael Drake, a
geochemist at the University of Arizona in Tucson, US.

But the research fails to explain why other asteroids do not show any
evidence of melting. Ceres, the largest known asteroid - 930 kilometres
wide - appears to be totally undifferentiated.

Drake thinks the difference may be down to timing. Previous research has
suggested asteroids were heated by the decay of radioactive aluminium-26
in the dusty disc from which the solar system condensed. That isotope
has a half-life of only 700,000 years. So if it was the main heat source
for the first asteroids, too little may have remained to warm those that
formed later, Drake says.

Journal reference: Nature (vol 435, p 916)