[meteorite-list] Hot Asteroids Make Earth-Like Planets More Likely

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:10:02 2004
Message-ID: <200304031900.LAA26016_at_zagami.jpl.nasa.gov>

http://www.abc.net.au/science/news/stories/s822757.htm

Hot asteroids make Earth-like planets more likely
Mark Horstman
ABC Science Online (Australia)
April 3, 2003

Young stars beyond our Sun can form rocky planets like
Earth from nearby gas and dust, suggests a new study of
meteorites which found terrestrial planets formed quicker
than previously thought.

In a report in today's issue of the journal Nature,
German and French geologists have reconstructed the
temperature histories of several meteorites that split
from a single ancient asteroid dating back to the birth of
our Solar System.

Known as 'thermochronometry', the technique also
indicates the asteroid was heated from the inside out.

Led by Dr Mario Trieloff of Mineralogisches Institut der
Universität Heidelberg in Germany, the team used an
ingenious array of 'radiometric clocks' - natural
radioactive isotopes, such as lead (Pb),
potassium-argon (K-Ar), and plutonium (Pu) that
decay at known rates - in order to measure the
asteroid's rate of cooling.

Asteroids have a layered structure rather like an onion.
One type was heated to incredibly high temperatures,
melted, and formed metallic cores of iron and nickel, with
an outer mantle of silicate minerals - similar to the
terrestrial planets Mercury, Venus, Earth and Mars.

Another class of 'undifferentiated' asteroids were not
heated enough to melt or form a metallic core. They
contain millimetre-sized droplets of rock that are the
melted dust grains of early solar nebula, before larger
bodies were formed more than 4 billion years ago. These
tiny time capsules are called 'chondrules', and the
meteorites in which they are found, 'chondrites'.

Meteorites are samples from different depths - and
therefore, temperatures - of an asteroid, reflecting the
history of its formation. The researchers used 'H-chondrites'
(H denoting 'high metallic iron'), that they knew were
derived from the same parent because they shared the same
composition of oxygen isotopes, oxidation, and
rock-forming minerals.

"Part of our problem was that we had meteorites, or small
fragments from this asteroid, but we didn't know exactly
from which depths they were excavated, Trieloff told ABC
Science Online. "Our study checked the models against our
experiments to reconstruct the thermal histories of the rocks
coming from different depths."

Radioactive decay

"Our dating methods use decay of natural radioactive elements
such as 40K (decaying to 40Ar), and 244Pu that was only alive
in the early Solar System, due to its relatively short
half-life of 80 million years. The 'ages' determined in this
way do not mean the rocks came into existence at that time,
but are 'cooling ages' when the rock fell below a specific
temperature."

For example, if the K-Ar 'age' of the feldspar in the sample
was 4.4 billion years, this would indicate when the mineral
was first heated to 280°C. At less than this temperature,
the natural radioactive decay product of 40K would be
retained in the mineral as 40Ar.

The energy source to heat up small asteroids on the inside -
enough to melt and form iron cores - has long been a mystery.
Theories included heat from the harsh glow of an early proto-Sun,
induction heating by 'wind' blasts of ion particles, or
collisions with larger bodies. However, none of these could
account for the necessary high energy levels required.

"Such energy sources would have yielded asteroids where the
exposed outer layers were heated most strongly and cooled most
rapidly," Trieloff said. "The insulated inner regions would have
been heated most weakly, and cooled most slowly. Our results did
not confirm such a cooling behaviour."

Instead, the researchers found a progression of different
exposures to heat, from the coolest on the outside to hottest in
the centre. This implied that the asteroid had been heated from
within.

"We used several meteorites in which the radiometric clocks had
not been disturbed by collisions. The cooling behaviour we
observed agrees with a mathematical model in which an asteroid is
heated by the 'decay energy' of 26Al." [A short-lived isotope of
aluminium with a half-life of 700,000 years that was present in
the early Solar System]

"This is the first time that the decay energy of 26Al has been
demonstrated as the energy source that heated one specific small
body in the early Solar System," said Trieloff. "These results also
imply that accretion of bodies of 100 km size occurred with a few
million years, otherwise 26Al would have been mostly decayed."

Combining all their information from the meteorites, the team
describes the parent asteroid: born in the first few million
years - the dawn of our Solar System - with a radius of about
100 km and an 'onion shell' structure, internally heated by 26Al
to a peak temperature of about 850°C, cooling over the next 160
million years to about 120°C, and more iron-rich than any rock
on Earth.

"If accretion was this fast in our early Solar System, then
formation of planets around other stars may occur similarly fast,
and the existence of other terrestrial planets is likely,"
Trieloff concluded.
 
Received on Thu 03 Apr 2003 02:00:38 PM PST