[meteorite-list] 4.567 billion years-- and a month.

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Sat Apr 2 22:31:32 2005
Message-ID: <33pu41leld9bep1e9hujepbur3n2ikurjh_at_4ax.com>

http://www.spaceref.com/news/viewpr.html?pid=16538

PRESS RELEASE
Date Released: Saturday, April 2, 2005
Source: NASA Astrobiology Institute

Using Isotopes to Probe the Earliest History of the Solar Nebula

By: David Morrison, NAI Senior Scientist

Members of the NAI UCLA team led by Ed Young are using high-precision analysis of tiny grains in
meteorites to probe the earliest history of the solar nebula. The age of the solar system is set at
4.567 billion years, and the new work traces some of the history of these small grains during about
300,000 years, before the formation of comets, asteroids, or planets. All the solid objects in the
solar system formed within a circumstellar protoplanetary disk called the solar nebula. Among the
oldest materials are tiny calcium- and aluminum-rich grains or inclusions (called CAIs) found in
some meteorites. The earliest history of these grains can be probed by studying their magnesium
isotopes, since part of this magnesium was produced by the rapid radioactive decay of aluminum-26,
an isotope with a half-life of less than a million years. Somewhat ironically, we have better
time-resolution on these earliest events than for most of the later history of the planetary system.

A recent paper by Ed Young and other members of the UCLA NAI team, done in collaboration with
colleagues in England, examines this early history of CAIs. The absolute ages of these oldest
grains, determined by the decay of uranium into lead, is 4567 million years (4.567 billion years),
which is usually defined to be the age of the solar system. Part of the new high-precision analysis
of meteorites reported in this paper leads to a re-evaluation upward of the amount of aluminum-26
present when the solar system formed. Knowing this initial quantity of radioactive aluminum allows
scientists to make better use of the aluminum-decay "clock" to understand events at the time dust
was condensing in the early solar nebula.

This new work looks at the history of the CAIs as revealed by the balance between aluminum and
magnesium, which can be altered by heating. They conclude that over about 300,000 years temperatures
were high enough for transfers to take place between the minerals melilite and anorthite. However,
these temperatures cannot have been elevated for that entire time. Rather, the grains must have
experienced many episodes of heating and cooling.

Two mechanisms are suggested for this episodic heating. One possibility is that the orbits of these
millimeter-sized grains within the solar nebula brought them very close to proto-sun perhaps as many
as 15 times, where temperatures up to 1600 K persisting for 10-20 years would have caused partial
melting of the grains. To achieve such high temperatures, the grains would have needed to come much
closer to the young Sun than the present orbit of Mercury. Alternatively, part of the grain heating
could have come about from shock waves generated by turbulence in the protoplanetary disk.

Studies like this, based on analysis of meteorites, can tell us something about the conditions in
the solar nebula before the planets, or even the comets and asteroids, were formed. These data can
also be compared with remote sensing of circumstellar disks seen today, where other planetary
systems are being born in our Galaxy.

The paper by Young et al., titled ?Supra-Canonical Al-26/Al-27 and the Residence Time of CAIs in the
Solar Protoplanetary Disk, was published on-line in Science Express for March 3, 2005.
Received on Sat 02 Apr 2005 10:39:15 PM PST