[meteorite-list] Fossils From A Disk-Jet Boundary?
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:41:59 2004
SKY & TELESCOPE'S NEWS BULLETIN - JANUARY 12, 2001
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ASTRONOMERS FLOCK TO SAN DIEGO
The 197th meeting of the American Astronomical Society was held this
past week San Diego, California. Sky & Telescope's on-the-scene
editors Rick Fienberg and Alan MacRobert filed the following reports.
FOSSILS FROM A DISK-JET BOUNDARY?
Meteorites have always carried mystery and fascination, and never more
so than now. They are lumps of ancient extraterrestrial history you
can hold in your hand. In particular, the common type of meteorites
known as chondrites come from a time when the Sun was a newborn T
Tauri variable star surrounded by a protoplanetary disk that later
became the solar system. Chondrites even contain interstellar grains
from before the Sun existed.
Chondrites pose a number of mysteries and apparent paradoxes, which
Frank H. Shu (University of California, Berkeley) described Monday to
a ballroom filled with many of the 1,800 professional astronomers
assembled for the American Astronomical Society meeting in San Diego.
(Also attending the meeting are some 1,200 members of the American
Association of Physics Teachers.)
All chondrites apparently come from the asteroid belt, about 2 to 3
astronomical units from the Sun. Most of their material clearly has
never been heated, since its water-bearing minerals and amino acids
would have been destroyed by heat. Yet embedded within this material
are little nodules of once-molten rock, all roughly a millimeter in
size. Among these, the "calcium-aluminum-rich inclusions," or CAIs,
must have been heated to 1,700 degrees Kelvin -- but only briefly, for
times measured in days, judging by their crystal structures and by
volatile constituents that did not have time to be driven off.
Similarly, "chondrules" of igneous rock, mostly magnesium-iron
silicates, were bought to somewhat lower melting temperatures -- but
for only tens of minutes.
Moreover, the CAIs somehow incorporated short-lived radioactive
material. And chondrules froze amid magnetic fields much stronger than
would be expected in open space near the asteroid belt. What are these
droplets telling us?
Shu and his colleagues think they know. Their theory centers on the
innermost edge of the solar system's early accretion disk, where the
disk presumably gave rise to magnetically driven jets or outflows.
The inner edge of such a disk will not always extend down to the
surface of a newborn star. Instead, the rotating star's strong
magnetic field can halt the inspiraling gas above the stellar surface.
Material piles up at this boundary, and hot gas entrained by the
magnetic field is flung away to form the observed outflows.
Shu's team proposes that the rocky material coming into this region
was bared to the full fury of the central protosun before being
carried away in the outflows. Droplets of molten rock would solidify
as they were carried away by gusts in the gaseous outflow. If large
enough, they would rain from the outflow back onto the face of the
disk farther out -- but only after the wind sorted them by size, like
wheat from chaff. Small nodules would fall back onto the disk far out
and large ones closer in. Those that fell into the region of today's
asteroid belt would all be about the same size, accounting for their
rough uniformity in meteorites.
The CAIs, Shu suggests, ventured closest of all to the early Sun --
and were melted by its direct heat. The chondrules could have been
melted more briefly a little farther out by the frequent flares seen
on newborn stars. The strong magnetic field in which the droplets
froze would cease to be a mystery. High-energy protons from the flares
could account for the radioactivity. After raining back into the disk,
the rock droplets mixed with the cold primordial material there to
form the meteorites we find today. Fully 80 percent of the solar
system's present rocky matter, Shu claims, may have been processed
through the inner disk this way.
Shu's picture met with objection from other meteorite experts in the
audience who have competing theories of their own. But it makes a
number of testable predictions -- for one, that chondrules will be
found in comets -- and represents an intriguing synthesis of current
observations of young stars far away and ancient geology close at
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Received on Fri 12 Jan 2001 04:38:39 PM PST