[meteorite-list] Giant Planet Birth Linked to that of Primitive Meteorites

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Mar 21 13:24:53 2005
Message-ID: <200503032043.j23KhLE05835_at_zagami.jpl.nasa.gov>

http://carnegieinstitution.org/news_releases/news_050303.html
 
Carnegie Institution of Washington

For Immediate Release

March 3, 2005

Contact Alan Boss at 202-478-8858, or boss_at_dtm.ciw.edu

Images available at:

http://www.dtm.ciw.edu/boss/ftp/chond/cal72efdcn17.jpg

http://www.dtm.ciw.edu/boss/ftp/chond/uniscene_new.pdf

Giant planet birth linked to that of primitive meteorites

Washington, D.C. Scientists now believe that the formation of Jupiter,
the heavy-weight champion of the Solar System's planets, may have
spawned some of the tiniest and oldest constituents of our Solar
System - millimeter-sized spheres called chondrules, the major component
of primitive meteorites. The study, by theorists Dr. Alan Boss of the
Carnegie Institution and Prof. Richard H. Durisen of Indiana University,
is published in the March 10, 2005, issue of The Astrophysical Journal
(Letters).

"Understanding what formed the chondrules has been one of the biggest
problems in the field for over a century," commented Boss. "Scientists
realized several years ago that a shock wave was probably responsible
for generating the heat that cooked these meteoritic components. But no
one could explain convincingly how the shock front was generated in the
solar nebula some 4.6 billion years ago. These latest calculations show
how a shock front could have formed as a result of spiral arms roiling
the solar nebula at Jupiter's orbit. The shock front extended into the
inner solar nebula, where the compressed gas and radiation heated the
dust particles as they struck the shock front at 20,000 mph, thereby
creating chondrules," he explained.

"This calculation has probably removed the last obstacle to acceptance
of how chondrules were melted," remarked theorist Dr. Steven Desch of
Arizona State University, who showed several years ago that shock waves
could do the job. "Meteoriticists have recognized that the ways
chondrules are melted by shocks are consistent with everything we know
about chondrules. But without a proven source of shocks, they have
remained mostly unconvinced about how chondrules were melted. The work
of Boss and Durisen demonstrates that our early solar nebula experienced
the right types of shocks, at the right times, and at the right places
in the nebula to melt chondrules. I think for many meteoriticists, this
closes the deal. With nebular shocks identified as the culprit, we can
finally begin to understand what the chondrules are telling us about the
earliest stages of our Solar System's evolution," he concluded.

"Our calculation shows how the 3-dimensional gravitational forces
associated with spiral arms in a gravitationally unstable disk at
Jupiter's distance from the Sun (5 times the Earth-Sun distance), would
produce a shock wave in the inner solar system (2.5 times the Earth-Sun
distance, i.e., in the asteroid belt)," Boss continued. "It would have
heated dust aggregates to the temperature required to melt them and form
tiny droplets." Durisen and his research group at Indiana have
independently made calculations of gravitationally unstable disks that
also support this picture.

While Boss is well known as a proponent of the rapid formation of gas
giant planets by the disk instability process, the same argument for
chondrule formation works for the slower process of core accretion. In
order to make Jupiter in either process, the solar nebula had to have
been at least marginally gravitationally unstable, so that it would have
developed spiral arms early on and resembled a spiral galaxy. Once
Jupiter formed by either mechanism, it would have continued to drive
shock fronts at asteroidal distances, at least so long as the solar
nebula was still around. In both cases, chondrules would have been
formed at the very earliest times, and continued to form for a few
million years, until the solar nebula disappeared. Late-forming
chondrules are thus the last grin of the Cheshire Cat that formed our
planetary system.

Boss's research is supported in part by the NASA Planetary Geology and
Geophysics Program and the NASA Origins of Solar Systems Program. The
calculations were performed on the Carnegie Alpha Cluster, the purchase
of which was supported in part by the NSF Major Research Instrumentation
Program. Durisen's research was also supported in part by the NASA
Origins of Solar Systems Program.

Caption for image at http://www.dtm.ciw.edu/boss/ftp/chond/cal72efdcn17.jpg

This image uses colors to represent high (red) and low (purple to black)
densities in the equatorial plane (midplane) of a gravitationally
unstable disk after 252 years of evolution from an initially nearly
uniform state. A strong shock front (sharp edge of black region) has
formed at about 12 o'clock, just outside of the inner boundary of the
disk at a radius of 2 Astronomical Units (2 AU; 1 AU is the Earth-Sun
distance = 93 million miles). The radius of the entire region shown is
20 AU. A solar-mass protostar is located at disk's center. Dust
particles rotating in the counterclockwise direction between 2 and 3 AU
encounter the shock front at about 20,000 mph. (Image courtesy of Alan
Boss, Carnegie Institution).

Caption for image at http://www.dtm.ciw.edu/boss/ftp/chond/uniscene_new.pdf

This series of diagrams depicts a unified scenario for the evolution of
solids in the inner solar nebula, the rotating disk of gas and dust in
which our planetary system formed. The first image, (a) at t=0, begins
at 4.6 billion years ago, which is the age of the calcium- and
aluminum-rich inclusions, or CAIs, the earliest solids in the Solar
System. CAIs are already present in the disk, formed close to the
protosun, and possibly lofted by the protosun's bipolar outflow to
greater distances (streamlines about and below the disk). The bulk of
the disk gas is magnetically dead because of the low ionization
fraction, while the surface of the disk is ionized and magnetically
active. The disk (b) is marginally gravitationally unstable, resulting
in the rapid inward and outward transport of CAIs and dust grains.
Spiral arms (c) form Jupiter-mass clumps as disk mixing and transport of
solids continues. The spiral arms and clumps (d) at 5 AU and beyond
drive strong shock fronts in the inner disk capable of thermally
processing precursor dust aggregates into chondrules. Jupiter and Saturn
(e) form either rapidly or slowly, but in either case continue to drive
shock fronts intermittently at asteroidal distances. Chondrules, CAIs,
and matrix-sized dust grains collide and form planetesimals and
planetary embryos in the inner disk. Within 3 million years (f) the
inner solar nebula is accreted by the protosun, leaving behind the rocky
bodies that will collide over the next ~ 30 million years to form the
terrestrial planets and the asteroid belt. (Image courtesy of The
Astrophysical Journal (Letters)).

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Received on Thu 03 Mar 2005 03:43:20 PM PST