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
Institution and Prof. Richard H. Durisen of Indiana University, is
published in the March 10, 2005, issue of The Astrophysical Journal

“Understanding what formed the chondrules has been one of 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.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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