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Giant planet birth linked to that of primitive meteorites

Press Release From: Carnegie Institution
Posted: Thursday, March 3, 2005

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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 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)).

The Carnegie Institution (www.CarnegieInstitution.org) has been a pioneering force in basic scientific research since 1902. It is a private, nonprofit organization with six research departments throughout the U.S. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science.

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