http://www.sciencemag.org/cgi/content/full/312/5780/1599
Science 16 June 2006:
Vol. 312. no. 5780, p. 1599
Letters
Stardust Mission Results: Hot in Cold
The News of the Week article "Minerals point to a hot origin for icy
comets" by R. A. Kerr (17 Mar., p. 1536) highlighted the recent
results from the Stardust mission, which sampled dust from comet
Wild 2. The subsequent finding that a major portion of the dust
was crystalline and had formation temperatures in excess of 1400 K
appears to be surprising, because comets are icy objects that
formed some 5 to 40 Astronomical Units (AU) from the sun and were
never exposed to such temperatures.
This "hot" in "cold" structure suggests that some solar nebula
material was processed in the hot, innermost regions of the solar
nebula and transported to the cooler outer regions. Supporting
evidence for this idea is provided by observations of young
stellar objects (YSOs), which indicate that similar crystalline
dust is formed in the inner disk regions, within 1 or 2 AU of a
star (1).
It was first suggested in 1990 (2) that crystalline olivine dust
could have formed in an early solar bipolar outflow and was then
transported to other regions of the solar nebula. These high-speed
jets are produced from and flow perpendicular to the inner regions
of the disks that surround young stars. They may exist for millions
of years and typically eject about 10% of the material that accretes
onto a star (3). A solar mass (M.) YSO will subsequently eject around
0.1 M. of material of which 10-3 M. will probably be "rock-like." If
only 10% of this rock-like material falls back to the nebula, then we
have 10-4 M. of high-temperature, processed material in the solar
nebula, an amount that is approximately equal to the total rock mass
of the Solar System (4). This argument (5) plus other lines of
evidence suggest that a significant portion of the dust in the solar
nebula may have been processed by a solar jet (6, 7).
The Stardust results are consistent with this jet flow model, which
may provide a potentially coherent and predictive framework for
understanding the formation and transport of rocky material in the
solar nebula.
Kurt Liffman
Centre for Fluid Dynamics
CSIRO/MIT
Post Office Box 56
Highett, VIC 3190, Australia
E-mail: [EMAIL PROTECTED]
References
1. R. van Boekel et al., Nature, 432, 479 (2004).
2. W. R. Skinner, Lunar Planet. Sci. XXI, 1166 (1990).
3. N. Calvet, in Herbig-Haro Flows and the Birth of Low Mass Stars
(IAU Symp. 182), B. Reipurth, C. Bertout, Eds. (Kluwer,
Dordrecht, Netherlands, 1997), pp. 417-432.
4. W. B. Hubbard, M. S. Marley, Icarus 78, 102 (1989).
5. K. Liffman, M. Brown, Icarus 116, 275 (1995).
6. K. Liffman, M. J. I. Brown, in Chondrules and the Protoplanetary
Disk, R. Hewins, R. H. Jones, E. R. D. Scott, Eds. (Cambridge
Univ. Press, Cambridge, 1996), pp. 285-302.
7. F. Shu, H. Shang, T. Lee, Science 271, 1545 (1996).
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