Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.489b&link_type=abstract
Meteoritics, vol. 30, no. 5, page 489
Other
1
Calcium-Aluminum-Rich Inclusions, Chondrites, Condensation, Nebular, Nebula, Solar, Volatile Depletion
Scientific paper
Chondritic meteorites contain components which have experienced widely varying degrees of thermal processing, yet eventually accumulated into the same planetesimal. The strong correlation of the depletion of volatile elements in the bulk compositions of CV3 carbonaceous chondrites with condensation temperature implies that either evaporation or incomplete condensation was responsible for volatile depletion [1]. The absence of any evidence for isotopic fraction of potassium in chondrites rules out evaporation of solids in a cool nebula, as nearly any conceivable evaporative process would have led to isotopic fractionation [2]. Survival of solids condensed during a relatively hot (about 1200 K) phase of nebular evolution seems to be required [1]. Flash heating to much higher temperatures (about 2000 K) is required to melt and thermally process chondrules [3]. Fluffy Type A calcium, aluminum-rich inclusions (CAIs) appear to have condensed from a hot (about 1400 K) nebular gas [4], while Type B CAIs seem to have been flash heated [5]. However, chondrules contain FeS, possibly implying that the ambient nebula temperature was less than 700 K during the flash heating event [3]. The retention of noble gases in presolar grains found in chondrite matrices similarly limits ambient temperatures to less than about 700 K at the time of their addition [6]. Regardless of the requirements of CAI and chondrule flash heating processes, we see that chondrites contain components that appear to have been formed when the ambient nebula temperature was about 700 K, 1200 K, and 1400 K -- hence the paradox. Two types of solutions to this thermal heterogeneity paradox are possible, namely spatial or temporal thermal variations. Provided that the midplane is hot enough [7,8,9], nebular temperatures spanning the range of 1400 K to 700 K (and considerably lower temperatures) could occur at increasing altitude at fixed orbital radius and at a given time. Considering that small dust grains may remain suspended at all altitudes throughout a turbulent disk [10], spatial thermal heterogeneity would be a conceivable solution if samples from a range of nebular altitudes can be preserved in a planetesimal. The nebula's midplane temperature (T(sub)m) may have dropped from about 1200 K to 700 K over radial distances of 2 AU to 3 AU [8], and provided that mixing of products from throughout this region was possible, the paradox could again be explained [2]. The other alternative, temporal variations, is perhaps the more traditional choice. CAIs are interpreted as the first condensates from an early, hot nebula, with the bulk of the chondritic material condensing later at somewhat lower nebular temperatures. The flash heating that melted the chondrules occurred when the nebula had cooled even further. Radiative hydrodynamical models [11] of temperatures in a nebula undergoing mass accretion at astronomically-inferred rates [12,13] imply that inner nebula temperatures are a strong function of the nebula mass. At orbital radii of 2 AU to 3 AU, a 0.04 M nebula has T(sub)m similar to 1400 K, a 0.02 M nebula has T(sub)m similar to 1200 K to 700 K, and a 0.01 M nebula has T(sub)m similar to 800 K to 500 K. An initial nebula mass of at least 0.04 M may be necessary, considering the inefficiency of the planet formation process. If the thermal history of the solar nebula can be represented by this sequence of models with decreasing nebula mass, then the full range of ambient nebula temperatures implied by the meteoritical data could be explained. References: [1] Palme H. and Boynton W. V. (1993) in Protostars and Planets III (E. H. Levy and J. I. Lunine, eds.), 979. [2] Humayun M. and Clayton R. N. (1995) GCA, 59, 2131. [3] Wasson J. T. (1993) Meteoritics, 28, 14. [4] Grossman L. (1980) Annu. Rev. Earth. Planet. Sci., 8, 559. [5] Stolper E. and Paque J. M. (1986) GCA, 50, 1785. [6] Ott U. (1993) Nature, 364, 25. [7] Morfill G. E. (1988) Icarus, 75, 371. [8] Boss A. P. (1993) Astrophys. J., 17, 351. [9] Cassen P. (1994) Icarus, 112, 405. [10] Dubrulle B. et al., Icarus, 114, 237. [11] Boss A. P. (1995) Astrophys. J., in preparation. [12] Hayashi M. et al. (1993) Astrophys. J., 418, L71. [13] Hartigan P. et al. (1995) Astrophys. J., in press.
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