Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.294s&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 294-295
Other
6
Scientific paper
Micrometeorites up to several hundred micrometers in diameter survive atmospheric entry without melting. A method to calculate the surface temperature of micrometeorites during atmospheric entry was developed by Whipple (1950). Flynn (1990) and Love and Brownlee (1991), using computer simulations of the Whipple method, concluded that large micrometeorites (>75 micrometers in diameter) survive unmelted only if their atmospheric entry velocities are near Earth escape velocity, suggesting main-belt asteroidal parents. This conclusion depends on the assumption of uniform internal temperature in the particles. Large micrometeorites recovered from polar ices have textures ranging from pristine through highly porous, scoriated and finally completely melted, indicating a range of alterations during atmospheric entry. Several 50- to 100-micrometer-diameter Antarctic micrometeorites contain pristine phyllosilicates intermixed with scoriated material, suggesting partial heating (Sutton et al., 1992). Substantial temperature gradients would be required to produce this range of textures by differential heating. We modeled the interior temperature of a 60-micrometer-diameter micrometeorite experiencing a thermal spike at its surface. For a thermal diffusivity of 1x10^-9 m^2/sec, the value measured for lunar soil in a vacuum (Cremers and Hsia, 1974), the entire particle reached the surface temperature within 0.5 seconds (Szydlik and Flynn, 1992). Since the entry heating pulse lasts several seconds (Flynn, 1989) significant temperature gradients would not be expected. We have extended our calculations using the Crank-Nicholson method to compute the internal temperature as a function of radial position and time for homogeneous, spherical micrometeorites experiencing the surface thermal pulse calculated using the Flynn (1989) entry heating simulation. Figure 1 shows the results for a 100-micrometer particle at normal incidence with an atmospheric entry velocity of 15 km/sec. For a thermal diffusivity of 1x10^-9 m^2/sec the peak central temperature is within 100 K of the surface temperature. When the thermal diffusivity is reduced to 1x10^-10 m^2/sec, significant temperature gradients result. The peak central temperature is 700 K lower than the peak surface temperature, and the duration of the heating pulse is much longer at the center than at the surface. Thermal diffusivities as low as 1x10^-10 m^2/sec do not seem appropriate for large Antarctic micrometeorites. The irregular, "unmelted" large micrometeorites are compact, with textures similar to chondritic meteorites. Thermal data for chondritic meteorites indicate thermal diffusivities from 6 to 18x10^-7 m^2/sec (Wasson, 1974), several orders of magnitude larger than that required to produce substantial temperature gradients. Thus, if the textural differences seen in the partially melted large micrometeorites are from differential heating, it is likely that another mechanism, such as local phase transformation of phyllosilicates, is responsible. REFERENCES: Cremers C.J. and Hsia H.S. (1974) Proc. 5th Lunar Sci. Conf., 2703-2708. Flynn G.J. (1990) Meteoritics, 25, 365. Flynn G.J. (1989) Proc. 19th Lunar Planet. Sci. Conf., 673-682. Love S.G. and Brownlee D.E. (1991) Icarus, 89, 26-43. Sutton S. R. et al. (1992) Lunar Planet. Sci. XXIII (abstract), 1391-1392. Szydlik P. P. and Flynn G. J. (1992) Lunar Planet. Sci. XXIII (abstract), 1399-1400. Wasson J. T. (1974) Meteorites, Springer-Verlag, 177-178. Whipple F. L. (1950) Proc. Nat. Acad. Sci., 35, 687-695. Figure 1, which in the hard copy appears here, shows temperature versus time and position [surface, center, and midway between (R/2)] for a 100-micrometer-diameter micrometeorite entering the Earth's atmosphere at 15 km/sec at normal incidence.
Flynn George James
Szydlik Paul P.
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