Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.504f&link_type=abstract
Meteoritics, vol. 30, no. 5, page 504
Computer Science
1
Asteroids, Heating, Atmospheric Entry, Interplanetary Dust Particles, Zinc Loss
Scientific paper
Large (>35 micron) interplanetary dust particles (IDPs) collected from the Earth's stratosphere are less severely heated during atmospheric entry, on average, than predicted. Evidence of severe heating, ie. magnetite rims, high He-release temperatures, and Zn depletions, is found in a substantial fraction (~20 to 50%) of the small IDPs (10 to 20 microns in size) from the stratospheric collections. Larger IDPs having the same density and velocity distribution should be more severely heated during atmospheric entry. However, Flynn et al. [1, 2] found low Zn contents in only 2 of 13 (15%) large IDPs. All four of these 13 IDPs examined, thus far, by TEM showed no magnetite rims [1]. Since these large IDPs generally appear to be less porous than the smaller IDPs, the observation that a lesser fraction are severely heated suggests the large IDPs have a lower mean atmospheric entry velocity than the smaller IDPs. Pulse heating experiments on 50 to 100 micron fragments of CI meteorite matrix demonstrate the onset of Zn loss by about 1300 K (3, 4), well below the 1600 K melting temperature of anhydrous silicates. For the 13 large IDPs the ratio of those heated between 1300 K and 1600 K (ie., unmelted but showing Zn loss) to those not heated to 1300 K is 2/11 = 0.18. The entry heating of large IDPs was modeled using the equations developed by Fraundorf (5) for the case of minimum entry velocity, 11.1 km/sec (Earth escape velocity from 100 km), and for higher velocities in 1 km/sec steps. IDPs were modeled as 36 micron spheres having a density of 2 gm/cc (consistent with the compact structure of large IDPs). Table 1 shows the fraction not heated above 1300 K, the fraction heated above 1300 K, and the fraction heated above 1600 K. The last line of the table is the ratio between the number of particles heated between 1300 K and 1600 K and those heated below 1300 K. This ratio is very low (0.20) for the minimum entry velocity case, but increases rapidly to 2.1 as the entry velocity increases to 13 km/sec. The ratio then becomes almost constant, varying between between 4 and 5 as the entry velocity increases to 19 km/sec, reflecting the loss of the most severely heated particles by melting. The ratio (0.18) observed for the 13 large IDPs is only consistent with an entry velocity distribution sharply peaked near 11.1 km/sec. This is different from the velocity distributions measured for radar and photographic meteors, which have mean values of 15 to >18 km/sec (6). More detailed consideration of the shapes of large IDPs is required, but the high fraction of large IDPs showing no indication of severe entry heating suggests a source with extremely low geocentric velocity, most likely main-belt asteroids, with little or no contribution from sources giving higher Earth encounter velocities. References: [1] Flynn G. J. et al. (1995) LPS XXVI, 407-408. [2] Flynn G. J. et al., this volume. [3] Klock W. et al. (1994) LPS XXV, 713-714. [4] Greshake A. et al. (1994) Meteoritics, 29, 470. [5] Fraundorf P. (1980) GRL, 10, 765-768. [6] Hughes D. W. (1978) in Cosmic Dust, 123-185, Wiley.
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