Mathematics – Logic
Scientific paper
May 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997m%26ps...32..349k&link_type=abstract
Meteoritics, vol. 32, pages 349-363
Mathematics
Logic
51
Scientific paper
Imaging of asteroids Gaspra and Ida and laboratory studies of asteroidal meteorites show that impacts undoubtedly played an important role in the histories of asteroids and resulted in shock metamorphism and the formation of breccias and melt rocks. However, in recent years, impact has also been called upon by numerous authors as the heat source for some of the major geological processes that took place on asteroids, such as global thermal metamorphism of chondrite parent bodies and a variety of melting and igneous events. The latter were proposed to explain the origin of ureilites, aubrites, mesosiderites, the Eagle Station pallasites, acapulcoites, lodranites, and the IAB, IIICD and IIE irons. We considered fundamental observations from terrestrial impact craters, combined with results from laboratory shock experiments and theoretical considerations, to evaluate the efficiency of impact heating and melting of asteroids. Results: Studies of terrestrial impact craters and relevant shock experiments suggest that impact heating of asteroids will produce two types of impact melts: (1) large scale whole rock melts (total melts, not partial melts) at high shock pressure and (2) localized melts formed at the scale of the mineral constituents (mineral-specific or grain boundary melting) at intermediate shock pressures. The localized melts form minuscule amounts of melt that quenches and solidifies in situ, thus preventing it from pooling into larger melt bodies. Partial melting as defined in petrology has not been observed in natural and experimental shock metamorphism and is thermodynamically impossible in a shock wave-induced transient compressions of rocks. The total impact melts produced represent a minuscule portion of the displaced rock volume of the parent crater. Internal differentiation by fractional crystallization is absent in impact melt sheets of craters of sizes that can be tolerated by asteroids, and impact melt rocks are usually clast-laden. Thermal metamorphism of country rocks by impact is extremely minor. Experimental and theoretical considerations suggest that single, disruptive impacts cannot raise the average global temperature of strength- or gravity-dominated asteroids by more than a few degrees; cumulative global heating of asteroids by multiple impacts is ineffective for asteroids less than a few hundred km in diameter; small crater size, low gravity and low impact velocity suggest that impact melt volume in single asteroidal impacts is a very small (~ 0.01-0.1 %) fraction of the total displaced crater volume; total impact melt volume formed during the typical lifetime of an asteroid is a small fraction (< 0.001) of the volume of impact-generated debris; much of the impact melt generated on asteroidal targets is ejected from craters with velocities > escape velocity and, thus, not retained on the asteroid. Conclusion: The inescapable conclusion from these observations and ca lculations is that impacts cannot have been the heat source for the origin of the meteorite types listed above, and recourse must be taken to processes other than impact, such as decay of short-lived radionuclides or electromagnetic induction during an early T tauri phase of the Sun, to explain heating and melting of the parent bodies of these meteorites.
Keil Klaus
Love Stanley G.
Scott Edward R. D.
Stoeffler Dieter
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