HR: 08:05h
AN: P41C-01 INVITED     [Abstracts]
TI: Whipple Lecture. Impacts, Porosity and Volatile Loss
AU: * Wasson, J T
EM: jtwasson@ucla.edu
AF: University of California, 405 Hilgard Ave., Los Angeles, CA 90095-1567 United States
AB: The planetesimals and first-generation asteroids and comets formed in the solar nebula were highly porous. Laboratory and modeling simulations suggest that typical porosities were about 90%. As a result, the products of their mutual impacts were not mainly bowl-shaped craters that lost high-temperature ejecta but rather cylindrical holes that confined most of the high-temperature solid products but that permitted the loss of gaseous products. The low ²⁶Al/²⁷Al ratios observed in chondrules in primitive chondrites imply that the meteoritic parent bodies could not be melted by heat produced by the decay of ²⁶Al (t1/2= 0.73 Ma); it appears that the ⁶⁰Fe abundance was also too low to produce melting. The only other plausible heat source is impact heating, and the con-version of kinetic energy to heat increases and the porosity increases. Melting in amounts larger than the projectile mass is expected at impact velocities >7 km s^-1. Impact heating is a fast process, with much of the key heat-transfer processes occurring on a time scale of minutes to days; in contrast, internal heating (e.g., by radionuclides) is a slow process, with time scales of millions of years or longer. During processing (including chondrule formation) in the solar nebula the most volatile elements formed thin surficial deposits. During impact heating fine grains and surficial deposits may be vapor-ized while grain interiors remain cool, with much of the vaporized materials escaping with the gases liberated by the impact. The range in moderately volatile element abundances in primitive chondrites is limited. For example, S/Si, Ga/Si and Ge/Si ratios vary by less than a factor of 10. The low values, particularly in Ge/Si, that are observed in some iron meteorites (e.g., group IVA) seem best explained in terms of impact volatilization. Large degrees of loss are more difficult to account for by internal heating because no carrier gases are liberated at temperatures high enough to introduce sizable fractions of these elements into the gas phase. Thus, impact processes may have done much more than produce the standard shock effects observed in meteorites. They may have produced small and large scale melting and volatile loss processes.
DE: 5205 Formation of stars and planets
DE: 6205 Asteroids
DE: 6240 Meteorites and tektites (1028, 3662)
SC: Planetary Sciences [P]
MN: Fall Meeting 2005