Regolith and megaregolith formation of H-chondrites: Thermal constraints on the parent body

Spectral reflectivity data and its location near an orbital resonance suggest that Asteroid 6 Hebe may be the source body for H-chondrites, the second largest meteorite group. Recent spacecraft images of asteroids and theoretical modeling indicate that, contrary to previous ideas, asteroids can retain thick regoliths. We model the thermal evolution of a Hebe-sized object coated with a thick insulating regolith and heated by Al-26 and other long-lived radionuclides. The heat conduction equations for spherically symmetric objects were solved using finite-difference approximations. We assumed a three-layer structure with regolith and megaregolith overlying a rocky core. The three layers differed in bulk density, porosity, and thermal conductivity. Interior peak temperatures were set to match metamorphic temperatures of H6 chondrites. The regolith has a major influence on thermal history, and the results are very different from those for a simple rocky body published by various authors. Regolith insulation produces a uniform interior peak temperature of similar to 1250 K and moves the petrographic type boundaries close to the surface of the parent body. Petrologic types 3-6 can be produced within 10 km of the asteroid's surface with only moderate (similar to 1 km) regolith thicknesses. The calculations indicate that H4-H6 formation would be consistent with the cooling rate estimates and Pb-Pb formation ages if the material originated in the near surface regions. We suggest that many if not all H-chondrites could have been formed in a megaregolith and thick regolith. Their observed properties are consistent with this environment, especially the abundance of regolith breccias and H-chondrites of all petrologic types with implanted solar wind gases. (C) Academic Press.