Regolith and Megaregolith Formation of H-Chondrites: Thermal Constraints on the Parent Body

• Published in: Icarus (New York, N.Y. 1962) Vol. 132; no. 1; pp. 185 - 195
• Main Authors: Akridge, Glen, Benoit, Paul H., Sears, Derek W.G.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01-03-1998
• DOI: 10.1006/icar.1998.5904
• EISSN: 1090-2643
• ISSN: 0019-1035

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 by26Al 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 ∼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 (∼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.