Space weathering on airless planetary bodies: Clues from the lunar mineral hapkeite

doi:10.1073/pnas.0401565101

Space weathering on airless planetary bodies: Clues from the lunar mineral hapkeite

^*Planetary Geosciences Institute, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996; ^‡Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow 119991, Russia; and ^§Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015

Contributed by H.-K. Mao, March 4, 2004

Abstract

Physical and chemical reactions occurring as a result of the high-velocity impacts of meteorites and micrometeorites and of cosmic rays and solar-wind particles are major causes of space weathering on airless planetary bodies, such as the Moon, Mercury, and asteroids. These weathering processes are responsible for the formation of their regolith and soil. We report here the natural occurrence of the mineral hapkeite, a Fe₂Si phase, and other associated Fe-Si phases (iron-silicides) in a regolith breccia clast of a lunar highland meteorite. These Fe-Si phases are considered to be a direct product of impact-induced, vapor-phase deposition in the lunar soil, all part of space weathering. We have used an in situ synchrotron energy-dispersive, single-crystal x-ray diffraction technique to confirm the crystal structure of hapkeite as similar to the structure of synthetic Fe₂Si. This mineral, hapkeite, is named after Bruce Hapke of the University of Pittsburgh, who predicted the presence and importance of vapor-deposited coatings on lunar soil grains some 30 years ago. We propose that this mineral and other Fe-Si phases are probably more common in the lunar regolith than previously thought and are directly related to the formation of vapor-deposited, nanophase elemental iron in the lunar soils.

Footnotes

↵ † To whom correspondence should be addressed. E-mail: anandm{at}utk.edu.
Abbreviation: np-Fe⁰, nanophase elemental Fe⁰.
↵ ¶ Anand, M., Taylor, L. A., Nazarov, M. A., Shu, J., Mao, H. K. & Hemley, R. J., 34th Lunar Planetary Science Conference, March 17–21, 2003, Houston, 1818 (abstr.).
↵ ∥ Anand, M., Taylor, L. A., Patchen, A. D., Cahill J. T. & Nazarov, M. A., 33rd Lunar Planetary Science Conference, March 11–15, 2002, Houston, 1653 (abstr.).
↵ ** Badjukov, D. D., 21st Lunar Planetary Science Conference, March 12–16, 1991, Houston, pp. 36–37 (abstr.).
↵ †† Badjukov, D. D. & Petrova, T. L., 22nd Lunar Planetary Science Conference, March 18–22, 1992, Houston, pp. 41–42 (abstr.).
↵ ‡‡ Taylor, L.A., Rossman, G.R. & Qi, Q., 26th Lunar Planetary Science Conference, March 13–17, 1995, Houston, pp. 1399–1400 (abstr.).
↵ §§ Bernatowicz, T. J., Nichols, R. H. & Hohenberg, C. M., 25th Lunar Planetary Science Conference, March 14–18, 1994, Houston, pp. 105–106 (abstr.).
↵ ¶¶ Taylor, L. A., The Moon Beyond 2002: Next Steps in Lunar Science and Exploration, September 12–14, 2002, Taos, NM, 3022 (abstr.).