Space weathering from Mercury to the asteroid belt

Abstract

Abstract. The variety of evidence bearing on the nature of space weathering is reviewed. The effects of space weathering include spectral darkening, reddening and subdued absorption bands, and the distinctive magnetic electron spin resonance caused by single-domain metallic iron particles. Ever since the Apollo missions, two paradigms have dominated the thinking of the planetary science community concerning space weathering: (1) the optical effects are caused by impact-vitrified glass in agglutinates, and (2) the submicroscopic metallic iron results from the reduction of ferrous iron by the impact melting of minerals whose surfaces have been saturated with hydrogen from the solar wind. However, studies carried out since the Apollo program showed that both of these paradigms are invalid. A hypothesis first suggested by the author and his colleagues 26 years ago, but not generally accepted at that time, now appears to be essentially correct: Both the optical and magnetic effects are caused by metallic iron particles smaller than the wavelength in ubiquitous vapor-deposited coatings on soil particle surfaces and inside agglutinates. The vapor is generated by both solar wind sputtering and micrometeorite impact vaporization and injected preferentially downward into the porous regolith. The iron is reduced by a physical process, the selective loss of oxygen that occurs during deposition of the vapor, and does not require heating, melting, or a reducing environment. A mathematical theory that describes the optical effects of the submicroscopic iron quantitatively is derived and applied to the regoliths of the Moon, Mercury and an S asteroid.

Surface Composition of Vesta: Issues and Integrated Approach

“…4 Telescopic spectrum of Vesta (circles) compared to laboratory spectra of different particle size separates for the howardite EET87503 (after Hiroi et al 1994) cal rocks? After decades of study, a principal cause (but not the precise origin) of the optical properties of lunar space weathering is now understood to be the development and accumulation of nanophase metallic iron, npFe 0 , on soil particles (Pieters et al 2000;Hapke 2001). A similar accumulation of nanophase opaque particles appears to be operative to a lesser extent on some chondritic asteroid surfaces altering the surface to what is now observed as an abundant class of "S-type" asteroids.…”

“…Laboratory experiments have various degrees of success in simulating these particular space weathering products along with their optical effects (e.g., Cassidy and Hapke 1975;Moroz et al 1996;Yamada et al 1999;. A model describing the optical effects of nanophase metallic iron was successfully developed by Hapke (2001). The question remains: to what extent do the processes active on the Moon also occur on asteroids of the inner solar system and how are they manifested?…”

The geology of 433 Eros

“…In the lunar regolith, as fresh crystalline material is altered by exposure to the space environment (space weathering), it becomes darker and redder and the mafic absorption feature near 1 ,um is diminished (McCord and Johnson, 1970;McCord and Adams, 1973). This optical alteration is attributed to submicroscopic metallic iron vapor deposited on grains, the vapor resulting from micrometeorite impact and solar wind sputtering (reviewed in Hapke, 2001;Pieters et al, 2001). Also important is the formation of relatively large aggregates of quenched melt and adhering regolith particles, called agglutinates, which tend to darken lunar soils.…”

“…Also important is the formation of relatively large aggregates of quenched melt and adhering regolith particles, called agglutinates, which tend to darken lunar soils. There is considerable debate over the rates at which space weathering (including agglutinate production) takes place in asteroid regoliths (Matson et al, 1977;Hapke, 2001;Pieters et a!., 2001). In the case of lunar-like space weathering the ponded material would correspond (on the basis solidified impact-melt pond or a planar exposure of bedrock) comes in the form of gouges made by low-velocity, low-angle impactors ( Fig.…”

Determination of iron metal optical constants: Implications for ultraviolet, visible, and near‐infrared remote sensing of airless bodies

“…Opaque iron metal and its various alloys are often found as native igneous minerals in meteorites and on planetary surfaces in the form of grains significantly larger than the wavelength of the incident light. However, nanophase iron metal grains are also a key by-product of space weathering [Keller and McKay, 1997;Pieters et al, 2000;Hapke, 2001;Noble et al, 2007] and introduce confounding effects on ultraviolet (UV), visible (VIS), and near-infrared (NIR) spectra as seen in laboratory and spacecraft data for airless bodies (e.g., Clementine, Moon Mineralogy Mapper (M 3 ), Lunar Reconnaissance Orbiter (LRO) Wide-Angle Camera (WAC), Dawn, Hayabusa, and MESSENGER). Sufficient knowledge of the optical properties of these metals is necessary for effective spectral characterization and theoretical modeling analysis of mixtures containing these constituents.…”