Solar System Research, Vol. 37, No. 1, 2003, pp. 31–35. Translated from Astronomicheskii Vestnik, Vol. 37, No. 1, 2003, pp. 34–39. Original Russian Text Copyright © 2003 by Noble, Pieters. Space Weathering on Mercury: Implications for Remote Sensing* S. K. Noble and C. M. Pieters Brown University, Dept. of Geological Sciences, Providence RI 02912 e-mail: [email protected] Received February 22, 2002 Abstract—By applying our understanding of lunar space weathering processes, we can predict how space weathering will effect the soil properties on Mercury. In particular, the extreme temperature range on Mer- cury may result in latitudinal variations in the size distribution of npFe0, and therefore the spectral properties of the soil. INTRODUCTION components of the soil, as well as to garden (overturn) Space weathering processes are very important on it. The products of these weathering processes include the Moon. These processes both create the lunar development of complex agglutinates as well as sur- regolith and alter its optical properties (Pieters et al., face-correlated products on individual soil grains, such 2000; Noble et al., 2001; Hapke, 2001). Like the Moon, as: implanted rare gases, solar flare tracks, and a variety Mercury has no atmosphere to protect it from the harsh of accreted components. Recent detailed microanalyti- space environment and therefore it is expected that it cal studies (Keller et al., 1998; Wentworth et al., 1999) will also incur the effects of space weathering (e.g. describe very thin (60–200 nm) patinas, or rims, of Hapke, 2001). However, there are many important dif- amorphous material developed on lunar soil grains ferences between the environments of Mercury and the (Fig. 1). The rims are created by both subtractive (radi- Moon. These environmental differences will almost ation damage) and additive (vapor deposition and solar certainly affect the weathering processes and the prod- wind sputtering) processes. ucts of those processes. It should be possible to observe The optical effects of space weathering are threefold the effects of these differences in Vis/Nir spectra of the (McCord and Adams, 1973; Fischer and Pieters, 1994). type expected to be returned by Messenger and Bepi As lunar soils mature they (1) become darker (lower Columbo, two upcoming missions which will explore albedo), (2) lose their spectral contrast (the strength of Mercury (Solomon et al., 2001; Grard and Mukai the absorption bands are reduced) and (3) develop a 2001). More importantly, understanding these weather- characteristic continuum for which reflectance ing processes and their consequences is essential for increases toward longer wavelengths. evaluating the spectral data returned from these and other missions in order to determine the abundance of Cassidy and Hapke (1975) first suggested that the iron and the mineralogy of the Mercurian surface. continuum slope might be due to inclusions of nanophase iron (npFe0). These tiny metallic iron parti- cles are now known to be ubiquitous in the rim material LESSONS FROM THE MOON (Keller et al., 1999). In the lunar case, formation of npFe0 in rims is largely created by vapor fractionation The lunar soil samples have provided a wonderful and sputtering of local FeO-bearing material. Neither opportunity to study the effects of space weathering process requires a H-saturated surface (Hapke, 2001). firsthand. “Space weathering” is an ill-defined phrase On the other hand, larger, but still sub-microscopic, Fe that is used to describe a number of processes that result is also found throughout lunar agglutinates resulting in the space environment from the continuous bom- from the reduction of FeO by interaction with solar bardment of the surface by energetic cosmic and solar wind H in a melt. The spheres of npFe0 found in agglu- rays, solar wind particles, and micrometeorites. These tinates are at least twice as large as those found in amor- processes occur on bodies that lack a substantial atmo- phous rims, averaging ~7 nm in diameter vs. ~3 nm for sphere. On the Moon, the constant flux of high energy the rims (Keller and Clemett, 2001). A second study particles and micrometeorites, along with larger mete- using backscattered election images suggests that the orites, act to comminute, melt, sputter and vaporize agglutinitic npFe0 may be significantly larger yet, aver- *This article is based on the paper presented at the 34th Interna- aging over 100 nm in diameter (James et al., 2001). The tional Microsymposium (Vernadskii–Brown) on Comparative size distribution of metallic Fe particles in a soil Planetology (October 8–9, 2001). strongly controls the effects on the Vis/Nir spectrum. It 0038-0946/03/3701-0031 $25.00 © 2003 åÄIä “Nauka /Interperiodica” 32 NOBLE, PIETERS composition suggest that the surface is low in iron. Using microwave data, Jeanloz et al., (1995) suggested that the surface of Mercury may have virtually no Fe2+. Grain McCord and Clark (1979) compared Vis/NIR reflec- tance spectra with lunar data and concluded that the Rim amount of Fe2+ present should be similar to lunar high- lands, roughly 6%. Recently, Hapke (2001) suggested that there should be about 3% FeO present based on the 0 shape of the continuum. Even for the extreme endmem- npFe ber case where the surface of Mercury has no native FeO, the iron brought in by meteorites should be suffi- cient to make the formation of npFe0 through vapor fractionation an important process on the planet. Lack- ing any large scale recycling mechanism (i.e., plate tec- Epoxy resin tonics), meteoritic components should make up several percent of the regolith. From trace elements, iridium in 100 nm particular, it has been estimated that 1–4% of the lunar soil is meteoritic contamination (Heiken et al., 1991). If × Fig. 1. TEM bright field image of a lunar soil grain with a the Meteorite flux at Mercury is 5.5 greater (Cintala, complex weathered rim containing npFe0. 1992), the surface soils may contain as much as 5–20% meteoritic components. Iron brought in by meteorites could account for as much as 1–5% FeO in the regolith. appears that larger nanophase Fe0 particles (those Amounts as small as 0.05 wt % npFe0 are enough to greater than ~10 nm in diameter) result in darkening of affect the optical properties (Noble et al., 2001). the soil (Keller et al., 1998; Britt and Pieters, 1994); The Mercurian environment is also unique in our while the smaller particles (<5 nm in diameter) are solar system because of its extreme temperature range. largely responsible for the more complex continuum- Due to its slow rotation and proximity to the Sun, equa- altering effects. torial regions of Mercury can achieve temperatures above 700 K during the day, while nighttime tempera- tures can dip below 100 K. These conditions will have MERCURIAN ENVIRONMENT important effects on diffusion in glass and crystal It will be a while before samples of Mercury’s soil growth processes. will be available to study, but by understanding the ways in which Mercury’s environment differs from the Moon’s, predictions can be made as to how space SPACE WEATHERING ON MERCURY weathering will effect Mercury. Because of its proxim- What, if any, effects might Mercury’s unique envi- ity to the Sun, Mercury has a flux of impactors 5.5 times ronment have on space weathering products? The pos- that of the Moon (Cintala, 1992). Also, its location in sibilities fall into two groups. (1) Formation processes: the solar system and greater mass require much faster What weathering products are formed on Mercury and velocity impacts. These factors combine to make Mer- how do they compare to those on the Moon? (2) Evolu- cury much more efficient than the Moon at creating tion processes: How do the products of space weather- melt and vapor. Per unit area, impacts on Mercury are ing change as they are exposed to the Mercurian ther- expected to produce 13.5 times the melt and 19.5 times mal regime? the vapor than is produced on the Moon (Cintala, 1992). Mercury has a magnetic field that helps to pro- tect its surface from charged particles, such that the Formation Processes solar wind flux at the surface is significantly less than in Melt products produced from micrometeorites that the lunar environment despite its proximity to the Sun impact on the nightside of Mercury are expected to look (e.g., Hartle et al., 1975; Killen et al., 2001). The com- similar to those observed in lunar soil. The major differ- bination of these factors means that melting and vapor- ence should be the rate of formation. As discussed ization due to micrometeorites will dominate space above, agglutinitic glass and vapor should be forming weathering on Mercury with little solar wind sputtering at a much faster rate (Cintala, 1992). In a mature lunar effects (Hapke, 2001). Furthermore, agglutinitic glass- soil, agglutinates make up as much as 50–60% of the like deposits and vapor deposited coatings should be soil. A mature soil on Mercury probably has little, if created much faster and more efficiently on Mercury. any, original crystalline material remaining. In addi- The nanometer-scale metallic Fe particles (npFe0) tion, Mercurian agglutinates should contain less npFe0 that are ubiquitous in the rims and agglutinates of lunar than their lunar counterparts assuming a lower initial soil (Keller and Clemett, 2001) should also be present amount of surface FeO available. Also, because the on Mercury. Most predictions of Mercury’s surface npFe0 in lunar agglutinates is believed to be created SOLAR SYSTEM RESEARCH Vol. 37 No. 1 2003 SPACE WEATHERING ON MERCURY 33 Mean grain diameter, nm 1day 1year 10 years 100 years 200 200 Moon Mercury Lower limit 150 Upper limit 150 100 100 50 50 FeO nanoparticle diameter, nm 0 500 1000 1500 2000 2500 (Time)1/3, s1/3 0 100 200 300 400 500 600 Heating temperature Fig.