Space Weathering on Phobos and Deimos
Carle M. Pieters Brown Univ/SSERVI Topics Why we suspect space weathering (SPWE) on Phobos & Deimos.
Review what we know of SPWE forms. Lunar Asteroids: S-type; Vesta What do we know of Phobos/Deimos environment? Summary and conjecture.. CM Pieters SPWE Phobos and Deimos variations are without obvious spectral features.
Rivkin et al (2002) Phobos 2 Murchie et al., 1999 CM Pieters SPWE All data are VIS-NIR scaled spectral reflectance.
Phobos and Deimos appear to have affinities with asteroid and lunar soils. Reflectance Spectra of Exploration Targets: Spectral range 450 2450 nm, scaled to unity Remote measurement of spectral properties. NEA Main Belt Asteroids All data are VIS-NIR scaled spectral reflectance. Just what IS Space Weathering?
SPWE Space weathering refers to an array of processes that measurably alter the character of surfaces with time as a result of exposure to the space environment. CM Pieters SPWE Just what IS Space Weathering?
MANY Processes! Many Products! CM Pieters SPWE Just what IS Space Weathering?
S. Noble V1 V4 SPWE CM Pieters SPWE Just what IS Space Weathering?
Lunar-like SPWE:accumulation of npFe0 on regolith grains. CM Pieters SPWE Just what IS Space Weathering?
Vesta-like SPWE: contamination and mixing of exogenic (CC) materials. CM Pieters SPWE Just what IS Space Weathering?
Which processes are most active on Phobos and Deimos? CM Pieters SPWE Observations & Ground Truth: The Moon
L. Keller, S. Noble et al. 1996-present RELAB With time, lunar soils Become darker across the visible Have weaker absorption bands Exhibit a red sloped scaled nearIR continuum CM Pieters SPWE Observations & Ground Truth: The Moon
L. Keller, S. Noble et al. 1996-present RELAB With time, lunar soils Become darker across the visible Have weaker absorption bands Exhibit a red sloped scaled nearIR continuum CM Pieters SPWE Evolution of thought Old View
Space weathering occurs on the Moon, and. agglutinates and glass are the cause of lunar optical alteration. Since agglutinates are unlikely on asteroids, space weathering has no effect. CM Pieters SPWE Evolution of thought Old View (incorrect!) Current Understanding
Space weathering occurs on the Moon, and. agglutinates and glass are the cause of lunar optical alteration. Since agglutinates are unlikely on asteroids, space weathering has no effect. Current Understanding Nano-phase opaques form on lunar grains by XX process. During soil evolution, npFe0 accumulates on grains while Fe0 particles become larger within glass. Absorptions weaken Albedo reduced in the visible Continuum lowers toward short wavelengths Formation of npFe0 requires mobilization of Fe atoms CM Pieters SPWE TEM Images of Lunar Soil Grains
npFe0 is observed as (sometimes multiple) deposits on rims Fe0 particles on the interior of agglutinates are larger. Plagioclase grain L. Keller et al., 1996-present Agglutinate grain Optical properties and amount of npFe0 are directly related
Contaminant 0.005 0.02 0.07 Low FeO 0.13 0.19 0.30 High FeO 2.1 Lunar Samples: Noble et al., 2001 after Morris 1977 CM Pieters SPWE Optical properties and amount of npFe0 are directly related
Contaminant 0.005 0.02 0.07 Low FeO 0.13 0.19 0.30 High FeO 2.1 Lunar Samples: Noble et al., 2001 Experimental: Noble et al., 2004; 2006 CM Pieters SPWE Lunar-like SPWE Current Understanding
Nano-phase opaques form on lunar grains by XXprocess. During soil evolution, npFe0 accumulates on grains while Fe0 particles become larger within glass. Absorptions weaken Albedo reduced in the visible Continuum lowers toward short wavelengths Formation of npFe0on grains requires mobilization of Fe atoms CM Pieters SPWE Is the same process likely or unlikely on asteroids?
CM Pieters SPWE S-Asteroids are ABUNDANT!
Ordinary Chondrites are the most abundant meteorites. S-typeV-typeC-type 40,000 asteroids Sloan Digital Sky Survey Asteroid Types S-type Distance from the Sun Wavelength NEO Observations & in-situ: Eros
S-Asteroid Ordinary Chondrite? [Abundant] NEAR S-Asteroids have weaker absorption bands .and a red sloped continuum. CM Pieters SPWE NEO Observations & in-situ: Eros
S-Asteroid Ordinary Chondrite? [Abundant] NEAR Results: Eros is a slightly (lunar-like) space weathered ordinary chondrite body. [Trombka et al., 2000; Clark et al., 2001; McCoy et al. 2001] Observations & Ground Truth: Itokawa
Hayabusa orbital and earth-based spectroscopy, suggested small amounts of lunar-like weathering for Itokawa. Binzel et al. 2001, MaPSpredict OC parent body. CM Pieters SPWE Observations & Ground Truth: Itokawa
Hayabusa returned regolith grains to Earth laboratories for analyses in 2010. From Hayabusa orbital and earth-based spectroscopy, Itokawa shows small amounts of npFe0 weathering. Furthermore, half of the grains studied contain np-opaque rims (both Fe and S). [Noguchi et al., 2011] Demonstrated Itokawa is a LL Chondrite [Nakamura et al., 2011] CM Pieters SPWE Moon & NEA summary. Prior to Dawns arrival at the main asteroid belt, aconsistent story of space weathering had evolved: Optical alteration is largely dominated by the accumulationof nano-phase opaques (npFe0 or npOpq) on the surface ofregolith grains. Both solar wind and micrometeorite processes areimportant (for the Moon, micro-impact vaporization & depositionmay be common). NEAs exhibit significantly less space weathering productsthan the Moon. npFe0is not the only phase to accumulate on regolithgrains [composition of the host appears to be important]. CM Pieters SPWE Moon & NEA summary. BUT.. What of Vesta, a main-belt asteroid??
Prior to Dawns arrival at the main asteroid belt, aconsistent story of space weathering had evolved: Optical alteration is largely dominated by the accumulationof nano-phase opaques (npFe0 or npOpq) on the surface ofregolith grains. Both solar wind and micrometeorite processes areimportant (for the Moon, micro-impact vaporization & depositionmay be common). NEAs exhibit significantly less space weathering productsthan the Moon. npFe0is not the only phase to accumulate on regolithgrains [composition of the host appears to be important]. BUT.. What of Vesta, a main-belt asteroid?? Observations & Ground Truth: Vesta
The HED meteorites (Howardites, Eucrites, Diogenites) have been associated with Vesta for decades (McCord et al., 1970). Telescopic data are well modeled by very fine-grained howardites,