Laboratory reflectance measurements of analogues to “dirty” ice surfaces on atmosphereless solar system bodies

Abstract

This article presents laboratory measurements of bidirectional reflectances from particle/ice mixtures representing “dirty” ice surfaces on atmosphereless Solar System bodies. The reflectance was monitored as ice sublimed under vacuum from the mixtures leaving behind porous refractory particle layers up to a millimeter thick. Similar layers or “dust-mantles” may evolve on cometary surfaces and on icy asteroids. The measurements cover a 40° to 90° phase angle at a fixed incidence angle of 20° from the surface normal. The spectral coverage is from the U band (350 nm) in the ultraviolet to the R band (720 nm) in the near infrared. Before significant ice sublimation, a 10% (by mass) colloidal silica in water ice sample was the brightest with reflectances in the 0.20–0.30 range. A similar mixture with the addition of 1% carbon had only 0.03 to 0.06 reflectance. A mixture of 1% carbon in water ice (without silica) had an intermediate reflectance of 0.05 to 0.20. Only carbon in ice showed marked angular dependence with a broad increase toward the specular angle. The initial reflectance spectra were slightly blue with reflectivity gradients (RG) < −1.4%/100 nm except for one carbon/silica/ice sample that was red in color with RG = 2.2%/100 nm. The formation of porous refractory layers or mantles through sublimation is coupled with significant changes in reflectance and color. The silica/ice sample more than doubled in reflectance to 0.53–0.78 and became bluer with RG > − 5%/100 nm. Reflectance from the carbon/ice changed to 0.05 with no phase angle dependence and became slightly red (RG < 1%/100 nm). The optical properties of the carbon/silica/ice samples are more complex and depend on the thermal history of the sample. These samples became more blue (RG < −0.67) as a mantle formed with increased reflectivities in the range of 0.07 to 0.18. The processed carbon/ice sample is optically most similar to dark icy Solar System bodies including C type asteroids (RG < 3%/100 nm, albedo ( P v) < 0.05) and the asteroid Chiron (RG < 1%/100 nm, P v ≈ 0.1). The sample is more neutral in color than D type asteroids (RG > 6%/100 nm, P v < 0.05) and Comet Halley (RG = 6%/100 nm, P v = 0.06). The carbon/silica/ices samples, with comparatively high reflectance and blue reflectivity gradients, are unlike asteroids or comets despite containing a mixture of silicate and carbonaceous material, a composition expected of these bodies. Differences in reflectance and reflectivity gradient between our samples and solar system bodies may reflect differing physical processes during mantle formation as well as differences in chemistry of silicate and carbonaceous materials. Further laboratory and theoretical work are needed to fully understand the optical consequences of mantle formation on atmosphereless icy Solar System bodies.