Spectral Properties of Soil Grains as Inferred from Images of the Optical Microscope onboard the Phoenix Mars Lander

The Optical Microscope (OM) onboard the Phoenix Mars Lander has returned high-resolution (4 μm/px) color images of soil particles at the landing site. Spectral information was acquired by illuminating the soil samples with four different types of Light Emitting Diodes (LEDs): UV (375 nm, weak emission at 705 nm), blue (B, 468 nm), green (G, 525 nm), and red (R, 636 nm) [Hecht et al., JGR 113, E00A22, 2008]. Analysis of images acquired in the visible channels (R, G, B) lead to the distinction of four different types of particles: (1) red fines (< 10 μm), (2) brown sand (20-100 μm), (3) black sand (20-100 μm), and (4) white fines (< 10 μm) [Goetz et al., JGR 115, E00E22, 2010]. Most particles (except white fines) were found to be substantially magnetic with a saturation magnetization in the range 0.5-2 A m2/kg. Different strategies were used to derive further information on these particles from the UV images. Their analysis is complicated due to the dual emission of the UV LEDs. Potential UV luminescent particles would be expected to belong to the brightest soil particle population, as dark (black) particles would auto-absorb luminescent emission. However, when taking into account the bright particles’ near-infrared reflectance (705 nm) that would be expected based on their known red reflectance (636 nm) analysis shows that particles with increasing brightness in the RGB color images do not become brighter in UV images. Statistical analysis of the UV images suggests that soil material as a whole does not show any measurable UV luminescence. Its potential quantum efficiency (QE) must be smaller than 0.01 %. An extensive search for particles that would shine up in the UV images was also unsuccessful. From these observations we conclude that the imaged soils do not show a measurable homogeneous level of luminescent emission. Furthermore we exclude the presence of strongly luminescent mineral grains in an (otherwise) non-luminescent soil matrix.