Microscopy analysis of soils at the Phoenix landing site, Mars: Classification of soil particles and description of their optical and magnetic properties

The optical microscope onboard the Phoenix spacecraft has returned color images (4 ?m pixel?1) of soils that were delivered to and held on various substrates. A preliminary taxonomy of Phoenix soil particles, based on color, size, and shape, identifies the following particle types [generic names in brackets]: (1) reddish fines, mostly unresolved, that are spectrally similar to (though slightly darker than) global airborne dust [red fines], (2) silt? to sand?sized brownish grains [brown sand], (3) silt? to sand?sized black grains [black sand], and (4) small amounts of whitish fines, possibly salts [white fines]. Most particles have a saturation magnetization in the range 0.5?2 Am2 kg?1 as inferred from their interaction with magnetic substrates. The particle size distribution has two distinct peaks below 10 ?m (fines) and in the range 20–100 ?m (grains), respectively, and is different from that of ripple soils in Gusev crater. In particular medium to large sand grains appear to be absent in Phoenix soils. Most sand grains have subrounded shape with variable texture. A fractured grain (observed on sol 112) reveals evidence of micrometer?sized crystal facets. The brown sand category displays a large diversity in color including shiny, almost colorless particles. Potential source regions for these grains may be the Tharsis volcanoes or Heimdal crater (20 km east of the landing site). The black grains are suggested to belong to a more widespread population of particles with mafic mineralogy. The absence of black/brown composite grains is consistent with different formation pathways and source regions for each grain type.