Thursday, September 04, 2008

Where's the soil moisture on Mars?

All the measurements of Martian soil made by a conductivity probe on the Phoenix Lander are consistent with extremely dry soil. There are no indications of thin films of moisture in the soil which is puzzling because the probe has sensed humidity rising and falling at the site. The Lander science team say measurements so far indicate soil that is thoroughly and perplexingly dry.

The team plans deeper tests of the soil, closer to the ice layer.

[right, conductivity probe. Credit, NASA, JPL-CalTech, UA, Texas A&M]


  1. I guess one of the key questions is whether it's even "soil"... My working hypothesis would be that it's only regolith, and the presence of organic matter (making it soil) is a bit unknown...

  2. That's a good point. The NRCS website posts two formal definitions of soil, that in some places allow it to be mineral OR organic, but in other places include biological components as seemingly necessary. It appears they never considered non-terrestrial situations - assuming biology will be pervasive in soil formation. From NRCS:

    What is Soil?

    This definition is from the Soil Science Glossary (Soil Science Society of America).
    soil - (i) The unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants. (ii) The unconsolidated mineral or organic matter on the surface of the earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time. A product-soil differs from the material from which it is derived in many physical, chemical, biological, and morphological properties and characteristics. photo of hand holding soil

    This definition is from Soil Taxonomy, second edition.

    soil - Soil is a natural body comprised of solids (minerals and organic matter), liquid, and gases that occurs on the land surface, occupies space, and is characterized by one or both of the following: horizons, or layers, that are distinguishable from the initial material as a result of additions, losses, transfers, and transformations of energy and matter or the ability to support rooted plants in a natural environment.

    The upper limit of soil is the boundary between soil and air, shallow water, live plants, or plant materials that have not begun to decompose. Areas are not considered to have soil if the surface is permanently covered by water too deep (typically more than 2.5 meters) for the growth of rooted plants.

    The lower boundary that separates soil from the nonsoil underneath is most difficult to define. Soil consists of horizons near the earth's surface that, in contrast to the underlying parent material, have been altered by the interactions of climate, relief, and living organisms over time. Commonly, soil grades at its lower boundary to hard rock or to earthy materials virtually devoid of animals, roots, or other marks of biological activity. For purposes of classification, the lower boundary of soil is arbitrarily set at 200 cm.