What is a soil? On Earth, this term refers to the top layer of fine-grained, weathered material, which has been modified by atmospheric, hydrolytic, and biotic effects. Because terrestrial soils contain organic carbon and have been produced at least in part by the activity of soil microbes, it has been argued that this term does not apply on Mars. However, the concept of soil is still valuable. The MER science team has Mars soil as "any loose, unconsolidated materials" as opposed to rocks. On Mars, as on Earth, the most important attributes of soil are that it is broken up and chemically modified by weathering.
Banin suggests that martian soils can be defined analogously to terrestrial soils, as the top non-consolidated layer of weathered or partly weathered material that has been exposed to atmospheric or hydrologic effects. To date, we have no evidence to suggest biotic agents on Mars.
We know about martian soils almost entirely as a result of the five landers that have operated on its surface. Some of the key measurements were made by the Viking landers nearly 30 years ago. The Viking chemical analysis showed that the soil was highly oxidizing and that if there were any organic (carbon) compounds, they were at a level far below that of any soils known on Earth. At that time the absence of organic compounds in the martian soil was considered definitive evidence against the presence of a microbial ecosystem. Compared to terrestrial soils, the martian materials are rich in sulfur and chlorine, depleted in carbonates, and rich in iron oxides. It is the ubiquitous iron oxide (rust) that gives Mars its characteristic red color.
The current Mars rovers Spirit and Opportunity have added greatly to our understanding of the minerals in the martial soil, which can contains many evaporite salts that appear to have been deposited in wet (or at least damp) environments. In local areas the minerals hematite and jarosite have been detected. The grey hematite found on Mars is considered an indicator of past water. On Earth it can collect in layers at the bottom of a lake or other pool of standing water. However, it can also be formed volcanically without liquid water. Jarosite is a mineal usually precipitated from highly acidic sulfate solutions; on Earth it is often associated with acid mine tailings.
A major question for interpreting the minerals on Mars is to decide whether or not they required liquid water for their formation. The production of soil on Mars seems to be a very slow process, and these minerals could have formed billions of years ago when the planet was warmer and wetter. However, there are alternate ways to weather martian rocks and produce some of the same minerals, such as interactions with acidic volcanic haze. The widespread evidence for acid conditions on Mars in now a factor that must be considered when we think about microbial life on that planet.
In his article, Banin also discusses the search for analog sites on Earth that can perhaps be used to study some of the processes that have formed the soil on Mars. The interesting analog on our planet seems to be in the Atacama Desert on the high Andes, which is one of the driest places on Earth. Some recent investigators have found areas in the Atacama that appear to be sterile - the only places on the surface of Earth where life does not seem to have taken hold. Other scientists, however, are finding some organic material and identifiable microbes. Even in this case, however, the Atacama remains the place on Earth that most nearly resembles our sister planet Mars.
Banin calls the Atacama Desert a unique spot on Earth. He suggests that the scientific community should control and preserve sections of this environment for future studies. Thus preserved, we may use it as a testing ground for sterilization and quarantine protocols for Mars missions as well as studying it for what we we can learn here about the history of the soil on Mars.
For an interesting discussion of terrestrial microbes and what they may tell us about the ancient history of Mars, see the following story:
Mars' life signs? Earthly microbes giving directions By Todd Neff, Scripps Howard News Service