It comes as no surprise that Mars is dry. However, because soil covers most of the planet, some scientists thought that aqueous conditions may have contributed to this soil’s formation. The Phoenix Mars Lander carried instruments that allowed researchers to performed detailed measurements on the distribution of particle sizes in the soil. Analysis of this distribution reveals that Martian soil bears more resemblance to lunar regolith than to Earth soil. Furthermore, the research provides strong evidence that the soil formed in the absence of water and that Mars is hostile to life.
I dug the trenches for lawn sprinkler systems at the last two houses I purchased. The fluvial (or river-related) deposits of my current home made the shovel-work relatively easy. However, the compacted, clay-like soil at my previous abode resulted in lots of frustration, a few blisters, and one very sore back. These laborious projects produced clear evidence that the two types of soils had formed from different processes even though both involved liquid water. Using more sophisticated tools, scientists can distinguish what processes produced different types of soil. Application of those tools to Martian soils provides additional evidence of the hostility to life of the Martian environment.
Soils can differ in the size of the particles that comprise them. Typically, mechanical processes (like wind or river erosion) produce larger-grained soils, whereas chemical weathering (where chemical reactions breakdown rocks) leads to smaller-grained soils. On Earth, chemical weathering usually occurs in aqueous environments and produces clay minerals. For reference, these clay-like soil particles have dimensions below a few micrometers (μm, 10–6 meters, roughly the size of bacteria or the finest human hair) and most commonly less than a μm.
NASA’s Phoenix lander uses two instruments to search for particles of this size in the Martian soil. An optical microscope resolves particles as small as 4 μm and an atomic force microscope finds details down to 0.1 μm.
Analysis of the Martian soil by these instruments revealed two different populations of particles. The first type of soil consisted of larger, round-bodied particles, most likely produced by mechanical processes not involving water. The second set represented the smallest particles found in the Martian soils and included particles 11 μm and smaller. Further investigation indicated that all the particles in the second set formed by the same physical processes. However, particles smaller than 2 μm (those associated with aqueous formation processes on Earth) comprised a very low fraction of the mass.1
The researchers used the sparse amount of small particles to place a limit on the amount of aqueous interaction in the soil over its history. A 6 percent by volume fraction of particles below 2 μm would require up to 400,000 years of interaction between rocks and water. The measured fraction from the Martian soil is less than 0.05 percent, which means the rocks have experienced no more than 5,000 years of aqueous activity since the soils formed—at least 600 million years ago. Since the soils around the Phoenix lander appear to be representative of the entire Martian surface, this places strong constraints on how much standing water ever existed on the Red Planet. Those constraints mean that no life (assuming Mars ever had life on its surface) could have remained until the present day.
Even the worst soil on Earth hosts an abundance of microbial life and usually multicellular life. While we might take this for granted, studies continue to reveal Earth’s rare, if not unique, friendliness to life.
1. W. T. Pike et al., “Quantification of the Dry History of the Martian Soil Inferred from in situ Microscopy,” Geophysical Research Letters 38 (December 21, 2011): L24201.