As space agencies get ready for a new series of Mars missions dedicated to the search for life (the first of two European ExoMars missions launches next Monday) one important question is where they should land to have the best chance of finding biosignatures on the Martian surface. Thinking about this problem led me to a paper published several years ago that linked mineral composition in soils to life.
Based on their analysis of the mineral content of Martian soils, Tom Pike and co-authors concluded in a 2011 paper in Geophysical Research Letters that the Red Planet went through an extremely dry period that lasted hundreds of millions of years. The researchers looked at soil samples from the Phoenix landing site, which had almost no clay minerals or any other chemical or mineralogical markers suggesting the interaction of soil and water. Pike and colleagues concluded that the Martian soil at that location is Moon-like, and has been exposed to liquid water for a maximum of 5,000 years. Earlier assessments of the Phoenix site had focused on the soil’s low acidity, which was benign enough to grow asparagus, feeding speculation that the Phoenix site may be suitable for life.
The finding by Pike and colleagues harkened back to the Viking life detection experiments in 1976, particularly the Gas Exchange Experiment, which indicated that Martian soil was highly reactive chemically when exposed to liquid water. And the discovery of perchlorates, both at the Phoenix landing site and at the Curiosity site in Gale Crater, indicate extremely dry conditions on Mars today. This does not bode well for any putative Martian microbes that might have thrived on a wetter and warmer Mars about four billion years ago.
Whether these very dry conditions prevail only in some places on Mars or all over the planet is unclear at this time, but of critical importance. Although the planet’s wind patterns distribute sediment from one location to the other, resulting in mixing on a global scale, Mars is still a large and heterogeneous world, and we see evidence for a diversity of environments. Remote sensing images taken from Mars orbit support the notion that there are places on the surface with much different environmental conditions than we have observed at any of the landing sites to date. And there are many places we have not yet visited due to engineering constraints having to do with how high, or rough, or far from the equator a particular site is. No lander has yet touched down in the deep canyons of Valles Marineris, for example, or the Southern Highlands of Mars. And the so-called Special Regions of Mars, where water and life may be more likely, are subject to strict planetary protection requirements that drive up a mission’s costs. All of these factors make landing site selection as difficult as it is important.