|In 2008 the Phoenix Mars Lander took all the fun out of astrobiology by finding a lot of perchlorate salts in Martian soil samples. It is reasoned that perchlorate compounds are too toxic to accommodate any life form. All that is left is finding out how the perchlorate gets there in the first place: It is very reactive and the two obvious precursors to perchlorate, ozone and chloride, are very scarce. Schuttlefield et al. took up the challenge and offer a new theory in a recent JACS publication (DOI).|
They argue that due to the thin Martian atmosphere, UV radiation is more efficient and perchlorates can form by heterogeneous photocatalysis at a mineral surface. But what mineral? In one experiment rutile and anatase were compared. Both are silicon oxide semiconductors and well known to photocatalysis: radiation pushes out an electron (leaving a hole) which then lends a helping hand in any redox reaction. In the present case that would be chlorine oxidation with water forming a perchlorate ion and reduction of oxygen to water. The nett reaction is one highly endothermic between the chlorine ion and 2 equivalents of oxygen to one equivalent of perchlorate.
Schuttlefield notes the theory has drawbacks. For one it relies on the presence of liquid water but as it happens perchlorates do a great job in freezing-point depression. It is also true that water may be scarce in the present but more liquid water could have been available in the Martian past. Another drawback is the lack of evidence of any silicon oxide mineral on Mars but other semiconducting minerals could fit the bill.