Free Astronomy Magazine July-August 2018

28 JULY-AUGUST 2018 ASTROBIOLOGY tion has been seen growing to about 0.6 parts per billion, while in subsequent win- ters that value has dropped to 0.2 parts per billion. The gas is certainly released from the surface or the subsoil, but we do not know if it is limited to the Gale Crater re- gion or if it involves larger territories. There are not many known processes through which this phenomenon can occur. Perhaps trapped methane is released by the melting of water ice crystals present in the subsoil, but it is unlikely that there still exists reserves of such a volatile gas near the surface after millions or billions of years. Perhaps a geological process called “serpentinization” could be involved, which is a chemical reaction between fer- rous rocks and liquid water that also pro- duces hydrogen and hydrogen sulphide, an association of elements that can easily be verified through future automatic mis- sions. Unavoidably, however, the most intriguing hypothesis, and perhaps the simpler one, is the one that associates the origin of sea- sonal variations of methane with the pro- liferation of colonies of microorganisms in summer, followed by their subsequent ac- T he ExoMars rover will travel across the Martian surface to search for signs of life. It will collect samples with a drill and analyze them with next-generation instruments. ExoMars will be the first mission to combine the capability to move across the surface and to study Mars at depth. [ESA]