Free Astronomy Magazine November-December 2015

PLANETOLOGY if they draw water from the atmo- sphere, they always materialize in the same places: the deposits of hydrate salts are the cause or the result of RSL? The scenario is for now rather hazy, and the same researchers argue that at the origin of the phenomenon there might be different mechanisms that intervene in dissimilar ways depending on the cir- cumstances. The only thing certain is that the RSL are the most striking demonstration of the presence of liquid water on Mars today. McEwen assesses its amount in several Olympic swimming pools (roughly several million cubic me- tres). The importance of RSL with re- gard to the ex- istence of Mar- tian life is rela- tive, since these are transient en- vironments, very salty and hence corrosive, in which no form of known life could proliferate. In having however been discovered several thousand of them, this requires to posi- tively re-evaluate the planet’s hydrological cycle, because even though the source of that water is not clear, it must come from somewhere and at its origin it may not be briny water at all. n atmosphere is in fact little known at ground level and may be too dry to provide the water va- pour necessary to account for the cyclical re- currence of RSL. An alternative m e c h a n i s m would involve infiltrations of salt water, associate with groundwater aquifers, but at the altitudes at which the rivulets generally activate make it difficult to explain how these sources could re- generate themselves over the years. More generally, the regions in which the RSL occur appear too dry for the water to come from the steep subsurface, but at the same time it is difficult to explain why, E volution of some RSL in- side Palikir Crater during 2 Martian years. [NASA/JPL, D.E. Stillman et al.] 50 m ➊ ➋ ➌ ➍ ➎ ➏