Free Astronomy Magazine November-December 2015

PLANETOLOGY crystal structure are embedded some water mole- cules. Chlorate and perchlorate also have hygroscopic properties, that is to say that they are very efficient in absorbing the water vapour mol- ecules present in the atmosphere surrounding them, and when such process exceeds a certain threshold, water condenses and changes from gas to liquid state, with ensuing dis- solution of the salts. Naturally, if it is on a slope, the resulting brine will start flowing. This is one of the mechanisms pro- posed by Ojha and colleagues for explaining the for- mation of RSL, or at least some of them. The Martian other were found too narrow to be ana- lyzed). In particular, the researchers’ at- tention focused on four different RSL sites: Palikir Crater (on the crater’s wall), Horowitz Crater (on the central peak), Coprates Chasma (on the canyon’s wall) and Hale Crater (on the central peak). In these sites, CRISM has revealed the pres- ence of mixtures of hydrate salts, among which prevail magnesium chlorate and perchlorate, and sodium perchlorate (it is the first time that perchlorates are de- tected from the Martian orbit). Their presence along the streaks is in itself evi- dence of the existence of liquid water, since these salts are formed due to the presence of water, such that into their T his sequence of images of Corozal Crater (8 km diameter) gives an idea of the relatively small size of the RSL, only a few metres across. [NASA/JPL, D.E. Stillman et al.]