Free Astronomy Magazine November-December 2015

PLANETOLOGY colder ones. Long a few hundred me- tres and only a few metres across, the streaks’ evolution seems thus to depend on soil temperature. Over the years, the number of struc- tures identified along slopes of dif- ferent nature (mainly canyon walls, peaks and rims of impact craters), becomes increasingly higher, such that researchers began to call them by the generic name of Recurring Slope Lineae (RSL). Their form and behaviour immedi- ately appeared to researchers comparable to that of a rivulet of water that in flowing downhill moistens (and thus darkens) the land that it crosses. The suggestion that the RSL were small streams of flowing water was also supported by another fact: their distribution favoured equatorial and middle latitudes, and such events recurred one year after the other on the slopes most exposed to the solar radiation. In short, the RSL have since the very beginning shown that they are activated by a heat T he video on the side shows the central peak of Hale Crater, one of the four sites where Ojha and McEwen’s team detected the pres- ence of hydrated salts. [NASA/JPL- Caltech/Univ. of Arizona] Below, schematic illus- tration summa- rizing some pos- sible activating mechanisms of RSL. [Chuck Car- ter, Alfred Mc- Ewen, Scientific American]