Free Astronomy Magazine January-February 2016

49 PLANETOLOGY olution Imaging Science Experiment (Hi- RISE) on NASA's Mars Reconnaissance Or- biter (MRO); it will thus be interesting studying the phenomenon in-situ. Besides the dunes’ migration process, researchers are also keen to investigate the mineral- ogical composition of the sand grains form- ing them. Since this sand is also the basic component of ancient sandstone rocks rath- er common on Mars, understanding the sand grains composition and evolution can help us to better interpret the sandstone internal structure and assess to what ex- tent its formation can be attributed to water. The relative abundance of certain minerals – such as olivine (fairly widespread in the solar system) – inside rock layers can pro- vide an idea of the role of water in a given scenario, although certain concentrations may, however, also have different origins. In particular, if the Martian wind is actually able to perform a kind of “preliminary sort- ing” of the minerals, according to the size surface has already been removed by the wind. The image is a mosaic of photographs taken on 3 De- cember with the Mars Hand Lens Imager (MAHLI). The captured area is just 3.3 × 2.5 cm. [NASA/JPL- Caltech/MSSS]