Free Astronomy Magazine January-February 2018

18 JANUARY-FEBRUARY 2018 SPACE CHRONICLES They also provide steep surfaces for active processes occurring in more recent times. For example, NASA’s Mars Reconnais- sance Orbiter iden- tified gullies on some of the steep slopes in Sirenum Fossae, along troughs and in the rims of impact craters. What material carves out the small channels is a topic of active re- search: they were initially thought to be related to flowing water, but recent proposals suggest that sea- sonal frozen car- bon dioxide – dry ice – flowing downslope may be responsible. T his image shows the relative heights and depths of a region in the southern hemisphere of Mars showing the Sirenum Fossae fracture system. As indicated in the key at top right, whites and browns/reds represent the highest terrain, while blue/purple is the lowest (values are marked on the scale). [ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO] the magma cham- ber emptied. It is also possible that each graben was associated with an ancient volcanic dike: a steep corri- dor within the rock along which magma from the interior of Mars once propagated upwards, causing cracking along the surface. In this case the graben could represent a giant ‘dike swarm’ extending from the volcanic cen- tre. Dike swarms are also seen on Earth, as in Ice- land where they are observed with surface fractures and graben sets in the Krafla fissure swarm. As with any geological feature that cuts C olour view of the Sirenum Fossae fracture system on Mars, located about 1800 km southwest of the vast Tharsis volcanic region. The images were acquired by the High Resolution Stereo Camera on Mars Express 5 March 2017. The ground resolution is approximately 14 m/pixel and the images are centered at 28°S / 215°E. [ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO] into the surface of the planet, the graben systems make for a good window into the subsurface. !