Free Astronomy Magazine September-October 2018

7 SEPTEMBER-OCTOBER 2018 PLANETOLOGY T he upper image of this composite is a ‘radargram’ from the Mars Advanced Radar for Subsurface and Ionospheric Sound- ing (MARSIS) on board ESA’s Mars Express. It shows data from the subsurface of Mars in the water-ice-rich layered de- posits that surround the south pole of the planet. The lower image shows the position of the ground track of the spacecraft (indicated by a white line) on a topographic map of the area based on data from the MOLA laser altimeter on board NASA's Mars Global Surveyor. The images are 1250 kilometers wide. The MARSIS radar echo trace splits into two traces on the left side of the image, at the point where the ground track crosses from the surrounding plains onto elevated layered deposits. The upper trace is the echo from the surface of the deposits, while the lower trace is interpreted to be the boundary be- tween the lower surface of the deposits and the underlying material. The strength of the lower echo suggests that the in- tervening material is nearly pure water ice. Near the image center, the bright lower echo abruptly disappears for unknown reasons. The time delay between the two echoes reaches a maximum of 42 microseconds left of center, corresponding to a thickness of 3.5 kilometres of ice. The total elevation difference shown in the topographic map is about 4 kilometres be- tween the lowest surface (purple) and the highest (red). [NASA/JPL/ASI/ESA/Univ. of Rome/MOLA Science Team] E SA’s Mars Express has used radar signals bounced through underground layers of ice to identify a pond of water buried below the surface. This image shows an example radar profile for one of 29 orbits over the 200 x 200 km study region in the south polar region of Mars. The bright horizontal feature at the top corresponds to the icy surface of Mars. Layers of the south polar layered deposits – layers of ice and dust – are seen to a depth of about 1.5 km. Below is a base layer that in some areas is even much brighter than the surface reflections, while in other places is rather diffuse. The brightest reflections from the base layer – close to the centre of this image – are centred around 193°E/81°S in all intersecting orbits, outlining a well-defined, 20 km wide subsurface anomaly that is interpreted as a pond of liquid water. [ESA/NASA/JPL/ASI/Univ. Rome; R. Orosei et al. 2018]