Free Astronomy Magazine September-October 2018

6 SEPTEMBER-OCTOBER 2018 PLANETOLOGY The search that detected what might be liq- uid water began upon MEO entering orbit in 2003, but the study itself is based on data collected from 2012 and 2015. MARSIS sends radar signals and measures the properties of the reflected beams − including the time it takes for signals to bounce and return and the difference in intensity of the sent signal and returned reflections. The time and in- tensity of the reflections reveal information about the surface − fast returns may mean solid rock immediately below, while inter- faces between rock and ice, or ice and water and rock, produce more complicated and delayed signals. Liquid water is a very differ- ent material than either rock or ice in terms of radar reflectivity, making its presence stand out brightly compared to the material around it. These patches were observed dur- ing early passes of the MEO, but were not always observed for reasons that have to do with how the data was processed. Radar measurements performed by the MEO before 2010 were processed by the onboard computer, involving data averaging before being sent to Earth. This is efficient, but re- sulted in only some of the data containing evidence for liquid water radar reflections − all due to the detected region being rela- tively small compared to the surveyed re- gion. Astrophotographers know the con- sequences of omission attributed to the pre- processing of images by onboard computers − raw image formats are always preferred from DSLR cameras over the compressed E SA’s Mars Express has used radar signals bounced through under- ground layers of ice to find evidence of a pond of water buried below the south polar cap. Twenty-nine dedicated observations were made between 2012 and 2015 in the Planum Australe region at the south pole using the Mars Advanced Radar for Subsurface and Iono- sphere Sounding instrument, MARSIS. A new mode of operations es- tablished in this period enabled a higher quality of data to be retrieved than earlier in the mission. The 200 km square study area is indicated in the left-hand image and the radar footprints on the sur- face are indicated in the middle image for multiple orbits. The greyscale background image is a Thermal Emission Imaging System image from NASA’s Mars Odyssey, and highlights the underlying to- pography: a mostly featureless plain with icy scarps in the lower right (south is up). The footprints are colour-coded corresponding to the ‘power’ of the radar signal reflected from features below the surface. The large blue area close to the centre corresponds to the main radar- bright area, detected on many overlapping orbits of the spacecraft. A subsurface radar profile is shown in the right hand panel for one of the Mars orbits. The bright horizontal feature at the top represents the icy surface of Mars in this region. 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, highlighted in blue, while in other places is rather diffuse. Analysing the details of the reflected signals from the base layer yields properties that correspond to liquid water. The brightest reflec- tions are centred around 193°E/81°S in the intersecting orbits, outlin- ing a well-defined, 20 km-wide zone. [Context map: NASA/Viking; THEMIS background: NASA/JPL-Caltech/Arizona State University; MAR- SIS data: ESA/NASA/JPL/ASI/Univ. Rome; R. Orosei et al 2018] jpeg formats that sacrifice image quality and fine detail for portability. In 2010, a change was made to the way the radar image data handled, with select data stored and re- turned to Earth unmodified. The techniques used to obtain this raw radar data from 2012 to 2015 are not just a feat of engineering,