Free Astronomy Magazine September-October 2018

10 SEPTEMBER-OCTOBER 2018 PLANETOLOGY at 0°C, the freezing point of seawater rests at -2°C, and a saturated salt water sample freezes at -21°C − all due to how the Na + and Cl - ions disrupt ice formation. The ice- disrupting behavior of salts can be en- hanced by mixing several different kinds of salts together. On Mars, the perchlo- rates of sodium (Na), magnesium (Mg), and calcium (Ca) have all been detected by the many rovers capable of analyzing the chemical make-up of surface samples. In the right concentrations, it is possible to depress the freezing point to as low as -74°C − slightly colder than the predicted temperature of the detected liquid zone. The necessary ingredients for greatly de- pressing the freezing point of water may all be present in the reflective zones both in terms of chem- istry and geology − although it must be noted that this liq- uid water would be anything but drink- able. All indications for present liquid water on Mars are much better de- scribed as very salty brines. On Earth, brines are used as a preservative − a liq- uid better suited for killing microbial enced by the Vostok fresh water. Addition- ally, thermal energy may contribute to the estimated -3°C temperature for Lake Vos- tok − an almost balmy temperature com- pared to the possible -68°C temperature for the detected Martian liquid water. Pressure from above is an obvious contrib- utor in the Martian case because we see the ice sheet − but it cannot be the only factor given what we know about the re- sponse of ice to pressure from above. A second contributing factor is difficult to measure directly for a liquid water body buried so deeply, but is the basis for our understanding of all forms of liquid water on the Martian surface today. To lower the freezing point of water at home, simply add table salt (NaCl). Fresh water freezes T his map shows the thickness of the south polar layered deposits of Mars, an ice-rich geologic unit that was probed by the Mars Advanced Radar for Subsurface and Ionos- pheric Sounding (MARSIS) on board ESA’s Mars Express. The map was generated by comparing the elevation of the bed as deter- mined by MARSIS with the high-resolution map of surface topography obtained by the Mars Orbiter Laser Altimeter (MOLA) aboard NASA’s Mars Global Surveyor. The thickness of the layered deposits is shown by colors, with purple representing the thinnest areas, and red the thickest. The total volume of ice in the layered deposits is equivalent to a water layer 11 metres deep, if spread evenly across the planet. [NASA/JPL/ASI/ESA/Univ. of Rome/MOLA Science Team/USGS]