Free Astronomy Magazine September-October 2018

SEPTEMBER-OCTOBER 2018 ment of high radar reflectivity aside, astro- geologists and new chemistry students alike might ask the same question − what possi- ble conditions could depress the freezing point of water so low? To begin, the de- tected liquid water at the south polar cap is below 1.5 km of ice. Water behaves differ- ently under pressure than most other mole- cules because of the way it forms crystals when it freezes − the average distance be- tween molecules actually increasing slightly compared to the liquid phase, producing a slightly less dense solid. You know this from experience − when a lake freezes over, even the largest pieces of ice float. If water be- haved like most other liquids upon freezing, ice would sink to the bottom. Frozen water under high pressure relieves the strain from above by having its lattice fall back into unstructured, more closely- spaced water molecules. This is how ice skaters are able to move smoothly around an ice rink − the pressure exerted by their body melts the ice directly in contact with their skates, producing a tiny pocket of liq- uid water over which the skates glide. An analogous high-pressure situation exists on Earth. Lake Vostok is the largest of the nearly 400 subglacial lakes discovered in Antarctica. At 250 km long by 50 km wide, averaging almost one-half km in depth, Vostok is a massive fresh-water liquid body buried under more than 4 km of ice. Lake Vostok lies between a massive ice sheet and solid rock − just the situation detected on Mars. The conditions that make Lake Vos- tok and other subglacial lakes liquid on Earth are easier to explain away using only a pressure argument − the pressure expe- rienced in Lake Vostok is a combination of 4.5 km of ice above and an Earth gravity three-times greater than that of Mars. The detected liquid water on Mars is under one-third as much ice on a planet with one-third the gravity, which produces only a small fraction of the pressure experi- E xoMars Trace Gas Orbiter showing the region where the ancient Uzboi Vallis enters Holden crater in the southern hemi- sphere of Mars. The valley begins on the northern rim of the Argyre basin and was formed by running water. The fluvial de- posits are clearly visible in the impact cratered terrain. The image was taken by the orbiter’s Colour and Stereo Surface Im- aging System, CaSSIS on 31 May 2018 and captures an approximately 22.7 x 6.6 km segment centred at 26.8ºS/34.8ºW. North is to the bottom left in this orientation. [ESA/Roscosmos/CaSSIS, CC BY-SA 3.0 IGO]