thumb|260px|Topographic map of Hellas Planitia and its surroundings in the southern uplands, from the [[Mars Orbiter Laser Altimeter|MOLA instrument of Mars Global Surveyor. The crater depth is below the standard topographic datum of Mars.]]
Hellas Planitia is a plain located within the huge, roughly circular impact basin Hellas located in the southern hemisphere of the planet Mars. Hellas is the fourth or fifth-largest known impact crater in the Solar System. The basin floor is around deep, deeper than the Moon's South Pole-Aitken basin, and extends about east to west. It is centered at This is 103% higher than the pressure at the topographical datum (610 Pa, or 6.1 mbar, or 0.09 psi) and above the triple point of water, suggesting that the liquid phase could be present under certain conditions of temperature, pressure, and dissolved salt content. It has been theorized that a combination of glacial action and explosive boiling may be responsible for gully features in the crater.
Some of the low elevation outflow channels extend into Hellas from the volcanic Hadriacus Mons complex to the northeast, two of which Mars Orbiter Camera images show contain gullies: Dao Vallis and Reull Vallis. These gullies are also low enough for liquid water to be transient around Martian noon, if the temperature were to rise above 0° Celsius.<!-- Heldmann cite talks of equatorial regions, not the 34 degree latitude region. Pathfinder at 19 degrees North found maximum of -8 degrees. -->
Hellas Planitia is antipodal to Alba Patera. It and the somewhat smaller Isidis Planitia together are roughly antipodal to the Tharsis Bulge, with its enormous shield volcanoes, while Argyre Planitia is roughly antipodal to Elysium, the other major uplifted region of shield volcanoes on Mars. Whether the shield volcanoes were caused by antipodal impacts like that which produced Hellas, or if it is mere coincidence, is unknown.
thumb|Elevation profiles along south to north transects across Mars's Hellas basin and the Moon's South Pole-Aitken basin, created with Lunar Quickmap and Mars Quickmap
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Wikiterracimmeriaboundaries.jpg|MOLA map showing boundaries of Hellas Planitia and other regions
Hellas basin topo.jpg|Geographic context of Hellas
Image:False color of Hellas Planitia.jpeg|This elevation map shows the surrounding elevated ring of ejecta
Image:Twisted Ground in Hellas.jpg|Apparent viscous flow features on the floor of Hellas, as seen by HiRISE.
Image:Twisted Terrain in Hellas Planitia.jpg|Twisted terrain in Hellas Planitia (actually located in Noachis quadrangle).
File:ESP 055080 1425twistedbands.jpg|Twisted bands on the floor of Hellas Planitia, as seen by HiRISE under HiWish program
File:ESP 055212 1420taffypull.jpg|Twisted bands on the floor of Hellas Planitia, as seen by HiRISE under HiWish program These twisted bands are also called "taffy pull" terrain.
</gallery>
Discovery and naming
Due to its size and its light coloring, which contrasts with the rest of the planet, Hellas Planitia was one of the first Martian features discovered from Earth by telescope. Before Giovanni Schiaparelli gave it the name Hellas (which in Greek means Greece), it was known as Lockyer Land, having been named by Richard Proctor in 1867 in honor of Sir Joseph Lockyer, an English astronomer who, using a refractor, produced "the first really truthful representation of the planet" (in the estimation of E. M. Antoniadi).
Possible glaciers
Radar images by the Mars Reconnaissance Orbiter (MRO) spacecraft's SHARAD radar sounder suggest that features called lobate debris aprons in three craters in the eastern region of Hellas Planitia are actually glaciers of water ice lying buried beneath layers of dirt and rock. The buried ice in these craters as measured by SHARAD is about thick on the upper crater and about and on the middle and lower levels respectively. Scientists believe that snow and ice accumulated on higher topography, flowed downhill, and is now protected from sublimation by a layer of rock debris and dust. Furrows and ridges on the surface were caused by deforming ice.
The shapes of many features in Hellas Planitia and other parts of Mars are strongly suggestive of glaciers, as the surface looks as if movement has taken place. Advances in orbital and climatic modelling have supported earlier arguments that viscous flow features present in the mid-latitudes of Mars like Hellas Planitia are related to geologically recent ice ages.
Select analysis of landforms in eastern Hellas Planitia suggests that the detected ice deposits are remnants of a complex history of glaciation and that the region has undergone at least two and possibly three, phases of glaciation. The presence of multiple overlapping glacial units indicates episodes of ice accumulation and flow, interrupted by periods of stagnation and burial under debris. Evidence recorded in the lobate debris aprons suggests that the region underwent a wider glacial period, while analysis of several glacier-like forms with several distinct structures indicative of flow and transportation of mass down-slope suggest additional subsequent more localised glaciation. The geologic process responsible for creating these features remains unresolved. Some calculations indicate that this formation may have been caused by ice moving up through the ground in this region. The ice layer would have been between 100 m and 1 km thick.