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Topographic map locating Hellas Planitia in the southern uplands
This elevation map shows the surrounding elevated debris ring
Hellas Planitia, also known as the Hellas Impact Basin, is a huge, roughly circular impact basin located in the southern hemisphere of the planet Mars. It is the second or third largest impact crater and the largest visible impact crater known in the Solar System. The basin floor is 3 km deeper than the moon's South Pole-Aitken basin, extends about 2,300 km east to west. It is centered at


With a diameter of about , it is the largest unambiguous impact structure on the planet, though a distant second if the Borealis Basin proves to be an impact crater. The basin is thought to have been formed during the Late Heavy Bombardment period of the Solar System, an unknown number of years ago, when a large asteroid hit the surface.

The altitude difference between the rim and the bottom is ~9 km (30,000 ft). The depth of the crater (~7 km (23,000 ft) below the standard topographic datum of Mars) explains the atmospheric pressure at the bottom: 1,155 Pa (11.55 mbar or 0.17 psi). This is 89% 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 would be transient (would evaporate over time) if the temperature would rise above .

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 would rise above 0 Celsius.

Hellas Planitia is antipodal to Alba Pateramarker, suggesting some connection, between the Solar System's (nearly) largest crater, and its largest volcano.

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 ('Greecemarker'), it was known as 'Lockyer Land', having been named by Richard Anthony Proctor in 1867 in honor of Sir Joseph Norman Lockyer, an English astronomer who, using a 6.25-inch (16-cm) refractor, produced "the first really truthful representation of the planet" (in the estimation of E. M. Antoniadi).

Possible Glaciers in Hellas Planitia

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 ~ 250 meters thick on the upper crater and ~ 300 and 450 meters 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.

See also



  • J. N. Lockyer, Observations on the Planet Mars (Abstract), Monthly Notices of the Royal Astronomical Society, Vol. 23, p. 246
  • E. M. Antoniadi, The Hourglass Sea on Mars, Knowledge, July 1, 1897, pp. 169-172.

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