| Hellas | |
|---|---|
NASA image of Hellas Planitia |
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| Planet | Mars |
| Region | Hellas quadrangle, south of Iapygia |
| Diameter | 2,300 km |
| Depth | 7152-meters |
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 about 7152-meters deep, 3 km deeper than the moon's South Pole-Aitken basin, and extends about 2,300 km east to west.[1][2] It is centered at
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With a diameter of about 2,300 kilometres (1,400 mi),[3] 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, approximately 4.1 to 3.8 billion years ago, when a large asteroid hit the surface.[4]
The altitude difference between the rim and the bottom is ~9 km (30,000 ft). The depth of the crater (7152-meters[5] (23,000 ft) below the standard topographic datum of Mars) explains the atmospheric pressure at the bottom: 1,155 Pa[5] (11.55 mbar, 0.17 psi, or 0.01 atm). 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 0 °C (32 °F).[6]
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.[7]
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 Planitia, the other major uplifted region of shield volcanoes on Mars. Whether the shield volcanoes were actually caused by antipodal impacts like that which produced Hellas, or if it is mere coincidence, is presently unknown.
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 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).[8]
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.[9] 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.
Also, the shapes of many features in Hellas Planitia and other parts of Mars are strongly suggestive of glaciers. The surface looks as if movement has taken place.
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