Hellas Basin

Hellas

Viking orbiter image mosaic of Hellas Planitia
Planet	Mars
Region	Hellas quadrangle, south of Iapygia
Coordinates	42°24′S 70°30′E / 42.4°S 70.5°E / -42.4; 70.5Coordinates: 42°24′S 70°30′E / 42.4°S 70.5°E / -42.4; 70.5
Diameter	2,300 km (1,400 mi)
Depth	7,152 m (23,465 ft)

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 third or fourth largest impact crater and the largest visible impact crater known in the Solar System. The basin floor is about 7,152 m (23,465 ft) deep, 3,000 m (9,800 ft) deeper than the Moon's South Pole-Aitken basin, and extends about 2,300 km (1,400 mi) east to west. It is centered at 42°24′S 70°30′E / 42.4°S 70.5°E / -42.4; 70.5. Hellas Planitia is in the Hellas quadrangle and the Noachis quadrangle.

With a diameter of about 2,300 km (1,400 mi), it is the largest unambiguous impact structure on the planet; the obscured Utopia Planitia is slightly larger. (The Borealis Basin, if it proves to be an impact crater, is considerably larger.) Hellas Planitia 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.

The altitude difference between the rim and the bottom is 9,000 m (30,000 ft). The depth of the crater (7,152 m (23,465 ft) ( 7,000 m (23,000 ft)) below the standard topographic datum of Mars) explains the atmospheric pressure at the bottom: 12.4 mbar (0.012 bar) during the northern summer . 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.