Hellas Planitia
Hellas Planitia, also known as the Hellas Impact Basin, is a roughly circular impact crater located in the southern hemisphere of the planet Mars. With a diameter of about 2,100 km (1304 Miles), it is the largest impact structure on the planet. The basin is thought to have been formed during the Late Heavy Bombardment period of the Solar System, about 3.9 billion years ago, when a large asteroid impacted Mars.
To get an idea of its size, if all the material excavated from it were spread evenly on the continental United States, there would be a 3.5 km (2 miles) thick layer of debris.
Due to its size and contrasting light colour, Hellas Planitia was one of the first Martian features discovered from Earth by telescope. It was at one point known as 'Lockyer Land' before Giovanni Schiaparelli gave the feature the name Hellas ('Greece'). Richard Anthony Proctor had named it in 1867 in honour 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).
The altitude difference between the rim and the bottom is 9 km (5.6 Miles). The depth of the crater (4 km (2.4 Miles) below the topographic datum, or "sea level" of Mars) explains the atmospheric pressure at the bottom: 840 Pa (8.4 mbar) (.25 InHG). This is 38% higher than the pressure at the topographical datum (610 Pa, or 6.1 mbar or .18 InHG). The pressure is high enough that water is speculated to be present in its liquid phase at temperatures slightly above 0 °C (32 F).
The floor of Hellas shows diverse landforms, some of which appear volcanic in origin (this assumes the basin filled with melt soon after the impact event). Among features believed volcanic in nature are linear ridges similar to the wrinkle ridges found in lunar maria.
References
- 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.