Hellas Planitia
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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, it is the largest impact structure on the planet. The basin was formed during the heavy bombardment period of the Solar System, about 3.9 billion years ago, when a large asteroid impacted Mars.
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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').
The altitude difference between the rim and the bottom is 9 km. The depth of the crater (4 km below the topographic datum, or "sea level" of Mars) explains the atmospheric pressure at the bottom: 840 Pa (8.4 mbar). This is 38% higher than the pressure at the topographical datum (610 Pa, or 6.1 mbar). The pressure is high enough that water is speculated to be present in its liquid phase at temperatures slightly above 0 °C.
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.
- A NASA site that writes of Hellas Planitia (http://rst.gsfc.nasa.gov/Sect19/Sect19_12.html)