|Moore, Patrick. The Guinness Book of Astronomy: Facts & Feats. Great Britain: Guinness Superlatives Limited, 1979: 81.||"The deepest basin on Mars is Hellas, which measures 2200 km by 1800 km and is about 3 km deep (reckoned from the level on Mars where the atmospheric pressure is 6.1 millibars)."||3 km|
|Mars. Encarta. 16 May 2003.||"Hellas Planitia is a giant impact basin in the southern hemisphere. The impact of a large asteroid formed the basin long ago. At 6 km (3.8 mi) deep and with a diameter of about 2,000 km (about 1,250 mi), it is the largest and deepest basin on Mars."||6 km|
|Morton, Oliver. Mapping Mars. New York: Picador USA, 2002: 103-104.||"Hellas is almost three times as wide as Argyre, to the west of it, and its floor is more than five miles lower than the level of the surrounding cratered plains."||> 8 km|
|Burgess, Eric. To the Red Planet. New York: Columbia UP, 1978: 158.||"Hellas is thought to have been formed by the impact of a mountain-sized body on Mars in the early days of the solar system. Its floor is some 3.7 miles (6 km) below the mean level of the Martian surface, and some parts may be deeper."||6 km|
|Moche, Dinah L. Astronomy: A Self-Teaching Guide. 4th ed. New York: Wiley, 1993: 236.||"The lowest point on Mars, 6 km (4 miles) below the mean surface, is the bottom of the circular Hellas Planitia basin."||6 km|
The surface of Mars exhibits two main types of terrain. The northern hemisphere contains an array of geologic features that include large volcanoes, a great rift valley, and extensive plains. Generally the landmasses in the northern hemisphere lie below the Martian "reference level," the altitude at which we call zero. On Earth, we use sea level for this purpose, but Mars has no sea level. Thus the reference level of Mars is where the atmospheric pressure is 6.1 millibars. The southern hemisphere is dominated by many impact craters and generally has a higher elevation than the northern hemisphere. The southern highlands are considered to be the oldest terrain on Mars.
The lowest point on Mars is roughly 6 km below the Martian reference level at the bottom of Hellas Planitia, located in the southern hemisphere. Hellas is by far the largest and deepest impact basin on Mars. Scientists believe that the impact of a large asteroid or meteor formed the Hellas basin about 3 billion years ago.
In 1976, the US Viking 1 space probe landed on Mars and was able to take pictures of the Hellas basin floor. Mist and frost usually surround the floor, creating an illusion that the floor is featureless. However these pictures showed otherwise. Lava from old volcanoes flowed from the rim down into the basin. Also fans of debris formed at the base of landmasses of higher elevation (i.e. mounds). These fans are similar to debris flows found at the edges of glacial regions on earth, which strengthen the idea that much of the ground on Mars is icy. Thus Mars could have had large amounts of water millions of years ago, even though almost none exists today.
Allison Chin -- 2003