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Hellas Planitia(hell -as) A vast, roughly circular impact basin in Mars' southern hemisphere, located in the planet's southern highland region at the areographic coordinates 40° S latitude, 290° W longitude (see areography). It can be seen as a bright area from the Earth and is the largest impact feature on the planet. It is thought to be the result of an asteroidal impact about 3900 million years ago during the heavy bombardment phase of the formation of the Solar System. The crater excavated by the asteroid is about 9 km deep and 2100 km in diameter and is girdled by a ring of material forming an elevated rim that reaches a height of about 1.5 kilometers above the level of the surrounding area. See Mars, surface features.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006