Mars also has the largest volcanoes in the solar system , Olympus Mons being one of them . The massive volcano , which is about 370 miles ( 600 km ) in diameter , is wide enough to cover the state of New Mexico . Olympus Mons is a shield volcano , with slopes that rise gradually like those of Hawaiian volcanoes , and was created by eruptions of lava that flowed for long distances before solidifying . Mars also has many other kinds of volcanic landforms , from small , steep-sided cones to enormous plains coated in hardened lava . Some minor eruptions might still occur on the planet today . The number of craters on Mars varies dramatically from place to place , depending on how old the surface is . Much of the surface of the southern hemisphere is extremely old , and so has many craters — including the planet ' s largest , 1,400-mile-wide ( 2,300 km ) Hellas Planitia — while that of northern hemisphere is younger and so has fewer craters . Some volcanoes also have just a few craters , which suggests they erupted recently , with the resulting lava covering up any old craters . Some craters have unusual-looking deposits of debris around them resembling solidified mudflows , potentially indicating that the impactor hit underground water or ice .