Olympus Mons is an extinct volcano on Mars. It’s the tallest mountain in the Solar System. This picture of the mountain was taken recently by the Indian Space Research Organization’s Mars Orbiter from an altitude of 8,387 km. The arrow indicates the summit which rises about 21 km above the mean surface level, so high that it’s effectively above the atmosphere. The white area around the mountain is a cloud formation.
This picture was taken on 25 February, 1995, when Mars was only a bit more than 100 million km away from the Earth. It is among the best pictures we have made from Earth or Earth orbit. Hubble resolves Martian surface features with a level of detail only exceeded by planetary probes, such as impact craters and other features as small as 50 km across.
Mars is cloudier than usual in this image. This means the planet was cooler and drier than normal, causing water vapor in the atmosphere to freeze forming ice-crystal clouds.
A crescent-shaped cloud just right of center identifies the immense shield volcano Olympus Mons, which is 550 km across at its base. Olympus Mons is the tallest mountain in the solar system. Warm afternoon air pushed up over the summit forming ice-crystal clouds downwind from the volcano. Farther to the east (right) a line of clouds forms over a row of three other extinct volcanoes which are from north to south: Ascraeus Mons, Pavonis Mons, Arsia Mons.