Mars is the fourth planet from the sun at about 228 million-km (141 million miles) and the last terrestrial planet from the sun. The next five planets in order from the sun are gaseous. Mars follows closely behind Earth but is comparatively smaller, with about half the diameter of Earth and about one-tenth of Earth s mass. Thus the force of gravity on Mars is about one-third of that on Earth. Though it is much smaller, Mars does have the same surface land area as Earth. Other than Earth, Mars posses the most highly varied and interesting known terrain in our solar system. The surface of Mars is a very hostile place however it is more like Earth s surface than any other planet in our solar system.
Much of the Martian surface is rough and cratered, but expansive flat plains and smooth hills can also be found. Unlike any other planet, there is a striking difference between the northern and southern hemispheres of Mars; one is extremely rough and old while the other is young and relatively smooth. The southern hemisphere is strewn with ancient craters of all sizes and is also elevated by a several kilometers creating a visible boundary. On the opposite end the northern hemisphere consists of a wider variety of geological features, but is obviously smoother and much younger. There are large volcanoes, a great rift valley, and a variety of channels.
Volcanism is a geological process that occurs on Earth today, and has on many planetary bodies throughout the history of the solar system. No volcanism is occurring on the surface of Mars today. In the past, however, volcanism was one of the main forces creating and reshaping the surface of the planet. All of the rocks that have been observed by the Viking landers and the Mars Pathfinder Rover are generally agreed to be volcanic in origin.
Tharsis is the largest volcanic region on Mars. It is approximately four thousand kilometers across, ten kilometers high, and contains twelve large volcanoes. The largest volcanoes in the Tharsis region are four sheild volcanoes named Ascraeus Mons, Pavonis Mons, Arsia Mons, and Olympus Mons. The Tharsis Montes (Ascraeus, Pavonis, Arsia) are located on the crest of the crustal bulge and their summits are about the same elevation as the summit of Olympus Mons, the largest of the Tharsis volcanoes. While not the largest of the Tharsis volcanoes, Arsis Mons has the largest caldera on Mars, having a diameter of one hundred twenty kilometers!
The largest of the volcanoes in the Tharsis region, as well as all known volcanoes in the solar system, is Olympus Mons. Olympus Mons is a shield volcano 624 km in diameter (approximately the same size as the state of Arizona), 25 km high, and is rimmed by a 6 km high scarp. A caldera 80 km wide is located at the summit of Olympus Mons. To compare, the largest volcano on Earth is Mauna Loa. Mauna Loa is a shield volcano 10 km high and 120 km across. The volume of Olympus Mons is about one hundred times larger than that of Mauna Loa. In fact, the entire chain of Hawaiian islands would fit inside Olympus Mons!
The main difference between the volcanoes on Mars and Earth is their size; volcanoes in the Tharsis region of Mars are ten to one hundred times larger than those anywhere on Earth. The lava flows on the Martian surface are observed to be much longer, probably a result of higher eruption rates and lower surface gravity. The less gravitational pull, the higher volcanoes can grow without collapsing under their own weight.
Another reason why the volcanoes on Mars are so massive is because the crust on Mars doesn t move the way it does on Earth. On Earth, the hot spots remain stationary but crustal plates are moving above them. The Hawaiian Islands result from the northwesterly movement of the Pacific plate over stationary hotspots producing lava. As the plate moves over the hotspot, new volcanoes are formed and the existing ones become extinct. This distributes the total volume of lava among many volcanoes rather than one large volcano. On Mars, the crust remains stationary and lava piles up in one, very large volcano.
Valles Marineris, or Mariner Valley, is a vast canyon system that runs along the Martian equator just east of the Tharsis region. Valles Marineris is 4000 km long and reaches depths of up to 7 km! For comparison, the Grand Canyon in Arizona is about 800 km long and 1.6 km deep. In fact, the extent of Valles Marineris is as long as the United States and it spans about 20 percent of the entire distance around Mars! The canyon extends from the Noctis Labyrinthus region in the west to the chaotic terrain in the east. Most researchers agree that Valles Marineris is a large tectonic crack in the Martian crust, forming as the planet cooled, affected by the rising crust in the Tharsis region to the west, and subsequently widened by erosional forces. However, near the eastern flanks of the rift there appear to be some channels that may have been formed by water.
The Tharsis bulge has a profound effect on the appearance, weather, and climate of Mars. It s enormous mass may have dramatically changed the climate by changing the rotation of Mars. Tharsis has many stress features but the most apparent is Valles Mariners, named after the U.S. spacecraft that discovered it. It is about the same length as the distance from New York to California and at it s widest is 700 km (440 mi). It also reaches depths of 7 km (4 mi).
Depending on its orbit, Mars is at times the fourth brightest object in Earth s sky, exceeded only by the Sun, Moon, and Venus. When one gazes at the planet glowing in the sky like a red danger signal, it is easy to understand why the ancient people identified this object with the god of war-the Romans with Mars, the Greeks with Ares. A Martian day or time it takes to rotate on its axis is about half an hour longer than Earth s and a Martian year or the time it takes to orbit the sun is about twice that of Earth s (687 Earth days).