Mars (planet)

V

The Surface of Mars

The surface of Mars would be a harsh place for humans, but it is more like the surface of Earth than that of any other planet. The temperature on Mars never gets much warmer than the temperature at Antarctica, and it is usually much colder. At the surface the average temperature is about -55°C (about -67°F) and at the extremes it ranges from about -140° to 15°C (about -225° to 60°F). The surface’s famous reddish color comes from iron oxide minerals in the dust, similar to rust on Earth. The most interesting surface features of Mars include two very distinct hemispheres, an enormous bulge called Tharsis littered with volcanoes and cut by an enormous rift valley, channels apparently carved by water, and polar ice caps similar to Earth’s.

A

Distinct Hemispheres

The northern and southern hemispheres of Mars have different characteristics. The southern hemisphere has many impact craters and has a generally much higher elevation than the northern hemisphere. The southern highlands are probably the oldest terrain on Mars, dating back to the early history of the solar system when large impact events were much more common than they are today. The southern highlands, with their pervasive craters, resemble the surface of the Moon.

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. A few other large basins and thousands of large craters can be found on the surface, mostly concentrated in the lunar-like southern highlands.

The northern hemisphere of Mars contains a much wider variety of geologic features, including large volcanoes, a great rift valley, and a variety of channels. The northern hemisphere also contains large expanses of relatively featureless plains. Radar and topographic studies of the northern hemisphere by Mars orbiters have revealed ancient impact craters beneath the plains, however, indicating that the underlying crust may be the same age as the southern highlands. Astronomers do not know why the northern and southern hemispheres of Mars are now so different; figuring out the reason is an important goal of Mars exploration.

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B

The Tharsis Bulge

Mars has an enormous bulge in its surface called Tharsis. Tharsis is 10 km (6 mi) high and 4,000 km (2,486 mi) wide, and contains giant volcanoes and valleys. The largest volcano in the solar system, Olympus Mons, is located in the Tharsis region. It is over 21 km (13 mi) high (more than twice as high as Earth’s Mount Everest) and covers an area comparable to the state of Arizona. Near it, three other volcanoes almost as large—Arsia Mons, Pavonis Mons, and Ascraeus Mons—form a line running from southwest to northeast. These four volcanoes are the most noticeable features of Tharsis. Another volcano, Alba Patera, is also part of the Tharsis bulge but is quite different in appearance. It is probably less than 6 km (4 mi) high but has a diameter of more than 1,600 km (1,000 mi). None of these volcanoes appears to be presently active, but there is some evidence of small eruptions in the last 100 to 350 million years, and perhaps as recently as 2 million years ago.

The Tharsis bulge has had a profound effect on the appearance of the surface of Mars. It includes many smaller volcanoes and stress fractures in addition to the large volcanoes. Its presence affects the weather on Mars and its formation may have changed the climate by changing the rotational axis of the planet. Valles Marineris (named for the U.S. Mariner spacecraft that discovered it) is the most notable stress feature associated with the Tharsis bulge. It is a great rift valley and interconnected canyon system extending from the Tharsis region to the east-southeast. Valles Marineris is about the same length as the distance from New York to California (about 4,000 km or 2,500 mi). This canyon system reaches widths of 700 km (440 mi) and depths of 7 km (4 mi) in some places. High-resolution spacecraft images have revealed a spectacular variety of layered landforms in and around the canyon system. These layers may represent different episodes of volcanic eruptions, or they may be sedimentary deposits laid down when the canyons were possibly water-filled. The origin of this enigmatic layering on Mars is presently unknown, but most astronomers agree that understanding it will be critical to understanding the history of the planet.

C

Water Channels

Two main types of channels, valley networks and outflow channels, can be found on Mars. Both were probably formed by the action of liquid water. These channels are unrelated to the “canals” thought to be seen in early telescopic views of Mars.

Valley networks are similar in general appearance to streambeds on Earth and occur in the southern highlands. These channels may date from a time early in Mars’s history when the atmosphere was thicker and liquid water could flow readily on or near the surface. High-resolution images reveal important differences between these Martian valley networks and terrestrial valley networks, however. Specifically, Martian valley networks do not appear to have formed from rainfall or surface runoff, but instead may have formed primarily from the action of underground liquid water. A small number of valley networks, however, observed at the highest resolution by the Mars Global Surveyor orbiter, look like they may have been formed from rainfall or surface runoff. Mars Global Surveyor images of Eberswalde Crater southeast of the Valles Marineris canyon system also show a fan-shaped deposit that closely resembles a river delta, further suggesting that water sometimes flowed for an extended period of time.

Outflow channels, formed by giant floods, occur primarily on the boundary between the southern highlands and the northern plains regions. Ares Vallis, where the Mars Pathfinder spacecraft landed in 1997, is one of these outflow channels. An important difference between outflow channels and valley networks is that outflow channels appear to have been formed quickly by the sudden and catastrophic release of enormous volumes of liquid water, with no particular requirements on climatic conditions.

Small-scale water events may still be occurring on Mars. Outflows of liquid water may have formed gullies seen on the walls of craters by the Mars Global Surveyor. Comparison of images taken between 1999 and 2005 showed fresh flows of bright material on the inner walls of several small craters. These flows could result from subsurface liquid water erupting onto the surface. Under current conditions of extreme cold and low air pressure, liquid water cannot exist for long on the surface. Water mixed with salts that lowered its freezing point or water erupting after being under pressure from overlying rock layers might flow for a short distance, however. New, higher-resolution images from the Mars Reconnaissance Orbiter suggest that these features might represent dusty avalanches rather than watery flows, however. Scientists are still actively debating the origin of these features.

D

Ice Caps

Mars has small, permanent ice caps at its north and south poles that increase in size with the addition of seasonal ice caps during the winter of each hemisphere. The polar caps in the north and south have important differences and similarities. The northern permanent ice cap is composed of water ice and is about 1,000 km (about 620 mi) across. A seasonal cap of frozen carbon dioxide adds to the northern ice cap in the northern winter. The southern permanent ice cap is one-third the diameter of the northern cap because summer in the southern hemisphere is warmer than in the north. The southern seasonal cap is larger than the northern cap—more carbon dioxide is frozen out in the south than the north because Mars is farthest from the Sun, and therefore coldest, in the southern winter. While carbon dioxide may also make up some of the southern permanent cap, it is now thought to consist largely of water ice, like the northern permanent cap. Radar on the Mars Express orbiter found evidence for deep layers of frozen water under the south pole. If the amount of ice apparently indicated were melted, it could cover the entire planet in almost 11 m (36 ft) of water.

Both polar caps and their surrounding deposits show spectacular, fine-scale striped layering of dust, rock, and ice to the limits of the resolution of the best available pictures. Like similar layering found in Earth’s polar regions, these Martian polar layers may provide evidence of both short-term and long-term changes in the planet’s climate. The true origin of the Mars polar layering is unknown at present, but it may have been caused by climate cycles similar to ice ages on Earth. Understanding the polar layering is yet another important motivator for continued exploration of the planet.

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