DEFINITION OF MARS

The fourth planet from the Sun, Mars is only 11 percent

of the mass of Earth. It has an average density of 3.9

grams per cubic centimeter and a diameter of 4,222 miles

(6,794 km). Mars orbits the Sun every 687 days at a distance

of 142 million miles (228 million km) and has a period of rotation

about its axis of 24 hours, 37 minutes, and 23 seconds.

When viewed from Earth, Mars shows several striking

surface features, including bright polar caps that consist mostly

of frozen carbon dioxide (dry ice) that change in size with

the seasons, almost disappearing in the Martian summer.

Some spectacular canyons are also visible, including the

2,485-mile (4,000-km) long Valles Marineris. This canyon

probably formed as a giant crack or fracture on the surface of

the expanding bulge in the Tharsis region. Its surface may

have later been modified by running water. Mars is prone to

strong surface winds that kick up a lot of dust and generate

dust storms that occasionally obscure the surface for long

periods of time, an observation that led early observers to suggest

that the planet may host vegetation and other life-forms.

Mars shows evidence for widespread volcanism in its past,

and its surface is covered with basaltic volcanic rocks, flows,

and cones and hosts several large shield volcanoes in the Tharsis

and Elysium regions. The Tharsis region is a huge, North

America-sized bulge on the planet that rises on average 6.2

miles (10 km) above the elevation of surrounding regions.

These volcanoes are huge compared to shield volcanoes on

Earth. The largest volcano, Olympus Mons, is 435 miles (700

km) across, and several others are 220 to 250 miles (350–400

km) across and rise 12.5 miles (20 km) over the surrounding

terrain. The northern hemisphere of Mars is made of rolling

volcanic plains, similar to lunar maria, but formed by much

larger volcanic flows than on Earth or the Moon. In contrast,

the southern hemisphere consists of heavily cratered highlands,

with a mean elevation several kilometers higher than the volcanic

plains to the north. It is estimated that the average age of

the highland plains is 4 billion years, whereas the age of most

of the volcanic plains in the north may be 3 billion years, with

some volcanoes as young as 1 billion years old.

Recent high-resolution images of the surface of Mars

have strengthened earlier views that water may once have run

across the surface of the planet. Outflow channels and runoff

channels are common. Runoff channels form extensive systems

in the southern hemisphere and resemble dried-up river

systems. Outflow channels may have formed during a catastrophic

flooding episode about 3 billion years ago in the early

history of the planet and are most common in equatorial

regions. Flow rates are estimated to have been at least 100

times that of the Amazon River. Since there is no longer any

water, or visible water ice on the surface, it has been suggested

that much of this water is frozen beneath the surface in a

permafrost layer, reflecting a severe global cooling since 4 billion

years ago.

The atmospheric pressure on Mars is only 1/150th that

of Earth, and carbon dioxide makes up most of the gas in the

atmosphere, with a few percent nitrogen, argon, oxygen, carbon

monoxide, and less than 0.1 percent water vapor. The

temperature rises from about –244°F at 62 miles (120°K at

100 km) above the surface, to about –10°F (250°K) at the

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