Mars is the fourth planet from the Sun in the Solar System, the next one beyond Earth's orbit. Its red color inspired the Greeks and Romans to name it after their god of war, Ares, or Mars. The distance of Mars from Earth, and hence its brightness, vary considerably. At times it is the third- brightest object in the night sky, surpassed only by the Moon and Venus.

The major constituents of the Martian atmosphere are carbon dioxide (95.3%), nitrogen (2.7%), and argon (1.6%). Minor amounts of oxygen, carbon monoxide, water vapor, and other trace constituents make up the rest. The average surface pressure of the atmosphere is less than 1/100th of the average surface pressure of Earth's atmosphere, and it varies with season and elevation. The Martian atmosphere undergoes dramatic daily and seasonal temperature changes. It averages about 220 K (-64 degrees F) and varies from 145 K (-199 degrees F) during the polar night to 300 K (80 degrees F) at the equator during midday at perihelion.

Although thin and frigid, the Martian atmosphere is very active and complex. Mars and Earth have similar global atmospheric circulation patterns because of their other similarities. In the Martian atmosphere, as in Earth's, warm air rises at the equator, moves poleward, deflects to the east, and then descends at middle latitudes and returns to the equator. At middle to high latitudes, winds blowing from the west contain narrow bands of high winds called jet streams, which produce storm systems near the surface.

In addition, Mars has seasonal climate changes driven by solar heating and by the exchange of carbon dioxide between polar ice and frost (discussed below) and the atmosphere. During each Martian hemisphere's fall and winter, carbon dioxide freezes out of the atmosphere to form a frost hood that stretches from the pole to nearly halfway to the equator. As spring comes, strong winds are produced by the temperature contrast, at the edge of the retreating polar cap, between the ice and the Sun- heated soil. Augmenting this effect is the hotter southern summer, when the planet is closer to the Sun. The strong southern summer winds lift vast amounts of dust that rise into great storms. These storms have been observed to cover the entire planet.

The orbit of Mars lies about 15 times as far away from the Sun as the Earth's does. The orbit is somewhat elliptical, so the planet's distance from the Sun varies from a minimum, at perihelion, of 206.7 million km (128.4 million mi.) to a maximum, at aphelion, of 249.2 million km (154.8 million mi.). Because Mars is farther from the Sun than Earth is, it takes longer to complete a revolution. Its year is 687 Earth days long. Mars speeds along at 24 km/sec (15 mi./sec) in the same counterclockwise direction (when viewed from above the planet's north pole) as all the other planets.

The Martian day is 24 hours, 37 minutes, and 23 seconds long. The present tilt of the planet's axis is about 25 degrees, producing seasonal changes similar to those on Earth. Because of the elliptical orbit of Mars, summer in its southern hemisphere occurs when the planet is nearest the Sun, as does winter in its northern hemisphere.

The planet has an average diameter of 6,780 km (4,213 mi.), making it about half the size of Earth but nearly twice the size of the Moon. Because of its rotation it is slightly flattened, having an actual diameter of 6,794 km (4,222 mi.) at the equator and 6,752 km (4,195 mi.) at the poles. The bulk density of the planet (3.9 g/cu. cm.) is lower than that of Earth (5.5 g/cu. cm.). In addition, no measurable magnetic field for Mars has been detected, which indicates that the core is solid and explains why Mars has no radiation belt. The total mass of the planet is only one-tenth that of Earth, and thus Martian gravity is only 38% as strong.

Mars is orbited by two irregularly shaped satellites. The larger is named PHOBOS ("fear") and the smaller DEIMOS ("terror"), after attributes personified in Greek mythology as sons of Ares. Each is only a few kilometers wide. The moons are heavily cratered and may be astroids captured by Mars, or the accumulated remnants of the materials that formed it.

The color of the Martian surface ranges from orange to brownish black. The darker materials are weathered basaltic rock, and the lighter are iron oxides. As seen from Earth, streaked areas of contrasting brightness commonly form within or around topographic features. Many of them change seasonally in form and size, indicating that most of the surface is covered by thin deposits of dust and sand that are easily transported by winds. Photographs of the Martian surface provided by the U.S. Viking landers confirm the presence of windblown deposits and also show pebbles and cobbles strewn across the surface. These observations may be typical of most of Mars, as demonstrated by various measurements by both Earth-based and spacecraft instruments.

Because Mars has no oceans and hence no sea level, elevations on the planet are referenced to an artificial datum, or average surface level. Using this datum, the topography of Mars can be broadly divided into southern cratered highlands that mostly range from 1 to 5 km (0.6 to 3 mi.) above the datum, and relatively smooth northern lowlands--covering nearly 40% of the surface--that range 0 to 3 km (1.9 mi.) below it. Superposed on the highlands is the Tharsis rise, more than 3,000 km (1,864 mi.) across, which reaches 10 km (6 mi.) in elevation and supports several huge volcanic shields. The smaller Elysium rise is as much as 5 km (3 mi.) higher than the surrounding lowland plains.

The canyon system of Valles Marineris, the largest and deepest known in the solar system, extends for more than 4,000 km (2,500 mi.) and has 5 to 10 km (3 to 6 mi.) of relief between its floors and the tops of the surrounding plateaus. Two immense circular basins within the southern highlands, Hellas and Argyre, are 1,500 and 800 km (about 900 and 500 mi.) across and 7 and 2 km (4.3 and 1.2 mi.) deep, respectively.

Each Martian pole is covered by layered deposits, forming a plateau 1,000 to 1,500 km (about 600 to 900 mi.) across and mostly 2 to 4 km (1.2 to 2.5 mi.) thick. The plateaus are partly capped by thin ice sheets. The northern ice cap covers about two-thirds of its plateau, whereas the southern cap covers only about one-fifteenth of its plateau.

View a close-up image of Mars.