Mars

fourth planet in the solar system in order of distance from the Sun and seventh in size and mass. It is a conspicuous, sometimes quite bright, reddish object in the night sky. Mars is designated by the symbol .

Blood-red in colour and sometimes called the Red Planet, Mars has long been associated with warfare and slaughter. It is named for the Roman god of war. As long as 3,000 years ago, Babylonian astronomer-astrologers called the planet Nergal for their god of death and pestilence. The planet's two moons, Phobos (Greek: “Fear”) and Deimos (“Terror”), were named for two of the sons of Ares and Aphrodite (the counterparts of Mars and Venus, respectively, in Greek mythology).

In recent times, Mars has intrigued people for more substantial reasons than its baleful appearance. The planet is the second closest to Earth, after Venus, and it is usually easy to observe in the night sky because its orbit lies outside Earth's. It is also the only planet whose solid surface and atmospheric phenomena can be seen in telescopes from Earth. Centuries of assiduous studies by earthbound observers, which have been extended by spacecraft observations since the 1960s, have revealed that Mars is similar to Earth in many ways. Like Earth, Mars has clouds, winds, a roughly 24-hour day, seasonal weather patterns, polar ice caps, volcanoes, canyons, and other familiar features. There are intriguing clues that billions of years ago Mars was even more Earth-like than today, with a denser, warmer atmosphere and much more water—rivers, lakes, flood channels, and perhaps oceans. By all indications, Mars is now a sterile frozen desert, but close-up images from the Mars Global Surveyor spacecraft of seemingly water-eroded gullies suggest that at least small amounts of water may have flowed on or near the planet's surface in geologically recent times and may still exist as a liquid in protected areas below the surface. The presence of water on Mars is considered a critical issue because, without water, life as it is presently understood cannot exist. If microscopic life-forms ever did originate on Mars, there remains a chance, albeit a remote one, that they may yet survive in these hidden watery niches. In 1996 a team of scientists reported what they concluded to be evidence for ancient microbial life in a piece of meteorite that had come from Mars, but most scientists have disputed their interpretation.

Since at least the end of the 19th century, Mars has been considered the most obviously hospitable place in the solar system beyond Earth both for indigenous life to have developed and for human exploration and habitation. At that time, speculation was rife that the so-called canals of Mars—complex systems of long, straight surface lines that very few astronomers had claimed to see in telescopic observations—were indeed the creations of intelligent beings. Seasonal changes in the planet's appearance that were seemingly related to the spread and retreat of vegetation added to the purported evidence for biological activity. Although the canals later proved to be illusory and the seasonal changes geologic rather than biological, scientific and public interest in the possibility of Martian life and in exploration of the planet has not faded.

During the past century Mars has taken on a special place in popular culture. It has served as inspiration for generations of fiction writers from H.G. Wells and Edgar Rice Burroughs in the heyday of the Martian canals to Ray Bradbury in the 1950s and Kim Stanley Robinson in the '90s. Mars has also been a central theme in radio, television, and film, perhaps the most notorious case being Orson Welles's radio-play production of H.G. Wells's novel War of the Worlds, which convinced thousands of unwitting listeners on the evening of October 30, 1938, that beings from Mars were invading Earth. The planet's mystique and many real mysteries remain a stimulus to both scientific inquiry and human imagination to this day.

Basic astronomical data

Mars moves around the Sun at a mean distance of 228 million km (140 million miles), or about 1.5 times that of Earth from the Sun. Because of its relatively elongated orbit, the distance between Mars and the Sun varies from 206.6 million to 249.2 million km (128.4 million to 154.8 million miles). Mars orbits the Sun once in 687 Earth days, which means that its year is nearly twice as long as Earth's. At its closest approach, Mars is less than 56 million km (35 million miles) from Earth, but it recedes to almost 400 million km (250 million miles) when the two planets are on opposite sides of the solar system.

Mars is easiest to observe when it and the Sun are in opposite directions in the sky—i.e., at opposition—because it is then high in the sky and shows a fully lighted face. Successive oppositions occur about every 26 months. Oppositions can take place at different points in the Martian orbit. Those best for viewing occur when the planet is closest to the Sun, and so also to Earth, because Mars is then at its brightest and largest. Close oppositions occur roughly every 15 years.

Mars spins on its axis once every 24 hours 37 minutes, making a day on Mars only a little longer than an Earth day. Its axis of rotation is inclined to its orbital plane by about 25°, and, as for Earth, the tilt gives rise to seasons on Mars. (See the diagram.) The Martian year consists of 668.6 Martian solar days, called sols. As a result, southern summers are shorter (154 Martian days) and warmer than those in the north (178 Martian days). The situation, however, is slowly changing such that 25,000 years from now the northern summers will be the shorter and warmer ones. In addition, the tilt of the axis is slowly changing on a roughly one-million-year timescale. During some epochs the tilt is close to zero, and so Mars has no seasons; in other epochs the tilt may be as high as 35°, resulting in extreme seasonal differences.

Mars is a small planet, larger than only Mercury and slightly more than half the size of Earth. It has an equatorial radius of 3,396 km (2,110 miles) and a mean polar radius of 3,379 km (2,100 miles), both values accurately determined by the Mars Global Surveyor spacecraft, which began its primary mission in orbit around the planet in 1999. The mass of Mars is only one-tenth the terrestrial value, and its gravitational acceleration of 3.72 metres (12.2 feet) per second per second at the surface means that objects on Mars weigh a little more than a third of their weight on Earth's surface. Mars has only 28 percent of the surface area of Earth, but, because about three-fourths of Earth is covered by water, the land areas of the two planets are comparable. For additional orbital and physical data, see the table.