Mars
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JoinBritannica's Publishing Partner Program and our community of experts to gain a global audience for your work!Mars, fourthplanet in thesolar system in order of distance from theSun and seventh in size and mass. It is a periodicallyconspicuous reddish object in the night sky. Mars is designated by the symbol ♂.
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 planetNergal for their god of death and pestilence. The planet’s two moons,Phobos (Greek: “Fear”) andDeimos (“Terror”), were named for two of the sons ofAres andAphrodite (the counterparts ofMars andVenus, respectively, inGreek mythology).
| Planetary data for Mars | |
|---|---|
| *Time required for the planet to return to the same position in the sky relative to the Sun as seen from Earth. | |
| mean distance from Sun | 227,943,824 km (1.5 AU) |
| eccentricity of orbit | 0.093 |
| inclination of orbit to ecliptic | 1.85° |
| Martian year (sidereal period of revolution) | 686.98 Earth days |
| visual magnitude at mean opposition | −2.01 |
| mean synodic period* | 779.94 Earth days |
| mean orbital velocity | 24.1 km/sec |
| equatorial radius | 3,396.2 km |
| north polar radius | 3,376.2 km |
| south polar radius | 3,382.6 km |
| surface area | 1.44 × 108 km2 |
| mass | 6.417 × 1023 kg |
| mean density | 3.93 g/cm3 |
| mean surface gravity | 371 cm/sec2 |
| escape velocity | 5.03 km/sec |
| rotation period (Martian sidereal day) | 24 hr 37 min 22.663 sec |
| Martian mean solar day (sol) | 24 hr 39 min 36 sec |
| inclination of equator to orbit | 25.2° |
| mean surface temperature | 210 K (−82 °F, −63 °C) |
| typical surface pressure | 0.006 bar |
| number of known moons | 2 |
In recent times Mars has intrigued people for more-substantial reasons than its baleful appearance. The planet is the second closest toEarth, afterVenus, and it is usually easy to observe in the night sky because itsorbit lies outside Earth’s. It is also the only planet whose solid surface and atmospheric phenomena can be seen in telescopes from Earth. Centuries ofassiduous studies by earthbound observers, extended byspacecraft observations since the 1960s, have revealed that Mars is similar to Earth in many ways. Like Earth, Mars hasclouds,winds, a roughly 24-hour day, seasonalweather 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, warmeratmosphere and much morewater—rivers,lakes, flood channels, and perhapsoceans. By all indications Mars is now a sterile frozen desert. However, close-up images of dark streaks on the slopes of somecraters during Martian spring and summer suggest that at least small amounts of water may flow seasonally on the planet’s surface, and radar reflections from a possible lake under the south polar cap suggest that water may still exist as a liquid in protected areas below the surface. The presence of water on Mars is considered a critical issue because life as it is presently understood cannot exist without water. 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 wateryniches. In 1996 a team of scientists reported what they concluded to be evidence for ancient microbial life in a piece ofmeteorite 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 hospitable place in the solar system beyond Earth both forindigenous life and for human exploration and habitation. At that time, speculation was rife that the so-calledcanals of Mars—complex systems of long, straight surface lines that very few astronomers had claimed to see in telescopic observations—were the creations of intelligent beings. Seasonal changes in the planet’s appearance, attributed to the spread and retreat of vegetation, added further to the purported evidence forbiological 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 popularculture. It has served as inspiration for generations of fiction writers fromH.G. Wells andEdgar Rice Burroughs in the heyday of the Martian canals toRay 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 mostnotorious case beingOrson Welles’s radio-play production of H.G. Wells’s novelWar 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.
