Mars Pathfinder view of the surface of Mars

Mars Pathfinder view of the surface of Mars. The scene includes bouldery ridges and hummocks of flood debris that range from a few tens of meters away to the "Twin Peaks" – modest-size hills – about a kilometer (five-eighths of a mile) away.

The planet Mars

Mars. The huge canyon system in the middle of the picture is Valles Marineris (Valley of the Mariners).

Comparison of Earth and Mars

Earth and Mars compared.

orbits of the inner planets

Orbits of the inner planets.

Mars as seen by the Hubble Space 

Mars as seen by the Hubble Space Telescope.

Mars is the fourth planet from the Sun. It is only about half as big as Earth and has only about one-quarter of our planet's surface area. But because Mars doesn't have any oceans or seas, the total area of the martian surface is about equal to that of Earth's dry land.


Mars is named after the Roman God of War, probably because of its red color which suggests blood and battle. It's also commonly known as the Red Planet. Its ruddy color is due to large amounts of red iron oxides (rust), including minerals such as hematite, in the surface rocks and soil.


Mars has two tiny satellites, Phobos and Deimos, both believed to be captured asteroids.



Mars orbits about one and half times farther from the Sun than Earth does, which is the main reason it's so much colder. The orbit of Mars is also noticeably more elliptical than Earth's. This means the amount of solar heat received during the day at any given point on the planet's surface varies widely between when Mars is closest to the Sun (perihelion) and when it's farthest away (aphelion). Big swings in temperature are the result.


Although the yearly average temperature over the whole surface is about -55°C (-67°F), the temperature ranges from as low as -133°C (-207°F) at the winter pole to a surprisingly balmy 27°C (80°F) at the equator on a summer day. These values are generally much lower than would occur on Earth if our planet orbited the Sun at the distance of Mars, due to the relatively feeble greenhouse effect of the thin martian atmosphere.


One martian year equals just under two Earth years because Mars takes 687 days to complete an orbit.


When Mars is opposite the Sun in our sky it is said to be in opposition. This is also when Mars and Earth are closest together and therefore a good time for making observations. Before the age of space exploration, much of our knowledge about Mars came from studies carried out at these favorable times. For more details, see Mars, oppositions.



Mars spins around on its axis once every 24.62 hours. This is the length of the martian day, which is known as a sol. One sol is just 39 minutes longer than an Earth day of 24 hours.



One of the most striking things about Mars is that its two halves are quite different. Most of the southern hemisphere is marked by ancient cratered highlands, similar to those on the Moon. The northern hemisphere, in contrast, is dominated by lava-filled plains that reveal a more complex history of activity and change. At the boundary between the two types of terrain is an abrupt shift in of several kilometers. Scientists still aren't sure about the origin of this sharp boundary and the two contrasting hemispheres. One idea is that Mars suffered a massive impact shortly it formed.


From Earth, the northern plains of Mars look paler than the rest of the planet and were once thought to be continents. Consequently, they were given names such as Utopia Planitia (plain of Utopia) and Arabia Terra (land of Arabia). The darker features in the south were believed to be seas and so were given names such as Mare Erythraeum, Mare Sirenum, and Aurorae Sinus. The largest dark feature on Mars seen from Earth, and the first permanent marking on any planet to be spotted through a telescope, is Syrtis Major.


Surface features

Mars has some outstanding surface features – quite literally. Among its lofty volcanoes is the biggest in the solar system, Olympus Mons. Mars also boasts a collection of volcanoes in the northern Tharsis region so huge that they make a noticeable bump in the planet's roundness; an impact crater in the southern hemisphere, Hellas Planitia, which is more than 6km deep and 2,000km wide; and a huge canyon system, Valles Marineris (Valley of the Mariners), long enough to stretch from Los Angeles to New York. Mars may also possess caves. For more about the major surface features on Mars, see the table below and the accompanying links.


Argyre Planitia

Argyre Planitia


An 800-kilometer-diameter impact basin in the southern hemisphere that is the youngest such structure on the planet, with an age of about 3.5 billion years.


Chryse Planitia

A relatively smooth, circular plain, possibly an ancient impact basin, in the north equatorial region, 1,600 kilometers across and 2.5 kilometers below the mean level of the planet's surface. It seems to have suffered water erosion in the past and was the site of the Viking 1 landing. More here.


Elysium Planitia

Chryse Planitia


The second largest volcanic region on Mars, after Tharsis Montes. It measures 1,700 by 2,400 kilometers and contains three large volcanoes: Elysium Mons, Hecates Tholus (NE of Elysium Mons), and Albor Tholus (SE of Elysium Mons). More here.


Hellas Planitia

Hellas Planitia


Formerly known simply as 'Hellas,' a near-circular impact basin, some 2,500 kilometers wide and about 6 kilometers deep. It is conspicuous by its color which often appears lighter than surrounding areas due to overhanging mists and cloud. More here.


Ma'adim Vallis

Ma'adim Vallis


One of the largest valley systems on Mars. Named after the Hebrew for 'Mars,' it is about 860 kilometers long, 8 to 15 kilometers wide, up to 2,100 meters deep, and located in the highlands of the southern hemisphere. Images sent back by Mars Global Surveyor suggest that Ma'adim Vallis formed some 3.5 billion years ago when a large lake, estimated to have been 1.1 million kilometers2 in area and 1,100 meters deep, overflowed a low point in its perimeter.


Olympus Mons

Olympus Mons: largest volcano in the solar system


The highest volcano in the solar system – three times higher than Mount Everest. Mons Olympus wouldn't be difficult to climb, though, because it rises very gently from the surrounding plain. The only exception to this is a steep escarpment that borders the summit. A shield volcano, similar to those in the Hawaiian chain but vastly larger, it measures 624 kilometers wide and 25 kilometers high. In the center is a caldera, 80 kilometers wide with multiple circular, overlapping collapse craters created by different volcanic events. The radial features on the slopes of the volcano were formed by overflowing lava and debris. Olympus Mons is found in the Tharsis Montes region near the martian equator. More here.


Syrtis Major

A conspicuous dark, roughly triangular marking, about 1,200 kilometers long and 1,000 kilometers wide, centered at about +10° N, 290° W; it was first noted by Christiaan Huygens in 1659. Formerly known as the Hourglass Sea or the Kaiser Sea, its present name is Greek for 'great sandbank,' which is appropriate since it appears to consist of a large area of wind-blown dust. More here.


Tharsis Montes

Tharsis Montes


An extensive upland region, from which rise three giant shield volcanoes, Ascraeus Mons, Pavonis Mons, and Arsia Mons, each to about 27 kilometers above the datum level for the planet. Tharsis Montes, also known as the Tharsis Ridge, extends for 2,100 kilometers and varies in height between about 9 kilometers and 11 kilometers above the datum level. More here.


Utopia Planitia

A vast, sparsely-cratered, sloping plain, about 3,200 kilometers across, centered at latitude 48° N, longitude 277°. Viking 2 landed in eastern Utopia Planitia, about 200 kilometers west of the crater Mie. More here.


Valles Marineris

Valles Marineris


The largest system of canyons in the solar system. Just south of the martian equator, it is about 4,000km long – as wide as the continental United States. The central individual troughs, generally 50 to 100 kilometers wide, merge into a depression as much as 600 kilometers wide. In places the canyon floor reaches a depth of 10 kilometers – six to seven times deeper than the Grand Canyon. The geologic history of the central canyon system is complex: first the surface collapsed into a few deep depressions that later became filled with layered material, perhaps as lake deposits. Then graben-forming faults cut across some of the older troughs thus widening existing troughs, breaching barriers between troughs, and forming additional ones. At that time the interior deposits were locally bent and tilted, and perhaps water, if still present, spilled out and flowed toward the outflow channels. Huge landslides fell into the voids created by the new grabens. More here


Mars also has permanent ice caps at both poles, which retreat and grow with the seasons. For more about them, see Mars, polar caps.



Although tectonism has clearly played a major role in the planet's development, it has not involved the lateral movement of sliding plates as on our own world, but instead only vertical movement of hot lava pushing up to the surface. The lack of plate tectonics, as on Mercury and the Moon, has resulted in hot-spots remaining fixed in certain locations beneath the crust. This, along with the lower surface gravity, may account for the Tharsis bulge and its enormous volcanoes. Although there's no evidence of current volcanic activity, observations by Mars Global Surveyor suggest that Mars may have been tectonically active early on, making comparisons with Earth all the more interesting. Lacking plate tectonics today, Mars can't recycle any of the carbon dioxide in its rocks back into its atmosphere and so can't sustain much of a greenhouse effect. Large, but not global, weak magnetic fields exist in various regions of Mars, probably remnants of an earlier global field that has since disappeared.

In many places, there is incontrovertible evidence of erosion and a watery past, including flood plains and river channels, which tantalize the astrobiologist (see Mars, water). Valles Marineris, however, was created not by running water but by the stretching and cracking of the crust associated with the formation of the Tharsis bulge.



Mars interior


Inside, Mars may have a core roughly 1,700 kilometers in radius which the planet's relatively low density suggests contains a higher ratio of sulfur to iron than that in the cores of the other terrestrial worlds. Overlaying this is probably a molten, rocky mantle, somewhat denser than the Earth's, topped with a thin crust, which, based on data collected by Mars Global Surveyor, is about 80 kilometers thick in the southern hemisphere and about 35 kilometers thick in the north.



Mars has a thin atmosphere, which is composed of mainly (95.3%) carbon dioxide. Other gases present include nitrogen (2.7%), argon (1.6%), oxygen (0.15%), water vapor (0.03%), and, intriguingly, methane. The discovery of a bit of methane in the martian atmosphere has excited scientists because they don't know where it has come from. One possibility is geological activity below the surface, another is microbes giving off the gas as part of their metabolism. For more, see Mars, atmosphere.


Life and water on Mars

People have long wondered if there might be life on the fourth planet. For more about this whole subject, see Mars, life. In the second half of the nineteenth century, speculation about Martians stepped up a gear when some observers reported seeing channels, or what came to be called "canals," on the surface. The seasonal waxing and waning of the martian polar caps was seen by early observers to be accompanied by surface changes at lower latitudes (see Mars, changes). The darkening of certain regions, following each spring thaw, prompted theories of inundation by floodwater and the growth of vast tracts of plant life. See the encyclopedia entries on the canals of Mars and Mars, vegetation.


Ever since then, questions about the existence of life and water on Mars have been entwined. Observations by spacecraft have shown that, in the remote past, conditions on the fourth planet were friendlier to life as we know it than they are today. Mars has been through periods when it was warmer and wetter than it is now (see Mars, past conditions). However, we still don't know if life managed to develop under those milder conditions. And, if it did, whether it survived to the present day.


Mars data
distance from Sun (mean) 227.9 million km (141.6 million mi., 1.52 AU)
distance from Sun (min.) 206.7 million km (128.5 million mi, 1.38 AU)
distance from Sun (max.) 249.1 million km (154.8 million mi, 1.66 AU)
equatorial diameter 6,786 km (4,217 mi)
equatorial diameter (Earth = 1) 0.532
mass (Earth = 1) 0.107
density 3.9 g/cm3
axial period 24.6 hours
axial inclination 23.9°
orbital period 686.98 days
orbital inclination 1.85°
orbital eccentricity 0.093
number of moons 2
atmospheric composition 95.3% CO2, 2.7% N2, 1.6% Ar, 0.15% O2, 0.03% H2O
surface temperature (mean) -55°C (-67°F)
surface temperature (equat. summer day) 27°C (80°F)
surface temperature (polar winter night) -133°C (-207°F)
surface gravity (Earth=1) 0.38
escape velocity 5.0 km/s (18,108 km/h, 11,254 mph)
albedo 0.16