Mars is the fourth planet from the Sun and the seventh largest in the solar system. The planet was named after the Greek God of War and probably got this name due to its red color. Mars is sometimes referred to as the Red Planet. (An interesting side note: the Roman god Mars was a god of agriculture before becoming associated with the Greek Ares; those in favor of colonizing and terraforming Mars may prefer this symbolism.) The name of the month March derives from Mars.
The first spacecraft to visit Mars was Mariner 4 in 1965. Several others followed including Mars 2, the first spacecraft to land on Mars and the two Viking landers in 1976. Ending a long 20 year hiatus, Mars Pathfinder landed successfully on Mars on 1997 July 4.
Mars' orbit is significantly elliptical. One result of this is a temperature variation of about 30 C at the subsolar point between aphelion and perihelion. This has a major influence on Mars' climate. While the average temperature on Mars is about 218 K (-55 C, -67 F), Martian surface temperatures range widely from as little as 140 K (-133 C, -207 F) at the winter pole to almost 300 K (27 C, 80 F) on the day side during summer.
Though Mars is much smaller than Earth, its surface area is about the same as the land surface area of Earth.
Except for Earth, Mars has the most highly varied and interesting terrain of any of the terrestrial planets, some of it quite spectacular:
Olympus Mons: the largest mountain in the Solar System rising 24 km (78,000 ft.) above the surrounding plain. Its base is more than 500 km in diameter and is rimmed by a cliff 6 km (20,000 ft) high.
Tharsis: a huge bulge on the Martian surface that is about 4000 km across and 10 km high.
Valles Marineris: a system of canyons 4000 km long and from 2 to 7 km deep.
Hellas Planitia: an impact crater in the southern hemisphere over 6 km deep and 2000 km in diameter. Much of the Martian surface is very old and cratered, but there are also much younger rift valleys, ridges, hills and plains.
The southern hemisphere of Mars is predominantly ancient cratered highlands somewhat similar to the Moon. In contrast, most of the northern hemisphere consists of plains which are much younger, lower in elevation and have a much more complex history. An abrupt elevation change of several kilometers seems to occur at the boundary. The reasons for this global dichotomy and abrupt boundary are unknown (some speculate that they are due to a very large impact shortly after Mars' accretion). Mars Global Surveyor.has produced a nice 3D mapof Mars that clearly shows these features.
The interior of Mars is known only by inference from data about the surface and the bulk statistics of the planet. The most likely scenario is a dense core about 1700 km in radius, a molten rocky mantle somewhat denser than the Earth's and a thin crust. Data from Mars Global Surveyor indicates that Mars' crust is about 80 km thick in the southern hemisphere but only about 35 km thick in the north. Mars' relatively low density compared to the other terrestrial planets indicates that its core probably contains a relatively large fraction of sulfur in addition to iron (iron and iron sulfide).
Like Mercury and the Moon, Mars appears to lack active plate tectonics at present; there is no evidence of recent horizontal motion of the surface such as the folded mountains so common on Earth. With no lateral plate motion, hot-spots under the crust stay in a fixed position relative to the surface. This, along with the lower surface gravity, may account for the Tharis bulge and its enormous volcanoes. There is no evidence of current volcanic activity, however. But there is new evidence from Mars Global Surveyor that Mars may have had tectonic activity in its early history, making comparisons to Earth all the more interesting!
There is very clear evidence of erosion in many places on Mars including large floods and small river systems (right). At some time in the past there was clearly water on the surface There may have been large lakes or even oceans. But it seems that this occurred only briefly and very long ago; the age of the erosion channels is estimated at about nearly 4 billion years. (Valles Marineris was NOT created by running water. It was formed by the stretching and cracking of the crust associated with the creation of the Tharsis bulge.)
Early in its history, Mars was much more like Earth. As with Earth almost all of its carbon dioxide was used up to form carbonate rocks. But lacking the Earth's plate tectonics, Mars is unable to recycle any of this carbon dioxide back into its atmosphere and so cannot sustain a significant greenhouse effect. The surface of Mars is therefore much colder than the Earth would be at that distance from the Sun.
Mars has a very thin atmosphere composed mostly of the tiny amount of remaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%). The average pressure on the surface of Mars is only about 7 millibars (less than 1% of Earth's), but it varies greatly with altitude from almost 9 millibars in the deepest basins to about 1 millibar at the top of Olympus Mons. But it is thick enough to support very strong winds and vast dust storms that on occasion engulf the entire planet for months. Mars' thin atmosphere produces a greenhouse effect but it is only enough to raise the surface temperature by 5 degrees (K); much less than what we see on Venus and Earth.
Mars has permanent ice caps at both poles composed mostly of solid carbon dioxide ("dry ice"). The ice caps exhibit a layered structure with alternating layers of ice with varying concentrations of dark dust. In the northern summer the carbon dioxide completely sublimes, leaving a residual layer of water ice. It's not known if a similar layer of water ice exists below the southern cap (left) since its carbon dioxide layer never completely disappears. The mechanism responsible for the layering is unknown but may be due to climatic changes related to long-term changes in the inclination of Mars' equator to the plane of its orbit. There may also be water ice hidden below the surface at lower latitudes. The seasonal changes in the extent of the polar caps changes the global atmospheric pressure by about 25% (as measured at the Viking lander sites).
Recent observations with the Hubble Space Telescope have revealed that the conditions during the Viking missions may not have been typical. Mars' atmosphere now seems to be both colder and dryer than measured by the Viking landers.
The Viking landers performed experiments to determine the existence of life on Mars. The results were somewhat ambiguous but most scientists now believe that they show no evidence for life on Mars (there is still some controversy, however). Optimists point out that only two tiny samples were measured and not from the most favorable locations. More experiments will be done by future missions to Mars.
Large, but not global, weak magnetic fields exist in various regions of Mars. This unexpected finding was made by Mars Global Surveyor just days after it entered Mars orbit. They are probably remnants of an earlier global field that has since disappeared. This may have important implications for the structure of Mars's interior and for the past history of its atmosphere and hence for the possibility of ancient life.
When it is in the nighttime sky, Mars is easily visible with the unaided eye. Its apparent brightness varies greatly according to its relative position to the Earth.
Mars/Earth Comparison
Bulk parameters
Mars Earth Ratio (Mars/Earth)
Mass (1024 kg) 0.64185 5.9736 0.107
Volume (1010 km3) 16.318 108.321 0.151
Equatorial radius (km) 3397 6378.1 0.533
Polar radius (km) 3375 6356.8 0.531
Volumetric mean radius (km) 3390 6371.0 0.532
Core radius (km) 1700 3485 0.488
Ellipticity (Flattening) 0.00648 0.00335 1.93
Mean density (kg/m3) 3933 5515 0.713
Surface gravity (m/s2) 3.69 9.78 0.377
Escape velocity (km/s) 5.03 11.19 0.450
GM (x 106 km3/s2) 0.04283 0.3986 0.107
Bond albedo 0.250 0.306 0.817
Visual geometric albedo 0.150 0.367 0.409
Visual magnitude V(1,0) -1.52 -3.86 -
Solar irradiance (W/m2) 589.2 1367.6 0.431
Black-body temperature (K) 210.1 254.3 0.826
Topographic range (km) 30 20 1.500
Moment of inertia (I/MR2) 0.366 0.3308 1.106
J2 (x 10-6) 1960.45 1082.63 1.811
Orbital parameters
Mars Earth Ratio (Mars/Earth)
Semimajor axis (106 km) 227.92 149.60 1.524
Sidereal orbit period (days) 686.980 365.256 1.881
Tropical orbit period (days) 686.973 365.242 1.881
Perihelion (106 km) 206.62 147.09 1.405
Aphelion (106 km) 249.23 152.10 1.639
Synodic period (days) 779.94 - -
Mean orbital velocity (km/s) 24.13 29.78 0.810
Max. orbital velocity (km/s) 26.50 30.29 0.875
Min. orbital velocity (km/s) 21.97 29.29 0.750
Orbit inclination (deg) 1.850 0.000 -
Orbit eccentricity 0.0935 0.0167 5.599
Sidereal rotation period (hrs) 24.6229 23.9345 1.029
Obliquity to orbit (deg) 25.19 23.45 1.074
Mars Observational Parameters
Distance from Earth
Minimum (106 km) 54.5
Maximum (106 km) 401.3
Apparent diameter from Earth
Maximum (seconds of arc) 25.7
Minimum (seconds of arc) 3.5
Mean values at opposition from Earth
Distance from Earth (106 km) 78.39
Apparent diameter (seconds of arc) 17.9
Apparent visual magnitude -2.0
Mars Mean Orbital Elements (J2000)
Semimajor axis (AU) 1.52366231 Orbital eccentricity 0.09341233 Orbital inclination (deg) 1.85061 Longitude of ascending node (deg) 49.57854 Longitude of perihelion (deg) 336.04084 Mean Longitude (deg) 355.45332
Martian Atmosphere
Surface Pressure: ~6.1 mb (variable) [6.9 mb to 9 mb (Viking 1 Lander site)] Surface Density: ~0.020 kg/m3 Scale height: 11.1 km Average temperature: ~210 K Diurnal temperature range: 184 K to 242 K (Viking 1 Lander site) Wind speeds: 2-7 m/s (summer), 5-10 m/s (fall), 17-30 m/s (dust storm) (Viking Lander sites) Mean molecular weight: 43.34 g/mole Atmospheric composition (by volume): Major : Carbon Dioxide (CO2) - 95.32% ; Nitrogen (N2) - 2.7% Argon (Ar) - 1.6%; Oxygen (O2) - 0.13%; Carbon Monoxide (CO) - 0.08% Minor (ppm): Water (H2O) - 210; Nitrogen Oxide (NO) - 100; Neon (Ne) - 2.5; Hydrogen-Deuterium-Oxygen (HDO) - 0.85; Krypton (Kr) - 0.3; Xenon (Xe) - 0.08
Satellites of Mars
Phobos Deimos
Semi-major axis* (km) 9378 23459
Sidereal orbit period (days) 0.31891 1.26244
Sidereal rotation period (days) 0.31891 1.26244
Orbital inclination (deg) 1.08 1.79
Orbital eccentricity 0.0151 0.0005
Major axis radius (km) 13.4 7.5
Median axis radius (km) 11.2 6.1
Minor axis radius (km) 9.2 5.2
Mass (1015 kg) 10.6 2.4
Mean density (kg/m3) 1900 1750
Geometric albedo 0.07 0.08
Visual magnitude V(1,0) +11.8 +12.89
Apparent visual magnitude (V0) 11.3 12.40
