The fourth planet from the Sun.

Has a thin atmosphere composed of 95% carbon dioxide, 2-3% nitrogen, and 2% argon. Mars has no magnetic field. Atmospheric pressure is 7 mbar, varying with an amplitude of 2 mbar as volatiles are vaporized and recondense at the poles. The south polar cap is permanent, and is composed predominantly of solid carbon dioxide with some water. It expands and contracts with the seasons as the dry ice vaporizes and sublimes. The north cap is seasonal, so all the carbon dioxide is vaporized in the summer, leaving a residual cap of water ice only. A weak greenhouse effect (approximately 1 K) is present because of the carbon dioxide. Mars is a desert planet swept by frequent dust storms. The storms occur most frequently when Mars is closest to the Sun. Its climate is affected by external causes such as comets, orbital changes, and obliquity fluctuations (small fluctuations with a period of 10⁵ y superimposed on large fluctuations with 10⁶ y period). Wind blows from the east in the summer hemisphere (retrograde) and from the west in the winter hemisphere (prograde).

The planet's reddish color comes from iron oxides in its soil. The south half of the planet is a heavily cratered highland 1-4 km above the mean level, while the north half is sparsely cratered plains containing the Tharsis and Elysium regions. Smith and Zuber (1996) found that the hemispheric dichotomy is not a fundamental feature of the planet's shape, but represents a 3 km offset between the center of mass and figure center. Shield volcanos are much more prevalent in the northern hemisphere. The Tharsis region contains the three mountains Arsia Mons, Pavonis Mons, and Ascraeus Mons. Just beyond the northwest edge is the 25 km high Olympus Mons, a huge shield volcano 600 km across which is rimmed by a 6 km high cliff. Mars has mountain and vast planes such as Utopia Planitia, Elysium Planitia, and Hellas Planitia. Mars's center of figure is offset from its center of mass by 2.5 km. What appear to be dry stream beds can also be found on the planets surface, and the Valles Marineris trench stretches 4000 km across in the equatorial region east of Tharsis. Temperatures on Mars vary from 290 K in summer to 185 K in winter. Mars's crust and mantle are composed of silicates; its core is thought to be iron sulfide.

Mars makes side-by-side approaches with earth (is at opposition) approximately every 780 days, its so-called synodic period. However, this number can vary considerable as a result of Mars's highly eccentric orbit. Mars's orbit also means that the closest oppositions occur only every 15 or 17 years. The closest have been in 1971; September 28, 1988; and 2003. Because of the eccentricity of Mars's orbit, opposition (when Mars is opposite the Sun in the sky) and closest approach do not occur at the same time. For instance, although the 1990 opposition was on Nov. 27, the planet was closest to earth on Nov. 20.

date	phenomenon	angular size	mag.	distance (106 km)	days since last opposition
07-10-1986	opposition
09-28-1988	opposition	23.6		59.2	811
11-20-1990	closest	18.1		77.3	--
11-27-1990	opposition	17.9		78.0
02-12-1995	opposition
03-17-1997	opposition				764

Mars's two moons are Phobos and Deimos.

SNC Meteorites