Mars

The planet Mars was well recognized in very ancient times because of its brightness and red color. About 1000 B.C. it was named Nergal by the Babylonians after their god of death and pestilence, and Horus the Red by the Egyptians. Later, the Greeks named it Ares for their god of battle (who supported the Trojans), and the Romans renamed it Mars for their god of war (thus the term, martial). The symbol for Mars is a shield and spear. Mars’ two moons, discovered in 1877, are known appropriately as Phobos (Fear) and Deimos (Panic) from the two horses of Mars’ warchariot. Perhaps because of their similar brightness, Mars and Venus are associated in mythology with Harmony, born of the union of strife and love; similarly, Ares and Aphrodite became the parents of Eros (Cupid). Mars has a mean equatorial diameter of 6794 km (0.51 times that of the Earth), a mass of 6.418 x 1026 g, a mean density of 3.93 g/cm3 (Earth has a density of 5.52 g/cm3), an elliptical orbit of 780 Earth days, a daily rotation rate of 24 h and 37 min, and perihelion and aphelion distances from the Sun of 1.381 and 1.666 AU, respectively (Table 1; Carr 1984). Mars’ present obliquity of 25.1° may have changed during the past because of waxing and waning of polar ice caps (Rubincam 1992). These glacial advances and retreats tend to change Mars’ dynamical flattening, which is out of phase with the Sun. This in turn produces an annual solar torque on Mars that varies the angle between the equatorial and orbital planes. Rubincam estimated that climatic changes in martian history were capable of secularly increasing the planet’s obliquity by 1 ° or 2° since the Solar System was formed. Such a change, in turn, would enhance the martian seasons. Additional change in the obliquity of Mars may have been caused by creation of the Tharsis bulge, an increase of + 7° (Ward 1979). Bills (1990), however, indicated that until we know the dynamical history of the planets we cannot know the exact effect that the bulge had on the obliquity of Mars.