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Mars
The Planet Mars -- in astronomy, 4th planet from the sun, with an orbit next in order beyond that of the earth.
Physical Characteristics
Mars has a striking red appearance, and in its most favorable position for viewing, when it is opposite the sun, it is twice as bright as Sirius, the brightest star.
Mars has a diameter of 4,200 mi (6,800 km), just over half the diameter of the earth, and its mass is only 11% of the earth�s mass. The planet has a very thin atmosphere consisting mainly of carbon dioxide, with some nitrogen and argon.
Mars has an extreme day-to-night temperature range, resulting from its thin atmosphere, from about 80�F (27�C) at noon to about -100�F (-73�C) at midnight; however, the high daytime temperatures are confined to less than 3 ft (1 m) above the surface.
Surface Features
A network of linelike markings first studied in detail (1877) by G. V. Schiaparelli was referred to by him as canali, the Italian word meaning �channels� or �grooves.� Percival Lowell, then a leading authority on Mars, created a long-lasting controversy by accepting these �canals� to be the work of intelligent beings.
Under the best viewing conditions, however, these features are seen to be smaller, unconnected features. The greater part of the surface area of Mars appears to be a vast desert, dull red or orange in color. This color may be due to various oxides in the surface composition, particularly those of iron.
About one fourth to one third of the surface is composed of darker areas whose nature is still uncertain. Shortly after its perihelion Mars has planetwide dust storms that can obscure all its surface details.
Astronomical Characteristics
The mean distance of Mars from the sun is about 141 million mi (228 million km); its period of revolution is about 687 days, almost twice that of the earth.
At those times when the sun, earth, and Mars are aligned (i.e., in opposition) and Mars is at its closest point to the sun (perihelion), its distance from the earth is about 35 million mi (56 million km); this occurs every 15 to 17 years.
At oppositions when Mars is at its greatest distance from the sun (aphelion) it is about 63 million mi (101 million km) from the earth. It rotates on its axis with a period of about 24 hr 37 min, a little more than one earth day.
Satellites of Mars
Mars has two natural satellites, discovered by Asaph Hall in 1877. The innermost of these, Phobos, is about 7 mi (11 km) in diameter and orbits the planet with a period far less than Mars�s period of rotation (7 hr 39 min), causing it to rise in the west and set in the east. The outer satellite, Deimos, is about 4 mi (6 km) in diameter.
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