MARS [Mars] in astronomy, 4th planet
from the sun, with an orbit next in order beyond that of the earth.
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.
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.
Photographs sent back by the Mariner
4 space probe show the surface of Mars to be pitted with a number of large
craters, much like the surface of our moon. In 1971 the Mariner 9 space probe
discovered a huge canyon, Valles Marineris. Completely dwarfing the Grand
Canyon in Arizona, this canyon stretches for 2,500 mi (4,000 km) and at some
places is 125 mi (200 km) across and 2 mi (3 km) deep. Mars also has numerous
enormous volcanoes—including Olympus Mons (c.370 mi/600 km in diameter and 16
mi/26 km tall), the largest in the solar system—and lava plains. In 1976 the
Viking spacecraft landed on Mars and studied sites at Chryse and Utopia. They
recorded a desert environment with a reddish surface and a reddish atmosphere.
These experiments analyzed soil samples for evidence of microorganisms or other
forms of life; none was found. In 1997, Mars Pathfinder landed on Mars and sent
a small rover, Sojourner, to take soil samples and pictures. Among the data
returned were more than 16,000 images from the lander and 550 images from the
rover, as well as more than 15 chemical analyses of rocks and extensive data on
winds and other weather factors. Mars Global Surveyor, which also reached Mars
in 1997, has returned images produced by its systematic mapping of the surface.
The European Space Agency's Mars Express space probe went into orbit around
Mars in late 2003 and sent the Beagle 2 lander to the surface, but contact was
not established with the lander. The American rovers Spirit and Opportunity
landed successfully in early 2004.
Analysis of the satellite data
indicates that Mars appears to lack active plate tectonics at present; there is
no evidence of recent lateral motion of the surface. With no plate motion, hot
spots under the crust stay in a fixed position relative to the surface; this, along
with the lower surface gravity, may be the explanation for the giant volcanoes.
However, there is no evidence of current volcanic activity. There is evidence
of erosion caused by floods and small river systems. The possible
identification of rounded pebbles and cobbles on the ground, and sockets and
pebbles in some rocks, suggests conglomerates that formed in running water
during a warmer past some 2-4 billion years ago, when liquid water was stable
and there was water on the surface, possibly even large lakes or oceans. Rovers
have identified minerals that only form in the presence of liquid water. There
is also evidence of flooding that occurred less that several million years ago,
most likely as the result of the release of water from aquifers deep underground.
Data received beginning in 2002 from the Mars Odyssey space probe suggests that
there is water in sand dunes found in the northern hemisphere.
Because the axis of rotation is
tilted about 25° to the plane of revolution, Mars experiences seasons somewhat
similar to those of the earth. One of the most apparent seasonal changes is the
growing or shrinking of white areas near the poles known as polar caps. These
polar caps, which are are composed of water ice and dry ice (frozen carbon
dioxide). During the Martian summer the polar cap in that hemisphere shrinks
and the dark regions grow darker; in winter the polar cap grows again and the
dark regions become paler. The seasonal portion of the ice cap is dry ice.
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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