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Mars is the fourth
planet from the Sun and the seventh largest:
orbit: 227,940,000 km (1.52 AU) from Sun
diameter: 6,794 km
mass: 6.4219e23 kg
Mars (Greek: Ares) is the god of War. The planet 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.
Mars has been known since prehistoric times. It is still a favorite of science fiction writers as the most favorable place in the Solar System (other than Earth!) for human habitation. But the famous "canals" "seen" by Lowell and others were, unfortunately, just as imaginary as Barsoomian princesses.
The first
spacecraft to visit Mars was Mariner 4 in 1965. Several others
followed including the two Viking landers in 1976 (left). Ending a
long 20 year hiatus, Mars Pathfinder landed successfully on Mars on
1997 July 4 (right).
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. Overall, the Viking landers found that Martian temperatures vary from 150 K (-220 F) to 295 K (70 F).
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
(right).
- 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 (far
right);
- 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 (left)
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). Recently, some scientists have begun
to question whether the abrupt elevation is real in the first place.
Mars Global Surveyor should resolve the issue.
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. 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; there is no evidence of 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. And though Mars may have been more volcanicly active in the past, it appears to never have had any plate tectonics.
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. Although its atmosphere is mostly carbon dioxide (like Venus'), the greenhouse effect on Mars is strong enough to raise the surface temperature by only 5 degrees (K).
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 (right) 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. ( more details from STScI)
The Viking landers performed experiments to determine the existence of life on Mars. The results were negative. 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.
A small number of meteorites (the SNC meteorites) are believed to have originated on Mars.
On 1996 Aug 6,
David McKay et al announced the first identification of organic
compounds in a Martian meteorite. The authors further suggest that
these compounds, in conjunction with a number of other mineralogical
features observed in the rock, may be evidence of ancient Martian
microorganisms (left?)
Exciting as this is, it is important to note while
this evidence is strong it by no means establishes the fact of
extraterrestrial life. Remember, "extraordinary claims require
extraordinary evidence." Much work remains to be done before we can
be confident of this most extraordinary claim.
Mars has a weak global magnetic field. This unexpected finding was made by Mars Global Surveyor just days after it entered Mars orbit. 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 naked eye. Its apparent brightness varies greatly according to its relative position to the Earth. Mike Harvey's planet finder charts show the current position of Mars (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program such as Starry Night.
Mars has two tiny satellites which orbit very close to the
surface.
Distance Radius Mass Satellite (000 km) (km) (kg) Discoverer Date --------- -------- ------ ------- ---------- ---- Phobos 9 11 1.08e16 Hall 1877 Deimos 23 6 1.80e15 Hall 1877