The Bringer of War
Mars is the fourth planet from the Sun and the seventh largest:
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 Mars 2, the first spacecraft to land on Mars and 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. While the average temperature on
Mars is about 218 K (-55 C, -67 F), Martian surface temperatures range widely
from as little as 140 K (-133 C, -207 F) at the winter pole to almost 300 K (27
C, 80 F) on the day side during summer.
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 (top of page);
- 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). Mars Global Surveyor.has produced a
nice 3D map of Mars that clearly shows these features.
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. Data from Mars Global Surveyor indicates that Mars' crust is
about 80 km thick in the southern hemisphere but only about 35 km thick in the
north. 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 at
present; there is no evidence of recent 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.
But there is new evidence from Mars Global Surveyor that Mars may have had
tectonic activity in its early history, making comparisons to Earth all the more
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 some sort of fluid on the surface. Liquid water is the obvious fluid but
other possibilities exist. There may have been large lakes or even oceans; the
evidence for which was strenghtened by some very nice images of layered terrain
taken by Mars Global Surveyor. 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
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. Mars'
thin atmosphere produces a greenhouse effect but it is only enough to raise the
surface temperature by 5 degrees (K); much less than what we see on Venus and
Mars has permanent ice caps at both poles composed of water ice and 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 seems likely that a similar layer of water ice exists below the
southern cap (left) as well. 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 somewhat ambiguous but most scientists now believe
that they show no evidence for life on Mars (there is still some controversy,
however). 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. There have also
been several contradictory studies published since the McKay paper. Remember,
"extraordinary claims require extraordinary evidence." Much work remains to be
done before we can be confident of this most extraordinary claim.
Large, but not global, weak magnetic fields exist in various regions of Mars.
This unexpected finding was made by Mars Global Surveyor just days after it
entered Mars orbit. They are probably remnants of an earlier global field that
has since disappeared. This may have important implications for the structure of
Mars' 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 unaided eye.
Its apparent brightness varies greatly according to its relative position to the
Earth. There are several Web sites that 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
("Distance" is measured from the center of Mars).
Bill Arnett; last updated: 2003 Feb 14