Mars

Information

Mars is the fourth planet from the Sun in our solar system. It is named after the Roman god of war Mars (Ares in Greek mythology) because of its apparent red color. This feature also earned it the nickname "The Red Planet". Mars has two moons, Phobos and Deimos, which are small and oddly-shaped, possibly being captured asteroids. The prefix areo- refers to Mars in the same way geo- refers to Earth � for example, areology versus geology. (However, areology is also used to refer to the study of Mars as a whole rather than just the geological processes of the planet.)

The astronomical symbol for Mars is ♂, a circle with an arrow pointing northeast. This symbol is a stylized representation of the shield and spear of the god Mars, and in biology it is used as a sign for the male sex.

The Chinese, Korean, Japanese, and Vietnamese cultures refer to the planet as 火星, or fire star, a naming based on the ancient Chinese mythological cycle of Five Elements.

Physical Characteristics

The red, fiery appearance of Mars is caused by iron oxide (rust) on its surface. Mars has only a quarter the surface area of the Earth and only one-tenth the mass, though its surface area is approximately equal to that of the Earth's dry land because Mars lacks oceans. The solar day (or sol) on Mars is very close to Earth's day: 24 hours, 39 minutes, and 35.244 seconds.

Atmosphere

Mars' atmosphere is thin: the air pressure on the surface is only 750 pascals, about 0.75% of the average on Earth. However, the scale height of the atmosphere is about 11 km, somewhat higher than Earth's 6 km. The atmosphere on Mars is 95% carbon dioxide, 3% nitrogen, 1.6% argon, and contains traces of oxygen and water. The atmosphere is quite dusty, giving the Martian sky a tawny color when seen from the surface; data from the Mars Exploration Rovers indicates the suspended dust particles are roughly 1.5 microns across. In 2003, methane was apparently discovered in the atmosphere by Earth-based telescopes and possibly confirmed in March 2004 by the Mars Express Orbiter; present measurements state an average methane concentration of about 11�4 ppb by volume. The thin atmosphere cannot hold heat and is the cause of the lower temperatures on Mars. The maximum temperature is roughly 20� (68�).

The presence of methane on Mars would be very intriguing, since as an unstable gas it indicates that there must be (or have been within the last few hundred years) a source of the gas on the planet. Volcanic activity, comet impacts, and the existence of life in the form of microorganisms such as methanogens are among possible but as yet unproven sources. The methane appears to occur in patches, which suggests that it is being rapidly broken down before it has time to become uniformly distributed in the atmosphere, and so it is presumably also continually being released to the atmosphere. Plans are now being made to look for other companion gases that may suggest which sources are most likely; in the Earth's oceans biological methane production tends to be accompanied by ethane, while volcanic methane is accompanied by sulfur dioxide.

Other aspects of the Martian atmosphere vary significantly. In the winter months when the poles are in continual darkness, the surface gets so cold that as much as 25% of the entire atmosphere condenses out into meters thick slabs of CO2 ice (dry ice). When the poles are again exposed to sunlight the CO2 ice sublimates, creating enormous winds that sweep off the poles as fast as 250 mph. These seasonal actions transport large amounts of dust and water vapor giving rise to Earth-like frost and large cirrus clouds. These clouds of water-ice were photographed by the Opportunity rover in 2004.

Recently, evidence has been discovered suggesting that Mars may be warming in the short term; however, it is now cooler than it was in the 1970s.

Geology

The surface of Mars is thought to be primarily composed of basalt, based upon the Martian meteorite collection and orbital observations. There is some evidence that some portion of the Martian surface might be more silica-rich than typical basalt, perhaps similar to andesitic rocks on Earth, though these observations may also be explained by silica glass. Much of the surface is deeply covered by dust as fine as talcum powder.

Observations of the magnetic fields on Mars by the Mars Global Surveyor spacecraft have revealed that parts of the planet's crust has been magnetized. This magnetization has been compared to alternating bands found on the ocean floors of Earth. One interesting theory, published in 1999 and reexamined in October 2005 in a publication by the same group, is that these bands could be evidence of the past operation of plate tectonics on Mars. However, this has yet to be proven or widely accepted and remains an area of active research.

Amongst the findings from the Opportunity rover is the presence of hematite on Mars in the form of small spheres on the Meridiani Planum. The spheres are only a few millimeters in diameter and are believed to have formed as rock deposits under watery conditions billions of years ago. Other minerals have also been found containing forms of sulfur, iron or bromine such as jarosite. This and other evidence led a group of 50 scientists to conclude in the December 9, 2004 edition of the journal Science that "Liquid water was once intermittently present at the Martian surface at Meridiani, and at times it saturated the subsurface. Because liquid water is a key prerequisite for life, we infer conditions at Meridiani may have been habitable for some period of time in Martian history". On the opposite side of the planet the mineral goethite, which (unlike hematite) forms only in the presence of water, along with other evidence of water, has also been found by the Spirit rover in the "Columbia Hills".

In 1996, researchers studying a meteorite (ALH84001) believed to have originated from Mars reported features which they attributed to microfossils left by life on Mars. As of 2005, this interpretation remains controversial with no consensus having emerged.

Topography

The dichotomy of Martian topography is striking: northern plains flattened by lava flows contrast with the southern highlands, pitted and cratered by ancient impacts. The surface of Mars as seen from Earth is consequently divided into two kinds of areas, with differing albedo. The paler plains covered with dust and sand rich in reddish iron oxides were once thought of as Martian 'continents' and given names like Arabia Terra (land of Arabia) or Amazonis Planitia (Amazonian plain). The dark features were thought to be seas, hence their names Mare Erythraeum, Mare Sirenum and Aurorae Sinus. The largest dark feature seen from Earth is Syrtis Major.

Mars has polar ice caps that contain frozen water and carbon dioxide that change with the Martian seasons &mdash. Each cap has surface deposits of carbon dioxide ice that form a polar "hood" during Martian winter, and then sublimate during the summer uncovering the underlying cap surface of layered water ice and dust. The southern polar cap differs from the Northern polar cap in that it appears to contain at least some permanent deposits of CO2, which are changing on the time scale of years.

The shield volcano, Olympus Mons (Mount Olympus), is at 26 km the highest mountain in the solar system. It is in a vast upland region called Tharsis, which contains several large volcanos. The Tharsis region of Mars also has the solar system's largest canyon system, Valles Marineris or the Mariner Valley, which is 4000 km long and 7 km deep. Mars is also scarred by a number of impact craters. The largest of these is the Hellas impact basin, covered with light red sand.

The difference between Mars' highest and lowest points is nearly 31 km (from the top of Olympus Mons at an altitude of 26 km to the bottom of the Hellas impact basin at an altitude of 4 km below the datum). In comparison, the difference between Earth's highest and lowest points (Mount Everest and the Mariana Trench) is only 19.7 km. Combined with the planets' different radii, this means Mars is nearly three times "rougher" than Earth.

The International Astronomical Union's Working Group for Planetary System Nomenclature is responsible for naming Martian surface features.

Other notes:

Zero elevation: Since Mars has no oceans and hence no 'sea level', a zero-elevation surface or mean gravity surface must be selected. The datum for Mars is defined by the fourth-degree and fourth-order spherical harmonic gravity field, with the zero altitude defined by the 610.5 Pa (6.105 mbar) atmospheric pressure surface (approximately 0.6% of Earth's) at a temperature of 273.16 K. This pressure and temperature correspond to the triple point of water.

Zero meridian: Mars' equator is defined by its rotation, but the location of its Prime Meridian was specified, as was Earth's, by choice of an arbitrary point which was accepted by later observers. The German astronomers Wilhelm Beer and Johann Heinrich M�dler selected a small circular feature as a reference point when they produced the first systematic chart of Mars features in 1830-32. In 1877, their choice was adopted as the prime meridian by the Italian astronomer Giovanni Schiaparelli when he began work on his notable maps of Mars. After the spacecraft Mariner 9 provided extensive imagery of Mars in 1972, a small crater (later called Airy-0), located in the Sinus Meridiani ('Middle Bay' or 'Meridian Bay') along the line of Beer and M�dler, was chosen by Merton Davies of the RAND Corporation to provide a more precise definition of 0.0� longitude when he established a planetographic control point network.

Canals

Mars has an important place in human imagination due to the belief by some that life existed on Mars. These beliefs are due mainly to observations by many in the 19th century popularized by Percival Lowell and Giovanni Schiaparelli. Schiaparelli called these observed features canali, meaning channels in Italian. This was popularly mistranslated as 'canals', and the myth of the Martian canals began. They were apparently artificial linear features on the surface that were asserted to be canals, and due to seasonal changes in the brightness of some areas that were thought to be caused by vegetation growth. This gave rise to many stories concerning Martians. The linear features are now known to be mostly non-existent or, in some cases, dry ancient watercourses. The color changes have been ascribed to dust storms.

Ice Patches

On 29 July 2005, the BBC reported that a visible "ice lake" had been discovered in a crater in the north polar region of Mars. Images of the crater, taken by the High Resolution Stereo Camera on board the European Space Agency's Mars Express spacecraft, clearly show a broad sheet of ice in the bottom of an unnamed crater located on Vastitas Borealis, a broad plain that covers much of Mars' far northern latitudes, at approximately 70.5� North and 103� East. The crater is 35 km (23 mi) wide and about 2 km (1.2 mi) deep.

According to the original HRSC/Mars Express feature, the height difference between the crater floor and the surface of the water ice is about 200 metres. ESA scientists have attributed most of this height difference to sand dunes beneath the water ice, which are partially visible. While scientists do not refer to the patch as a "lake", the water ice patch is remarkable for its size and for being present throughout the year. Deposits of water ice and layers of frost have been found in many different locations on the planet.

The Moons of Mars

Mars has two tiny natural moons, Phobos and Deimos, which orbit very close to the planet and are thought to be captured asteroids.

Mars has two tiny natural moons, Phobos and Deimos, which are thought to be captured asteroids.

From the surface of Mars, the motions of Phobos and Deimos appear very different from that of our own moon. Speedy Phobos rises in the west, sets in the east, and rises again in just 11 hours, while Deimos, being only just outside synchronous orbit, rises as expected in the east but very slowly. Despite its 30 hour orbit, it takes 2.7 days to set in the west as it slowly falls behind the rotation of Mars, and as long again to rise.

Both moons are tidally locked, always presenting the same face towards Mars. Since Phobos orbits Mars faster than the planet itself rotates, tidal forces are slowly but steadily decreasing its orbital radius. At some point in the future, when it approaches Mars closely enough, Phobos will be broken up by these tidal forces. Several strings of craters on the Martian surface, inclined further from the equator the older they are, suggest that there may have been other small moons that suffered the fate expected of Phobos, and also that the Martian crust as a whole shifted between these events. Deimos, on the other hand, is far enough away that its orbit is being slowly boosted instead, as in the case of our own moon.

Both satellites were discovered in 1877 by Asaph Hall, and are named after the characters Phobos (panic/fear) and Deimos (terror/dread) who, in Greek mythology, accompanied their father Ares, god of war, into battle. Ares was known as Mars to the Romans.

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Specifications

Orbital Characteristics

Semi-Major Axis   227,936,637 km (141,632,976 Miles) (1.523 662 31 AU)
Orbital Circumference   1.429 Tm (888,005,041 Miles) (9.553 AU)
Eccentricity   0.093 412 33
Perihelion   206,644,545 km (128,402,710 Miles) (1.381 333 46 AU)
Aphelion   249,228,730 km (154.863,243 Miles) (1.665 991 16 AU)
Orbital Period   686.9600 days (1.8808 a)
Synodic Period   779.96 days (2.135 a)
Avg. Orbital Speed   24.077 km/s (53,820 Miles / Hour)
Max. Orbital Speed   26.499 km/s (59,220 Miles / Hour)
Min. Orbital Speed   21.972 km/s (49,140 Miles / Hour)
Inclination   1.850 61� (5.65� to Sun's equator)
Longitude of the Ascending Node   49.578 54�
Argument of the Perihelion   286.462 30�
Number of Satellites   2

Physical Characteristics

Equatorial Diameter   6,804.9 km (4228.4 Miles) (0.533 Earths)
Polar Diameter   6,754.8 km (4197.2 Miles) (0.531 Earths)
Oblateness   0.007 36
Surface Area   1.448�10⁸ km² (55.907 Million Square Miles) (0.284 Earths)
Volume   1.638�10¹¹ km³ (0.151 Earths)
Mass   6.4185�10²³ kg (0.107 Earths)
Mean Density   3.934 g/cm³
Equatorial Gravity   3.69 m/s² (0.376g)
Escape Velocity   5.027 km/s (11,232 Miles / Hour)
Rotation Period   1.025 957 days (24.622 962 h)
Rotation Velocity   868.22 km/h (539.49 Miles / Hour) (at the equator)
Axial Tilt   25.19�
Right Ascension of North Pole   317.681 43� (21 h 10 min 44 s)
Declination   52.886 50�
Albedo   0.15
Min. Surface Temp.   133 K (-140 C)
Mean Surface Temp.   210 K (-63 C)
Max. Surface Temp.   293 K (20 C)
Adjective   Martian

Atmospheric Characteristics

Atmospheric Pressure   0.7-0.9 kPa
Carbon Dioxide   95.32%
Nitrogen   2.7%
Argon   1.6%
Oxygen   0.13%
Carbon Monoxide   0.07%
Water Vapor   0.03%
Nitric Oxide   0.01%
Neon   2.5 PPM
Krypton   300 PPB
Xenon   80 PPB
Ozone   30 PPB
Methane   10.5 PPB

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