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Mars' History Mars is the fourth planet from the Sun and the seventh largest. 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 Mars may prefer this symbolism). Also 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, imaginary at the time.
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).
Click here to see the big picture! Surface and Atmosphere of Mercury One week, the sky is pink and cloudless, filled with windblown dust raised from the rusty Martian surface. By Martian standards, it's warm, about minus 40 degrees Fahrenheit. Then, in a matter of days, the dust is swept from the atmosphere, temperatures plummet 40 degrees, and brilliant water ice clouds appear against a dark blue sky. Dramatic weather changes like these may not seem very different from a batch of severe thunderstorms passing through your home town, but for Mars these changes can sweep over the entire planet every week. It appears that Mars' roller coaster-like weather is more chaotic and unpredictable than scientists first thought. Observations by the Hubble Space Telescope and the National Radio Astronomy Observatory (NRAO) radio telescope at Kitt Peak, Ariz., show that the atmosphere of Mars is more complex and variable than the picture revealed by the Viking and Mariner 9 orbiters. These spacecraft collected information from the planet in the 1970's and painted a fairly one-dimensional picture of Mars' climate. Images snapped by the orbiters revealed huge dust storms spreading throughout the entire atmosphere when Mars was closest to the sun (perihelion). These dusty conditions continued to dominate the planet's climate when it was farthest from the sun (aphelion). (Perihelion and aphelion occur every Mars year, which equals two Earth years. Aphelion occurs in northern summer, perihelion in southern summer.) But information captured by Hubble and NRAO show that Mars is more often cloudy than dusty, experiencing abrupt planet-wide swings between dusty and hot and cloudy and cold. A state of emergency would be declared on Earth if an ice or dust storm blanketed the entire planet. These shifts in climate are driven by three important factors: Mars' thin atmosphere, its elliptical orbit around the sun, and strong climatic interactions between dust and water ice clouds in the atmosphere. Mars' atmosphere is so thin that it weighs less than 1 percent of Earth's atmosphere. Because Mars' atmosphere is so paper-thin and there are no oceans to store up heat from the sun, the planet's temperatures respond more quickly and intensely to surface changes and atmospheric heating by the sun. There are also much larger annual changes in sunlight falling on Mars than on Earth, because Mars' distance from the sun varies by 20 percent in its orbit around the sun every two years. Mars' elliptical orbit leads to planet-wide changes in atmospheric and surface temperatures over the course of a Mars year. During perihelion, when Mars is closest to the sun (summer in the southern hemisphere), the planet receives 40 percent more sunlight than during aphelion, when it is farthest from the sun (summer in the northern hemisphere). This annual variation in sunlight causes 35-degree Fahrenheit increases during southern summer (perihelion), forcing continental-scale dust storms at the planet's surface. The dust is swept aloft to altitudes of tens of miles, where it spreads globally, absorbs light from the sun, and heats the entire atmosphere by another 30 to 50 degrees Fahrenheit. This dusty perihelion climate was observed by Viking and Mariner 9 and by NRAO in 1992, 1994, and 1996. But what the 1970's orbiters did not identify was the very distinctive Mars aphelion climate, with its planet-wide belts of water ice clouds. These clouds are as striking as the perihelion global dust storms. During the aphelion climate, surface dust raised by low dust storms is confined to low altitudes (about 10 km or 6 miles), and is eventually swept to the ground by water ice clouds. These clouds surround the planet at altitudes of 3 to 10 km (2 to 6 miles). It is the cold atmospheric conditions of Mars during aphelion, when the sun is much weaker, that stimulate the formation of these water ice clouds. The clouds further reduce atmospheric temperatures by forming around the dust. Without sunlight, the dust freezes and falls to the ground. This strong competition between dust heating and cloud cooling drives sweeping annual and short-term regional changes in Mars' climate. 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 dayside 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, such as: 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 maps of Mars that clearly shows these features like the picture below.
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 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 interesting!
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 Earth. 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).
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. 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 bee 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'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 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' SatellitesMars 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 Mars Statistics: Discoverer: Unknown Discovery Date: Prehistoric Distance from Earth Minimum (106 km) 54.5 Maximum (106 km) 401.3 Mass (kg) 6.4185 * 1023 Volume (km3) 16.318 * 1010 Equatorial radius (km) 3397 Ellipticity (flattening) 0.00648 Mean density (km/m3) 3933 Surface Gravity (eq.)(m/s2) 3.69 Escape Velocity (km/s) 5.03 GM (x 106 km3/s2) 0.04283 Mean distance from the Sun (km) 227,920,000 Rotational period (Earth hours) 24.6229 Orbital period (Earth days) 686.980 Mean orbital velocity (km/sec) 24.13 Orbital eccentricity 0.0935 Tilt of axis (degrees) 25.19 Orbital inclination (degrees) 1.850 Bond albedo 0.16 Visual geometric albedo (reflectivity) 0.150 Visual magnitude V(1,0) -1.52 Solar irradiance (W/m2) 589.2 Moment of inertia (I/MR2) 0.366 J2(x 10-6) 1960.45 Mean surface temperature -63.15°C or 210 K Maximum surface temperature -31.15°C or 242 K Minimum surface temperature -89.15°C or 184 K Atmospheric composition Carbon Dioxide (CO2) 95.32% Nitrogen (N2) 2.7% Argon (Ar) 1.6% Oxygen (O2) 0.13% Carbon Monoxide (CO) 0.08% Trace amounts of (ppm) Water (H2O) 210 100 2.5 Hydrogen Deuterium Oxygen (HDO) 0.85 Krypton (Kr) 0.3 0.08 Highest point on surface Olympus Mons (24km above surrounding lava plains) **Don't understand the measurements? Click here to get definitions and notes. Open Issues
Picture 1 | Picture 2 | Picture 3 | Picture 4! | Picture 5 | Picture 6 | Picture 7 | Picture 8 |
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).
