Mars is the fourth planet from the Sun and the seventh largest.
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.
227,940,000 km (1.52 AU) from Sun
6,785 km (4,217 miles)
Mass : 0.64x10^24 kilograms (0.11 x Earth's)
Density : 3,933 kg/m^3
Minimum Distance from Sun : 205 million km
(128 million miles)
Maximum Distance from Sun : 249 million km
(155 million miles)
Minimum Distance from Earth : 35 million miles
Rotation Period about Axis : 24.6 hrs
Revolution Period about the Sun : 1.88 years
Tilt of Axis : 25 degrees 12"
Surface Gravity : 3.7 m/s^2 (0.37 x Earth's)
Temperature : -82o C to 0o C
( -116o F to 32o F)
Average Surface Temperature (K) : 218K
Name in Roman/Greek Mythology : Mars/Ares
Satellites : 2
Mars visited by many spacecrafts beginning with Mariner 4, 6, 7, and 9 in the period 1965-1971, and the Viking 1 and 2 probes in 1976.
Mariner 4, 6, 7, were flyby missions that could only photograph small regions of the Martian surface. They saw portions of the surface that suggested Mars was drab and cratered like the Moon, and geologically dead. This was changed profoundly by the orbiter Mariner 9, which went into orbit around Mars in late 1971. When it arrived the entire Martian surface was engulfed in a dust storm that left almost no surface features visible.
When the dust storm finally subsided in early 1972, Mariner 9 discovered that we had been badly misled by the earlier flyby missions that had seen only small (in retrospect, unrepresentative) portions of the surface. Mariner 9 found evidence for a planet having many interesting geological features:
1-Meteor craters and volcanic plains (the largest crater is Hellas, which is 2000 km across).
2-Huge volcanic cones (left).
3-Gorges larger than the Grand Canyon here on Earth (the feature in the center of the adjacent image is a canyon system, Valles Marineris, that extends over a region the width of the United States).
Vast sedimentary deposits in the Polar regions.
4-Valleys that looked as if they could be water-formed (but these don't coincide with the "canals" that people erroneously thought they saw from Earth in earlier times).
No spacecraft in the history of space exploration has more profoundly changed our view of a planet than Mariner 9.
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.
The atmosphere and (probably) the interior of Mars differ substantially from that of the Earth. The atmosphere is much less dense and of different composition, and it is unlikely that the core is molten.
The Martian Atmosphere
The atmosphere has a pressure at the surface that is only 1/200 that of Earth(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).
The primary component of the atmosphere is carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%).
Seasonal heating drives strong winds that can reach 100 mph or more, stirring up large dust storms. Clouds form in the atmosphere, but liquid water cannot exist at the ambient pressure and temperature of the Martian surface: water goes directly between solid and vapor phases without becoming liquid.
(Mars is unable to recycle any of this carbon dioxide back into its atmosphere and so cannot sustain a significant greenhouse effect cause of lacking Plate Tectonics).
The preceding image shows the variation of the surface temperature over a period of 50 Martian days at the Viking 1 landing site.
Notice the large variation between night and daytime temperatures (associated with the low density of the atmosphere) and the almost constant high and low temperatures for this period.
Compare this, for example, with the daily temperature variations for Nome, Alaska (note however that the Nome plot is in degrees Fahrenheit, not Celsius).
Here is a graph of Martian atmospheric temperature variations as recorded over a period of days at the Pathfinder (1997) landing site compared with data from the Viking 1 site over a similar period in 1976 (in the these graphs a Sol is a Martian day, which corresponds to 24 hours and 37 minutes of Earth time).
At the time of these observations, the night temperatures drop to around -90 degrees celsius, but at the Pathfinder site the day temperature approaches a relatively balmy -10 degrees celsius at it peak.
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.
The Martian Interior
The density of Mars is about 25% less than that of the Earth, suggesting a proportionally larger concentration of lighter materials relative to the core. It is probably intermediate in composition between the Earth and the Moon.
Though Mars is probably at least partially differentiated, there is little evidence for large-scale tectonic motion (but there is smaller scale motion such as that responsible for the Valles Marineris system).
The core is thought to be iron sulphide; the absence of any detectable magnetic field even though the rotation period is comparable to that for Earth suggests that the core is probably not liquid.
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).
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 miles (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.
The surface of Mars is therefore much colder than the Earth would be at that distance from the Sun.
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. 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, which 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.
In 1976 the Viking 1 and 2 landers undertook searches on the Martian surface for the chemical evidence of present or past life on Mars.
The images shown below give a picture of one of the backup landers, and two different views of the Martian surface as photographed from Viking 1.
In addition to photgraphing the surface, the Viking landers undertook a series of experiments at two points on the surface to find evidence for life.
The 4 basic experiments that the Vikings carried out to search for evidence of life were:
Gas Metabolism: look for changes in the atmosphere induced by metabolism in the Martian soil.
Labeled Release: Look for release of radioactive carbon dioxide by metabolism from organic material labeled by radioactive carbon.
Pyrolytic Release: Search for radioactive compounds in soil by heating soil exposed to radioactive carbon dioxide.
Mass Spectrometer: Search directly in Martian soil for organic compounds known to be essential to Earth life.
These experiments were built around the hypothesis that if there were life on Mars it would have a similar metabolism to life on Earth, and that it would have a similar biochemistry based on the same organic compounds important to life on Earth.
The results of these experiments were complex. The first three gave positive results, but the complete absence of any organic compounds in the Martian soil according to the mass spectrometer experiment suggests that the positive results for the first three were not evidence for life, but rather evidence for a complex inorganic chemistry in the Martian soil. Thus, the Viking verdict was that there was no evidence for present or past life on Mars.
Renewed Interest in Martian Life
This issue has been given renewed impetus by the recent claim (see also this and this) that a meteorite found on the Earth was once part of Mars (because of detailed chemical composition), and that there may be evidence in this rock for past organic activity. However, this is a very open topic at the moment, since there potentially are other explanations of the meteorite's content. We will have to wait on further evidence to clarify this issue.
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 may be composed of ordinary ice or of dry ice (frozen carbon dioxide) and are thought to be only a few inches thick.
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.
Mars has a rotational period of 24 hours and 37 minutes, a period for revolution about the sun of 687 days, and a diameter of 6800 km (about half that of Earth). Its average density is 3.9 g/cc, which is considerably less than the 5.2-5.5 g/cc characteristic of Mercury, Venus, and the Earth.
This density gives it a mass about 11% of that for Earth. It is most easily observed from Earth when it is at opposition. Even then, it was difficult in the past to observe from Earth because of turbulence in the Martian atmosphere and ours.
Earth based observations concluded that Mars
1-Has a reddish hue over 3/5 of the planet, which we now known to be caused by red dust and rocks on the surface of the planet.
2-Has polar ice caps waxing and waning with the seasons that we now know to be composed both of dry ice (frozen carbon dioxide) and water ice.
3-Has surface markings that some originally thought looked like "canals" from Earth. These are now known to be features like the edges of mountain ranges.
4-Has areas of changing color that were once thought to correspond to vegetation. We now believe these regions of changing color to be due to blowing sand, not vegetation.
5-Has an atmosphere with clouds.
Mars has many interesting geological features on its surface that first became apparent with Mariner9, were subsequently studied by the Viking missions, and many of which now are visible from the Hubble Space Telescope.
Enormous Shield Volcanoes
The volcanoes on Mars are now extinct, but they indicate a preceding period of significant Martian volcanism. Such volcanoes are called shield volcanoes, because they look like shields. The largest volcano on Mars is not one of the three shown previously. It is called Olympus Mons, and is illustrated below.
(Olympus Mons: is 600 km across its base and about 25 km above the surrounding plain). A perspective model is shown below ***
For reference, the largest shield volcano on the Earth is Mauna Loa, which is only about 200 km across its base, and the 25 km height of Olympus Mons is about 2 1/2 times the height of Mount Everest. This raises the interesting issue of why Mars in its past developed a few very large shield volcanoes, while on the Earth the more normal pattern is for volcanoes to develop in strings of smaller volcanoes.the answer is thought to lie in plate tectonics.
Absence of Plate Tectonics
There is no evidence on Mars for large-scale plate tectonics as we find on Earth. This is believed to be responsible for the different character of Martian and Terrestrial volcanoes.
On the Earth, as crustal plates move over subsurface chambers of molten rock the lava tends to come to the surface in a line of places, producing strings of volcanoes (for example, volcanic island chains like the Hawaiian Islands).
On Mars, with no horizontal motion of crustal plates the same point in the crust sits over subsurface chambers of molten rock and a few very large volcanoes are built. Here is a more extensive discussion of volcanism on Mars.
Large Canyon Systems
The Martian surface has some large canyon systems. The largest is Valles Marineris, which extends for about 5000 km, is 500 km wide in the widest portions, and as much as 6km deep. The adjacent image shows a portion of the Valles Marineris.
This enormous system of connecting canyons appears to have been formed mostly by local tectonic activity (local motion of surface) rather than by erosion, though as we will discuss below there is some evidence for fluid erosion in portions of it.
Running Water Erosion
There are channels on Mars as much as 1500 km long and 200 km wide that appear to have been cut by running water.
Under present atmospheric conditions on Mars (low pressure), water cannot exist as a free liquid on the surface (it must be gas or solid). Thus, evidence for water erosion suggests that the Martian atmosphere may have been more dense in the past.
The following two images show portions of the Martian surface where the erosion patterns have regions that are very similar to those found for erosion by surface water on the Earth.
the age of the erosion channels is estimated at about nearly 4 billion years.
The atmosphere of Mars is thin (about 1/200 of the pressure of the Earth's atmosphere), but this atmosphere supports high velocity seasonal winds that are correlated with solar heating of the surface and that produce duststorms that lead to a lot of surface erosion.
Mars has polar caps that wax and wane with the Martian seasons.
These polar caps appear to be partially composed of frozen carbon dioxide ("dry ice") and partially composed of frozen water.
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.
Mars has two tiny satellites which orbit very close to the surface:
||Distance (km) 000 km
1-Why are the northern and southern hemispheres of Mars so different? Why are the northern and southern polar caps different?
2-Is there still active volcanism on Mars?
3-What exactly caused the erosion patterns that look so much like stream beds on Earth?
4-How much subterranean ("sub-martian"?) water is there?
Mars remains at the top of the list of possible life-bearing planets. The Viking probes found little evidence of life on Mars. But they sampled only two isolated locations. Is there life elsewhere or was there life at some time in the past on Mars? The recent meteoric evidence needs to be confirmed. Ultimately, a sample return mission will be necessary.
The future of Mars exploration is more hopeful than for the other planets. NASA's Mars Global Surveyor an orbiter which includes most of the science instruments from the ill-fated Mars Observer is now in orbit around Mars. Mars Pathfinder, which includes a lander and mini-rover landed successfully on Mars on 4 July 1997. Several more robotic missions are planned by NASA and others. But no one seems willing to put any money toward a manned expedition.
(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).
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Last Modified : 5 Sep. 2001
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