The planet Mars

The adjacent image shows a Viking 2 image of Mars which is still of great interest to us, not the least reason being that it may once have harbored conditions favorable to the evolution of life.

General features

The animation to the right shows a sequence of Hubble Space Telescope images of the Martian surface. The image at the bottom of the page shows several still views from the same source.

Earth based observations concluded that Mars

• 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.

• 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.

• 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.

• 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.

• Has an atmosphere with clouds.

Three views of Mars. Click on the image for a larger versionand here is a further description of the images (Ref)

Increased understanding of Mars had to await the results of space probes, beginning with Mariner 4, 6, 7, and 9 in the period 1965-1971, and the Viking 1 and 2 probes in 1976.

Mariner 9

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 (the three round features at the left of the adjacent image are volcanic cones).
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).
4. Vast sedimentary deposits in the Polar regions.
5. 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).

Surface features

Enormous Shield Volcanoes

In the previous section we saw an image of Mars with 3 large volcanoes on the left limb of the image. 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 (Ref).

Olympus Mons is 600 km across its base and about 25 km above the surrounding plain. A perspective model is shown below

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, as illustrated in the following animation.

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 (the full system is shown in the preceding section). 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.

Regions with possible evidence for watererosion on the Martian surface

Wind Erosion

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. The following image shows a local dust storm on the Martian surface (lower left).

A local dust storm on the Martian Surface (source)

Here is another example of a Martian dust storm. At times, such local dust storms grow and merge until essentially the entire surface of the planet is covered by a dust storm. These periods are correlated with times of maximum solar heating of the Martian surface.

Polar Caps

Mars has polar caps that wax and wane with the Martian seasons. The following image shows the North Polar Cap.

The Martian polar caps (source)

These polar caps appear to be partially composed of frozen carbon dioxide ("dry ice") and partially composed of frozen water. An MPEG movie (775 kB) of the rotating Mars animation is also available.

Atmosphere and Interior

The Martian Atmosphere

The atmosphere has a pressure at the surface that is only 1/200 that of Earth. The primary component of the atmosphere is carbon dioxide (95%), with the remainder mostly nitrogen. 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.

Variation in temperature at the Viking 1 landing site

The preceding image shows the variation of the surface temperature over a period of 50 Martian days at the Viking 1 landing site (data source). 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, notCelsius).

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.

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.

Viking: Search for Life

Viking lander (Ref) and two views of the Martian surface from Viking 1 (Ref)

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 Experiments

The 4 basic experiments that the Vikings carried out to search for evidence of life were:

1. Gas Metabolism: look for changes in the atmosphere induced by metabolism in the Martian soil.
2. Labeled Release: Look for release of radioactive carbon dioxide by metabolism from organic material labeled by radioactive carbon.
3. Pyrolytic Release: Search for radioactive compounds in soil by heating soil exposed to radioactive carbon dioxide.
4. Mass Spectrometer: Search directly in Martian soil for organic compounds known to be essential to Earth life.

The Results

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 andthis) 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.

The Newest Martian Missions

• Pathfinder which explored the Martian surface last year. The image to the left is from Pathfinder on Mars.

• Mars Global Surveyor: arrived in Martian orbit September 12, 1997. Here is a summary of its trajectory from Earth to Mars.

The moons

Mars has two small moons that are illustrated in the following figures, Phobos and Deimos.

Phobos (Ref)	Deimos (Ref)

These are examples of what are called minor satellites: small chunks of rock in orbit around planets as compared with large satellites like the Earth's Moon. As the adjacent images show, they are very irregular in shape. Phobos is 27 km long in its longest dimension and Deimos is 15 km long in its longest dimension.

Both are cratered and orbit the planet in rather low orbits. Phobos is only about 3000 miles above the Martian surface and orbits in a little over 7 hours (thus it makes more than 3 orbits in a single Martian day). Deimos is a little further out and orbits in about 30 hours.

The figure to the right (Ref) shows a rather spectacular image taken by the Viking 2 orbiter from an altitude of about 8000 miles above the Martian surface. The image looks down on a Martian shield volcano (Ascraeus Mons) which is about 200 miles across and in the center. The object down and to the left of the volcano is Phobos, which is 5000 miles below the orbiter, and 3000 miles above the Martian surface! (The regular horizontal rows of dots seen in the image are an artifact associated with the imaging; they don't correspond to real features.)

These moons of Mars were not formed in the same way as the Earth's Moon. They are probably fragments of larger objects broken apart in a collision. Such moons may be formed from collisions of objects originally in orbit around the planet, or they might also have been captured gravitationally at some point in the past. The adjacent image (Ref) compares the appearance of several minor satellites of the Solar System (Phobos, Deimos, Gaspra, and Ida). We shall encounter many such small moons around the giant planets like Jupiter and Saturn.