General Remarks

Diameter6,792 km (half of Earth's)
Mass11% of Earth's
Density3.94 g per cc (4/5 that of Earth's)
Surface gravity38% of Earth's
Escape speed5.0 km/s
Reflectivity15 - 30% at surface
Rotation period24 hrs 33 min
Sidereal period1.8809 years
Avg distance from the Sun1.523 AU

Mars is considered to be the planet most like Earth, and therefore is the highest priority target in the solar system for searching for extraterrestrial life. Its thin atmosphere makes it ideal for exploration by both orbiters and landers.

Mars has a rich history of activity in evidence on its surface: the largest volcanoes, impact basins, and canyonlands in the solar system exist on Mars.

Mars's early history is thought to be similar to that of the Earth's Moon. The planet accreted from rocky material through collisions and gravity. There followed an era of intense impacts, then radioactive decay of elements interior to Mars heated it up to the point where it become molten. Heavy elements then sunk to Mars's interior and light elements rose to the surface, a process called "differentiation".

Outgassing produced a substantial atmosphere that allowed for liquid water to exist on the surface. Mars's size allowed it to remain volcanically active for 1-2 billion years. Then an unknown process led to there being terrain at very different altitudes: the northern hemisphere is largely younger, lowland regions that in some ways resemble an ocean bed. The southern hemisphere is largely highlands, heavily cratered and therefore older. A large upwelling from Mars's center raised a large, equatorial upland called the Tharsis plateau.

Mars is smaller than the Earth and so cooled more rapidly. Volcanic activity eventually ceased and much of Mars's atmosphere either escaped or fell frozen to the surface. Regolith from meteorite impacts is an excellent place to store water, so there may be an extensive layer of subsurface permafrost. Large flood basins appear to have been caused by the catastrophic release of subsurface water.

Today, wind (erosion and deposition) and impact cratering, are the two processes shaping the surface of Mars.

Exploration of Mars

Mars has been host to a bevy of missions, the surprising result being that the surface of Mars is the best-mapped surface in the solar system, better even than that of the Earth.

The Mariner fly-by missions and orbiters returned early images of the surface of Mars in the late 1960s and early 70s. The Mars missions of orbiters and landers were flown in the mid-1970s and collected information on the Martian atmosphere and surface.

The highly-successful Viking 1 and 2 missions from 1976 each landed a spacecraft on the surface that were able to collect soil samples and analyze them for atomic content, and test for organic molecules, and subject the soil to a number of tests designed to indicate life-like activities, such as respiration and photosynthesis.

The Mars rovers program has been highly successful, with each mission lasting well beyond its design limitations. The Sojourner (1996), Opportunity (2003), and Spirit (2003) rovers had the ability to travel from the landing site. The Sojourner rover (part of the Pathfinder mission) was largely an engineering mission designed to test the capabilities of a light-weight rover. The following missions were scientific missions, and included an array of imaging, atmospheric, and geologic probes.

Orbiters include the Mars Global Surveyor (1996), Mars Odyssey (2001), and Mars Express (2003). These spacecraft are designed to map the surface in exquisite detail and at a variety of wavelengths, to learn both about surface structure and composition.

The history of Mars missions is a peculiar one: one-third of all Mars missions have failed. Some failures were the result of human error, e.g., the Mars Climate Orbiter (1998) was lost due to a conversion error between imperial and metric units. Other failures are completely without explanation, characterized by a loss of contact from the spacecraft.

Overall Features

Some features on Mars are viewable in small telescopes, particularly light-reddish-colored areas indicative of surface dust, and darker areas indicative of underlying rock. Large dust storms move the dust around and consequently the shapes of the light and dark colored areas change.

Dust storms on Mars are seasonal. The fanciful interpretation of these changing features from the early 20th century was that the dark areas were vegetation, and we were seeing their proliferation during Martian spring times. Mars, and its imagined inhabitants, inspired a lot of speculation in the genres of fiction and non-fiction alike. The most famous of these episodes is H.G. Wells' "War of the Worlds" (1898) and its realization in the dramatic radio interpretation by Orson Welles (1938). I have heard a recording of this broadcast and recommend it to anyone with an interest in radio plays and who wants to savor the delight of the pre-television world.

Wells' book was inspired by, among other things, the close approach of Earth to Mars in 1894. Earth and Mars experience a close approach about every 780 days. The position of Mars at closest approach is called "opposition", when Mars is located exactly 180° from the Sun on the celestial sphere. Because of Mars's elliptical-shaped orbit, some oppositions are closer than others. Oppositions that occur in August are the closet of all. During these times, Mars's features are best viewable from the Earth.

The Italian word "canale" means "channel". It was introduced by the Italian astronomer Secchi in the 1860s to describe some features he saw on Mars. Another Italian, Schiaparelli, claimed to have seen "channels" on Mars in 1877 and he published a map of them in 1878. The "canale" was mis-translated as "canal". There suddenly followed stories of the poor residents of Mars building canals to bring water from the north and south poles to irrigate their drought-ridden equatorial regions.

Surface Elevations

Mars's low gravity allows for a larger range of elevations than on Earth. When rocky structures become very large, they will deform under their own weight. The maximum height of rocky structures on Earth is about 10-15 km, compared with 25 on Mars.

Mars's northern hemisphere is mostly lowland, has fewer craters and is therefore younger, and in some ways resembles an ocean floor. The southern hemisphere is mostly highland, is more heavily cratered and therefore older. The youngest surface of Mars occurs on the Tharsis plateau, a highland region about the size of North America. The largest mountain in the solar system, Mount Olympus, sits on the Tharsis plateau.

Surface features

Mars's surface has been shaped by volcanic, water, wind, and impact processes.

Surface features: cratering

Cratering on Mars is heavier than on Earth, due to Mars's thinner atmosphere; there is only 0.01 atm of pressure in the lowest areas. The craters themselves resemble lunar craters, with a central peak, flat crater floor, high surrounding wall and ejecta blanket. However, martian ejecta blankets are more extensive and smoother than those on the Moon. Smooth ejecta blankets are called "pedestals". They indicate that the ejecta blanket was laid down by the flow of ejecta along the surface, and not the airborne deposition of material like on lunar craters. This flow-like structure is most likely due to the presence of water (solid or liquid) in the soil at the time of impact.

The largest impact basin is called Hellas, and is located in the southern hemisphere. It is 1800 km in diameter and 6 km deep. In fact, the floor of this basin is one of the lowest altitude places on Mars. Another large basin is Argyre, also located in the southern hemisphere.

The density of cratering across Mars's surface indicates a range of surface ages, from 4 to 0.2 billion years. The lack of small craters in the oldest regions of Mars indicates that the protective atmosphere was once much more substantial than it is today.

Today, craters are worn down by wind. Wind can erode crater walls and deposit dust in crater floors, very slowly erasing them from the surface of Mars.

The images from the Mars landers show us boulder-strewn landscapes. Most of Mars's surface is formed of this loose rock, which has been laid down by a long history of impacts. This loose, shattered rocky layer is called "regolith".

Surface features: volcanic action

The four largest volcanoes on Mars sit on the Tharsis plateau. They are called Mount Olympus, Ascraeus, Pavonis, and Arsia; there are hundreds more. Olympus is the largest, rising to 25 km above the plateau, and ringed by a gently sloping shield of diameter 700 km. In comparison, the largest shield volcano on Earth is Mauna Loa, with a diameter of less than 10 km.

Shield volcanoes are characterized by a single summit. They are formed when material flows outward from their centers, and the location of the central flow does not change. Were Mars to have had moving continental plates like on Earth, the location of the active vent of the volcano would have changed over time. In this way we know that Mars did not experience a period of tectonic plates.

The Tharsis plateau is a large uplift region that formed from 3-2 billion years ago, probably as a result of liquid rock rising from the interior. The volcanoes formed afterward. The region surrounding Tharsis was stretched and fractured as a result of the uplift, forming large canyonlands. The canyons were further shaped and widened by wind and water action. These canyons all point outward from Tharsis.

The largest canyon is the Mariner Valley. It is 4,000 km across, 500 km wide, and 3-6 km deep. Its length would span the distance from Seattle to NYC. Its north-western section is a series of small intersecting canyons called the Noctis Labyrinth ("Labyrinth of the Night").

Water features

There are four pieces of evidence for liquid water having existed on Mars's surface in the distant past:

  1. pedestal craters,
  2. dry river beds,
  3. presence of frozen water at the north and south poles,
  4. presence of minerals that can form in the presence of liquid water.

Pedestal craters were already discussed above.

Water features: dry river beds

Dry river beds on Mars are identified by their shape, and there are three types: runoff channels, outflow channels, and narrow gullies.

Runoff channels are characterized by smaller, tributary channels coming together to form fewer, larger channels. These channels were formed by long-term water runoff from the surface or from springs. The channels all drain into lower lying areas, and are all located in the southern highland regions. The lack of channels on the Tharsis plateau indicates that there channels are about 4 billion years old.

Outflow channels, by contrast, were formed during single catastrophic flood events, like water breaking out from behind a dam. They are characterized by a single, long and wide channel, within which we see several smaller parallel channels, streamlined islands, and sandbars. These outflow channels can be 100s of km in length, and a few 10s of km wide. They tend to run from south to north, to drain into the northern lowlands. For example, the Ares Valley emptied onto the Chryse Plain on the border of Tharsis. The closest terrestrial analog is the Columbia Plateau in the states of Washington and Montana.

These outflow channels appear to have been formed by the sudden release of underground water. The headlands of the channels feature rounded hills where soil would have been removed from underneath the surface by tremendous volumes of flowing water. These channels date back about 3.5 billion years.

The cause of the sudden outflows is unknown. Among the theories are (i) the sudden melting of subsurface ice by a plume of rising heat in Mars's interior, (ii) a chemical release-process Martian clay, (iii) large-scale flow of underground water during the uplift of Tharsis.

Gullies are small water channels often appearing in cliff sides. These features show no evidence of wind erosion and therefore must be recently formed. Their presence raises the possibility that there might be a lot of subsurface ice on Mars today, which occasionally flows as a liquid due to some heating process.

Water features: minerals

A crystal is a substance whose atoms are arranged in a repeating pattern. A mineral is an inorganic crystal with a specific composition. It is characterized by a unique hardness, color, and texture. For example, quartz of a crystal of oxygen and silicon atoms, where there are 2 oxygen atoms for each silicon.

Jarosite, goethite, olivine, hematite, and others have been found on Mars by rovers. These crystals are of particular interest because they contain iron, and their abundance contains information about the history of water on Mars. Jarosite, goethite, and hematite can be formed in water. Olivine is the exception: it dissolves in water. The crystal found so far do not form exclusively in water, and can be formed by volcanic processes. The mineral-based evidence for liquid water existing in Mars's past is not well-founded.

Polar ice caps

White and highly reflective, easily viewable through a small telescope, the regions surrounding Mars's north and south poles are covered in frozen material. Mars gets cold enough, at 150 kelvin (-123° C), freeze both water and carbon dioxide. Carbon dioxide is the major constituent of Mars's atmosphere.

The appearance of the polar caps is seasonal: over the course of a given hemisphere's winter, the polar cap will grow in extent. Due to Mars's elliptical orbit shape, southern winters are longer and colder than northern winters. The southern polar cap can creep up to 55° south latitude, compared with a maximum extent to 65° north latitude for the nothern cap.

In the summers, neither cap completely disappears; there is a permanent component to each cap. The low atmospheric pressure of the atmosphere prevents most liquids from existing on the surface. During the summer, carbon dioxide ice will sublimate directly into the gas phase. The reverse process occurs during the winter.

The seasonal ice cap grows to be about one meter thick at the poles, decreasing to only a few cm at the edge. The permanent ice caps at the northern and southern poles are different from each other in both size and character. The southern permanent cap is smaller, 350 km across, compared with the north at 1,000 km across. The temperature of the southern cap never rises above 150 kelvin, so must be composed primarily of carbon dioxide ice. The northern cap can rise to a temperature of 200 kelvin, at which point most of the carbon dioxide must have evaporated away. The northern polar cap must be composed primarily of water ice. Why this difference in composition?

The reason may lie with Martian dust storms. These storms always occur during southern summer. Some of this dust would be deposited on the northern cap while this cap is forming. The dust will give the ice cap a darker color, thus enable it to absorb sunlight better, thus raising its temperature above the sublimation point of carbon dioxide. Water stays frozen.

Although there is a lot of water at the north pole, most of Mars's water is suspected to exist as subsurface permafrost. Only at the poles can this frozen layer be maintained above the surface.

The permanent polar caps themselves have a peculiar, spiral pattern of etched and darker terrain, apparently the result of sun-facing layers of darker material that heat up and locally vaporize carbon dioxide.


Wind-related processes operate on Mars today. Wind can erode and soften sharp edges, it can cover areas with dust, and it can give rise to interesting phenomena like dust devils.

The atmospheric pressure at Mars's surface is only 1% (0.01 atm) that of the Earth's. Its primary constituents are carbon dioxide (CO2, 95%), nitrogen (N, 2%), argon (Ar, 2%), and trace amounts (much less than 1%) of oxygen (O), carbon monoxide (CO), water (H2O), neon (ne), krypton (Kr), ozone (O3), and xenon (Xe).

The air temperature at the surface, near the equator, during the day approaches but does not exceed 0° C. Most of the planet is cooler than this, dropping to -100° C at night; it is coldest at the poles.

The thin air can blow about quite a bit. Speeds range from a few to about 10 m/s (36 km/h) in normal conditions. In a dust storm, wind speeds can reach 30 m/s (108 km/h).