Table 7.2Significant space probes to Mars since 1996
Probe
Country of origin
Date launched
Notes
Mars Globe Surveyor
USA
1996
Mapped surface
Mars Pathfinder
USA
1996
Lander on surface
Mars Odyssey
USA
2001
Orbiter 3 years
Mars Explore Rovers
USA
2003
Two landers
Mars Express
USA
2003
Orbiter
Mars Recon. Orbiter
USA
2005
Orbiter
Phoenix
USA
2007
Lander
Mars Orbiter
India
2013
General stud
MAVEN
USA
2013
Study atmosphere
In November 2013, MAVEN (Mars Atmosphere and Volatile EvolutioN) was successfully launched, the first space probe devoted to understanding Mar’s upper atmosphere. It entered orbit around Mars on September 22, 2014. The probe has already found that solar particles bury more deeply into the atmosphere than previously thought.
In 2016 NASA’s Insight mission will place a lander on the planet. Insight will measure the seismic activity and internal heat flows on Mars. ESA and Russia are working on a two-pronged mission in 2016 and 2018, both looking for evidence of life past or present.
Position and Orbit
Mars orbits the Sun in an elliptical orbit that has the third highest eccentricity of all the planets’ orbits. Its mean distance from the Sun is just over 228 million km, placing it about one and a half times further from the Sun than Earth. At perihelion, Mars is 208 million km from the Sun, while at aphelion it is 249 million km. The difference between the two is about 41 million km, whereas on Earth the difference between perihelion and aphelion is only 5 million km. This has a major influence on Mar’s climate and results in a wide range of seasonal temperatures.
Mars orbits the Sun with a velocity of about 86,868 km/h and takes 687 Earth days to complete the trip. The planet takes 24.6 h (1.029 Earth days) to rotate once on its axis, which is tilted at an angle of 25.2° to the vertical.
About every 780 days Mars passes through a point in its orbit where it appears opposite the Sun in the sky (opposition). Because of its eccentric orbit, Mars distance at opposition varies, so its apparent size and brightness also change. The most favorable opposition is when Mars is closest to both the Earth and the Sun (this occurs about once every 17 years). Oppositions occurred on 8 April 2014 and 22 May 2016. Mars can be studied easily from Earth using a telescope of moderate power. Dark markings and the white polar ice caps may be seen on the surface, depending on the distance Mars is from Earth.
Density and Composition
Even though Mars is more than half the diameter of Earth, it has only about one tenth of its mass. Details about the interior of Mars are limited because of the lack of seismic data.
The average density of Mars is the lowest of the terrestrial planets (3.95 g/cm3 compared to Earth 5.52 g/cm3). This suggests the iron-bearing core of Mars is smaller than Earth’s core.
In fact the core is thought to have a radius of only 1100 km (but some estimates have it as high as 2000 km). The core makes up only about 6 % of the planet’s mass, compared to Earth’s core, which makes up about 32 % of its mass. A weak magnetic field suggests the core is no longer liquid or that currents within it are slow. Surrounding the core is a molten rocky mantle about 2200 km thick that is less dense than the core. The outer crust of the planet varies in thickness from about 20 km to 150 km (Fig. 7.5).
Fig. 7.5The interior structure of Mars.
The strength of gravity on Mars is about a third less of Earth’s gravity. A 75 kg person on Earth would weigh 735 N, but on Mars they would only weigh 270 N.
The Surface
Although Mars is much smaller than Earth, its surface area is about the same as the land surface area of Earth. Our first view of the surface of Mars was obtained from the Viking 1 lander in July 1976. Pictures revealed a rocky, desert-like terrain. Two weeks later Viking 2 set down on the opposite side of Mars, and images returned to Earth showed the same rocky surface as revealed by Viking 1. Both landing sites contained rocks ranging from pebbles to boulders in an orange-red, fine-grained soil.
Much of the Martian surface is very old and cratered, but there are also much younger rift valleys, ridges, hills and plains. There is a highly varied terrain on Mars with highlands in the southern hemisphere and lowlands in the northern hemisphere. The highlands are the oldest terrain (about 3 billion years old), and many parts are heavily cratered. The oldest terrain also contains small channels that may have been carved by flowing water or dry ice. Smooth plains between the cratered areas are volcanic in origin.
The lowlands in the north contain mostly plains with few craters, indicating they probably formed after the period of bombardment by meteorites. The region between the highlands and lowlands is marked by an escarpment or long cliff. The reason for this abrupt elevation change could be due to a very large impact during Mar’s past. A three-dimensional map of Mars that clearly shows these features was produced by the Mars Global Surveyor space probe.
The planet’s western hemisphere contains a distinct bulge about 10 km high and 8000 km long, called the Tharsis Rise. This region contains the greatest concentration of volcanic and tectonic activity on Mars. Many volcanoes, fractures and ridges, and the enormous Valles Marineris canyon system, are linked to this rise. Valles Marineris was named after the Mariner 9 probe that discovered it. The canyon is about 8 km deep and 4500 km long. Smaller tributary canyons are as large as the Grand Canyon on Earth. Valles Marineris probably formed from rifting, or the pulling apart of the Martian crust, at the same time as the Tharsis Rise formed (see Figs. 7.1 and 7.6).
Fig. 7.6A canyon in Valles Marineris on Mars filled with dense ground fog photographed from orbit by Mars Express (Credit: ESA).
Mars also contains the largest volcanic mountain in the solar system, Olympus Mons. This mountain rises to a height of 24 km above the surrounding plains, is more than 500 km wide and is rimmed by a cliff 6 km high. The volcano’s summit has collapsed to form a volcanic crater or caldera about 90 km across (see Fig. 7.7).
Fig. 7.7Olympus Mons is the largest volcanic mountain in the solar system (Credit: NASA).
There are three other prominent volcanoes on Mars: Arsia Mons, Pavonis Mons and Ascraeus Mons (see Fig. 7.1 left limb). Each is over 20 km high and forms part of a volcanic chain near the centre of Tharsis Rise. Alba Patera is a low-relief volcano about 2 km high and 700 km across situated near the rise’s northern edge. Most of the volcanoes on Mars are in the northern hemisphere, while most of the impact craters are in the southern hemisphere.
The Martian surface contains many large basins formed when large meteors or asteroids have hit the surface. The largest basins formed by impacts are Hellas (with a diameter of about 2000 km), Isidis (1900 km) and Argyre (1200 km) (see Fig. 7.8).
Fig. 7.8Simplified map of geological features on Mars. The shaded area is the northern plain. The lower area is the southern cratered terrain.
The soil on Mars was analysed by the Viking landers and found to be slightly magnetic, indicating it contains iron. Further analysis showed the rocks at both landing sites were rich in iron, silicon and sulfur. As a result the Martian soil is described as an iron-rich clay. The soil is also rich in chemicals that effervesce (fizz) when moistened. Unstable chemicals called peroxides exist in the soil and these break down in the presence of water to release oxygen gas. Mars Pathfinder was able to identify the presence of conglomerates like those that are formed by running water on Earth. This evidence suggested Mars might have had a warmer past in
which liquid water was stable.
Like Mercury and the Moon, Mars appears to lack active plate tectonics at present since there is no evidence of recent horizontal motion of the surface. Although there is no current volcanic activity, the Global Surveyor space probe showed that Mars might have had tectonic activity in its early history. Mars does not appear to have a crust made up of several large plates, like Earth has—it may be a single-plate planet.
Mars Global Surveyor found many landforms on Mars seem to have been formed or altered by running water. The channels that form valleys in the cratered highlands are similar to those formed by water on Earth. Some eroded valleys are huge, for example the Kasei Valles cuts over a kilometre deep into the volcanic plains of the Tharsis Rise, and is over 2000 km long. It is thought that some of the water flows in the past were huge in volume and occurred when internal heat or meteorite impacts released groundwater in sudden floods. Such flows were brief since Mars does not have enough water to sustain continuous flow. Some scientists believe that many of the geological features that appear to have been caused by liquid water, may have instead been formed by pyroclastic flows similar to those that occur during a volcanic eruption or by dry ice. In July 2014, NASA scientists using MRO data reported that gullies on Mars were likely to be formed by seasonal freezing of dry ice rather than liquid water.
If there is evidence of water in Mar’s past, where is it today? Most of the water is believed to exist as permafrost in the northern lowlands and maybe underground in the heavily fractured and cratered highlands, and as ice at the poles. In December 2006, scientists comparing photographs taken in 1999 and 2005 from the Mars Global Surveyor’s orbiting camera, discovered that water had flowed down the walls of a crater during this period.
Mars has two prominent polar ice caps, which can be seen through telescopes from Earth. The two polar regions of Mars are mostly covered with layered deposits of solid carbon dioxide (dry ice), with some dust and water ice. The mechanism responsible for layering is thought to be due to climatic changes. The Viking landers found that seasonal changes in the extent of the polar ice caps changes the atmospheric pressure by about 25 %.
During summer in the northern polar region, carbon dioxide returns to the atmosphere, leaving a cap of water ice. The southern polar region reduces its size during summer but stays as frozen carbon dioxide.
Fig. 7.9A lake of water ice 200 m deep was discovered in an impact crater on Mars in 2005 by ESA’s Mars Express. The crater is 35 km wide and has a maximum depth of 2 km (Credit: ESA).
Fig. 7.10The north polar cap of Mars has spiral shaped structures and a large canyon (Chasm Borale) that MRO radar images have shown to be caused by strong winds which blow from the top of the ice cap (Credit: NASA/MRO).
Life may exist in the permafrost or under the polar ice caps of Mars. On Earth, algae, bacteria, and fungi have been found living in ice-covered lakes in Antarctica, and so future explorations of Mars’s ice may reveal life forms.
On 4 August 2011, NASA announced that MRO had found evidence of flowing salty water on the surface or subsurface of Mars.
In December 2013, scientists using Mars Express images, completed a topographical map of the Martian surface.
The Curiosity probe that landed in Gale crater found that rivers and streams once flowed over the crater floor, it also found evidence that a lake once existed in the region. A detector on Curiosity has made the first measurements ever of radiation on the surface of Mars. The detector found that Galactic cosmic rays and solar eruptions bombarded Mars, and their high-energy particles break the bonds that allow organisms to survive. The radiation would almost certainly be damaging to any microbial life on the surface and just below it. Many scientists on the Curiosity team believe that such radiation would damage the carbon compounds on Mars, and that this is a major reason why it has been so difficult to identify organics on the surface. In December 2014, Curiosity’s instruments detected methane, the simplest organic compound, in both the atmosphere and surface of Mars. This indicates microbial life could live beneath the planet’s surface.
The Martian Atmosphere
The Martian atmosphere is very different from Earth’s, but there are some similarities. Mars has a very thin atmosphere composed of 95.3 % carbon dioxide, 2.7 % nitrogen, 1.6 % argon, and less than 0.2 % oxygen. The atmosphere is near its saturation point with water vapour (0.03 %). In the past the Martian atmosphere may have been denser but it is now one-hundredth the density of Earth’s atmosphere. This low value is partly due to the low gravitation field. Recent, isotopic studies of Mar’s atmosphere for several elements, including hydrogen, argon, and carbon, suggest that the planet has lost between 25 % and 90 % of its original atmosphere. Data returned by the Phobos space probe suggests that the solar wind is carrying away the weakly held atmosphere at a rate of 45,000 tonnes per year.
Losing so much atmosphere would have left Mars much colder and drier, and liquid water would not have lasted on the surface. In 2014, the Maven space probe began to investigate the Martian atmosphere with a view to providing answers as to how the planet has been losing its atmosphere over time. Instruments on Maven detected comet dust in Mars’ atmosphere and UV auroral glows (caused by particles from the solar wind). Maven also generated a map of a layer of ozone in the lower atmosphere of Mars and detected an ionosphere between 120 km and 480 km altitude.
The average air pressure on Mars is about seven-thousandths of Earth’s, but it varies with altitude from almost nine-thousandths in the deepest basins to about one-thousandths at the top of Olympus Mons.
The minute traces of water vapour can at times form clouds, particularly in equatorial regions around midday. Early morning fogs also appear in canyons and basins. Temperatures around the poles are often low enough for carbon dioxide to form a thin layer of cloud.
The atmosphere is thick enough to support strong winds and dust storms that sometimes cover large areas of the planet. At times, the dust storms can hide surface features from Earth view. Such storms occur most often during the southern hemisphere’s spring and summer. In 1971 Mariner 9’s view of Mars was obscured by a dust storm that lasted for 2 weeks. In 1977, 35 dust storms were observed, and two of these developed into global storms. Global storms spread rapidly, eventually enshrouding the whole planet in a haze that can last a few months.
Mar’s thin atmosphere produces a greenhouse effect but it is only enough to raise the surface temperature by 5°, which is much less than increases on Earth and Venus.
In December 2014, the Curiosity rover inside Mars’ Gale crater, measured a ten-fold spike in methane in the atmosphere around it, and detected other organic molecules in a rock-powder sample collected by the rover’s drill. This temporary increase is believed to be from some localized source.
Temperature and Seasons
The two Viking landers functioned as weather stations for two full Martian years. Their data, together with information from the orbiters, has given us a good picture of the weather on Mars.
Mars has a greater average distance from Earth and because of this it has a lower average surface temperature (−60 °C). At perihelion, Mars receives about 45 % more solar radiation than at aphelion. As a result there is a large variation in surface temperatures during the Martian year. The coldest temperatures of −125 °C occur in winter at the south pole; this temperature is the freezing point of carbon dioxide. The warmest temperatures, of around 22 °C, occur during summer in southern mid-latitudes. The large difference between equatorial temperatures and polar temperatures produces a brisk westerly winds and low-pressure systems, similar to cyclonic systems on Earth.
Mars is tilted on its axis at 25.2°, which is similar to Earth’s tilt of 23.5°, and so it experiences four seasons. Each season lasts about twice as long as Earth’s because Mar’s orbit is much larger and more elongated.
On Mars the Sun appears about half the size as it does on Earth.
In Mar’s northern hemisphere, spring and summer are characte
rised by a clear atmosphere with little dust. White clouds may be seen at sunrise near the horizon and at higher elevations. During winter, falling temperatures around the northern polar ice cap cause carbon dioxide from the atmosphere to condense to renew the ice cap. The carbon dioxide ice comes and goes with the seasons, but a permanent ice cap of water ice remains.
The southern hemisphere summer occurs when Mar’s is closest to the Sun and so southern summers are hotter than northern summers and winters are colder. During summer the southern polar ice-cap shrinks but a core of water ice remains.
Magnetic Field
Data from space probes indicates that Mars does not have an internal dynamo capable of generating a large global magnetic field. However, Mars may once have had such a dynamo. This is mainly supported by observations from the Mars Global Surveyor probe, which from 1997 to 2006 measured the magnetic field of Mars using a small magnetometer from an altitude of 100–400 km above the planet’s surface. These measurements showed the existence of magnetic crustal fields on the planet’s surface. However, the strength of the crustal magnetic field varies from place to place on the planet.
The weak magnetic field overall suggests that the core of Mars is no longer liquid or that currents in the core are slow. A computer model produced by scientists in 2009 suggests Mars’s magnetic field may have been slowly weakened or knocked out by several large meteoroid impacts.
The Solar System in Close-Up Page 13