Mars is a cold, dry and hostile desert planet. It is the second smallest planet in the solar system (behind Mercury). It is named after the Roman god of war; perhaps appropriate in view of its blood-red hue. The Greeks called it Ares (son of Zeus and Hera), their own god of war.
Mars has the most elliptical orbit of all the planets. At aphelion (furthest point from the sun) it is 249.2 million kilometres away; at perihelion (the closest point) it is 207.3 million kilometres away. Because of this Mars receives 45% more heat from the sun at perihelion than it does at aphelion, which causes a wide range in surface temperatures: from as low as -143°C at the winter pole to as high as 27°C in summer. The average surface temperature is around -60°C.
Its axial tilt fluctuates greatly due mainly to Jupiter’s gravitational pull: from 25° to as much as 60°. But this occurs over a very long climate cycle, tens of thousands of years or longer. These shifts range from cold to relatively warm periods and affect the winter-ice distribution. Its axial tilt is currently 25.19°, similar to that of Earth’s 23.45°.
The planet’s mean distance from the sun is 227.9 million kilometres, with an orbital speed of 24 kilometres a second. The earth’s is average distance is 149.6 million kilometres, with an orbital speed of 29.8 kilometres a second.
In 2003, Mars made its closest approach to Earth in 70 000 years: a distance of 55.76 million kilometres. This will not happen again for another 60 000 years!
A Martian day is only 40 minutes longer than Earth’s. At 24 hours 37 minutes, it is probably the only familiar environmental feature to humans that should at least not disturb sleep rhythms.
It is a very difficult telescopic object to observe in detail. It is only about half the apparent size of the much more distant, but much larger planet Jupiter. Mars is also often too close to the sun to be observed by Earth-based telescopes.
Surface features
Mars is the outermost of the four rocky planets. It has the deepest canyons and highest volcanoes in the solar system. Liquid water once flowed on its surface. Giovanni Schiaparelli (1835-1910) was the director of the Brera Observatory in Malan. In 1877 he studied the planet and drew maps, naming the light and dark areas after historical and mythological figures. He also noted the linear features on the planet’s surface he called “canali” which, in Italian, means “channels”. This was misinterpreted as suggesting canals had been created by intelligent life. Having said that, there is now direct chemical evidence of transient saltwater flowing on the surface (see below).
The surface features of Mars are characterised by cratered uplands, lowland plains, ice caps, giant volcanoes, clouds and planet wide dust storms. Once such volcano, Olympus Mons in the north, is about three times the height of Everest and has ten times the volume of Earth’s largest volcano, Mauna Kea.
Except for the polar regions, the northern and southern hemispheres are quite different. The surface of the planet in the north is generally below the mean topographical level. It has far fewer craters and is characterised by flat, widespread, relatively smooth low-lying sandy plains and towering volcanoes.
The southern hemisphere is a much older landscape. The surface is mostly one to two kilometres above the mean topographical level and heavily cratered. It resembles the lunar highlands and the planet Mercury.
These differences have yet to be explained by planetary geologists.
Just south of the equator is Valles Marineris, a system of valleys of over 4 000 kilometres in length with an average depth of eight kilometres. Olympus Mons is the largest volcano in the solar system. It is about 500 kilometres in diameter and 27 kilometres in height. It formed over thousands, if not millions of years. The lack of plate tectonics allowed the volcanic flows to build up unhindered, releasing the normal pressure of a volcano gradually so it never blew its summit. Other northern monster features include the Tharsis volcanic area, home to Arsia Mons (9km high), Pavonis Mons (7km high), and Ascreus Mons (18km high). The Elysium area, also in the north, is over 2 000 kilometres in diameter with volcanic residents: Elysium Mons (14km high), Hecates Tholus (4.8km high) and Albor Tholus (4.1km high).
Mars has almost the same amount of landmass as Earth: 145 million kms2. The surface of the earth is, of course much greater, about 511 million kms2. But 71% is ocean, leaving Earth with a landmass of 148 million kms2.
Core
Mars is full of red iron oxide. Its low density suggests the iron core may also contain lighter elements such as sulphur in the form of iron sulphides. The thick mantle is composed of solid silicate rock and was once the source of volcanic activity, but which is now dormant. The rocky crust is 80 kilometres thick in the southern hemisphere, and just 35 kilometres thick in the north.
It is thought that Mars once had a molten iron core. When it lost its internal heat the core solidified and shut down the magnetic field. There is still residual magnetism in the rocks in the southern hemisphere. Why there is zero magnetism in the north is again a puzzle still to be explained by planetary geologists. The magnetic field in the south is extremely weak (almost negligible). It measures 1 500 nanotesla. For comparison, Earth’s magnetic field is running at 65 000 nanoteslas. The upshot is that Mars had no defence against the solar winds, which stripped the planet of most of its atmosphere. The planet’s weak gravity (only 0.38 of the earth’s) also played a part. In turn, this may have led to water boiling off into space.
Water
Nothing can live without water. So scientists are trying to find traces of water in the solar system. In 1999, the space probe called Lunar Prospector found possible signs of water on the moon. Scientists also think that there is water on Europa, one of Jupiter’s moons. A mission in 2003 looked for signs of water beneath the surface of Mars.
Scientists using observations from NASA’s Mars Reconnaissance Orbiter, taken in 2011, say they’ve found powerful evidence that briny water routinely flows on the Martian surface. This particular water source is so salty that it would probably be too harsh for life as we know it.
The atmosphere on Mars is cold and thin. This means that any pure water on the surface would freeze or immediately evaporate, depending on the temperature. But if that water is contained in salts, it would be more stable in liquid form. It is thought that the surface of Mars formed from sedimentation at the floor of an ocean. These sediments in the dried-up beds are called lacustrine and may contain salts. Super brine water (with salts of potassium and magnesium, etc) stay liquid at -20°C, even down to -60°C.
There are also millions of tonnes of water ice below the polar caps; although the white caps themselves are frozen CO2.
One theory is that life started on Mars sooner than on Earth as it was further from the sun and conditions were suitable. Mars (like Earth) was once wet and cold. There was a period of liquid water very soon after formation of the planet. But it is thought that there was insufficient temperature for this water to remain as a liquid, so it froze out leaving CO2 as the primary component in the atmosphere.
Atmosphere
Mars has an ultrathin carbon dioxide atmosphere that carries a faint pink hue from iron oxide dust. The air is about 100 times less dense than the atmosphere around Earth. Such as it is, the atmosphere is made up of carbon dioxide (95%), Nitrogen (2.7%) and Argon (1.6%). Atmospheric pressure at the surface is just 0.6% that of the earth. Thin clouds at high altitude comprise frozen carbon dioxide and water ice. At the polar regions, these clouds cause ground frost.
Unlike the earth, the Martian atmosphere is usually free of obscuring clouds. One exception is the cold region surrounding the winter pole that may be covered by water vapour or even carbon dioxide clouds. Another exception occurs during periods of widespread dust storm activity, usually in southern spring and summer. Warm winds blow northwards stirring up clouds of dust to an altitude of as high as a thousand metres, which can leave the surface obscured for weeks on end.
In the equatorial regions near the surface, the atmospheric flow is dominated by the Hadley circulation that transports air from the cold winter hemisphere southwards across the equator. (A rapid transition to northward winds at this altitude occurs as the season changes to southern spring.) Because the equator rotates at a faster rate than other parts of the planet, this leads to a trade wind-like pattern of easterlies in the winter hemisphere and westerly’s in the summer hemisphere. Strong westerlies are also apparent in the region of the polar night while light easterlies are prevalent in the vicinity of the summer pole.
Meteor bombardment
Craters formed during an intense period of bombardment 3.9 billion years ago. This was mainly in the southern hemisphere, which is geologically older than the north. The boundary between the two is an imaginary line tilted by about 30° to the equator. The planet’s major tectonic features are found within the region that extends roughly 30° either side of the equator. This region contains the main volcanic centre, the Tharsis region and the Valles Marineris.
With its thin atmosphere, even then, Mars had no defences. The Herschel Impact Crater is about 300 kilometres across and most of the floor is a thousand metres in-depth, with the highest parts of the rim reaching three thousand metres. Mars also boasts the largest impact crater in the solar system, the Hellas Planitia. It is the most dominant feature in the southern hemisphere with a diameter of a whopping 2 200 kilometres. It is so big it is called a basin, rather than a crater. It was made very early on in the life of Mars, probably four billion years ago. The second largest crater, Isidis Planitia and the third, Argyre Planitia, are also classified as basins because of their sheer size. All three testify to the planet’s early weather and erosion dynamics with features changed by lava flow, wind, water and fresh crater formation.
Geological Terminology
- Chaos – broken or jumbled terrain
- Chasma – deep, elongated, steep-sided depression
- Fossa – long, narrow and shallow depression (trough)
- Grabens – blocks of crust dropped between two faults
- Horsts – crustal blocks that remain in place and are thrown up
- Mare – large, smooth plain of low albedo (low reflectance)
- Mons (also Montes) – mountain
- Patera – irregular crater
- Planitia – low plain
- Rupes – scarp
- Terra – extensive land mass; rougher regions than Planitia (chaos terrain)
- Valles (also Vallis) – valleys, canyons
Moons
Mars has two moons, Phobos and Deimos. They are both captured asteroids. Phobos is only 9 380 kilometres from Mars and there is speculation it may eventually collide with the planet. The smaller moon, Deimos, may well escape its Mars orbit under the pull of external gravitational forces.
They were discovered in 1877 by US astronomer Asaph Hall.
There are almost 60 known meteors from Mars that have landed on Earth. The most interesting is ALH84001. It landed in the Antarctic some 13 000 years ago. The find was misclassified for six to eight years. But finally, after a two-year study, green coloured mass found in the materials was confirmed as having come from Mars; and, further that it contained proof of possible early biology. It has been dated at about 400 million years old. Other known Mars meteors are Shergotty, which was found in India in 1865; and Nakhla, which was found in Egypt in 1911. Others have been found in the US, Brazil, Nigeria, Antarctica, Morocco and Algeria. A Martian meteorite retrieved in northwest Africa (NWA 7034) was found to have ten times the water content of other previously found specimens.
| Moons of Mars | Diameter | Surface temperature | Orbital Period |
|---|---|---|---|
| Deimos | 6 kilometres | -40°C | Once a Mars day |
| Phobos | 22 kilometres | -40°C | Twice a Mars day |
Exploration
There have been over fifty attempts to send spacecraft on missions to Mars since the 1960s, with about half meeting with success: firstly as fly-bys, then orbiters and finally landers. The first successful flyby was by Nasa’s 1964 Mariner 4. It followed this up with Mariner 6 and 7 in 1969. Nasa’s Mariner 9 was the first to orbit the planet in 1971 to begin mapping the surface. Russia’s Mars 2 and Mars 3 also achieved orbit a few weeks later.
The first landers came in 1976 with Nasa’s Viking 1 and 2 missions. While the orbiters sent back images the landers descended to two different sites and sent back analyses of soil and atmosphere as well as images. They confirmed Mars as a cold, dry, dead place. Nasa’s Mars Global Surveyor arrived in 1997 to map the entire planet at high resolution. Meanwhile, Nasa’s Pathfinder also deployed a stationary lander, with a free-ranging robot called Sojourner (the first rover to visit Mars), which sent back images and analyses of soil samples.
ESA (the European Space Agency) successfully sent its Mars Express to orbit the planet (although its Beagle lander failed to make it). Since 2003 Mars Express has been imaging the entire surface of Mars as well as mapping its mineral composition and studying its atmosphere. The agency’s Rosetta later conducted a successful flyby in 2007.
Nasa’s Spirit and Opportunity rovers both landed successfully in 2004, followed by the Mars Reconnaissance Orbiter in 2006. Nasa’s Phoenix landed in 2008 at the northern pole. It ceased operations in 2010 following ice damage to its solar panels. Nasa’s Dawn made a successful flyby in 2009. Curiosity landed in 2012 and is still operational, as is Nasa’s Insight which landed in 2018. It sent back the first image from Elysium Planitia.
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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar
Last updated: Thursday, 6th February 2020