Using academic literature and appropriate web sites, account for the geomorphological processes operating (and suspected to be operating) on the surface of Mars and compare their effectiveness comparative to earth.’

20/02/2002 James Alexander

‘Despite the first significant imaging of the Martian surface occurring in the 1960’s, planetary scientists from many disciplines have been able to speculate about the processes operating on the red planet by applying the knowledge of known terrestrial science. Using academic literature and appropriate web sites, account for the geomorphological processes operating (and suspected to be operating) on the surface of Mars and compare their effectiveness comparative to earth.’

The surface of Mars has been examined for centuries using telescopes from earth, but recent satellite missions (Mariner, 1964 and Viking 1976- 80) have revealed more about the red planet than could have ever been imagined. Mars, unlike other planets in the solar system has geomorphological processes very similar to that of planet earth’s. Aeolian, volcanic, fluvial, periglacial and mass wasting processes occur or have occurred on the planet’s surface, although their effectiveness comparative to earth differ significantly. The processes can be put into two categories, processes that have occurred in the past, such as fluvial and volcanic, and processes that are occurring now, namely mass wasting and Aeolian processes. Due to the differing geology, gravity and atmospheric pressure of Mars compared to Earth the effectiveness of the processes in the altering of the landscape is bound to be different.

It is now almost universally accepted that liquid water once flowed on Mars, supported by the discovery by the Mariner 9 mission. The Viking missions also identified this in more detail, with images of outflow channels as much as 100km wide and 2000km in length. It is the formation of these outflow channels that have been open to controversy, suggestions include erosive agents such as water, ice, wind, debris flows and lava are the cause. The most convincing argument is that of Baker, who believes that they were formed by cataclysmic flooding. When comparing the surface of Mars to the morphology of the Channelled Scabland of eastern Washington and Oregon USA (areas of cataclysmic flooding) the Martian channels have a striking resemblance. These outflow channels on Mars are much larger tan the ones in Oregon which can be explained by the lower surface gravity and atmospheric pressure observed there. Atmospheric pressure is 6 millibars compared to that of 1013 millibars on earth and gravity is 37% less than terrestrial gravity. This lower surface gravity would allow larger material travel as suspension, rather than bed load as it would be on earth, allowing sediment concentration of up to 60- 70 per cent by volume of channel. Erosion of the riverbed in times of catastrophic flood would be rapid compared to that on earth, where this degree of erosion would take years rather than days and weeks on Mars. Although the theory of cataclysmic flooding is generally thought of as the reason behind the formation of outflows, it is still open to question. This is because a source of such amounts of water cannot be found. Ideas such as the melting of permafrost by volcanic activity have been suggested but this is thought to be an implausible way of producing large amounts of water spontaneously enough. Cataclysmic flooding is not the only important process that took place on Mars in its past, volcanic action was just as significant in its geomorphologic history.

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The main difference in volcano complexes on Mars compared to the ones on earth is the sheer size of them. For example Olympus Mons is 27km in height and 600km across, with Alba Patera in the same complex being a few kilometres high but 1700km in diameter. Compare this to similar shield complexes on earth such as the Hawaiian volcanoes, which are generally less than 120km in diameter and 9km high. This great difference in diameter and height is due to the lower surface gravity making the volcanic material weigh less, as well as the lower atmospheric pressure giving the lava less viscosity, meaning it travels further. On earth volcanic activity can be divided into two basic types, eruptions that occur repeatedly from the same conduit and slowly build roughly circular mountains, and eruptions from any widely spaced vents, usually fissures, that create extensive lava plains. Both types are found on Mars. On Mars there have also been explosive eruptions in the past. This is not seen on earth, and may occur on Mars because magma penetrating frozen, water saturated regolith exploding. Both fluvial and volcanic activity occurred in the planet’s history, but aeolian action is the most important factor in shaping the landscape of Mars today.

Due to the absence of vegetation to hold the planet surface in place, combined with fast wind speeds, aeolian action is very effective. On earth the maximum particle speed is 30- 40 m sֿ¹ compared to 140 m sֿ¹ on Mars, although average particle speeds on Mars rarely go in excess of 75 m sֿ¹ . The atmosphere of Mars is composed predominantly by carbon dioxide, meaning it is of very low density compared to earth, this makes threshold drag velocities about 16 times higher than on earth. High threshold drag velocities mean larger particles can be picked up and therefore there are extremely high rates of eolian erosion on Mars compared to earth. Due to the lack of granite on the Martian surface there may be little quartz sand present and it is thought that the sand dunes present on Mars are made up of basalt from volcanic plains as they are very dark. Dunes on Mars are largely formed from sand sized aggregates formed from the electrostatic bonding of this finer material. However, unlike on earth, these sand like aggregates have a very short lifespan because of the kamikaze effect, whereby aggregates smash into rocks at high speed, breaking into smaller particles. Compared to earth then dunes are composed of basalt, are much larger and have a shorter lifespan (mainly because of the lack of vegetation to hold sand in place).

Although there is no evidence to suggest glaciers existed on Mars, periglacial processes are thought to be taking place. The term periglacial refers to environments that have low temperatures above and below freezing. This would mean at some point in the history of Mars it is warmer than it is today, as surface temperatures average -60ºC. Freeze thaw and frost creep may have occurred when water was present on the surface, but presently water is only present in the regolith of equatorial regions. Periglacial processes have thought to have occurred on Mars because of the evidence of patterned ground and thermokarst landforms. The patterns of permafrost are similar to the ones seen on earth but are much larger in size. On earth the ice- wedge polygons making up the permafrost are 1- 100metres across while Martian polygons are 5- 10km in diameter. This would suggest that they have been formed by temperature changes that have happened over a longer duration than on earth, extending deeper within the permafrost. Although this patterned ground is believed to have been caused by this, other more convincing theories suggest that it was formed from the contraction and extension of cooling lava. It is these tectonic processes that are thought to be responsible for thermokarst landforms of chaotic terrain. Chaotic terrain is thought to be formed by the melting of once heavily saturated permafrost causing large- scale ground collapse, on a scale never seen on earth. Thermokarst landforms may also be because of bolide impacts, a process that rarely occurs on earth, but is frequent on Mars.

Bolide impacts may result in the shattering of bedrock, explaining the thick, non-cohesive regolith, which features in Martian landslides. The mass movement of this unconsolidated material may be caused not only by bolide impacts, but the process of sapping, whereby the undermining of free faces occurs by the evaporation of ground water or ice within he permafrost. This process is thought to occur on the many high escarpments of Mars, causing Mass Movement on a larger scale than seen on earth.

Aeolian, volcanic, periglacial, fluvial and mass movement are geomorphic processes that presently occur, or have occurred in the history of Mars. Due to the lower surface gravity and lower air pressure, liquids such as lava have different properties than on earth and rocks have different weights, meaning geomorphic processes associated with these properties are different to that on earth. Wind speed, periglacial processes and vegetation growth are also affected directly or indirectly by these factors. In the main processes on Mars are similar to what happen on earth but are exaggerated by the different conditions and physical laws present there. In conclusion geomorphic processes on Mars are generally thought to be or have been in the past, more effective than similar processes that occur on earth.

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Baker, V.R (1985) Models of fluvial activity on Mars in Woldenberg, M.J (1985) ‘Models in Geomorphology’ Chap. 13 London: Allen & Unwin

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NASA (accessed 19/02/2002) Mars climate FAQ: liquid water? ‘’