Introduction
MARS is the fourth planet of our solar system, orbiting the sun every 687 days. Its highly elliptical orbit is just outside that of the Earth and we overtake our half size cousin every 26 months, coming to within 55 million km from the red planet, at which time it appears as a bright star in the night sky. It was its red colour that arose to its name - from the Roman war god Mars. Since the advent of telescopes mars has been intriguing to humans, leading to imaginative speculations about life elsewhere in the universe. Dark spots and blurry shapes fueled thoughts of seas and canals, and the discovery of ice caps projected images of fragile life, struggling for survival.
These hopes, however, were soon dashed by the advancement of science, Earth based spectra showed Martian atmosphere to be icy and chiefly carbon dioxide with very low air pressures. Space probes in the late 60's sent pictures of a desolate planet, covered by impact craters. Mariner 9 was the first probe to achieve an orbit but initial photographs showed the planet covered in a cloud of dust caused by seasonal storms with very high winds. As the dust began to settle, the first things seen were four dark spots as they began to emerge from above the cloud. These turned out to be the huge craters, 40 - 80km in diameter, of four giant shield volcanoes. Arsia, Pavonis, and Ascraeus Montes, as well as Olympus Mons - a feature commonly seen from Earth.
Surface of the Planet
The Viking probes were the first to land on the Martian surface, sending back photos of a barren, rocky landscape devoid of life. These probes conducted mass spectrometry on the Martian soil sending back information on the chemical makeup of the planets surface. It showed that it was 44% Silicate, 17.5% Iron Oxides, giving the planet its red colouration, 7% Aluminium, 6% Magnesium, 6% Calcium, there were particularly high proportions of Sulphur Oxide with 7%. These point to the fact that the surface is made up chiefly of basalts and other mafic rocks which would have erupted as low viscosity lava flows and hence lead to the low gradients on the surface of the planet. The other interesting thing is that there was no evidence of Potassium pointing to the fact that these magmas were mantle melts that had undergone very little chemical evolution.
The surface of the planet itself is chiefly made up of lava plains, which cover about 60% of the planet's surface (80% in the northern hemisphere). These represent huge outflows of primitive fluid magma with very large discharge rates. These lava plains cover the uplifts of Tharsis and Elysium and fill basins like Chryse and Amazonis and the northern plains of Vastitas Borealis. These lava plains are just as widespread as the basaltic mare on the moon's near side or the ocean floor on Earth. The expanse of the lava is so great that the plains show subdued swells dozens of km long from compression and brickling.
Shield Volcanoes
The volcanoes found on Mars are almost all shield volcanoes. These are created by convection currents beneath the surface of the planet, hot, buoyant mantle rises to the surface as hot spots. This occurs on the Earth as well, in areas such as Hawaii and Bermuda. Martian volcanoes do resemble Hawaiian shield volcanoes in many ways, their eruptions are effusive and therefore quiet, they are basaltic in nature showing that the magma has reached the surface quickly and has not had long enough to partially crystallise to any extent. Shield volcanoes have calderas and typically long lava flow channels. In fact the main difference between shield volcanoes on Earth and those on Mars is size, Martian volcanoes are 10 - 100 times larger.
Hot spots do not just form volcanoes, if they occur under a thick continental plate, as they most commonly do, the forces are sometimes sufficient to tear the crust apart forming rifts or graben. These faults occur when crust moves in opposite direction, leaving the land in the middle to drop down. On Earth this can be seen in the East Africa Rift, on Mars there is Valles Marineris.
Valles Marineris
On Mars, rifting has occurred, creating an immense valley like a scar on the Martian landscape, out of proportion to anything on Earth. It runs East to West across Mars, just below the equator. At its greatest depth, the main chasm is 7000m deep, this is 3 times the depth of the Grand Canyon. Its width exceeds 500km and from end to end is 4000km - the width of the United States.
In the above photo, constructed from the many images sent back by the Viking probes, Valles Marineris can clearly be seen cutting across the planet. To the left hand side of the photo the volcanoes of the Tharsis region can be seen.
Volcanoes
The main volcanic regions of Mars are located in the uplifted areas of Tharsis and Elysium.
The Tharsis dome is 4000km across and up to 10km in height and serves as a pedestal for the 3 giant volcanoes that tower over the region, all located on the North West flank, Acraeus Mons, Pavonis Mons and Arsia Mons.
The image shows Arsia Mons on the lower left, Pavonis Mons in the middle and Ascraeus Mons in the upper right. Olympus Mons lies off image to the upper left. Each of these Tharsis volcanoes is 700km apart and they all reach almost to the height of Olympus Mons at about 25000m but they are not as tall because they stand on Tharsis which itself is 10km high. But they are still about 15km high, 1.5 times the height of the highest volcanoe on Earth. It seems that they were all active at the same time though arsia was probably formed first, then Pavonis and then Ascraeus, the hot spot beneath them shifting North over time, similar to the way the Hawaiian islands formed.
Click here for a map showing the topography of the region.
Click Here for an image of the summit of Ascraeus Mons.
Click Here for an image of the Pavonis caldera.
Click Here for an image of Arsia Mons.
Elisium is another uplift similar to Tharsis though slightly smaller. The flanks of Elysium Mons are steeper at about 10 degrees suggesting that the lava was more chemically evolved when it came to the surface. Some of the mafic minerals had crystallised out, leaving the remaining magma more viscous, and hence leading to the steeper slopes. Elysium Mons possesses a single caldera and several large channels to the North West, possibly carved by flowing mud and water released by the heat of an eruption. Elysium Mons can be seen below.
Hecates Tholus, is another volcano of the region, similar in size to Elysium, it is located near to the crest of the uplift. Albor Tholus Is a volcano located about 2700km South East of Elysium Mons.
Beyond the North West edge of the Tharsis uplift is Olympus Mons, the tallest volcano in the Solar System. It is 27000m high and 20 times wider than it is high, its average width is 550km. It is close to 500,000km in area and if you include the basal lava plain, the complex is roughly the size of Spain. By comparison, the largest volcano on earth is Mouna Loa in Hawaii, if measured from the sea floor it is 9000m high and 120km across. The comparison can be seen below:
The volcano is rimmed by a 6km high scarp, though in some places it is hidden under lava flows, this feature is unique to Olympus Mons, it is not found anywhere else on Mars. It is thought to have been formed due to landslides though there is no evidence for this theory. Apart from this feature the rest of the volcano is very flat, in fact if your were to climb it at times it would be hard to tell which way was up because the peak would remain beyond the horizon. The caldera itself is 80km in diameter, and standing on the edge of it the far side would be far below the horizon.
Why such large volcanoes?
The answer to that question is that nobody really knows why the volcanoes on Mars have grown to such extreme proportions and a number of theories have been put forward to attempt to explain it. Probably the most likely reason is that Mars has no plate tectonics, so the surface of the planet does not move over the mantle below, therefore hot spots which cause such volcanoes as these, are allowed to remain under one area for a very long time, so the volcanoes keep growing.
Another question is whether or not these volcanoes are still active or not, many of the volcanoes on Mars have many meteorite impact craters. By judging the age of these it is possible to tell how long it has been since the last eruption. According to this scale, Arsia Mons erupted 700 million years ago, Pavonis Mons 300 million years ago, Ascraeus Mons 100 million years and Olympus Mons 30 million years. That is not a long time in planetary history, 100My is about 2% of the age of a planet, so these have been erupting for 98% of the age of Mars, why should they stop now? Certainly much more work will be done on the subject in the future.
Joe Appleby 29/4/97 ja6147@bris.ac.uk
