Mars is the fourth planet out from the sun and is one of Earth’s close neighbours. Mars is around half the size of the Earth and has a similar internal structure – a solid crust with a hot nickel-iron core.
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Mars as seen by the Viking 1 Orbiter Credit: NASA
However unlike Earth’s, Mars’ mantle is solid and cold (Earth’s is still hot and semi-molten). Mars’ interior cooled much faster than Earth’s because of its small size. This difference in size meant that Mars had much less thermal energy in its interior to begin with and so its crust and mantle solidified more quickly (Mars only has 1/8th the volume of the Earth and so had much less radioactive material in its interior, it is the decay of such materials that is thought to help keep the interiors of rocky planets molten). As Mars has little heat internal heat remaining it is said to be geologically inactive (or dead) – this means that Mars has no active geological features such as volcanoes however, Mars was once a very different place… (and may still stir occasionally, as some evidence of activity have been detected on a few rare occasions)
Early in its formation Mars did have enough thermal energy to ‘power’ mass volcanism, and indeed Mars’ volcanic features are absolutely massive. The dead Martian volcano Olympus Mons is the largest volcano in the solar system. It is 26km (16 miles) high – that is three times taller than Mount Everest! But how can a volcano grow so tall?
As Mars is much smaller and thus has less mass (‘weighs’ less) it has a lower gravity than Earth. On Earth the acceleration due to gravity is around 9.81 ms2 (a ball dropped from rest will increase its velocity by 9.81 ms every second – until it hits the ground or reaches terminal velocity)whilst on Mars it is just 3.42ms2 (i.e about a 1/3 as ‘strong’). This means that with less force trying to pull material back to the surface of the planet geological features can grow much taller than what is possible on Earth. Combined with Mars’ very thin atmosphere, ( only 1% the thickness of the Earth’ and composed of 95% carbon dioxide, 3% nitrogen and traces of oxygen and water) meaning that weathering (the process that wears down mountain of Earth) occurs at a much slower rate than on Earth, so such features can exist for many millions of years. Further more, as the volcano is not being pushed down on as firmly due to the lower atmospheric pressure as it is being formed, it is able to get higher faster.
Mars also has another geological property; unlike Earth, Mars has no tectonic plates. This means that as the surface of the planet is rigid and not moving this in turn allows volcanoes to build up in exactly the same place over the millennia further increasing the maximum size. This property also created one of the planet’s most viable surface features – Valles Marineris. This is a massive rift valley in the planet’s southern hemisphere, it stretches over a third of the way round the planet and is over 4000km (2500 miles) long and in places it is four times as deep as the Grand Canyon! It gets its name for the Latin for valley – Valles – and the probe that first observed it Mariner 9- Marineris.
The rift formed when the Tharsis Plateau (containing the volcanoes - Arsia Mons, Pavonis Mons, and Ascraeus Mons, Tharsis Tholus, and the mighty Olympus Mons) pushed up. This caused massive strain on the planer’s rigid crust which ripped open causing the rift to its east.
Mars has two small moons: Phobos and Deimos. They take their names from the Greek gods of fear and panic respectively. They are both very small rocky bodies only a few kilometres in diameter and both are thought to be captured asteroids partly because both are tidally locked to Mars (the same side always faces the planet). The closer orbiting Phobos has a decaying orbit and is expected to be ripped apart by tidal forces within the next few million years and will crash through Mars’ thin atmosphere in fragments before impacting its surface.
Mars has long been a target for human curiosity. The Greeks were one of the first civilisations to attribute it to a deity; Ares – the god of war. The Romans also attributed it to their god of war thanks to its blood red colour – Mars.
Mars has also been a favourite for ideas about extraterrestrial life, ‘Martians’ if you will. Whilst the chances of finding intelligent life on Mars have all but vanished (despite all the exploration on the planet has found no sign of intelligent life) the discovery of abundant water ice, indeed even small amounts of transient liquid water offer new hopes of finding microbial life on the ‘Red Planet’.
(For any fans of the X-box 360Playstation 3PC Mass Effect series of games several locations in the game are taken from actual locations on Mars – Ellysium for example. There is also a region on Mars named Cerberus – it gets its name from a region of darker surface material in the shape of a barking dog.)