Below is a piece I wrote published in Rockwatch (the club for young geologists run by the Geologists Association) in 2011/12, Issue 59, page 12: http://www.rockwatch.org.uk/magazine.html
Jane Robb, several times a Rockstar competition winner, has graduated and is now a professional geologist. Here she explains some of the features of the planet that is most like ours – Mars.
Think of the Earth.
Think of its colours, continents, massive deep blue oceans and the rivers that feed them. The images of our ‘blue planet’ that we see in our minds conjure up thoughts of life and vast, thriving ecosystems.
Now think of Mars, our neighbouring ‘red planet’. What do you see?
I see a planet once covered by vast oceans, scoured by winds and sculpted by volcanoes.
Early observers of Mars such as Percival Lowell, once thought that the features on Mars’ surface were artificial canals built to tap the drying planet’s polar ice caps. His perceptions, although wrong, fuelled new efforts to discover more of the Solar System’s fascinating secrets.
Since then, we have evidence of natural channelling across large areas of Mars’ surface. The landscapes they form are often referred to as ‘chaotic terrain’, with steep sided troughs and slumping of channel walls. These features could be the result of removal of underground material from release of water and ice pressure from microscopic holes inside the rocks. They are not thought to be from long term flow of liquid water as they are several times the size of similar channels on Earth. The image adjacent  shows what is referred to as a ‘starburst spider’ from its shape. It is thought to be formed from CO2 (carbon dioxide) gas and rock dust escaping from underground through an opening at the surface and spreading out in a fan shape. Other features can be seen, caused by the movement of liquid water under the surface of Mars, called sapping, that have since collapsed in on themselves forming the channels we see today.
Across the surface of Mars features termed ‘sinuous ridges’ can be found that are 100’s of kilometres in length and 100’s of metres wide. These ridges can be thought of as river ‘trace fossils’. Imagine a fast flowing river in which lots of sediment is deposited on the river bed, including large boulders. Eventually, the river dries up and millions of years of erosion go by. Next to the river are soft sediments like clays and muds, while inside the river are sands and boulders. The clays and muds erode away faster than the river, so that the old river bed is now a long hill, just like a country scale trace fossil! The same ridges can also form from lava flows!
Mars is also home to the largest volcano in the entire Solar System at 27,000m high – Olympus Mons. Mars has not been visibly active in our lifetimes, although it is assumed that there has been recent volcanic activity (geologically speaking!). The main form of volcanism is basaltic, but more gas accumulates in the lava than expected due to a lack of atmospheric pressure. This means that normally runny magmas on Earth can erupt violently on Mars. The lack of gravity also means that magmas are less buoyant (do not float), so they stay in the mantle for longer in large magma chambers. So, when the magma eventually gets hot enough to escape, the eruptions last much longer with larger volumes of lava. Therefore, with larger eruptions, Mars can build larger volcanoes!
‘Fretted terrains’ form at a break in the surface such as a fault or crater, whose edges have eroded to form smooth, flat lowlands. The material removed from the edges does not seem to show evidence of fluvial (water) erosion, so it could have been transported by aeolian (wind) erosion. Vast expanses of Mars’ surface have been altered by wind erosion, like these dune fields , and after initial large scale fluvial, tectonic or igneous activity .
Images  and  show some more beautiful scenes of the Martian landscape. The layers seen in  are opal (silica, SiO2) and iron sulphate (FeSO4). These are formed from aqueous alteration (water erosion) of basalt lavas. There is substantial evidence for large volumes of liquid water on the surface of Mars at least early in its history, around 3.5 to 4 billion years ago. At present, only a fraction of that water remains trapped in polar ice caps, indicating that at one point Mars was warmer and wetter than at present. In , an ancient delta in a crater that once held a lake can be seen. The enhanced colours show clay minerals (in green) which are known to trap and preserve organic matter on Earth.
Now, the chemical and geomorphological evidence in favour of vast volumes of liquid water in early Martian history, at least some of which remains today, points scientists in the direction of the crucial question: ‘Is there life on Mars?’.
“Are they worlds, or are they merely masses of matter? Are physical forces alone at work there or has evolution begotten something more complex, something not unakin to what we know on Earth as life? It is in this that lies the peculiar interest of Mars.” Percival Lowell.