The Geology of Mars

  Professor N C Wickramasinghe

At the moment our neighbouring red planet, Mars, is the focus of three orbiting spacecraft and has three robotic vehicles busily examining its surface. The study of this planet has come on in leaps and bounds since the time when Schiaparelli's illusionary linear surface features were mistranslated as canals.

To give us a true picture of the planet we had invited Dr John Bridges from Leicester University who led us through the various ages of its geological history. Initially he gave us an overview of the planet's vital statistics.

To the naked eye Mars appears as a bright reddish-orange dot of light. It appears this colour as its surface is covered in iron (III) oxide, better known in everyday terms as rust. Mars has only a tenth the mass of the Earth and presently has no magnetic field and only a very thin atmosphere mainly composed of carbon dioxide. If you totalled all the area of dry land on the Earth this would roughly equal the Martian surface area.

The geography (or more correctly 'areography') of Mars is split into two distinct regions: the young northern plains flattened by lava flows and its ancient southern highlands peppered with ancient impact craters. It has a number of unique features including Olympus Mons which is the largest volcano in our Solar System, measuring 26km in height (or three Mount Everests). Nearer the Martian equator is a gigantic canyon system called Valles Marineris measuring 4,000km in length and 7km deep, extending a fifth of the way around the Martian surface. In contrast our Grand Canyon is a paltry 446km long and 2km deep.

Usually a planet's surface is dated by counting the number of craters appearing on it; the more ancient areas having more craters. Following this method, the earliest geological era on Mars is called the Noachian Epoch which was from 3.8 to 3.5 billion years ago. By this time the planet had lost its magnetic field and so the solar wind steadily stripping its early atmosphere. This was also the time when a large amount of debris was still circling around the Solar System and most of the terrestrial planets such as Mars took a pounding adding to the atmospheric destruction. It is thought that at this time Mars could have been relatively warm and wet with a thick atmosphere despite the many impacts. The latest research suggests that the warm and wet period was relatively short.

The next era is the Hesperian Epoch from 3.5 to about 1.8 billion years ago. In this time period the atmosphere was thinning and there was extensive volcanic eruptions creating great plains of lava. The final era is the Amazonian Epoch from 1.8 billion years ago to the present day when sedimentary rock layers appear and the landscape is scarred by glaciers moving from the polar regions to the equator. The latest robotic mission to the red planet is NASA's Phoenix Lander which will probe the history of liquid water that may have existed in the north polar region. It will analyse the chemistry and mineralogy of the soil and ice and hopefully give us a new understanding of the history of water on Mars.