Ashleigh Edmondston, of Ardgowan School, asks :-

Why is Mars red?

Jim Cole, a geologist at the University of Canterbury, responded.

Mars is a fascinating planet which has been a subject of wonder since earliest recorded time. In 1965 and 1971 spacecraft photographed the planet and showed just how spectacular it is. There is, for example, an enormous volcano (Olympus Mons) 700 kilometres across and a vast canyon (Valles Marineris) which extends for over 4000 kilometres. Then in 1976 the two Viking spacecraft landed on the surface of Mars and provided important clues to earlier questions.

Mars is further from the Sun than Earth, and so is very cold (the average surface temperature is -63 Celsius). It also has a very thin atmosphere that is mainly carbon dioxide. This means that surface processes are very different to those on Earth. There can be no surface water, although there may have been water in the past. The main rock type is basalt lava, similar to that forming the Auckland volcanoes. This lava has weathered over billions of years to produce a surface 'soil' which is now largely an iron-rich clay. The iron, originally forming part of the chemical make-up of minerals in the basalt, has over time become oxidised, in the same way that iron or steel will rust if not painted. This gives it a red or brown colour. Even though the atmosphere is thin, winds develop which create dust storms shifting soil into vast desert areas, leaving other areas more rocky. This accounts for the varying intensity of colour that can be seen when Mars is viewed with a telescope.