The Three Phases of Mars
The European Space Agency's Mars Express spacecraft has found three distinct phases in Mars' history. The orbiting satellite has used infra-red light to identify minerals that have formed on the surface. Although Mars is currently a frozen desert, we've known for decades that water must have been present at some point; a secret revealed by dried up river beds and underground permafrost. It's only now that we know the timescales involved. Just after Mars formed around 4.6 billion years ago, it boasted a dense atmosphere and plenty of liquid water. This water pooled on the surface and formed the clay beds discovered by Mars Express. However it seems that Mars' arid fate was sealed even in those early days. Radioactive elements slowly heated the planet for 600 million years until Mars erupted in a cataclysmic episode of volcanism. This released tonnes of sulphur dioxide into the atmosphere which reacted with the water to form acid rain and sulphate minerals. The volcanism died down as the planet cooled over the next 500 million years, but the planet also lost its protective magnetic field. This allowed charged particles blown out from the Sun to strip away Mars' atmosphere. For the last 3.5 billion years Mars has been a desert world with the iron in the rocks slowly rusting to create the red planet we see today. This seems a bleak place for life to try and survive, but if there is life on Mars then it will most likely be found in the ancient clay beds.