The Mars Express satellite belonging to the European Space Agency (ESA) has helped researchers identify three distinct geological periods on Mars. The OMEGA instrument located evidence of water in the planet's distant past - around four billion years ago - but this water had disappeared within half a billion years. The Mars Express satellite identified Phyllosilicates (clay), which are formed by the action of water and correspond to some of the oldest rocks on the red planet. Two subsequent periods show firstly the emergence of sulphates in an acidic environment from 3.5 billion years ago. Finally, the last period which extends to today, shows the emergence of dry ferric oxides and weathering. This final period is responsible for the planet's red colour. The information, published in the latest volume of Science, is a collaboration between researchers based at 21 different institutions around France, Italy, Germany, Russia and the US. The Express probe had already identified evidence of water erosion on the surface of the planet, but 'was water-driven activity on its surface transient or persistent, the latter being a prerequisite to sustain habitable surface environments?' asks the report. In one Martian year of orbit, the OMEGA instrument has mapped 90 per cent of the planet's surface, and has had the opportunity to link with the Mars Rover to explore specific areas in the Terra Meridiani region. The Rover uncovered sulphates formed in the presence of water in areas identified by OMEGA. The earliest period, the 'phyllosian' era, occurred between 4.5 and 4.2 billion years ago, soon after the planet formed. The researchers believe that the environment was warm and moist, leaving large clay beds which have been picked up by Express. This era could, in theory, support forms of primitive life. The second period, the 'theiikian' era, which took place 4.2 to 3.8 billion years ago, was dry, characterised by significant volcanic activity which altered the planet's climate. Sulphurous eruptions from the volcanoes would have reacted with any remaining water in the atmosphere to fall as highly acidic rain, which reacted with the rocks as it fell, giving this era the characteristic sulphurous composition. The final period, the 'siderikian' era, is by far the longest, stretching from around 3.5 or 3.8 billion years ago through to today. This era is also the least interesting for geologists, as it is characterised by weathering over huge time-periods. Professor Jean-Pierre Bibring from the Institut d'Astrophysique Spatiale (IAS) in Orsay, France, led the research. He acknowledges that there is another, possibly less exciting explanation for the clay beds. 'Hydrothermal activity below the surface, the impact of water-bearing asteroids, even the natural cooling of the planet, could all have promoted the formation of clay below Mars' surface. If so, the surface conditions may always have been cold and dry.' This explanation reduces the possibility of life on Mars considerably. 'If living organisms formed, the clay material would be where this biochemical development took place, offering exciting places for future exploration because the cold Martian conditions could have preserved most of the record of biological molecules up to the present day,' says Professor Bibring. So the answer to the question 'Is there life on Mars?', posed by researchers, romantics and David Bowie, remains tantalisingly unanswered - for now.
Germany, France, Italy, Russia, United States