Life on Mars?
Box 1 | Mars: Earth's cool cousin
If life exists elsewhere in the Solar System, the most likely place is on Mars. Our closest neighbour, Mars contains water, carbon dioxide, nitrogen and a small amount of oxygen – the basic ingredients of life as we know it.
There are similarities between Mars and Earth that make the possibility of life there even more plausible. Although much smaller than Earth, Mars has a similar density and a similar mineral composition. The day length on Mars is only just over 24 hours, and the angle that the axis of rotation forms with the orbital plane is currently nearly the same on both planets, causing both to have seasons.
The hitch is that Mars is cold. The average temperature at the surface of the planet is about -63°C at the equator and as low as -110°C at the poles. Although living things can survive on Earth under extreme conditions, life is unlikely to thrive at these temperatures.
Nevertheless, the existence of life or former life cannot be ruled out. There may be 'hotspots' under the surface where heat produced by the planet's core creates temperatures warm enough for microbial life.
In addition, Mars may once have been much warmer than it is today. Scientists have detected evidence that water and carbon dioxide have escaped the planet into space. If these gases were once present in the planet's atmosphere in large enough quantities, they could have created a 'greenhouse' effect similar to the one which keeps Earth warm. Indeed, it is possible that about 4 billion years ago the conditions on both Earth and Mars were suitable for life.
Some big questions thus remain unanswered. Was the beginning of life on Earth an extraordinary fluke, or is the process so easy that it also occurred on our nearest planetary neighbour at about the same time? Could life have hitched a ride on a meteorite between the two planets, as some scientists suggest? If life did start on Mars, what happened to it?
Boxes
Box 2. Cyanobacteria: The simple things of life
Box 3. ET, can you speak up?
Related sites
Mars 101 (The University of Arizona, USA)
Mars fact sheet (National Space Science Data Centre, NASA, USA)
Posted November 2008.