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Hello Mars, is anyone at home?

Why Mars? Not just, scientists now argue, because (like Everest) it's there. After the discovery last August of possible traces of fossilised past life in a Martian meteorite, the question now is: could life have once existed there, and might it still do so?

The area where the Mars Pathfinder spacecraft rolled to a stop on Friday evening was once a river valley, with huge flows of water back and forth along it. "Early Mars may have been like early Earth," said Matthew Golombek, one of the mission scientists. "Mars may have once been much warmer and wetter, and enveloped in a much thicker atmosphere."

Given that we have found evidence that life existed on Earth about 4 billion years ago - just 500 million years after it formed from the floating debris of the primeval solar system - it seems reasonable that it could have formed on Mars too, given the right conditions. Those conditions are simply warmth and water, according to biologists who in the past 20 years have made many surprising discoveries, including bacteria that can live in far hotter and more airless environments than had previously been thought possible.

The last visits to Mars, by the Viking spacecraft in 1976, proved disappointing to a world public which had come to expect bigger and better things from the space programme, after the delights of the Moon landing in 1969 and the increasingly extravagant missions that followed it.

The disappointment was that Viking's onboard laboratory found no signs of any life in the rock and soil samples it examined. For many, that quashed their hopes that we were not alone. Space became a duller place.

But the scientific suggestion that there may have been life on Mars - and that there may even be deep beneath the ice crust of Europa, one of Jupiter's moons - has put the spring back into many space scientists' step.

The Mars Pathfinder and Mars Sojourner Rover have many sophisticated measurement instruments on board, able to measure characteristics of the rocks. Together with the pictures, this will reveal what kinds of minerals the Martian crust is made of, and what kind of geological processes are happening there. Rocks on Mars are unique: some date back to the first billion years of the planet's existence, whereas on Earth those have long ago been eroded.

The Lander will analyse the atmosphere, take pictures and record the weather - which yesterday never rose above a chilly 0 degrees Centrigrade, and is expected to fall to around minus 88 Centrigrade during the Martian night, which roughly coincides in time with the British night.

The six-wheeled Sojourner (a considerably smaller answer to the Moon buggy, weighing just 22 pounds) is designed to venture several hundred feet, travelling at less than a half-inch per second, carrying cameras and a device able to analyse the chemical composition of the soil and rocks.

The Rover - assuming it is able to get off the Lander, where it remained stuck late on Friday, held back by an airbag - will be a triumph of communications. It will be driven by a human on Earth, wearing a virtual reality headset. There will be a huge time-lag between initiating a movement and anything happening, because the signal must travel the 309 million miles - undergoing an 11-minute delay - and then make the same trip back. The Rover is expected to survive at least seven days on its batteries. The Pathfinder's lander could survive at least a month.

But the mission's objective is not to look for life. It is meant to find suitable sites as the first of several landings. Putting man on Mars remains far off, it being thought preferable to let robots face extraterrestrial dangers.