Is there life on Mars?
For more than a century, people have speculated about whether there is life on Mars. Within five years we may have an answer.
In the late 19th Century, the American astronomer Percival Lowell thought he saw canals on the surface of Mars.
He suggested that this was evidence that our near planetary neighbour might not only harbour life, but could possibly be home to an advanced civilisation.
This fired the public's imagination - prompting HG Wells's novel, The War of the Worlds, several comics and numerous Hollywood science fiction films depicting Martian invasions.
What did the first missions find?
There was a mixture of disappointment and some relief when the existence of canals was finally disproved by orbiting spacecraft sent to the Red Planet in the 1960s and 70s.
Instead, they showed a cold desolate world. Even so, people watched in awe as Nasa's Viking landers sent back the first pictures from the surface of this eerily beautiful place.
And there was eager anticipation for the results of Viking 1's tests on soil samples. One of them indicated what was interpreted as a signature for life - but was soon discounted as a bogus result.
And so for the best part of 20 years, Mars was seen as a dry dusty planet devoid of life.
When might life have emerged on Mars?
There were also indications that Mars might have ice under the surface in some places and may once have had a magnetic field. Subsequent missions gradually built up a picture of a planet that was once not unlike our own 3.8 billion years ago. The data gathered suggested that the Red Planet might once have had a thicker atmosphere as well as lakes and oceans.
These are the conditions that could have enabled life to emerge - just as it did on Earth at about the same time.
But then something catastrophic happened: Mars lost its magnetic field and with it most of its water and atmosphere.
Is it likely there is still life on Mars?
The prospect of it still being there is slim but not impossible.
For there to be life, there needs to be liquid water. Evidence for that has been growing and earlier this week Nasa seemed to have the the strongest evidence yet that some is still there - albeit in small amounts.
Fifteen years ago, Nasa discovered gullies inside craters and on slopes. It looked like they might have been caused by running water. But the temperatures are below freezing so many dismissed the idea.
Scientists then noticed that these features increased and decreased over a period of a few months, so whatever was causing the gullies was happening now.
The latest study reported earlier this week has found evidence of different kinds of salts in dark streaks on the slopes of peaks and crater walls. That's important because salt can melt ice - enabling it to flow.
The discovery confirms that the Red Planet is still geologically active and, tantalisingly, it increases the possibility that it may currently harbour simple living organisms.
Current life on Mars is a remote possibility because Mars is harsh, dry and bombarded with radiation, but it is a prospect that will whet the appetite of those planning future missions to a world that becomes more intriguing with each new set of results.
Prof Andrew Coates, who is deputy director of the Mullard Space Science Laboratory at University College London, says: "It was a good time for life on Mars to have developed 3.8 billion years ago, which was about the time that life was starting on Earth. But then conditions changed which could have killed it off.
"But life is tenacious and once it has a foothold it is able to survive extreme conditions."
The race to find life
Nasa, Europe and Russia all have missions planned for the next few years to discover whether there was once life on Mars and whether it still exists. We may have the answer within five years.
The missions will follow up on a sensational result that came in 2004 from an orbiting European Space Agency spacecraft, Mars Express. This was the detection of the gas methane.
It was a surprise because there are limited ways it could be produced: by volcanoes and associated geological activity or by living organisms.
What's more, methane is a short-lived gas in the atmosphere - it is broken down by sunlight. So whatever produced the methane must have done so relatively near in time.
The implications of the discovery were so profound that some in the scientific community thought that the instruments on the spacecraft were faulty.
Even though methane has been detected again by Nasa's Curiosity rover last year, some remain unconvinced. In 2020, Nasa will send another rover to the Martian surface which will seek signs of ancient biology.
The European and Russian space agencies plan a pair of missions called Exomars to resolve the issue. Next year, they will launch an orbiting spacecraft that will try to confirm the presence of methane.
If life does exist on Mars, it is most likely to be below the surface. That is why two years later the Russians and Europeans will send the Exomars rover to to drill two metres into the planet. Just as the Viking lander did nearly 40 years ago, the Exomars rover will take a soil sample and test it for signs of life.
Dr Matthew Balme of the Open University believes that this will be one of the most important experiments carried out in human history.
"If we find life on Mars and it can be shown to be of a different origin to that on Earth, then that essentially means that the Universe is teeming with life. It seems almost impossible that life could spring up by chance on two adjacent planets if life was rare."
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