The question of whether life ever existed on Mars has inspired scientists – and songwriters – for decades. The Red Planet is currently an arid, icy desert where no sign of life remains, but was it always this way?

It is widely recognised that living entities have three basic requirements: standing water, an energy source and the five chemical elements, carbon, oxygen, hydrogen, phosphorus and nitrogen. Then a very long time for the chemical soup to stew. The rover Curiosity has found evidence of all three in certain areas of Mars, namely the Gale Crater, but were these available for long enough for life to develop?

John P. Grotzinger of Caltech, the project scientist for the Curiosity mission, has now revealed at a recent press conference that these three factors were most probably available for tens of millions of years, creating plenty of potential for life to form. It is thought that the crater was filled with lakes and rivers, the water from which would have remained underground even when the surface water dried up.

“As a science team, Mars is looking very attractive to us as a habitable planet,” Dr. Grotzinger said in an interview. “Not just sections of Gale Crater and not just a handful of locations, but at different times around the globe.”

Another member of the Curiosity team, Roger Summons of M.I.T., says that findings from that rover and previous missions suggest that early Mars may have borne similarities to an early Earth.
Dr. Summons believes that, for the first billion years, both planets had stable environments that could have supported life for extensive periods of time. Most scientists believe that life on Earth first developed around 3.8 billion years ago and the same could have happened on Mars.

Despite these promising signs, no organic compounds have ever been detected on Mars, though Curiosity does not carry a "life detector" as such as scientists are not sure exactly what this would consist of. Whether life did ever evolve on Martian soil is therefore still unknown, but what is certain that something cataclysmic happened to the climate which would have eradicated every life form on the planet.Why the climate changed in the way that it did remains a mystery, but scientists believe that the evolution of the Martian climate could help to provide clues for the future of Earth. Climate change is already a major issue on our planet, but conditions are still a way off the frosty wasteland that is now the norm on the Red Planet.

So what happened on Mars and could the same thing happen here on Earth?

It seems that, after the first billion years, Mars lost the majority of its protective atmosphere.

“I think the short story is the atmosphere went away and the oceans froze but are still there, locked up in subsurface ice,” said Chris McKay, an astrobiologist and Mars expert at NASA’s Ames Research Center.

The mission to find out why the atmosphere disappeared has now been taken on by a new spacecraft called Maven, the Mars Atmosphere and Volatile Evolution mission, which has been sent into orbit around the planet. Various theories already exist, the first being that the atmosphere was degraded away by solar radiation and wind. Maven is designed to test that theory by calculating how quickly Mars is losing its atmosphere today, which will hopefully provide an insight into the temperatures and precipitation experienced on the planet four billion years ago.

Mars originally had a magnetic field that, like Earth’s, created an umbrella effect, deflecting the rain of energetic particles shed by the sun. Earth’s magnetic field is generated by a dynamo generated by convection in the planet’s molten iron core. Once temperatures on Mars began to cool down, the dynamo and the magnetic field would have stopped and the solar wind could have begun eating away at the Martian atmosphere. Weakened by ultraviolet radiation from the sun which would ionize atoms in the upper atmosphere, making them vulnerable to magnetic forces in the solar wind, and they would erode away into space an atom or two at a time.

“A little bit every few hours,” said Dr. Brain a Colorado astronomer and Maven team member," and “suddenly you can change an entire planet.”

Asteroid impacts could also have produced shorter periods of near-freezing temperatures; there are many craters on Mars that would substantiate this theory. The impact that created the huge crater called the Hellas basin, for example, would have hurled vast amounts of vaporized rock into the sky leading to decades or centuries of hot rain and flash floods, said Brian Toon of the University of Colorado.

“What we are learning about are planetary atmospheres in general,” said David Brain,. “It’s really fascinating to think that the planet changed in such a large way.”

Before temperatures cooled, however, the general scientific consensus is that Mars was much wetter than Earth, as there is considerable evidence of water channels and water environments. Mars may have had a thick atmosphere with large amounts of carbon dioxide which would have increased temperatures in the same way as that "greenhouse gas" is doing on Earth. There is a flaw in the carbon dioxide theory, however, as scientists are not sure where it went; there is no evidence of the chemical left to support this theory.

“The holy grail of Mars,” said Dr. Jakosky of the Maven team, is to find the carbonate deposits that should have formed from its atmosphere. “We haven’t found them,” he said.

But the deposits could have been lost to space, and this is where particle measurements from Maven may help to clarify the question.

If the amount lost is substantial — “a couple of bars of CO2,” continued Dr. Jakosky, describing it in units of the atmospheric pressure on Earth — “would tell us that Mars must have been warmer in the past.” If losses are trivial, he said, that would sound the death knell for the early greenhouse theory, and the great Martian arguments would continue.

Whether Mars can help to provide some insights into our planet’s future remains to be seen, but Maven should help scientists to determine what happened to life, if any, on the Red Planet’s surface.

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