Seasoned Mars observers could be forgiven for a feeling of déjà vu yesterday when they read about the discovery of running water on the surface of the Red Planet.
Such announcements come with frequent and rather confusing regularity from the US National Aeronautics and Space Administration (Nasa).
The reason, of course, for this obsession with liquid water on Mars is that it raises the prospect of life on a planet that has captivated generations of schoolboys and science fiction writers. Planetary scientists are agreed that extraterrestial life almost certainly requires liquid water to exist, just as it does on Earth.
In 2000, Nasa announced that it had discovered deep-sided gullies running down the dusty slopes of Martian craters. The immediate suggestion was that these channels were cut by running water, although when this was supposed to have happened was open to speculation as the gullies could have been created at any time over the past few million years.
Then, in 2006, the American space agency revealed to an eager global audience that it had found evidence of running water on the present-day Martian surface. It had taken two sets of satellite images of the same Martian craters, one in 1999 and the other in 2006, and witnessed the appearance of lightly shaded patches which indicated the sudden flow of underground water.
Nasa's lead scientist on its Mars exploration programme, Dr Michael Meyer, said at the time that these patches suggested the presence of liquid water on the Red Planet today. "These observations give the strongest evidence to date that water still flows occasionally on the surface of Mars," he said in December 2006.
Life may not automatically follow from the discovery of flowing water on Mars, but it certainly brings the prospect closer. And the discovery of life on another planet, no matter how primitive and microbial, would certainly amount to one of the greatest scientific finds of all time.
It would mean, for example, that life has originated at least twice in a single solar system – provided we can eliminate the possibility that any Martian life forms were somehow carried there from Earth. Two planets with life in a single solar system would indicate that the origin of life is a fairly common event, and that our galaxy, composed of billions of solar systems, must therefore be teeming with extraterrestial life forms.
A universe where life is so common would presumably be governed by the same rules of Darwinian evolution that produced intelligent, conscious human beings here on Earth. Discovering even the simplest life form on Mars, therefore, would almost certainly mean the existence of advanced, intelligent aliens in other solar systems with civilisations comparable to our own on Earth.
Ultimately, this is what lies behind Nasa's apparent obsession with the possibility of water on Mars. If liquid water exists, then so could primitive Martian life. And if Martian microbes exist, then intelligent aliens can no longer be confined to the realm of science fiction. As Charles Boden, the head of Nasa, said regarding this week's announcement: "Nasa's Mars exploration programme keeps bringing us closer to determining whether the Red Planet could harbour life in some form, and it reaffirms Mars as an important future destination for human exploration."
The search for life on Mars is nothing new, of course. It began in earnest at the end of the 19th century, when the Milanese astronomer Giovanni Schiaparelli observed long, straight lines on the planet's surface. These channels, or "canali" as he called them, became known through mistranslation as "canals", and people quickly assumed they were created artificially by intelligent Martians. Indeed, an American astronomer called Percival Lowell reasoned that as Mars is an arid place, the canals must have been built to divert this most precious resource.
"With this leap of the imagination, Lowell created one of the most enduring tropes of science fiction: Mars as a dying planet. It would live on in the works of H G Wells, Edgar Rice Burroughs, Leigh Brackett and many, many others," said the science writer Oliver Morton, author of Mapping Mars. In Wells' novel War of the Worlds, written in 1894, we are introduced to the idea of Martians from a water-starved planet invading Earth to take home the only thing they lack: that essential aqua vitae.
This week's announcement brings us closer to knowing whether Mars does indeed hold water. The downhill features identified by the HiRISE camera on board Nasa's Mars Reconnaissance Orbiter show much narrower gullies than the ones identified in 2000, and, crucially, these gullies appear to be seasonal, fading in winter and appearing again in spring and summer.
The gullies observed in 2000 now appear to be caused by the defrosting of carbon dioxide, whereas the latest finger-like streaks running down the slopes of Mars bear all the hallmarks of being caused by the flow of salty water, which freezes far below 0C and could easily exist in liquid form in the bitterly cold Martian temperatures.
"The best explanation for these observations so far is the flow of briny water," explained Alfred McEwen of the University of Arizona, the principal scientific investigator on the mission.
"The flows are not dark because of being wet. They are dark for some other reason. It's a mystery now, but I think it's a solvable mystery with further investigations and laboratory experiments," Dr McEwen said.
So the search for water on Mars continues, although final, unequivocal proof of its existence may come sooner rather than later. Nasa already has many instruments monitoring the Red Planet, and it will have even more later this decade with the launch of joint US-European probes.
More importantly, by following the water, Nasa hopes to achieve its ultimate goal – finding life on Mars.
Five thousand years of wondering what's up there
3,000 BCE or earlier Ancient Egyptian astronomers identify the planet, followed in due course by the Babylonians, who called it Nergal; by the Greeks, who called it Ares; and the Romans, who decisively named it Mars.
1543 Nicolaus Copernicus publishes On the Revolutions of the Celestial Spheres, arguing from the movements of the planets, including Mars, that the Earth is not at the centre of the Universe.
1609 Johannes Kepler establishes his first law of planetary motion, on the basis of Tycho Brahe's records of the motion of Mars. In the same year, Galileo Galilei is the first man to see Mars through a telescope.
1660 Christiaan Huygens uses a telescope to make the first detailed drawings of the planet.
1726 Jonathan Swift mentions two as-yet-undiscovered moons of Mars in Gulliver's Travels.
1877 Giovanni Schiaparelli observes channels ("canali") on the surface of Mars.
1877 Amateur astronomer Asaph Hall discovers Mars's two moons, Phobos and Deimos.
1895 Percival Lowell publishes the first of three books on Mars popularising the idea that Schiaparelli's "canals" are relics of a lost civilisation.
1898 HG Wells's The War of the Worlds describes a Martian invasion of Earth. The book later spawns a 1938 radio broadcast by Orson Welles – and, more recently, at least five films, and several computer games.
1899 Nikola Tesla observes patterns of atmospheric radio noise that he thinks might come from Mars.
1916 Mars is described as "The Bringer of War" in the first part of Gustav Holst's The Planets suite.
1923 Aelita, by AN Tolstoy, describes a Soviet expedition to Mars.
1948 Marvin the Martian makes his first appearance as a Looney Tunes character.
1950 Ray Bradbury publishes The Martian Chronicles, about the colonisation of Mars. Arthur C Clarke's novel The Sands of Mars and Isaac Asimov's The Martian Way would follow in the next two years.
1959 The film The Angry Red Planet revolves around a mission to Mars. By 2011, more than 20 films had been made using Mars as a subject.
1965 Nasa spacecraft Mariner 4 takes the first close-up pictures of the planet.
1970-71 The Nasa space orbiter lands on Mars and sends back more than 7,000 pictures of the planet.
1971 David Bowie releases Hunky Dory featuring "Life on Mars". Mars also features in Elton John's "Rocket Man", released the following year.
1976 A photograph from the Viking 1 orbiter seems to show the shadowy shape of a face on the planet's surface.
1977-78 Craig Raine's poem A Martian Sends a Postcard Home launches the influential "Martian School" of poetry.
1988 Two Soviet Phobos probes fail to achieve landings on the planet.
1990 Mars features in the film Total Recall, starring Arnold Schwarzenegger (below), based on the storyWe Can Remember It for You Wholesale by Philip K Dick.
1996 Mars Attacks!, a film by Tim Burton, showcases a stylised version of Martian invaders.
1997 Nasa's Pathfinder mission lands a base station and roving probe on the planet.
2003 Mars passes within 34.6 million miles of the Earth – the nearest it has been in 60,000 years.
2007 A paper explaining the gap in height between two shorelines on Mars reignites the debate over whether there was ever an ocean on the planet.
2008 Nasa's Phoenix lander exposes samples of ice under the planet's crust.
2011 Nasa reports it may have found evidence of flowing water on some of the planet's slopes. Images from the Mars reconnaissance orbiter show lineations in spring and summer which appear to fade away in winter, possible evidence of briny flowing water.
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