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Fire & ice: What really happened to water on Mars
23 May 2008
From New Scientist Print Edition.
Stuart Clark

Enlarge Phoenix lander probes for ice on mars

Enlarge Different worlds

Since the Viking orbiters beamed back the first tantalising images of water-cut features on Mars in the 1970s, NASA's mantra for the Red Planet has been simple: "follow the water". Working out when Mars had liquid water on its surface, and where that water went, they reason, will provide vital clues about whether Mars could once have harboured life, and whether life could cling on today.

Since then a long line of orbiters, landers and rovers have searched the Martian surface for signs of where water once flowed. The latest, NASA's Phoenix lander, is scheduled to reach the surface of Mars on 25 May. If all goes well, it could be the first lander to actually hold Martian water in its robotic hands, and will answer some long-standing questions about the planet's hydrological history.

According to the conventional view, from soon after its formation about 4.5 billion years ago until 2 to 2.5 billion years ago, Mars was a watery world like Earth, with luxuriant seas, perhaps even an ocean, that might have supported life. These large bodies of water were gradually lost through climate change, caused by a decline in volcanic activity and the whittling away of the planet's atmosphere by radiation from the sun. Some water remained frozen in the polar caps, but most drained downwards into the rocks and froze.

While there can be no mistaking that water did exist on Mars in large quantities, as new information from Mars's many probes and landers comes in, it is looking increasing likely that this simple tale isn't true. Instead, the "warm, wet phase" of Mars, when life might have originated, was actually quite short-lived, lasting less than a billion years, and was followed by a series of extreme conditions unlike any experienced on Earth.

If this is true, Mars's reputation as a once-habitable planet that may still harbour microscopic life is at stake. A billion years might well have been long enough for life to emerge - indeed it appeared within that time frame on Earth - but the speed with which those conditions vanished would have made it much more difficult for that early life to truly establish itself.

The main evidence for this new view of Mars comes from recent observations of the planet's surface. Gerhard Neukum, from the Free University of Berlin, Germany, used images from the High Resolution Stereo Camera on the European Space Agency's Mars Express probe to date the Martian surface, using the number of craters present as a yardstick. Since planets were most severely bombarded early in the formation of the solar system, and since lava from Mars's volcanoes can cover evidence of bombardment, Neukum assumed that areas with fewer craters are younger.

Neukum's analysis found no evidence to support the conventional view that Mars lost its water once, very slowly over billions of years. Instead, it seemed that the water disappeared within a billion years of the planet's formation and then reappeared five times following major volcanic upheavals. The earliest of these mega-eruptions took place 3.5 billion years ago, with repeat performances 1.5 billion, 800 million, 200 million and 100 million years ago.

During these episodes, major releases of lava partially resurfaced the planet. This sudden outpouring of internal heat thawed frozen reserves of underground water and drove it upwards to the surface. These events may not have lasted more than a few tens of thousands of years, but they have left ample evidence of water on the surface of the planet in the form of outflow channels, river beds and even shorelines. "We see evidence for these episodes everywhere on Mars," says Neukum.

This violent past may not be the only secret Mars has been hiding. Other researchers have found that a quirk in Mars's behaviour might be crucial to understanding the current distribution of water ice on the planet. The quirk is this: Mars wobbles.

Tipping point

In fact, to say it wobbles is an understatement: the entire planet can virtually fall over. This is because the planet sits right where the gravitational pull of Jupiter can exert enough influence to tip it over and stand it back up again. Mars is pulled onto its side this way every 10 million years or so. Computer simulations of the gravitational interplay between Mars and Jupiter suggest that this tipping over must have happened many times in the past, but until recently no one had considered what effect it would have on the distribution of water. François Forget, a Mars climatologist from Pierre and Marie Curie University in Paris, France, decided to find out. He had already developed a climate model of Mars for the present day, and wondered what would happen if he tweaked the obliquity - the amount of tilt in the planet's axis of rotation - to represent Mars at an earlier stage in its history.

Perhaps unsurprisingly, the simulation showed that as the planet tipped, and more sunlight struck the polar regions, the ice caps became unstable. In the northern hemisphere, ice began to sublime into the atmosphere and the resulting vapour moved towards the cooler regions of the planet, which were now on the equator (see Diagram). When Mars stood up again, the ice returned through the atmosphere to settle back at the poles.

This much they expected. But there were some surprises too. The model predicted that as the planet tipped onto its side, ice would accumulate on the sides of the great volcanoes that straddle the equator. This was particularly exciting because images from Mars Express, showing what looked like glaciated features on these volcanoes, had previously stumped planetary scientists.

What's more, the model predicted similar features in the Hellas Basin, the largest impact crater on Mars. Here too, scientists were seeing puzzling evidence for glacial erosion where they least expected it.

Between Neukum's volcanic episodes and Forget's tipping of the rotation axis, it is starting to look as if the story of Martian water is more complicated than anyone thought, with a climate driven by factors totally alien to those experienced on Earth. "We are certainly seeing more than wet Mars followed by dry Mars," says Forget.

There are plenty of mysteries left to solve. A major stumbling block is reconciling the amount of water that used to exist on the planet with the amount left today. Geological features in the outflow channels, such as erosion patterns that date to between 2.5 and 3 billion years ago, have led planetary scientists to estimate that the volume of water that flowed through them was enough to cover the planet to a depth of 500 to 1000 metres.

Yet when the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on Mars Express surveyed the south polar region, it revealed that a planet-wide ocean created by melting the water locked in the ice cap here would be just 11 metres deep. The north polar cap probably holds a similar amount of water. So what happened to the rest?

The most popular explanation involves Mars's water coming under a two-pronged attack. Solar ultraviolet radiation would have broken up some of the water molecules in the Martian atmosphere: the atoms of hydrogen escaped into space, whilst the heavier oxygen atoms sank to the surface and oxidised the rocks there, turning them red. At the same time, the constant stream of electrically charged particles from the sun gradually stripped away the Martian atmosphere, including its water vapour, by knocking individual atoms and molecules into space.

But not everyone is convinced that this can account for all the water that was lost. Stephen Clifford, a planetary scientist from the Lunar and Planetary Institute in Houston, Texas, thinks that these processes are nowhere near efficient enough to strip Mars of its water. He estimates that over the past 4 billion years, enough water has been lost in this way to cover Mars to a depth of just 100 metres, amounting to a fifth at most of what is estimated to have existed on the planet. "If most of the water has disappeared into space, it is by processes that we can't yet identify," he says.

Clifford believes that the water went not up, but down. He is a leading proponent of the idea that the water is lurking beneath the planet's surface, probably as permafrost or in giant ice-capped aquifers. As recently as 2005, many planetary scientists were taking it almost for granted that just such a large-scale underground reservoir network existed on Mars. Various observations from NASA spacecraft, including Mars Global Surveyor and Mars Odyssey, seemed to back this up. In 2002, patterns in gullies seen from orbit made it look as if water was leaking onto the surface, although some researchers now point out that these may have been caused by dust avalanches.

Then there were the observations from the neutron detector instrument aboard the Mars Odyssey orbiter, which spots neutrons emitted when water molecules buried up to 1 metre deep are struck by cosmic rays. It showed large permafrost ice fields perhaps just a few centimetres beneath the surface in the northern plains. It seemed inevitable that underground lakes of ice or even liquid water lay waiting to be discovered, and the instrument that was going to do it was MARSIS. Designed to probe more than a kilometre beneath the surface of Mars, its radar beams would bounce back from the boundaries between alternating layers of rock and water to build up a map of this network of underground lakes.

But so far, MARSIS has found no evidence of large quantities of ice hidden underground. "We have no confirmed detections yet," says Roberto Orosei of the Institute of Space Astrophysics and Cosmic Physics in Rome, Italy. He points out that the analysis is ongoing and that the water might still be there, but deeper than MARSIS can penetrate.

Forget's work might throw another spanner in the works for the underground aquifer theory. In his simulations, when Mars rights itself following a period of high obliquity and the sublimed ice moves back to the poles, some of it tends to fall back to the ground in the mid-latitudes, around 60 degrees from the equator. This was exactly where the Mars Odyssey neutron detector showed large quantities of ice in 2002. This suggests that if large quantities of water are being moved around Mars, it could be via the atmosphere rather than an interconnected network of underground lakes.

It's a controversial notion. "There isn't much water in the atmosphere of Mars presently, so you have to be sceptical about this idea," says Peter Smith of the University of Arizona's Lunar and Planetary Laboratory in Tucson, and principal investigator of Phoenix, NASA's next Mars lander. Instead, he still favours the idea that vast quantities of water passing through the outflow channels could have supplied the northern plains with their quota of ice.

Phoenix could settle the matter. It is going to land smack bang in the ice fields of the north, found by Mars Odyssey. Once there, it will begin digging down to around 1 metre in its search for ice.

If it succeeds, Phoenix will be able to measure the ratio of hydrogen to its isotope deuterium in the water molecules. It will also perform the same measurement for the water vapour in the atmosphere. The result could prove decisive because, if the ice plains were built by the atmospheric transportation of water vapour, then these ratios will match. If however, the ice in the plains was deposited billions of years ago by the outflow channels, then it will have a very different isotope ratio, dating from a time before the erosion of the planet's atmosphere. Either way, the result will confirm that Mars has a complex history all of its own, marking it out as distinctly different from our world, and all the more interesting because of it.

And the excitement may not be over yet. Tempting as it is to think that Mars has ossified into a perpetually desolate state, Neukum does not rule out the possibility of a new volcanic episode. "The interior of the planet is not cold. The planet could become active again," he says.

If he is right, rising magma will thaw any hidden water and drive it to the surface once again. When this tremendous event might happen, no one knows. It is highly unlikely to be in our lifetimes or those of our children or grandchildren. Nevertheless, the chance remains that future planetary scientists may see with their own eyes the lakes and seas that today's researchers can only dream about.

Stuart Clark (www.stuartclark.com) is a science journalist based in the UK. His latest book, Deep Space, is published by Quercus

From issue 2657 of New Scientist magazine, 23 May 2008, page 35-39

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