Shape up: did liquid water or carbon dioxide gas sculpt the martian landscape? Credit: NASA/HUBBLE HERITAGE TEAM

When it comes to deciding whether or not liquid water once flowed over the surface of Mars, most geologists are happy to believe the evidence of their own eyes. The surface of the red planet is marked by channels and gullies that are virtually identical in form to water-created landscapes on Earth1. As the saying goes, if it looks like a duck and quacks like a duck, it must be a duck.

But when Nick Hoffman, a geophysicist from La Trobe University in Victoria, Australia, discusses the arguments for water on Mars, he rounds off his presentations with an image of a duck-billed platypus. It is a ruse that draws chuckles from planetary scientists, but Hoffman is not joking. According to his hypothesis, water has had little or no importance in shaping the martian landscape. Instead, his 'White Mars' model contends that gurgling, frothing and sometimes explosive venting of carbon dioxide gas from beneath the surface created the features.

With 25 years of Mars research focusing on water, Hoffman is having to work hard to get his ideas taken seriously. His initial publication, in August last year2, raised eyebrows but won him few converts. Since then, however, his theory has gained momentum. Researchers who saw him present his ideas on a recent tour of the offices of NASA and the US Geological Survey say they were impressed. And data from NASA's Mars Odyssey orbiter, which will arrive at the red planet next week, should add to the debate. Scientists might not want to give up on the liquid-water model, but Hoffman's hypothesis seems to be emerging as a credible rival.

Don't duck the issue: Nick Hoffman is challenging his peers to abandon their assumptions about Mars.

“I would say that Nick has generated an outrageous hypothesis,” says Jeff Kargel of the US Geological Survey in Flagstaff, Arizona. “But I don't mean that in a negative way. The outrage is to an existing theory. Even if it's totally wrong, it makes you think about Mars in a different way.”

According to conventional wisdom, Mars has witnessed at least two periods during which liquid water existed on its surface3. Present-day Mars, where average temperatures are below freezing and the atmospheric pressure is less than 1% of that on Earth, is incapable of supporting liquid water. But frozen water is present on and below the surface, and the atmosphere contains a small amount of water vapour. If the martian atmosphere had been denser in the past, the greenhouse effect could have allowed liquid water to flow across the surface and carve out the distinctive valleys and channels.

The first of these wet periods is thought to have occurred around four billion years ago. Valleys, similar to those created on Earth by rivers, criss-cross areas of Mars that are believed to date from this time.

At least one similar period is thought to have occurred since then, probably around one billion to three billion years ago. Mars' vast flood channels, which are orders of magnitude bigger than those on Earth, probably formed during this time. Researchers think that water flowed from equatorial regions towards the planet's northern hemisphere, although the source of the water and the details of the water cycle remain unclear.

What is clear is that Mars must have gained and lost an atmosphere on at least two occasions for these wet periods to have occurred. How this happened is the subject of speculation, but researchers have identified several potential mechanisms3. Heat-trapping gases, mainly carbon dioxide, are thought to have been released from frozen underground stores by surges of heat from inside the planet. But the resulting atmosphere may have been inherently unstable, with carbon dioxide being removed and trapped in rocks and ice. A change in Mars' magnetic field may also have played a role, exposing the planet to the stream of charged particles known as the solar wind and allowing it to strip away the atmosphere.

Martian mimic: in the 'White Mars' model, explosions of gas created gullies and channels that resemble those generated by water on Earth. Credit: NASA/JPL/MALIN SPACE SCIENCE SYSTEMS

The images of gullies and channels, together with suggestions as to how the planet could have gained and lost an atmosphere, provide a good argument for the presence of liquid water on Mars. But not all of the data fit with the idea. NASA's Mars Global Surveyor, which is currently in orbit around the planet, has generated many new images, some of which have been touted as evidence of water on Mars. For every new piece of this evidence, however, there are complications.

Whither water?

Mars Global Surveyor has, for example, been unable to detect any carbonate minerals — a necessary product of liquid water interacting with Mars' abundant carbon dioxide2. The craft's high-resolution images have also failed to reveal the expected finer features of water, such as the tributaries that would have fed the larger drainage features, and the shorelines of putative lakes and oceans4. And the flood channels lack obvious sources, as there is no evidence of large lakes or other reservoirs at the heads of the channels3. “There are a lot of problems with the water model,” admits Michael Carr of the US Geological Survey in Menlo Park, California.

Problems like these leave room for debate, and Hoffman, for one, believes that the liquid-water model needs replacing. In his theory, Mars may always have had a cold, low-pressure atmosphere. Rather than being shaped by water, he claims that the distinctive features were formed by eruptions of carbon dioxide.

In the case of the flood channels, Hoffman proposes that the process began when steep slopes on the martian surface were heated from inside the planet. This melted underground deposits of frozen carbon dioxide, which is thought to make up around 5% of the upper layers of Mars. The melting weakened the slopes, which cracked under their own weight, exposing solid carbon dioxide within the rocks to the atmosphere.

Hoffman compares the creation of these cracks to the opening of a can of fizzy drink that has been violently shaken. Because the pressure of the martian atmosphere is so low, the carbon dioxide would have flashed violently into a gas, bursting from the rocks as the fizzy drink bursts from the can.

Together, the cracking of the slope and the explosion of carbon dioxide would have created an avalanche of gas and tumbling debris. Rocks within this turbulent flow would have ground against each other, exposing more carbon dioxide stores and prompting further explosions. The flow would eventually have ground to a halt when the stores of frozen carbon dioxide were exhausted, by which point, Hoffman argues, it could have travelled thousands of kilometres.

These frothing flows may sound odd, but Hoffman backs them up with established facts about the planet. Orbiting spacecraft have directly observed large amounts of solid and gaseous carbon dioxide on Mars3. Except for some ice at the poles and a tiny amount of water vapour in the atmosphere, the same cannot be said of water. And the melting and exploding of carbon dioxide can be explained in terms of the pressures and temperatures of the present-day martian atmosphere2.

Closer to home

Such facts make Hoffman's ideas seem plausible, but they offer no evidence as to whether the flows actually occurred. It is this gap in the story that has generated the most scepticism. But doubt is starting to diminish now that Hoffman has presented his ideas on what he claims are analogous processes on Earth.

Volcanic eruptions provide one example. Hot ash and lava can travel tens to hundreds of kilometres on a cushion of gases released from volcanoes. Undersea avalanches can also trigger flows of mud, sand and boulders which, supported by the surrounding water, can travel thousands of kilometres.

These analogies seem to have helped Hoffman's cause. “He made a major forward stride when he gave a talk at the US Geological Survey and showed a submarine flow,” says Kargel. “It's something that we can't dismiss.”

Hoffman's ideas may also explain an aspect of the martian landscape that has stumped advocates of the liquid-water model — the absence of headwaters at the sources of the flood channels. The source areas look like blocky, jumbled landscapes where the underlying ground suddenly lost volume and crumbled. Other researchers have shown that pressurized carbon dioxide exploding out of the ground could have left behind just such a landscape5.

Out in the cold

Since his original publication, Hoffman has extended his model to include another martian feature that is commonly associated with water — the gullies seen in the walls of craters. Michael Malin and Kenneth Edgett of Malin Space Science Systems in San Diego, California, attracted substantial press interest last year when they argued that the gullies, which are found in high-latitude craters in the planet's southern hemisphere, look sharp-edged and newly cut — perhaps just years or tens of years old6. Malin and Edgett suggest that the gullies are the result of very infrequent conditions that allow frozen ground-water to seep from the tops of the craters.

But Hoffman says that this theory shows just how entrenched and irrational the liquid-water model has become. “We know from the atmosphere of Mars that there are not multiple sources pumping water vapour into the atmosphere all over the southern hemisphere,” he says. He also points out that it is difficult to explain why the gullies are found only on the colder, poleward-facing slopes of craters instead of on the sunnier sides, where there would be more energy available to melt the underground water.

Hoffman's explanation, which is similar to one by Donald Musselwhite and colleagues at the University of Arizona in Tucson7, contends that the gullies were etched out by a miniature flow of gas and debris akin to that which created the flood channels. In his model, the process begins when sunlight heats up the carbon dioxide snow. Just like water snow on Earth, this melts from the bottom upwards, because sunlight shines through it and warms the ground underneath. On Mars, the melting generates carbon dioxide gas, which builds up under the snow, eventually bursting out when the pressure becomes too great. “If you were there you'd see this thing go by like a little miniature avalanche,” claims Hoffman. “It's a fluidized flurry of debris and gas boiling down the hillside into a channel.”

He backs up his argument by showing a series of images of the same gully in different seasons. In winter, the whole scene is covered in carbon dioxide snow. Dark patches appear as spring nears and the snow sublimes away to reveal the ground. But at one critical point, the shaded gullies can been seen cutting across the thinning snow. That, says Hoffman, is the moment at which they are active. The gullies appear only on poleward-facing slopes, he argues, because more snow accumulates there. The snow on the southern slopes, in contrast, is too thin to generate avalanches.

Staking a claim

What a gas: Victor Baker says it would be amazing if carbon dioxide created Mars' features. Credit: JAY M. ROCHLIN, UNIV. ARIZONA ALUMNI OFFICE

As Hoffman develops his arguments, planetary researchers are beginning to admit that they cannot simply dismiss them. “No one could come up with an argument that could really drive a stake into the heart of it,” concedes Carr, who heard Hoffman discuss his ideas at a meeting in August at NASA's Lunar and Planetary Institute in Houston, Texas.

“If water wasn't involved and it was all created by carbon dioxide, that would be an amazing discovery,” agrees Victor Baker of the US Geological Survey's Flagstaff office. But, he cautions, it would also entail an astounding mimicry of water-created features.

Hoffman suggests that some of the resistance stems from a desire to view Mars as an Earth-like place. In the early twentieth century, the idea that Mars was criss-crossed with canals made by intelligent beings was popular. Ever since then, says Hoffman, people have been viewing the planet through “blue-tinted glasses”, writing about and studying a Mars defined by terrestrial imaginations, rather than the Mars it actually is.

But others in the field are less impressed with Hoffman's ideas, and say that their objections are based on science, not tradition. “It doesn't strike me as plausible,” says Bruce Jakosky, a planetary scientist at the University of Colorado in Boulder.

Big picture: Bruce Jakosky argues that the liquid-water model provides a more coherent overview. Credit: FRAN BAGENAL

He points to good evidence that Mars contains a substantial amount of water3 and that analysis of some martian meteorites8 shows that it may have had periods of warm and wet climate. Water vapour in the atmosphere contains more deuterium, a heavier isotope of hydrogen, than water found on the surface, as water molecules containing the heavier isotope are less likely to be lost to space. But analysis of some martian meteorites suggests that these higher levels were also present during some periods on the surface, indicating that atmospheric water vapour may have condensed and fallen as rain to the surface.

Perhaps Jakosky's most important criticism is that Hoffman is attempting to explain individual features one at a time, rather than incorporating his explanation into a larger model, as has been done with the liquid-water theory. “The history of the atmosphere, history of geology, history of the crust — that's the way to do it,” says Jakosky.

At the moment, it seems that both models fit with the information available. Those data, however, are likely to be significantly enhanced in the coming year, as Mars Odyssey starts its work, looking for water-altered minerals and analysing the composition of volcanic rocks. All of this could say a lot about the role, or lack of one, that liquid water had in sculpting the planet's surface.

Hoffman says his theory would be bolstered if Mars Odyssey fails to find large amounts of carbonates. But not finding these minerals is unlikely to make researchers drop the liquid-water model. Like much of the data, analysis of carbonates provides only circumstantial evidence. And because definitive experiments, such as drilling deep into Mars in search of liquid water, are technologically and financially unfeasible, the arguments seem set to run and run.

In the meantime, Hoffman may have to be content with using his duck-billed platypus to persuade planetary scientists to re-evaluate their theories. Whether or not his outrageous hypothesis is correct, pushing researchers into this unfamiliar territory will benefit everyone's understanding of Mars. And that, for most, makes his ideas worth considering.