IN SPACE, a year can be a long time. Back at the beginning of 2004, the idea that the Red Planet had once been covered with rivers, lakes and seas was just a theory. Now abundant evidence on the ground has turned it into established fact. A year ago it was scientific heresy even to talk of the possibility of life existing today on Mars. But with the proof of past water, plus evidence that there was methane in the air not so long ago, it is now a subject for serious discussion.
It could so easily have been otherwise. After a year of stunning successes by NASA’s twin Mars rovers Spirit and Opportunity, it is hard to remember just how much tension surrounded the two landings. The missions were put together in three years – an exceptionally ambitious timescale – and the 150 NASA scientists and engineers involved had hardly a moment’s rest during that hectic time. When the craft arrived at Mars, a complex sequence of events had to work perfectly, with split-second timing. Each craft had to hit the atmosphere right, deploy its parachute, turn on retrorockets and inflate airbags just before touchdown. The consequences of failure of any one of these were all too apparent with the loss of the European Beagle 2 probe just days earlier.
First to arrive was Spirit on 3 January 2004, followed by Opportunity three weeks later. Hurtling into the Martian atmosphere, the craft were completely beyond the control of the team in Pasadena, California. Radio signals took 10 minutes to reach Mars – far too long to allow commands from Earth to correct anything that might have gone wrong. More than half the probes NASA had previously sent to Mars had failed. All the team could do this time was listen for the musical tone that would signal a successful transmission from the surface – and proof of an intact landing.
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When the tone finally came through, there was pandemonium in the mission control room. Chief scientist Stephen Squyres first sank to his knees and then hugged everyone in sight. Both Spirit and Opportunity’s landings were virtually flawless.
It has pretty much been that way ever since. Everything has worked better than expected, with just a few glitches along the way to keep it from looking too easy. The success of the missions has been far beyond what any of the team’s scientists could have dared hope for.
Designed to last for 90 days, both rovers have now gone about four times that long and show no signs of stopping any time soon. In fact, with the Martian winter over and dust mysteriously getting cleaned off the solar panels, the rovers are actually gaining power and should continue to do so for a few more months (see “Keep on roving”).
And the places they have been scouting out get more and more interesting. Before this mission, Martian bedrock had never been observed close up. Now it has been seen repeatedly at both landing sites: on the walls of craters at Opportunity’s site in Meridiani Planum, and in the hills that Spirit is now climbing in Gusev crater, on the other side of the planet from its twin. The rovers’ findings are proving a geologist’s dream. As hoped, clear stratification – the layering that records geological history – can be seen in the rock. And those strata have answered the question that has driven Mars exploration for the last two decades: was Mars once a wet planet?
Both landing sites had been specifically chosen in the hope they would yield evidence for water. But the proof didn’t come quite as expected; nothing in planetary exploration ever does.
Earlier images of Gusev crater taken from the air seemed to scream with evidence of an ancient lake bed. The NASA team had hoped Spirit would find equally clear signs on the ground: things like the sort of rounded pebbles or boulders found in terrestrial river beds, ripples in the sand, or rocks built up over time by the deposit of sediments from water. But no such luck. The first few months at Gusev produced not one clear indicator. All Spirit’s cameras could see was volcanic rock and wind-blown dust in every direction, even in the subsurface layers inside craters. Basalt, basalt everywhere, and not a drop of water in sight.
“Opportunity’s results represent a sea change in how we see Mars. The evidence for water is compelling and unambiguous”
Luckily the view did not remain so bleak. Almost from day one, the team was itching to get Spirit over to nearby hills which, from a distance, showed hints of stratification. After six months of hard driving, the rover reached Columbia Hills, named in memory of the ill-fated space shuttle and its crew. And everything changed. Crossing a line as sharp as if it had been drawn with a ruler, though there was no obvious visual difference, Spirit moved from pure, hard basalt to an area where every rock seemed to have been altered by water.
Squyres and other team members suggest that the layered deposits they saw on these rocks could have formed from volcanic dust that either mixed with water vapour or acquired a thin coating of water at some later stage. What strengthened the case for water was the fact that the spectrometer detected high concentrations of sulphur and chlorine, elements that are soluble in water. The amounts of each mineral detected matched what was expected from their solubility, suggesting that the minerals were repeatedly leached out and redeposited by water that may have been around only briefly and in small amounts.
“It’s a water story, but not the water story we were looking for,” says Squyres. As Spirit climbs further into the hills and thus into different geological epochs, the story could change again.
On the other side of the planet, the signs of water that led to Meridiani Planum being chosen as a landing site were much subtler, making it more of a gamble. The decision was based on spectral measurements by the orbiting Mars Global Surveyor, which showed an unusual concentration of haematite, a form of iron oxide. On Earth, haematite usually forms in water, but not always; one type forms at the high temperatures found in volcanoes. If Opportunity had found that, the whole site might have had nothing to do with water at all.
But the gamble paid off. Signs of water jumped out almost immediately, and just six weeks after Opportunity landed, NASA held a press conference in Washington to announce definitive proof that water once existed on Mars.
Nobody knew what the haematite would look like, or what kind of form it might show up in. It turned out to be the main component of the most obvious and unusual thing in the whole scene: tiny spherules that littered the plains as far as the camera could see, as if hundreds of strings of pearls had broken and scattered their beads across a flat floor. The spherules, which the team called blueberries, showed clear signs of having been built up layer by layer within sediments, just like the pearls in our oceans.
“Mars is not the same everywhere. Some sites are way more interesting than others. Please, no more dull Viking or Pathfinder lander sites!”
One by one, every other non-watery explanation for such perfect spherules was ruled out by a combination of spectroscopic and morphological results. What eventually won over even the most diehard sceptics were images of the random way the beads were embedded in the layers of rock rather than sorted by size, as they would have been by wind or waves.
Mars, it becomes clearer all the time, is a very different place from Earth. Rather than being a standing sea slowly accumulating sediment, Meridiani seems to have the characteristic chemistry of transient lake beds in a terrestrial desert, or salt flats on a seashore. Lakes appeared and then dried out, leaving a flat expanse of salty residue. The cycle repeated over and over again, building up a series of crusty, crumbly layers.
The minerals in the rock show that the water was not just salty, it was also extremely acidic. The environment was similar to old mines on Earth that are contaminated with sulphur and acid, says Jeffrey Kargel of the US Geological Survey in Flagstaff, Arizona.
The vast, flat plain of Meridiani has also revealed characteristic patterns associated with waves in a shallow body of water. The upturned lines that look like little smiles are quite unlike the shapes produced by sediments laid down by wind. But these patterns present a bit of a puzzle: unlike Gusev crater, where the raised rim would have allowed a deep lake to form, Meridiani Planum has no apparent edge to the north, nothing to keep water from draining away.
Perhaps the ancient shore has simply eroded away. But there is another possibility. Some researchers, including Tim Parker of NASA’s Jet Propulsion Laboratory in Pasadena and Jim Head of Brown University in Providence, Rhode Island, have suggested that much of the Martian northern hemisphere was once a vast ocean. Some features around the northern plains appear similar to terrestrial seashores, while the plains themselves are among the smoothest areas ever found in the solar system. Meridiani lies on that ancient “shoreline” and may once have been a continental shelf, cycling between dry and wet phases as the ocean’s level changed.
Opportunity then headed for a 190-metre-wide impact crater called Endurance. The layers of exposed rock on the crater’s slopes offer an extensive geological record, and in the 20-metre-deep layers it examined the rover found every one contained essentially the same water-deposited stuff it had found earlier in the tiny Eagle crater where it landed. It is always blueberries in a sandy matrix, and always rich in salts. “Everything so far looks like it was deposited wet, and dried out later,” says Squyres. “That surprised me.”
The environment must have been watery for a significant length of time, though making firm estimates is still impossible. Assuming the layers built up from fresh sediment every year, Kargel estimates that the layers in Endurance crater must have taken at least 250,000 years to form. And there is evidence of a later episode of water as well, in the material that filled cracks between broken layers of rock.
“Haematite is like a beacon saying ‘Land here!’and we did, and were very favourably surprised”
Opportunity went as far down Endurance crater as it dared before emerging in mid-December, but it may still have a chance to study even deeper sediments. It is now on its way toward sloping, eroded terrain a few kilometres south of the crater. Here it should encounter quite different geological formations, and perhaps they will reveal something of the conditions that preceded the shallow rolling seas. The rover might even find clues to some of the massive climate changes in the Red Planet’s past.
Now that we know there were long-lasting bodies of water, planetary scientists are starting to tackle the big question: what about life? Water is a prerequisite for life as we know it, and the rovers’ discoveries have improved the odds that life might have had a chance to get started. Every indication on Earth seems to show that, once life starts, it is awfully hard to keep down: the most inhospitable places are almost always found to harbour some kind of hardy organism.
So could there be living things on Mars today? Ever since experiments carried out by NASA’s two Viking landers failed to find convincing signs in 1976, few scientists have dared to risk their reputations by talking seriously about the possibility, or how to test the idea. Yet not everyone viewed the Viking findings as a failure.
Gilbert Levin was the lead scientist for one of Viking’s “life detectors”, which sniffed for gases given off when a nutrient soup was added to samples of Martian soil. He has always maintained that the results of the experiment can only be explained by living organisms in the soil metabolising the nutrients, and that evidence in recent years has only strengthened his case. Needless to say, his findings were hugely controversial, with most scientists insisting that the gases came from highly reactive chemicals in the soil. Yet Levin says that a new mathematical analysis of the original experiment has revealed that more gas was released during the day than at night, and he hopes to publish the results shortly. Could this perhaps be proof of organisms on the surface feeding during the warmth of day and resting at night? Few have taken Levin’s claims seriously, but that may change now that Mars’s wet past has been revealed – and that there are possible signs of bacteria breathing.
“We looked at the pictures NASA took with filters and compared them to spectra of liquid water, and they match”
Over the last year three teams, one using instruments on the Mars Express orbiter and two using ground-based telescopes, say they have detected methane in the Martian atmosphere. Back in the 1960s, when Viking’s experiments were being designed, some scientists suggested that finding methane would in itself be strong evidence of life. As tantalising as the detection is, more analysis is needed to pin down the source of the gas, with geothermal effects and comet impacts still a possibility (see “A whiff of life”).
Nowadays nobody is laughing off the idea of Martian life, and that change in perception could affect the design of future Mars probes. Given what we now know, we must assume that Martian life exists until we learn otherwise, Kargel wrote in Science last month (vol 306, p 1689). Any craft that lands on Mars, or might crash there, must be carefully and thoroughly sterilised lest it contaminate the planet with earthly microbes, rendering it impossible to distinguish between domestic and imported organisms.
It is too big a question to mess up. If life did evolve independently on Mars, its discovery “would complete the Galilean revolution that removed Earth and its life from the centre of the universe”, says Kargel. And if it didn’t, despite having an environment very similar to Earth’s, then our planet “might be seen as the only land of the living for light years around”. Either way, it’s heady stuff.
Thanks to the flood of new information, people like Kargel are daring to use the L-word in public when talking about Mars. It is likely to dominate the study of the Red Planet for many years to come.
