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Mars: What flows beneath
25 July 2007
NewScientist.com news service
Ivan Semeniuk

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Peter Smith has a personal hope about what he'll see when he gazes out once again across the windswept northern plains of Mars - or rather, what he won't see. He hopes it won't look like the view from his home town, which is what confronted him back in 1997. As a planetary scientist at the University of Arizona, Smith was running the stereo camera on board the NASA Mars Pathfinder mission, and was struck by how similar the panoramas coming back from the Pathfinder landing site were to the terrain near Tucson where he grew up. "It looked like Arizona without the cactus," says Smith. "Next time I'd like to land somewhere that really does look alien."

Smith will soon get another chance. These days he is principal investigator for NASA's Phoenix mission, which heads to Mars next month and is scheduled to land there in May or June of 2008. If it succeeds Smith will be delighted, no matter how featureless the landscape. The odds are that Phoenix will show us a side of the Red Planet completely unlike anything we've seen before. It could also be the first spacecraft in history to explore the habitable zone of another world.

Latitude is what separates Phoenix from the five successful Mars landings that have preceded it. Unlike Vikings 1 and 2, Pathfinder and NASA's indefatigable Mars rovers, Spirit and Opportunity, the Phoenix mission is designed to touch down above the Martian arctic circle during the northern summer. Operating under perpetual sunlight, the lander will use its robot arm to scrape away at the frigid dust of a northern desert so broad and bland that planetary cartographers have christened it Vastitas Borealis, or "widespread northern lowlands" (see Diagram).

If NASA's predictions are correct, Phoenix will strike Martian permafrost, achieving the first of its mission objectives. No one will be surprised when this happens, but the moment will still be a huge milestone in Mars exploration. After years of debate about how much water Mars had in the past and where it all went, Phoenix should finally bring that water within researchers' grasp. "'Follow the water' has been the mantra of Mars exploration," says Smith. "But no one has got near any water yet. We're actually going to touch the water and do analysis on it."

It is tantalising to imagine what that analysis could reveal. At the very least, Phoenix will validate decades of modelling and remote sensing that suggest Mars has a vast store of water locked away as ground ice just below the surface of its polar reaches.

More exciting still is the mission's second objective: searching for signs of habitability at the interface between Martian soil and the underground ice. This will be particularly interesting if, as some researchers suggest, that ice has not been inert for the past 3 billion years but instead melts from time to time just enough to create a water-soaked subterranean environment where Martian microbes can flourish between epochs of icy slumber.

This scenario is not as far-fetched as it may seem. Today Mars's poles are tilted by 25.2 degrees with respect to the plane of its orbit about the sun - remarkably similar to Earth's own 23.5-degree tilt and likewise small enough to keep its polar regions permanently frozen. But while Earth's orientation is stabilised by our large moon, the tilt of Mars is surprisingly variable.

Calculations reveal that the Red Planet has undergone significant shifts in the past, with a tilt that has averaged closer to 45 degrees over the past 20 million years. Such extreme shifts should periodically warm the higher latitudes and vaporise ground ice near the surface. Under the right conditions, liquid water might temporarily appear in thin films between ice crystals and mineral grains.

It is a far cry from the raging floods that are thought to have carved the now dry river valleys of Mars billions of years ago, but it could be just enough to sustain a colony of Martian bacteria from the past. Such bacteria would remain dormant most of the time, but spring into action when the planet's tilt warmed the ice.

Even if only a residue of former microbial life exists in the northern soils of Mars, Phoenix is well equipped to spot it (see Diagram). Not since the Viking missions of the 1970s has there been a lander on Mars capable of detecting organic molecules. The Viking landers surprised researchers when they discovered that the top 10 centimetres of Martian soil contain oxidants that would sterilise any life there by breaking down organic molecules. Phoenix is built to get under this sterile layer with a robotic arm that can dig down half a metre in search of a friendlier environment for carbon-based chemistry.

Material scooped from the subsurface will be placed in Phoenix's Thermal and Evolved Gas Analyzer experiment (TEGA), which contains eight miniature ovens that can bake small particles of soil in a controlled fashion. As the temperature of the sample rises to up to 1000 °C, any gases the soil emits will be identified by a mass spectrometer. Thus, TEGA will directly sense for the first time the presence of water, plus any organic compounds that may be there as well.

Significantly, it will be able to spot a class of highly stable chemicals known as polycyclic aromatic hydrocarbons (PAHs), the simplest of which is anthracene, used to make insecticides. Studies suggest that some of the more volatile organics associated with biological activity will slowly degrade into PAHs given enough time. "We're looking to see if conditions are hospitable enough for organic molecules to survive," says Bill Boynton, who developed and oversees TEGA at the University of Arizona in Tucson.

Boynton stresses that neither PAHs nor any other organic molecules would be unambiguous evidence for life on Mars. After all, such molecules have been found in meteorites, and impacts could easily have deposited them on the Red Planet's surface. Still, the appearance of organic molecules in and around a ready supply of water would tick all the boxes for a habitable zone - a place where life as we know it might conceivably gain a foothold.

Even if Phoenix does not see organic material, it has the potential to reveal volumes about not just the history of water on Mars, but also the larger question of whether the Red Planet has ever been a haven for life. By looking at the physical transition between the soil and ground ice at its landing site, as well as ratios of isotopes within the ice, Phoenix will help pin down whether the ice represents the remains of an ancient ocean or an accumulation of water vapour transported through the atmosphere.

Once Phoenix lands, it will only be able to analyse what lies directly within its grasp. This may seem unfortunate, given the ongoing success of the Mars rovers, which landed in relatively safe locations and then trundled over to the most interesting geologic features. However, for Phoenix such mobility is irrelevant, says Smith. "Our mission is all about going down." That underscores another key feature that sets Phoenix apart from previous missions to Mars. By concentrating on digging rather than roving, it will effectively take the exploration of Mars into a third dimension.

Given that so much of the mission depends on landing within easy reach of ground ice, what are Phoenix's chances of doing so? The planners are confident: the presence of the ice has been hypothesised for decades, partly because climate models suggest it should be there and partly because images at high latitudes show canyons and crater rims whose soft edges suggest they have been worn down by ice. Even stronger evidence comes from the gamma-ray spectrometer on board NASA's Mars Odyssey orbiter. In 2002, Boynton and colleagues released the results of a planet-wide survey that indicated the presence of hydrogen-rich soil near the Martian poles. The only plausible way of storing hydrogen in the quantities inferred by Mars Odyssey is as water ice in the ground. In the polar regions, that ice should make up, on average, more than half the volume of the top metre of the planet's surface.

NASA has decided that a flat, boulder-free landing site due north of the giant volcano Olympus Mons offers Phoenix the best opportunity to make its mark. No site to the east or west offers a better chance of survival. Going further south risks missing the ice because of uncertainties about its extent, and going further north reduces the amount of solar energy available to the spacecraft.

Once the mission is under way, the pace will be gruelling. Phoenix will be the first Mars lander not to rely on direct contact with Earth; instead it will receive commands and relay its data through the three functioning orbiters currently circling the Red Planet. The information will then be sent to Earth, typically when Phoenix is approaching the end of its working day, and the polar sun is too low to offer much in the way of power.

At that point each day - Mars's day is just 40 minutes longer than ours - the science team will have about 8 hours to assess what the lander is telling them, hammer out their priorities and decide what it should do next. This will be complicated by the limited power available from the lander's solar panels and competing demands on its suite of scientific instruments. A second team will then have another 8 hours or so to translate these decisions into instructions for the spacecraft in time for the next uplink. "We'll be on a tight time line," says Chris Shinohara, who manages the mission's science operation centre at Tucson. "If we don't get stuff sent up, we'll lose that day on Mars."

Losing just a single day would be significant. Unlike the rovers, which are still working after more than three years on Mars, Phoenix will have a far shorter lifespan. Its nominal mission time is 90 Martian days. After that, a steadily dropping sun will make it difficult to keep the spacecraft's electronics warm enough to function. Eventually the sun will begin setting, and before long a perpetual polar night will settle in - the same pattern experienced near Earth's poles. Once darkness falls for good, Phoenix will be entombed in a seasonal layer of solid carbon dioxide taller even than the spacecraft's meteorology mast. "We definitely won't survive that," says Smith.

As on Earth, the polar regions of Mars are not easy on would-be explorers. In 1999, the ill-fated Mars Polar Lander went silent while trying to touch down on curious layered terrain found near the Martian south pole. An investigation later revealed a design error as the most likely cause of the failure.

Phoenix carries some of the experiments developed for Polar Lander, and will use the same landing system: its name is meant to symbolise a new probe rising from the ashes of the Polar Lander disaster. This continuity may give Phoenix a much better chance of success, since the system has now had seven additional years of testing and reworking. It has also strengthened the bonds between team members, some of whom have been working towards this mission for more than 15 years, says Shinohara.

In the science operation centre is a room that houses a fully functioning duplicate of Phoenix. During the mission this Earth-bound Phoenix will be used, among other things, to test approaches for digging in different kinds of Martian soil. Nearby are large sacks of fine red powder purchased from a California quarry, which will stand in for Mars dust. Shinohara explains that members of the public will be able to tour through the centre and watch activities while the mission is under way.

Even now the centre is a popular spot for visiting schools. In one open sack is the perfect outline of a child's handprint embedded in the red powder. The urge to touch another planet starts young. For some, like Smith and Shinohara, it never goes away.

From issue 2614 of New Scientist magazine, 25 July 2007, page 42-45

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