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Phoenix uncovers history of liquid water on Mars
13 August 2008
From New Scientist Print Edition.
Rachel Courtland

Enlarge Phoenix – the mission of surprises

Ever since NASA's Phoenix lander gently touched down on Mars's bleak northern plains on 25 May, it has been unearthing tantalising hints about the planet's potential for life.

News reports have been full of its baby steps: the probe's first tentative scoops of soil, struggles with oven doors on one of its instruments, and efforts to unload sticky, icy soil for testing. Then last week, the Phoenix team announced a surprising discovery: they had detected the chemical perchlorate in two samples of Martian soil.

The announcement was prompted by a stir surrounding rumours that the Phoenix team had briefed the White House about life on Mars. The rumours were wrong, but after all the confusion, where do we really stand?

The picture is mixed. Some say the perchlorate could be a boon for studying the Martian climate, as its abundance and location may help clarify whether liquid water there once interacted with the atmosphere. However, the implications for life, past or present, are both good and bad.

Phoenix was commissioned to study the planet's water history and assess whether it is or was once habitable. It is the sixth successful mission to make it to the Red Planet (see box), and the first to find water ice. Many planetary scientists anticipate the data Phoenix is collecting could revolutionise our understanding of the planet. More big findings may be yet to come, as the Phoenix team plans to use a 40-day mission extension to employ the lander's contingency instruments to test more soil, and possibly pure water ice.

The perchlorate was found by a suite of instruments on Phoenix called MECA - the microscopy, electrochemistry and conductivity analyser. This uses "wet chemistry" to detect ions in the Martian soil. The first perchlorate signal came from a sample of soil taken from the top 2 centimetres of a trench that Phoenix dug dubbed Rosy Red.

Perchlorate ions comprise one chlorine atom and four oxygen atoms and are highly soluble in water, so deposits are common only in dry areas, such as the Atacama desert of Chile - often used by researchers as an analogue of Mars on Earth.

The discovery could be an important piece of the Martian habitability puzzle, and is especially timely as NASA is planning to send a 2016 mission explicitly to look for evidence of past or present life. This would be the first such project since the Viking missions of the 1970s.

However, sending a dedicated astrobiology mission depends on finding an appropriate site. NASA's advisory Mars Exploration Program Analysis Group says the best locations must have three qualities: evidence of liquid water, nutrients, and an energy source.

The perchlorate find is a boost in that some Earth bacteria use it as an energy source. "It's like a mineral form of pure oxygen," says Phoenix principal investigator Peter Smith of the University of Arizona in Tuscon. However, it may have a downside.

In the coming months, team member Carol Stoker of NASA's Ames Research Center in California will use Phoenix data to make a firm estimate of the probability of finding life at the site. Her approach is based on the famous Drake equation, which estimates the abundance of technological civilisations in the universe, based on various prerequisites for their existence. The Phoenix site already ranks higher than all previously explored sites, largely because it has water ice, but Stoker says the site's top billing may not survive.

It all depends on the levels of perchlorate, since too much can be toxic. Phoenix team members are still estimating the concentration, but preliminary results suggest there is too much for life. "It's at apparently very high concentrations, sufficiently high that there's a question of its toxicity," says Stoker.

Even if the site turns out to be uninhabitable, it could still yield a bounty of information about the recent history of the planet. "So far in my mind, they haven't released what will be the major results of this mission," says Bruce Jakosky, a planetary scientist at the University of Colorado in Boulder. "This is the first time we've been able to get access to ice near the surface, and this ice has the characteristic that it may have been liquid water in the recent geologic past."

At present, the Phoenix landing site is too cold to support liquid water, but that may not always have been the case. The tilt of Mars's axis varies widely over millions of years, in part because the planet lacks the stabilising presence of a large moon. Sometimes the sun shines mostly over the equator. Other times, it heats the polar regions, changing the global distribution of ice.

No one knows how this obliquity cycle affects buried ice, which may be insulated by the Martian soil. At the Phoenix site this soil layer is just centimetres thick, and might have been wet during warmer times. Phoenix aims to investigate this possibility by measuring salts and looking for the presence of clays, which can only form where there is liquid water.

Perchlorate might also provide evidence of the past presence of liquid water. Because the chemical is so soluble, liquid water might concentrate perchlorate at lower depths, where the water would collect. Phoenix is poised to look for exactly such differences. So far it has been digging on top of a pentagon-like hillock, where ice sits roughly 5 centimetres below the surface. But now the lander has moved to the edges of the pentagon, where soil is expected to be much deeper.

Some researchers say the lander's true value may not be in finding evidence of liquid water from millions of years ago, but in looking at the interaction of the ice with the present-day Martian atmosphere. Mars is thought to have a complex water cycle, in which ice vaporises at the polar caps in the summer and condenses elsewhere directly onto the ice underneath the soil. Phoenix's Thermal Evolved Gas Analyzer instrument will measure certain isotope ratios in the ice from the site, which may reveal whether it actively exchanges water vapour with the atmosphere. If it does, it would bolster the idea that this cycle dominates Mars's atmospheric chemistry.

"Where this mission is poised to make the best contribution is more insight about the atmosphere," says David Des Marais of the Ames centre. He notes that trace elements in ice might indicate the presence of previously undetectable materials, which could be a sign of ongoing volcanic activity.

Phoenix is also using its laser rangefinder to measure the altitude of clouds and dust in the atmosphere. Ultimately the results could help build better atmospheric models for Earth as well as Mars, says Ray Arvidson of Washington University in St Louis, Missouri, team leader for Phoenix's robotic arm.

From issue 2669 of New Scientist magazine, 13 August 2008, page 6-7

Will Phoenix rise above previous missions? Phoenix is the first Mars probe to find water ice, and may reveal much about recent conditions on the Red Planet. Can it match the successes of five previous missions that have built up a picture of what Mars was like in its youth? Vikings 1 and 2 were the first NASA missions to the surface, landing on 20 July and 3 September 1976 on the northern hemisphere of the planet. The landers gave the world its first high-resolution panoramas of Martian terrain, and tested for evidence of life, though they found none. Mobile exploration of the planet's surface began on 4 July 1997 when Mars Pathfinder landed in the Ares Vallis region, carrying an 11-kilogram rover called Sojourner. It found semi-rounded pebbles, bolstering the idea that the area was in fact an ancient floodplain. NASA's Spirit and Opportunity rovers, which landed in January 2004, are still operating and have collectively traversed more than 19 kilometres. The rovers have snapped a wide range of geological features, including explosive volcanic deposits and "blueberries" - haematite spherules that may have formed in ancient pools of water. Not all Mars landings have been soft though. NASA's first polar probe, the Mars Polar Lander, was meant to explore the planet's southern pole but crashed on 3 December 1999. The next mission slated to reach the Martian surface is NASA's Mars Science Laboratory, which is scheduled to launch in September 2009. The lab is a beefed-up rover that will be powered by radioactive fuel.

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