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Because the surface of Mars today is bone-dry and frozen all year round, it鈥檚 difficult to find any place on Earth that is truly Mars-like. But two locations, Antarctica鈥檚 Upper Dry Valleys and the hyper-arid core of Chile's Atacama Desert, come close. They have become magnets for scientists who want to understand the limits of life on Earth and the prospects for life on Mars.

Jocelyne DiRuggiero, an associate professor of biology at Johns Hopkins University in Baltimore, Maryland, studies samples from both locations. She鈥檚 interested in the similarities and the differences between the microbial communities that live in these two extreme desert regions. In both places, very little liquid water is present. In the core of the Atacama, years can go by between one rainfall and the next, but it is warm, so when there is precipitation, a significant amount of liquid water is available for a very short time. In University Valley, one of Antarctica鈥檚 Upper Dry Valleys, the availability of liquid water is limited in a different way. University Valley receives more regular precipitation than the Atacama, but it鈥檚 so cold there that any precipitation falls in the form of snow and remains frozen.

鈥淲hat we do in those environments is try to understand who is there, what those organisms might be doing, how they are distributed,鈥� and whether the organisms are 鈥渞eally active metabolically,鈥� or if instead they鈥檙e 鈥渏ust sitting there, because they鈥檝e been brought by the wind.鈥�

DiRuggiero鈥檚 primary tool is DNA sequencing. Working with soil samples that weigh one- to two-tenths of a gram each (about a teaspoonful), she extracts the DNA from any microbes present in each sample. She then sends the DNA off to a lab for sequencing.

Sample preparation is a difficult process because there aren鈥檛 many microbes in her samples. Each gram of soil contains perhaps one hundred to one thousand, an extremely low number. The same size sample of ordinary soil typically contains ten million to a billion organisms.

Because the microbial populations she鈥檚 working with are so small, contamination is a serious problem. She has to be careful not to let skin cells or hair fall into her samples. Sneezing or coughing on them could pollute them. So DiRuggiero does her work under a special hood that prevents contact with outside air. And even then she has problems, because some of the silica filters she uses to extract DNA from her samples arrive from the manufacturer with microbial cells clinging to them.

Although she has had more time to work with samples from the Atacama, DiRuggiero says the University Valley samples are particularly interesting. Because University Valley is both near the South Pole and more than a mile above sea level, the ground there stays frozen even in summer. There are few places in the world where this is true. 鈥淚t鈥檚 about 40 degrees Celsius colder than the Atacama soil,鈥� she says. That鈥檚 about 70 degrees Fahrenheit colder.

That temperature difference results in a significant difference in habitability. There are more microbes in University Valley soil than in Atacama soil.

鈥淩ight now the only parameter 鈥� we have measured that differentiates the populations, Antarctica and the Atacama, is the temperature,鈥� DiRuggiero says. In both locations, 鈥渢he soils are very dry, the soils are very low in organics, they contain a fair amount of salt. The big difference is the temperature.鈥�

鈥淲e don鈥檛 really know what it means yet.鈥�

It may seem odd that microbes are happier in sub-freezing conditions than in a warm desert. 鈥淭his is counter to human experience but makes sense for microbes,鈥� Chris McKay, a planetary scientist at NASA Ames Research Center in Moffett Field, California, wrote in an email. 鈥淐old allows them to sleep, which is a good survival mechanism,鈥� he explained, adding that 鈥渢his result bodes well for life in the cold deserts of Mars.鈥� McKay heads the NASA-funded IceBite team, which is testing a prototype coring drill for possible use on a future Mars mission. The IceBite team obtained the University Valley samples that DiRuggiero studies.

So far DiRuggiero has been working with University Valley samples collected during the IceBite team鈥檚 first season in the field, in 2009. She鈥檚 looking forward to getting her hands on more-extensive samples collected at the end of 2010, samples that are still making their way back from Antarctica.

Beneath the dry soil layer in University Valley is 鈥渨hat we call ice-cemented ground, which is basically frozen mud. And that mud has been frozen for thousands and thousands of years,鈥� says DiRuggiero. 鈥淪o the question is, Is there any water available for the micro-organisms, and do we see a difference in the microbial community between the soil above and this ice-cemented ground right underneath?鈥�

There is some evidence, based on climate data collected last year by the IceBite team, that at the interface between the dry soil and the frozen mud, 鈥渢here might be some melting in the summer,鈥� says DiRuggiero. 鈥淭here might be water available at least part of the time鈥� and microbes might be 鈥渁ctively growing and metabolizing at least during a small portion of the year.鈥�

鈥淢elting,鈥� in this case, doesn鈥檛 mean the soil gets soggy or muddy, or that the temperature gets above freezing. Rather, it means that thin layers of liquid water can form between the sand grains that make up the soil and the ice below it. But that鈥檚 plenty of water for microbes. They鈥檙e small. They don鈥檛 need a lot of water.

鈥淎t temperatures above -20潞C (-4潞F) there is a layer of unfrozen water between the sand grains and the ice. These layers can support microbial life at least [down] to -15潞C (5潞F),鈥� McKay explained.

鈥淥n Mars today the temperatures of the ground ice are much too cold for this effect to be useful,鈥� he wrote. But Mars wobbles. At present Mars is tilted on its axis at about the same angle as Earth鈥檚. Five million years ago, however, Mars leaned over at an angle of about 45潞, and for nearly half of each martian year (equivalent to about one Earth year), the polar regions received constant sunlight. Back then 鈥渢he ground ice at the polar regions,鈥� like the site where NASA鈥檚 Phoenix spacecraft landed in 2008, 鈥渨ould have been much warmer. We think it would have been in the range of -15潞C to -20潞C. So liquid water layers鈥� in the past were 鈥渁 possibility.鈥�

The question then is this: If life ever took hold on Mars, back when the planet was warmer and wetter, did a few hardy microbes evolve a survival strategy that let them go into a deep sleep, and then every 10 or 20 million years, when the ground warmed up to -20潞C or so, wake up and put on a little growth spurt?

The answer will have to wait until a follow-up mission to the martian polar regions can dig deeper than Phoenix did. It is just such deep polar drilling that McKay鈥檚 IceBite project is working to make possible.

In the meantime, DiRuggiero will have no problem staying busy. There is still much left to learn about the dry limit of life, in both Antarctica and the Atacama.