If aliens from another planet sent a probe to Earth in search of life, their most promising target would be the tropics, where the greatest density and diversity of life are found. But on other nearby worlds, planets and moons within reach of present-day spacecraft, tropics are hard to come by. Most likely, if we find life elsewhere in our solar system, we'll find it in ice.

That view is what motivated Jen Eigenbrode, a geobiologist at NASA's Goddard Space Flight Center, in Greenbelt, Md., to establish SLIce (Signatures of Life in Ice), a project designed to study both how life survives in ice and what clues it offers to signal its presence. "Our goal," Eigenbrode says "is to understand how we might be able to detect life living in surface ice [on Earth], in preparation for going to the surface of another planet."

The group's first field test took place in August of 2008 in Svalbard, an archipelago that lies north of Norway in the Arctic Ocean.

A combination of climatic and geologic features make Svalbard one of the most useful Mars-like sites on Earth, and the Arctic Mars Analog Svalbard Expedition (AMASE) has traveled there for the past several summers, testing various life-detection instruments and evaluating their suitability to future Mars missions.

During its field work, the AMASE team lives onboard the research vessel Lance, an icebreaker that ferries the scientists from site to site. In 2008, SLIce was an integral part of AMASE.

Eigenbrode's approach is multidisciplinary. Researchers who study life in ice often focus on cataloguing psychrophilic (cold-loving) microorganisms. "We're not getting to that kind of detail," Eigenbrode says. "What we are looking for is more general signals of life because that is what's important for investigations of life elsewhere in our solar system."

So, for example, the SLIce team looks for ATP (adenosine triphosphate), because all living cells use ATP as an energy source for constructing cellular components. ATP cannot be stored, though; an organism must use it shortly after it is created. So if SLIce researchers find it in their samples, Eigenbrode says, it means "we are detecting the [active] metabolism of [living] organisms. It doesn't matter what organism it is."

The team also looks for lipids, components of cells walls.

In the 2008 field work, tests for ATP and lipid abundance were performed in situ. Researchers armed with sterile swabs and handheld detectors trudged across Arctic ice fields to promising locations, collected samples, popped them into the battery-powered instruments and dutifully recorded the results in their notebooks.

The highly sensitive instruments, used in the food industry and in biotech labs to guard against bacterial contamination, were valuable in the field for their ability to detect even trace amounts of biological material.

For other tests, which could not be performed on-site, the SLIce team collected large ice cores, which they brought back to the ship, melted and filtered for further study, fifteen cores in all, each core weighing 35-40 pounds, all of them carried by hand from the field to the ship.

The three sample sites from which they were gathered were chosen for their different environmental characteristics: one near Svalbard's only town, Longyearbyen, which churns out environmental contaminants, the other two in more-remote and more-pristine locations.

In addition, researchers collected samples of both fresh and old snow, meltwater, seawater and even sediments found in the ice at the sample sites.

Some of these additional experiments were performed onboard the ship; still others will take place over the coming months in laboratories at NASA and other research institutions. One group of scientists is looking for the presence of airborne hydrocarbons that settle in the ice, some originating in ancient rocks, others the waste products of industrial activity. Another group is focused on inorganics: compounds like ammonia, and trace minerals - copper, cobalt, molybdenum, nickel - required by microbes to build enzymes. Others are peering at cells through microscopes, or looking for specific biological molecules, such as amino acids and lipids.

By collecting a broad set of samples and probing them with a variety of analytical approaches, Eigenbrode hopes to construct a systemic picture of the environment in which the organisms live:

+ how plentiful is life at various locations?

+ what factors make one icy site a better habitat than another?

+ what do the organisms eat?

+ are certain abiotic indicators typically associated with the presence of life?

+ what evidence do the organisms themselves leave behind?

+ and, most importantly for future missions to icy worlds, which instruments are best at detecting that evidence, at teasing out signs of life against the chemical background noise of geology?

And then there's the question of contamination. Before being put into service, scoops and scrapers and test tubes and coring equipment are all put through a seven-step scrubbing and rinsing process involving a hospital disinfectant, plus bleach, plus hydrogen peroxide.

In some cases, ethanol is also used. Because SLIce is looking not only for living cells, but also for scraps of biological detritus and non-biological organic material present by chance in the environment, it's not enough to kill whatever microbes are living on the sampling equipment. All the residues that microbicide leaves behind must be gotten rid of as well. "We went overkill on our procedure," Eigenbrode concedes. Over time, she hopes, it will be possible to simplify the process.

Eigenbrode doesn't expect SLIce to provide immediate answers. Nailing down the best approach to finding extraterrestrial life in ice could take years.

"We really have no idea what we're going to encounter in the ice," she says. "Even on Earth, it's relatively unexplored territory. We don't understand how life lives there. If we don't understand it on Earth, we're never going to understand it on another planet. So we're starting here. And eventually we're going to apply what we've learned to selecting instruments that are more appropriate for detecting life elsewhere. In ice."

SLIce is funded by NASA's Exobiology and Evolutionary Biology and Mars Fundamental Research programs; AMASE is funded by NASA's ASTEP (Astrobiology Science and Technology for Exploring Planets) program. Jen Eigenbrode is the principal investigator for SLIce. Andrew Steele of Carnegie Institution of Washington (CIW) is the lead scientist for AMASE; for the 2008 field season, Marilyn Fogel of CIW was the project's science lead.