Frozen Life Cubes
Scientists dream about bringing ice sample from Mars or Europa back to Earth for study. A robotic unmanned mission would return a small sample – possibly a five-pound block – that would have to be decontaminated to remove any terrestrial organisms on its surface that could confound the search for alien microbes. But how would researchers handle the sample when it was returned?
Answers to that question could come from Antarctica’s Lake Vostok, which is buried nearly two miles beneath the southern polar ice sheet. In 1998, a US, French and Russian expedition conducting paleoclimatological studies drilled into this glacier ice, and returned an intriguing ice sample from just 200 meters above the surface of Lake Vostok.
The ice comes from the water of the lake, deposited long ago by an unusual water cycle. In the northern region of the lake, ice from the overlying glacier slowly melts, adding water to the system. In the southern part of the lake, surface water slowly freezes, accreting to the bottom surface of the glacier. This latter process creates something of a time capsule, as the accretion of ice produces layers that date to perhaps as far as 15 million years in the past. The ice contains bacteria that were trapped at the time it formed. The result is a sort of inverse stratigraphy. In normal geological or archeological samples, the bottom layer is the oldest. In the accreted ice, the bottom layer is in fact the youngest.
Craig Cary, who is a professor of marine biosciences at the University of Delaware, has been preparing these samples for a genetic analysis of the entrapped bacteria, to be carried out by colleagues at the University of California at Riverside and the U.S. Department of Energy's Joint Genome Institute. The procedure he has developed to decontaminate those ice samples could one day be used on ice cores returned to Earth from extraterrestrial environments.
The accreted ice offers an unusual opportunity for researchers to study these ancient life forms, because bacteria from many eras can be found, each trapped in its own layer and flanked by older and younger microbes that have been frozen in place throughout thousands of millennia. The microbes are of particular interest to astrobiologists, who believe that extraterrestrial life may exist in similar locales – sequestered away from harsh surface environments, such as in Europa’s subsurface ocean.
Cary says that the Lake Vostok ice sample represents an unusual challenge. At about two liters of volume, there is precious little of it. Two liters may sound like a lot, but life is very sparse in Lake Vostok. “This ice is very precious,” he says. Cary estimates that the ice contains only 10-100 microbes per milliliter. By comparison, most lakes contain about a million microbes per milliliter.
Researchers need to be confident that the decontaminated samples contain only authentic ancient microbes at their core, but that presents a conundrum. Every step taken to increase confidence in the authenticity of the sample leads to a reduction in the sample’s volume. For example, use of a blowtorch to melt the outer veneer of the ice could eliminate contaminating bacteria on the surface, but would result in the loss of a large fraction of the sample. “It can be quite catastrophic, and you only have one shot at this,” says Cary. Worse, the accreted ice has a unique prismatic structure that is very difficult to create artificially, so Cary’s team had nothing to practice with.
Cary decided to proceed by washing the sample’s surface with bleach, followed by ultra-pure water. To prevent fractures from forming, Cary conducted the surface washes with liquid that was pre-cooled to the temperature of the ice. They then placed the sample in a meticulously clean, specially designed chamber.
Once the decontamination procedure was completed, Cary needed to determine if it had succeeded. Species of modern bacteria are estimated to number in the millions, and only a small fraction has been characterized. An unknown modern contaminant would be difficult or impossible to distinguish from the ancient bacteria trapped within the ice. To combat the problem, before he began the decontamination protocol Cary chose to deliberately contaminate the ice surface with a variety of known modern organisms. After the decontamination protocol, his team used ultra-sensitive genetic analyses to look for even the smallest trace of the known bacteria. When the sample proved to be free of any of the modern control bacteria, Cary could be confident that the procedure had successfully eliminated all modern contaminants.
Cary’s colleagues will use novel “‘whole genome amplification” techniques to study the genetic diversity of the bacteria, which should provide clues to how microbial life can survive in such an extreme low-energy environment.
Cary’s procedure could readily be applied to ice samples from the poles of Mars or Europa’s subsurface ocean. Any robotic mission would likely carry back only a very small payload, perhaps similar in size to the Lake Vostok core sample, “so I think the scale of what we’ve done would be appropriate,” says Cary.
The work could well guide future researchers handling extraterrestrial ice samples, although modern bacteria contaminants would actually be somewhat less of a concern, says Conley. That’s because extraterrestrial microbes would likely be genetically distinct from modern Earth bacteria, making them easier to identify during the analysis stage. Bacteria on Mars, Europa or elsewhere could be related to Earth bacteria, given the possibility that meteor or comet impacts could have carried bacteria from one planetary body to another. But in that scenario, the common ancestor would be at least 65 million years old, and possibly billions of years old, says Conley. As a result, the bacteria would have little genetic similarity to modern terrestrial microbes and could be easily distinguished.
In fact, scientists will have to place greater emphasis on the reverse problem – preventing extraterrestrial organisms from contaminating Earth and wreaking ecological havoc. Conley admits that such an event is unlikely to cause widespread damage, but nevertheless, prevention is of utmost importance. “That is the paramount thing. We protect [extraterrestrial environments] for the sake of science, but we also have to protect the Earth,” says Conley. Any samples brought back from an extraterrestrial environment will have to be placed in a containment facility equivalent to what is used for the study of the Ebola virus.