If extraterrestrial life exists in the solar system, there is a good chance that it will be discovered as frozen remains trapped in a block of ice. Mars’s ice caps are one possible location. Another is the surface ice of Jupiter’s moon Europa, which scientists believe harbors an ice-capped ocean.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.
