That's because genetic material is severely damaged or destroyed by exposure to so-called "cosmic radiation flux", he said. Falkowski is co-director of the two-year study of frozen glacial microbes, conducted in conjunction with Boston University, and published in the Proceedings of the National Academy of Sciences.

The researchers were mainly interested in whether genetic material from the microbes, which they identified as different types of bacteria, could have mixed with that of other organisms in the Earth's ancient oceans, and influenced evolution, Falkowski said. The Rutgers study refutes at least part of the "panspermia hypothesis" - a theory from the Greeks, and popular among many scientists since the 19th century - that micro-organisms and biochemicals were carried to the planet by comets, meteors and asteroids. Other scientists in New Jersey said that they were intrigued by the Rutgers study, but suggested there might be ways some organic material could survive long-term rides on a comet.

"The only question I'd have is whether the radiation can penetrate into the interior of a comet," said Dale Gary, an astrophysicist at the New Jersey Institute of Technology in Newark. Comets are called "dirty snowballs", which implies there is a certain amount of rocky material at their centre which could provide a shield for travelling DNA, Gary said.

"Certainly anything on the surface of comets would suffer radiation damage," he said. Gary, chairman of physics at NJIT, had not seen the study. "Perhaps they (Rutgers) have done some calculation of the penetration of these cosmic rays through ice, and concluded that, for a certain radius, it can destroy DNA deep inside. "However, we don't know everything there is to know about the interior of comets," he said. A large comet might have enough rock in its core "to keep DNA material rather pristine and safe", Gary said.

Radiation might be a problem for microbes, but not for very basic organic material, said Kevin Conod, an astronomer and manager of the Dreyfuss Planetarium at the Newark Museum. "I think the theory of panspermia is not about microbes from space, but amino acids, the building blocks of life," Conod said. "Radiation wouldn't necessarily affect those enough to kill pieces of protein."

The Rutgers researchers thawed five microbial samples taken from ice between 100,000 years and eight million years old, and were able to grow several organisms in liquid media, said Kay Bidle, a Rutgers marine microbiologist and oceanographer. They also wanted to know how long organisms could live over extended geologic periods, Bidle said. "This is of interest to whether there is life on Mars," he said, as the site in Antarctica resembled icy regions on the Red Planet.

Microbes might survive a trip from Mars if encased in a meteorite, Falkowski of Rutgers said. "So we could all be Martians," he said.