John Rummel, formerly in charge of the planetary protection program at the United States space agency Nasa, said: "We don't know if there are organisms out there, so we have to take precautions."
Now based at the Marine Biological Laboratory in Massachusetts, he said that the odds on discovering sites where Earth-like life could grow had increased greatly. "It appears that life is a natural product of planetary evolution," he said. "So we have to be ready for surprises."
An international space conference in Birmingham was told that Nasa had allowed insufficient time to design quarantine systems for samples returned from the Moon during the Apollo missions, leaving a risk of contamination being brought back. "They spent $24m on something that, in the end, satisfied almost nobody," Dr Rummel said.
Such systems would have to be better designed when dealing with samples from Mars, which had a far greater potential for harbouring life, he said.
The space and life scientists reviewed plans in place to protect the Earth from contamination by any life-form that space travel might reveal, whether on the Moon, Mars or even comets - which are thought by some to have seeded life on Earth.
Scientists told the conference that 12 meteorites had landed on Earth from Mars, revealing that millions of years ago conditions on the planet could have allowed life to develop.
In The Andromeda Strain, by Michael Crighton, a space capsule returns to Earth having picked up a deadly micro-organism which subsequently kills almost all the inhabitants of a town. The problem for any real-life scientists battling against such an organism would be recognising it.
The scientists, who met yesterday in Birmingham at the start of a six- day conference which is expected to attract 1,500 delegates, are understood to urge a cautious approach in returning rocks and other samples from Mars and Europa, one of Jupiter's moons. "Until we find life somewhere else, we don't really know what we know," Dr Rummel said.
Scientists have long been aware of the possibility that interplanetary life might be dangerous to human or other Earth life. But the worries about contamination of one planet by another also extend the other way: Nasa's designs now include precautions to ensure that life from Earth - such as bacteria - is not spread to Mars by spacecraft. Such contamination could easily lead to the exciting but false "discovery" of life on the planet.
However, the risks from other worlds remain low. So far, the only death caused by material from Mars is that of an Egyptian dog, in the last century. It was hit by one of the 12 meteorites.Reuse content