But, scientists believe, it is more likely to be giant white worms than little green men. Later this year, the first of a 10-year-long series of space probes will be launched to the Red Planet to hunt for fossil evidence of life on Mars.
A scientific conference in London will hear this week that life may have begun on Mars at almost the same time as it started here on Earth - some 3.8bn years ago. But scientists believe that as Mars - far more distant from the heat of the Sun - started to freeze, living organisms may have retreated deep into the planet's interior seeking the warmth of volcanic "hot springs" to supply them with the energy to sustain life.
It is even possible that life here on Earth might have started as a "cross- contamination from Mars", as Professor Paul Davies of Adelaide University will tell the conference, organised by the Ciba foundation.
In 1911, a piece of Mars known as the Nakhla meteorite fell to Earth in Egypt - killing a dog - and more than 500 tons of Martian material falls on to the earth each year. Primitive bacteria could have hitched a ride on rock fragments crossing interplanetary space and seeded life on Earth, he says.
Jack Farmer, a senior scientist with the NASA-Ames Research Centre, said: "We are returning to Mars for the first time in 20 years. This year is the 20th anniversary of the Viking missions which looked for evidence of extant life."
The Viking probe which landed on Mars did not detect "a single organic molecule", Dr Farmer said, even though it was sensitive enough to detect one part in a billion.
After the failure of the Viking probe to detect signs of life, the consensus was that surface conditions on the Red Planet "were not conducive for life as we know it," added Dr Farmer. Mars was too dry, any water was frozen in permafrost, and intense ultraviolet radiation bathes and sterilises the surface.
But some 3bn to 4bn years ago there was liquid water on the surface of Mars, which has left its mark in channels and valleys on the surface.
This represented "a window of opportunity for life to originate. At the same time as it started here on Earth, it could have originated on Mars," Dr Farmer said. But he warned that researchers may have to drill from 100m to a kilometre under the surface to find living organisms on Mars, and this would be beyond the technology of the space probes currently planned.
What has changed scientists' views over the 20 years since the Viking mission has been the discovery here on Earth of primitive forms of life which do not need the heat of the sun to sustain themselves but which live in volcanic hot springs and around ocean thermal vents. Bacteria thrive in the hot sulphurous environment of these "black smokers" on the sea bed, and strange white worms graze upon them.
According to professor Karl Stetter, of Regensburg University in Germany, these archaic bacteria form the very roots of the tree of life itself - they may be the ancestors of us all.
Similar creatures may have evolved to live on hydrothermal springs on Mars, according to Malcolm Walter of Macquarie University, Australia.
Professor Walter explained that early life on Earth would have formed "gelatinous colonies of bacteria encrusting columns on the sea floor".
However, during the Earth's early history, "they built up bacterial reefs - stromatolites - and some of the biggest reefs on earth today were built by bacteria, not coral", he said. These stromatolites represent the earliest fossil remnants of living creatures on the Earth.Reuse content