This frighteningly realistic adaptation of HG Wells's The War of the Worlds brought its producer, Orson Welles, to international prominence. Though life on Mars may not exist in the form envisaged by either Wells or Welles, some people feel that microscopic organisms may be present there. To test the idea, Nasa is planning missions to bring bits of Martian rock back to Earth.
Will the rocks contain hitch-hiking bacteria that could threaten life on Earth with a "Martian plague"? This question was examined at a meeting of astronomers and biologists in Hawaii last month. Scientists at the Bioastronomy '99 conference discussed the origin of life on Earth, planets of other stars, and new strategies for identifying signals from intelligent aliens. Mars is at the forefront in the search for extraterrestrial life and is currently the object of an invasion from Earth.
The curtain-raiser was Mars Pathfinder, which landed in a dried-up Martian river bed in 1997, and dispatched its small rover, Sojourner, to sniff around the surrounding rocks. Later this month - on 23 September - Mars Climate Orbiter will arrive at the Red Planet. Then, on 3 December, the Mars Polar Lander will settle down in the chilly area of the planet's south pole.
In addition, for two years it has been scrutinised by the orbiting Mars Global Surveyor space probe. Jim Head, a planetary geologist at Brown University in Rhode Island, amazed delegates at the conference with extraordinary news from the Global Surveyor mission. While its cameras can pick out objects as small as a car in the desert below, Global Surveyor has also been building up a three-dimensional map of Mars's surface, using an altimeter that bounces off infrared light.
Mars, it turns out, has two very different hemispheres. The southern half is generally higher and very rough; here high mountains and volcanoes are contrasted with deep valleys and craters. The peak of the highest volcano towers 100,000ft above the bottom of the deepest crater - far higher than Mount Everest stands above the Earth's deepest ocean trough.
The northern half is one vast, low-lying plain. "Several years ago it was hypothesised that there was once an ocean here," says Head, "but the idea was controversial. I've always been wary of the idea of large-scale standing bodies of water on Mars. With the Global Surveyor altimetry data, we are now in a position to test the oceans hypothesis."
As the orbiter swung round Mars, it crossed the border between the northern lowlands and the surrounding highlands more than 1,000 times. Head checked the altitude of this boundary, the so-called Contact 2, at each of these crossings. To his astonishment, it was at the same height all the way round the northern plain. It is hard to think of any reason for this, unless Contact 2 marks the shoreline of an ocean.
The Mars Global Surveyor could also measure how rough or smooth the surface is. "Below Contact 2, everything is smoother, at all scales," Head says. "That's consistent with the surface having been formed by sedimentation at the floor of an ocean."
Delving deeper into Global Surveyor's scans of Mars, Head has traced the way in which the terrain rises at Contact 2. It forms a series of long, low steps. To a geologist's eye, these look like the raised beaches we find on the shore of a terrestrial ocean when the sea level has changed abruptly.
At the two lowest regions of the great northern plains, Head has found, the surface is fractured by what look like huge mud-cracks - just where you would expect to find the last traces of moisture evaporating from the ocean-floor sediments as water from the ancient ocean dried out. In a computer model, Head then "refilled" the northern lowlands with water. It starts as two separate small seas, which merge as more water is "poured in". To fill the lowlands up to Contact 2, Head has to add 14 million cubic kilometres of water - roughly equal to the volume of Earth's Arctic Ocean. And, like the Arctic Ocean, Mars's polar seas would have been covered with a thick layer of ice. The shores of the ancient ocean could have provided the ideal habitat for life to start.
But the surface of Mars is no place for life today. It is frozen solid and its thin atmosphere is no shield against the Sun's deadly ultraviolet radiation. Jack Farmer, of Arizona State University, argues that any primeval living cells would have migrated deeper underground as the planet cooled. They could now be thriving in microscopic cracks deep down. Present unmanned spacecraft cannot dig that deep. "We'll have to wait until humans can get there with drilling rigs," Farmer says.
Meanwhile, he advocates looking for fossils of ancient life, which may be no more complex than the remains of microscopic cells. While some scientists expect to break open any old rock on Mars and find fossils, Farmer points out that only 1 per cent of fossils on Earth get preserved. "It requires special conditions of temperature, salinity, acidity, desiccation and oxygen," he says, "and so we should look for certain kinds of minerals - such as phosphates and silicates."
Future probes should be aimed at the oldest regions of Mars, where fossils have the best chance of having survived later geological activity. "It's good to have a site where old water channels flow into a basin and deposit sediments," Farmer says. "Another choice would be `chaotic terrain' around old volcanoes, where interior heat has melted the surface ice. We need to send a rover to Mars with a hand-lens."
In 2003, an America Delta rocket will launch a Mars probe with a rover, to select suitable samples of Martian rock and launch them in a canister to orbit around Mars. Two years later, a European Ariane rocket will send another mission. Arriving in Mars's orbit, it will capture the orbiting container and pack it into another rocket to return the sample to Earth. "In 2008 the sample will land at Dugway, Utah, where it will be collected as soon as possible," says Don DeVincenzi, of Nasa's Ames Research Center.
Just in case the sample contains living Martian cells, the rocks will be held in a quarantine laboratory as secure as those used to investigate the ebola virus. "Here we will test for life by checking for organic chemicals and attempting to culture living organisms from the sample. We'll also follow standard biohazard protocols, by attempting to infect cells, tissues and chicken eggs."
Is all this elaborate and expensive containment really necessary? Rich Terrile, of Nasa's Jet Propulsion Laboratory, points out that huge amounts of Mars fall on Earth every year as meteorites. "We don't suffer infections; we should just educate the public that the risks are minimal," he says.
However, Nasa's wonderfully named "Planetary Protection Officer", John Rummel, takes the opposite view: "We must involve the public from the beginning," he stresses. "Nasa must do it right."
Getting the public and the media involved in the discussions from the beginning is the best way of avoiding another media panic about an invasion from Mars.
Heather Couper and Nigel Henbest's latest book, `The Space Encyclopaedia', is published by Dorling Kindersley, price pounds 20Reuse content