The Martian question that everyone wants answered - did the Antarctic meteorite that Nasa has claimed contains signs of ancient life really come from the red planet? - could be solved by a piece of technological wizardry being developed at the Open University.

Colin Pillinger, who has developed the device for a European space mission, is now trying to persuade Nasa to land it on Mars in 2003. Professor Pillinger published a paper in 1989 saying that another Martian meteorite contained tentative evidence of life.

When Nasa scientists announced last week that they had evidence that there had been life on Mars several billion years ago, several holes were picked in their conclusions.

The most significant, in Professor Pillinger's eyes, is that the meteorite, found in Antarctica last century, may not have come from Mars.

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Although the meteorite contains air pockets whose composition matches the air measured on Mars by the 1976 probe, scientific precision has come a long way since then. "We need to prove without any doubt that the rocks came from Mars," Professor Pillinger said.

The key to doing so lies in analysing the chemical constituents of Martian rock, in particular the proportions of isotopes - different versions of the same element that differ slightly in weight, for example ordinary carbon (carbon 12) and radioactive carbon (carbon 14). Unlike Earth, Mars is "radioactively dead" so its rock would contain no carbon 14 atoms.

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The trouble is that to measure such isotopes a mass spectrometer is needed, which is very heavy, with a magnet weighing five tonnes. "The isotopes are very minutely different in weight," he said. To get the resolution required you need a huge mass spectrometer.

It is possible to amplify the differences between the isotopes by chemical reactions but these are of the explosive type. For example, to deal with the oxygen in silicon requires heating the rock to 700 degrees centigrade in fluorine - a highly reactive gas that no one would transport to Mars, said Professor Pillinger.

Pillinger's breakthrough is to make a solid, benign form of fluorine which will release enough in the gaseous form to react with the oxygen while any excess is reabsorbed. An automatic system carries out all this chemistry and the product can be tested in a much tinier mass spectrometer. The whole system, including the solid reagent, weighs three kilograms.

The device will fly in the European Space Agency's Rosetta mission, to land on a comet. Professor Pillinger is confident that if the test was used on Mars and the isotope findings match tests on the Martian rock then the proof of its validity would be strengthened.

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Others have suggested another weak link - that the meteorite could have picked up its famous organic material while hanging around in the Antarctic, which it did for thousands of years.

But Pillinger said: "The problems of contamination are well addressed now by the people who have been doing the work." The reason Nasa made its announcement last week, he said, was because "we have been through the long and tortuous process of demonstrating this".