Organic molecules were expected to be present in the martian surface strata even if there was no evidence of existing or fossil life, because organic materials are found in meteorites and they must be deposited on the martian surface as they are on Earth. If life has evolved on Mars it may continue to exist in isolated 'oases' where conditions are particularly good, such as near the equator and in deep depressions. It is obvious that the debate about the existence or non-existence of life on Mars will continue to rage until samples are taken from many different areas of the planet's surface, including beneath the surface. The landers made one momentous discovery about the atmosphereit contains 2.7 per cent of nitrogen, one of the essential elements of life. The passage of cold fronts, in which warm, moister air is lifted over a wedge of colder, denser air, was observed many times.