The discovery of possible fossils of nanobacteria in a meteorite from Mars is both an exciting development and a considerably challenge for future work. The meteorite bearing the possible fossils, ALH84001, has been on Earth for over ten thousand years, and thus the possibility that the `fossils' are terrestrial contamination must be considered. We suggest that the only way to fully resolve the issue of possible martian life is to study directly- sampled pieces of Mars, either using in situ instrumentation or via sample return. The small size of the possible fossils, and their relatively low abundance in bulk rock samples make in situ analysis difficult and indirect. We suggest that addressing the issue of ancient life on Mars will require sample return, probably assisted by in situ screening by landers/rovers. Our study of ALH84001 confirms the observation of McKay et al. of the existence of `fossils' in ALH84001, and we find that they are highly abundant on all the carbonate nodules we examined. Examination of lunar meteorites and two other martian meteorites, with terrestrial and laboratory histories very similar to that of ALH84001, shows that nano-`fossils' are absent, suggesting that the features in ALH84001 are probably not terrestrial contamination.