The difference here, as I understand it, is that the measurements done on mars were done by looking for samples of specific carbon compounds. This is indeed vul

The difference here, as I understand it, is that the measurements done on mars were done by looking for samples of specific carbon compounds. This is indeed vulnerable to contamination, since the compounds could be deposited on at any time. In the case of diatoms, they were looking for fossilized structures embedded in the rock. These aren't subject to contamination (at least not trivially), because contamination would have to actually change the structure of the rock. The results for both are wrong -- we didn't find life -- but in principle the methods of looking are subject to different kinds of errors. In this case, the issue is that the author of the original paper has done two major things wrong: - He hasn't shown that the meteor is actually a meteor. - What he claims to be fossils embedded in the rock aren't fossils.