The shergottites, nakhlites and Chassigny, collectively known as the SNC meteorites, seem to define a petrological and geochemical sequence1. Together with their young radiometric solidification ages of only ∼1,300 Myr (refs 2-9), this has been interpreted as evidence for magmatic activity on their parent body late in its history. Such late magmatism is most easily envisaged for planetary-sized bodies and it has been suggested that SNC meteorites are derived from Mars (see resf. 10 and refs therein). This suggestion is supported by the apparent similarities between the noble gas and nitrogen composition of the present martian atmosphere11,12 and gases trapped in shock-produced glass phases of the shergottite EETA 79001 (refs 13, 14). Our analyses of trapped argon, krypton and xenon in Shergotty, Nakhla and Chassigny reported here show, however, that these meteorites do not contain unadulterated noble gases from the martian atmosphere, fractionated or not. They indicate that for Nakhla and Chassigny, in particular, a martian origin is only possible if the martian atmosphere contains a higher fraction of radiogenic 129Xe and of fissiogenic xenon than does xenon in martian rocks. This is the opposite of the situation on Earth.
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