Data on the composition of the Martian atmosphere obtained by instruments aboard the Viking spacecraft are not of sufficient accuracy to address important questions regarding the composition and history of Mars. Laboratory analyses of gases trapped in glassy phases of shergottite meteorite EETA 79001 yield precise data, but it remains to be ascertained that these gases constitute unfractionated Martian atmosphere. Return from Mars of a gas sample for laboratory analysis appears preferable to another in situ measurement, especially if rocks of documented origin will become available for gas analysis as well.