The underlying data here comes from the Ptolemy instrument, a gas chromatograph mass-spectrometer (ion trap, in this case) which can measure ratios of molecules in a range of 14 to 140 Da. (It might also be able to identify somewhat larger species by inducing fragmentation and measuring the resulting pattern of smaller molecules within the effective mass range, but I haven't found if this capability was included).<p>Some more info here:
<a href="http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1937.pdf" rel="nofollow">http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1937.pdf</a><p>and: <a href="http://sci.esa.int/rosetta/31445-instruments/?fbodylongid=896" rel="nofollow">http://sci.esa.int/rosetta/31445-instruments/?fbodylongid=89...</a><p>The instrument has its own Twitter feed, of course: <a href="https://twitter.com/philae_ptolemy" rel="nofollow">https://twitter.com/philae_ptolemy</a>