First measurements of comet’s water ratio.  Rosetta’s measurement of the deuterium-to-hydrogen ratio (D/H) measured in the water vapour around Comet 67P/Churyumov–Gerasimenko. The measurements were made using ROSINA’s DFMS double focusing mass spectrometer between 8 August and 5 September 2014.  The graph displays the different values of D/H in water observed in various bodies in the Solar System. Credit: ESA/ATG medialab/Rosetta/NavCam; Data: Altwegg et al. 2014