sentences:  It is yet unclear what source of heat may have caused melting of some of the comet's water ice.Nevertheless, on 10 December 2014, scientists reported that the composition of water vapor from comet Churyumov–Gerasimenko, as determined by the "Rosetta" spacecraft, is substantially different from that found on Earth.That is, the ratio of deuterium to hydrogen in the water from the comet was determined to be three times that found for terrestrial water.