Water’s origin story
New evidence suggests that comets may not have delivered water to Earth. Water detected in comet 67P/Churyumov–Gerasimenko’s hazy atmosphere isn’t a chemical match for Earth’s oceans, as Ashley Yeager reported in “Ocean water may not be from comets” (SN: 1/10/15, p. 8).
The Rosetta spacecraft searched for a heavy form of hydrogen known as deuterium around 67P and found that the comet’s water had a deuterium-to-hydrogen ratio about three times as high as that of Earth’s water. “Might it not be true, though, that this comet and others have periodically had lighter molecules swept away by the