The two forms are naturally found in predictable ratios. Thus the investigators theorize that if there is an excess of HDO now, there must also have been more H20 in the past than there is now, and that this was lost.
The researchers distinguished the chemical signatures of the two types of water from the W. M. Keck Observatory in Hawaii, NASA’s Infrared Telescope Facility, also in Hawaii, and from the European Southern Observatory’s Very Large Telescope in Chile.