A trip past the sun may have selectively altered the production of one form of water in a comet known as Lovejoy — an effect not seen by astronomers before, a new NASA study suggests.
The findings could shed new light on how much comets might have contributed to Earth's water compared to asteroids.
"Comets can be quite active and sometimes quite dynamic, especially when they are in the inner solar system, closer to the sun," said co-author of the study Michael Mumma, Director of NASA's Goddard Center for Astrobiology.
NASA scientists observed the Oort cloud comet C/2014 Q2, also called Lovejoy, when it passed near Earth in early 2015.
Through NASA's partnership in the W. M. Keck Observatory on Mauna Kea, Hawaii, the team observed the comet at infrared wavelengths a few days after Lovejoy passed its perihelion – or closest point to the sun.
The team focused on Lovejoy's water, simultaneously measuring the release of H2O along with production of a heavier form of water, HDO.
Water molecules consist of two hydrogen atoms and one oxygen atom. A hydrogen atom has one proton, but when it also includes a neutron, that heavier hydrogen isotope is called deuterium, or the "D" in HDO.
From these measurements, the researchers calculated the D-to-H ratio — a chemical fingerprint that provides clues about exactly where comets (or asteroids) formed within the cloud of material that surrounded the young sun in the early days of the solar system.
Researchers also use the D-to-H value to try to understand how much of Earth's water may have come from comets versus asteroids.
The scientists compared their findings from the Keck observations with another team's observations made before the comet reached perihelion, using both space- and ground-based telescopes, and found an unexpected difference.
After perihelion, the output of HDO was two to three times higher, while the output of H2O remained essentially constant, showed the findings published online in the Astrophysical Journal Letters.
This meant that the D-to-H ratio was two to three times higher than the values reported earlier.
"If the D-to-H value changes with time, it would be misleading to assume that comets contributed only a small fraction of Earth's water compared to asteroids," lead author of the study Lucas Paganini, a researcher with the Goddard Center for Astrobiology, said.