spaceSpace and Physics

Comets’ Water Signature Might Change As They Orbit The Sun


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockMar 1 2017, 17:34 UTC

Comet Lovejoy. MarcelClemens/Shutterstock

American Astronomers have looked at Comet C/2014 Q2, known as Lovejoy, and discovered something quite unusual: The amount of a rare type of water within the comet increased as the ice rock moved past its closest point to the Sun.

You see, water comes in different forms and it depends on the isotope of the elements that make it. Normal water, H20, is made of oxygen and hydrogen, but if we switch hydrogen for deuterium atoms we have heavy water, HDO.  


By measuring the amount of HDO and H20 the researchers were able to estimate the deuterium-to-hydrogen (D-to-H) ratio, a chemical ID that tells astronomers where comets formed in the Early Solar System. The team measured the D-to-H value just after the comet’s perihelion – the point of the orbit of a planet or comet that is nearest to the Sun – and then compared this result with previous observations from when the comet was further from the Sun.

It appears that the D-to-H value was two to three times higher after passing near the Sun than before, with the comet releasing more HDO while the amount of H20 remained unchanged. This result, published in the Astrophysical Journal Letters, was unexpected.

“The change we saw with this comet is surprising, and highlights the need for repeated measurements of D-to-H in comets at different positions in their orbits to understand all the implications,” lead author Lucas Paganini, a researcher with the Goddard Center for Astrobiology, said in a statement.


“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, especially, if these are based on a single measurement of the D-to-H value in cometary water.”

This finding might reopen the debate on the origin of Earth’s water. Geological evidence suggests that the water mostly came from the mantle and the D-to-H ratio of comets is too high, compared to Earth’s water. The researchers suggest that the higher ratio (and the changes) might be due to the high-energy process from the Sun, as well as chemically altering mechanisms on the comet’s surface.

Normal water is 1,500 times more abundant than HDO on Comet Lovejoy, so gauging the exact cause of the change is difficult. Comets may or may not have contributed to the hydrosphere, and it’s clear that we need to know much more to make a judgment.  

spaceSpace and Physics
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  • deutrium,

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