Hartley 2, which comes from the distant Kuiper Belt, is home to icy, rocky bodies including Pluto, other dwarf planets and innumerable comets.
“Our results with Herschel suggest that comets could have played a major role in bringing vast amounts of water to an early Earth,” said co-author Dariusz Lis, senior research associate in physics at the California Institute of Technology in Pasadena.
“This finding substantially expands the reservoir of Earth ocean-like water in the solar system to now include icy bodies originating in the Kuiper Belt,” he added.
Scientists theorize that Earth started out hot and dry, so that water critical for life must have been delivered millions of years later by asteroid and comet impacts.
Until now, none of the comets previously studied contained water like Earth’s. However, Herschel’s observations of Hartley 2, the first in-depth look at water in a comet from the Kuiper Belt, paint a different picture.
Herschel detected the signature of vaporized water in the comet’s coma, or thin, gaseous atmosphere, and, to the surprise of the scientists, Hartley 2 possessed half as much “heavy water” as other comets analyzed to date.
The ratio between heavy water and light, or regular, water in Hartley 2 is the same as the water on Earth’s surface.
By tracking the path of Hartley 2 as it swoops into Earth’s neighborhood in the inner solar system every six and a half years, astronomers know that it comes from the Kuiper Belt.
The five comets besides Hartley 2 whose heavy-water-to-regular-water ratios have been obtained all come from an even more distant region in the solar system called the Oort Cloud. This swarm of bodies, 10,000 times farther afield than the Kuiper Belt, is the wellspring for most documented comets.
Given the higher ratios of heavy water seen in Oort Cloud comets compared to Earth’s oceans, astronomers had concluded that the contribution by comets to Earth’s total water volume stood at approximately 10 percent.
Asteroids, which are found mostly in a band between Mars and Jupiter but occasionally stray into Earth’s vicinity, looked like the major depositors.
The new results, however, point to Kuiper Belt comets having performed a previouslyunder-appreciated service in bearing water to Earth.
The findings are published in Nature.