Bergin is a co-investigator on HiFi, the Heterodyne Instrument for the Infrared on the Hershel Space Observatory. With measurements from HiFi, the researchers found that the ice on a comet called Hartley 2 has the same chemical composition as our oceans. Both have similar D/H ratios. The D/H ratio is the proportion of deuterium, or heavy hydrogen, in the water. A deuterium atom is a hydrogen with an extra neutron in its nucleus.
This was the first time ocean-like water was detected in a comet.
“We were all surprised,” Bergin said.
Six other comets HiFi measured in recent years had a much different D/H ratio than our oceans, meaning similar comets could not have been responsible for more than 10 percent of Earth’s water.
The astronomers hypothesize that Hartley 2 was born in a different part of the solar system than the other six. Hartley most likely formed in the Kuiper belt, which starts near Pluto at about 30 times farther from the sun than the Earth is. The other six hail from the Oort Cloud more than 5,000 times farther out.
Caption: This is the comet Hartley, as imaged by NASA’s EPOXI spacecraft.Credit: Image courtesy of NASA
For decades, the source of Earth’s volatiles, especially water with a deuterium-to-hydrogen ratio (D/H) of (1.558 ± 0.001) × 10^−4, has been a subject of debate. The similarity of Earth’s bulk composition to that of meteorites known as enstatite chondrites1 suggests a dry proto-Earth with subsequent delivery of volatiles by local accretion or impacts of asteroids or comets. Previous measurements in six comets from the Oort cloud yielded a mean D/H ratio of (2.96 ± 0.25) × 10^−4. The D/H value in carbonaceous chondrites, (1.4 ± 0.1) × 10^−4, together with dynamical simulations, led to models in which asteroids were the main source of Earth’s water, with less than 10 per cent being delivered by comets. Here we report that the D/H ratio in the Jupiter-family comet 103P/Hartley 2, which originated in the Kuiper belt, is (1.61 ± 0.24) × 10^−4. This result substantially expands the reservoir of Earth ocean-like water to include some comets, and is consistent with the emerging picture of a complex dynamical evolution of the early Solar System