We were all surprised, said Ted Bergin, a University of Michigan astronomy professor.
Scientists believe Earth's oceans formed about 8 million years after the planet itself.
Water is the source of life on Earth. Without it man would die of thirst in less than a week.
Life would not exist on Earth without liquid water, and so the questions of how and when the oceans got here is a fundamental one, Bergin said. It's a big puzzle and these new findings are an important piece.
For decades researchers have been debating where the planet's water came from. Planet-formation models have shown that early Earth was much too dry in order to bear liquid water on its surface. Hence, that is why the origin of Earth's oceans a mystery.
Scientists have theorized that comets slammed into Earth after the planet cooled. However, in the 1980s, measurements of the ratio of normal to semiheavy molecules - the D/H ratio - in comet water led to questions about this speculation. Since then researchers found that several comets in our solar system have D/H ratios that are much different from what's in Earth's water.
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.
In semiheavy water molecule, one hydrogen atom (H) is replaced with a heavier type called deuterium (D). All water has a D/H ratio. Deuterium is a very stable atom and this ratio can go unaffected for years.
Study leader Paul Hartogh, an astronomer at the Max Planck Institute for Solar System Research in Germany, told National Geographic that those results show that possibly only about 10 percent of Earth's water could've come from comets.
In January, researchers have noted that comets may have also supplied water to the Moon.
When astrophysicists led by James Greenwood of Wesleyan University in Connecticut examined lunar rock samples, they found that minerals on the moon had similar atomic signatures to comets, according to research published in Nature Geoscience.
Planetary researchers such as Lawrence Taylor, from the University of Tennessee, Knoxville, and his colleague Yang Liu, wrote that the current study might suggest the solar system may have more water with ocean-like qualities anticipated.
This claim, if substantiated with additional measurements in future, suggests significant modifications on current models of early solar system dynamics and origin of water in inner solar system bodies, the researchers wrote in an email to IB Times. They were not involved in the current study. It is imagined that during the early formation of the Earth-Moon system, just after the Giant Impact, the Earth already had most of its water, but the Moon has none, the researchers wrote. It is imagined that during the early formation of the Earth-Moon system, just after the Giant Impact, the Earth already had most of its water, but the Moon has none.
Bergin is also a co-investigator on HiFi, the Heterodyne Instrument for the Infrared on the Hershel Space Observatory. He and other researchers were able to find that the ice on Hartley 2 had similar composition to the Earth's oceans based on measurements from HiFi.
Both have similar D/H ratios, researchers said.
Only six other comets HiFi measured in recent years had a much different D/H ratio than Earth's oceans. This means that similar comets couldn't have been responsible for more than 10 percent of Earth's water, according to researchers of the current study published in the journal Nature.
Therefore, the astronomers hypothesize that Hartley 2 originated from a different part of the solar system than the other six comets.
Researchers said Hartley most likely formed in the Kuiper belt. This starts near Pluto at about 30 times farther from the sun than the Earth is. The other six comets come from the Oort Cloud more than 5,000 times farther out.
The results show that the amount of material out there that could have contributed to Earth's oceans is perhaps larger than we thought, Bergin said.