Researchers shed light on source of Earth’s water
By Marc Kaufman,
Water is everywhere on Earth, but nobody has ever been able to determine conclusively how it got here. Scientists know that the early Earth was far too hot to hold water or water vapor, but then, in relatively short geological time, the oceans appeared.
In a discovery that researchers say sheds important new light on that age-old question, a European team reported Thursday that it has found a very cold reservoir of water vapor in spacethat could explain where the water came from.
The region they discovered is at the outer reaches of a dusty disk surrounding a star 175 light-years away. The star and disk are in the early stages of forming planets, much as Earth was formed some 4.5 billion years ago.
The scientists’ conclusion from the new finding: Life-giving H2O was almost certainly delivered to Earth via comets and asteroids known to originate in these cold but water-filled zones, which were assumed to also be present when our solar system was forming.
“Our observations of this cold vapor indicates enough water exists in the disk to fill thousands of Earth oceans,” said astronomer Michiel Hogerheijde of Leiden Observatory in the Netherlands.
Hogerheijde is the lead author of a paper describing these findings in the Oct. 21 issue of the journal Science.
“Scientists have long suspected there were these reservoirs of cold water vapor hiding in the outer regions of planet-forming disks, but until now we’ve only found signs of water vapor in hot regions closer to the suns,” Hogerheijde said in an interview. “Since the comets and cold asteroids are formed in the outer reaches, this was a problem for the theory that comets delivered the water to Earth. But now we have the cold reservoir in the region where comets are formed, and so the theory gets considerably stronger.”
The logical extension, he said, is that water has also been delivered to some of the billions of exoplanets known to exist beyond our solar system, meaning there are likely to be many “ocean worlds” throughout the galaxies.
Hogerheijde said the 10 million-year-old star his team examined, TW Hydrae, is the closest planet-forming star yet identified.
Signs of the cold water vapor were detected by the Herschel Space Observatory, a European Space Agency satellite that looks for infrared light in the galaxy using the Heterodyne Instrument for the Far-Infrared, or HIFI. Efforts to find the cold water vapor in the past all failed because the instruments were not powerful enough to pick up the faint spectroscopic signals.
NASA also partially funds Herschel, and American researchers were part of the team.
“It is a testament to the instrument-builders that such weak signals can be detected,” said Paul Goldsmith, NASA project scientist for the Herschel Space Observatory at the agency’s Jet Propulsion Laboratory in Pasadena, Calif. “These are the most sensitive HIFI observations to date.”
According to Hogerheijde, the cold water vapor detected is a small portion of the “ice reservoir” existing in the region. The ice crystals, which cover the widespread dust particles, form in conditions reaching 400 degrees below zero. But ultraviolet light from the star warms the ice enough to briefly release the vapor that was detected, Hogerheijde said.
The announcement comes weeks after a related finding that the water in some comets has the same chemical composition of Earth’s oceans. Previous detections had found water with a different isotopic make-up, suggesting that no more than 10 percent of the Earth’s water was delivered by comet.
But data from the comet Hartley 2 found the ratio of heavy hydrogen (deuterium) to ordinary hydrogen to be almost exactly what it is in Earth’s oceans.
“Now, in principle, all the water [in Earth’s oceans] could have come from comets,” said principal investigator Paul Hartogh of Germany’s Max Planck Institute.
Planetary scientists have determined that huge amounts of graphite and silicon dust surround stars as they form. That material over time binds together to form larger bodies such as comets and asteroids and — around many stars — ultimately planets.
Earth formed about 4.5 billion years ago, they have concluded, and was then too close to the sun to hold much water or water vapor. But around 4.1 billion years ago began a period of “heavy bombardment,” when Earth was pummeled by comets and cold meteorites — both of which carry water — from the outer reaches of its disk.
The bombardment ended about 3.8 billion years ago, and at that point much of the Earth’s water was in place. The earliest forms of microbial life detected lived some 3.6 billion years ago, a relatively short geological period after the oceans had filled.
While there is growing evidence for the explanation that comets and wet asteroids delivered our oceans, some researchers hold that the water came primarily from other sources. For instance, water is believed to have been released from early volcanos that belched up molten material from deep within the planet, including H2O. Water could also leak out of certain minerals in rocks as the planet cooled.
Hogerheijde said that while this early “outgassing” most likely played a role in making early Earth wet, the evidence is now persuasive that much of the water for our oceans was later delivered from afar.