Our Ocean’s Cosmic Origin
Most of the Earth’s surface is covered in water, half of which may be older than the Sun itself.
The origin of the Earth’s water – the source of life on our planet and likely off of it – is a wellspring of debate. For a long time, our abundant oceans were attributed to comets, which may have delivered water to the surface after our planet cooled. Recently, astronomers at JPL concluded that comets are an unlikely source for the oceans, leaving asteroids or even small planets at the outskirts of the solar system as the top contenders for water delivery to early Earth. However, that doesn’t answer the more fundamental question: where did that water come from?
“We sought to understand where the water in comets and elsewhere came from originally,” said astronomer Ilse Cleeves, lead author on the paper in Science. “The Earth likely formed ‘dry’ – without water initially – but was later supplied with water ice by asteroids and perhaps some amount of cometary material. We wanted to understand where that reservoir of water originated.”
The cosmic origin of Earth’s water came to light after Cleeves, whose work focuses on the molecules the make up the Solar System, met cosmochemist Conel Alexander. Alexander studies asteroids that have fallen to Earth in order to understand the materials that make up the galaxy. Together, Cleeves and Alexander went on a quest to sort out the origin of the Solar System’s H20 – which, it turns out, is more like HDO.
Normal hydrogen, H, is a lonely proton. Heavy hydrogen, D, is a proton and a neutron. When heavy hydrogen joins with oxygen, the result is heavy water, HDO. According to Cleeves’ results, there is more HDO in comets and on Earth than can be accounted for by the formation of our planetary disk.
“The clue here,” said Alexander, “is the D/H ratio of the water. The D/H ratio of water in comets, meteorites, the Earth, Saturn’s Moons Enceladus and Titan are all much more deuterium-rich that the bulk solar composition.”
This ratio of heavy hydrogen to light hydrogen (D/H) in various places throughout the Solar System allowed Cleeves and Alexander to make a model of our water’s origin that explains what we see today when we study oceans, asteroids and comets. The model hinges on an important difference: where heavy water should be found versus where it actually is found.
Extremely cold temperatures like those at the edge of Solar System drive the formation of heavy water, while warmer climates near the planets leave the odds of finding light water, H20, much higher. According to Cleeves, strong solar winds should have shielded large portions of the coalescing Solar System, making the formation of heavy water near Earth and the gas giants much less likely. Yet, when we look, the heavy water is here, there and everywhere. The conclusion: our heavy water didn’t arise from the same disk that gave rise to the Sun, but rather travelled here from afar.
“We found that there simply weren’t enough energetic ionizing sources in the cold gas of disks to synthesize heavy water,” said Cleeves, “It had to have come from elsewhere, and the only other source is the cold interstellar gas that formed the Sun. “
By starting with an estimate of how much heavy water could have formed around our young Sun and moving forward in time, Alexander and Cleeves’ model proved that some, if not a majority, of our water didn’t come from around here.
“We estimated that ≥7 % and as much as 30-50% of the Earth’s water could have been interstellar,” said Alexander, “while for comets it is ≥14% and as high as 100%.”
How and when heavy water that pre-dates the Solar System travelled here has yet to be determined, but some good data may soon emerge. In a few weeks time, the Rosetta mission is set to directly sample a comet with its lander Philae. If that mission succeeds, before the year is out we may have direct evidence of how much HDO a comet contains. This will hone our understanding the predictive native of models like Cleeves’ and Alexanders’. For now, estimates provided by their paper suggest that perhaps all of the water in comets is cosmic – derived from molecular clouds in-between solar systems, rather than local in origin.
So what does this mean for life outside the Solar System?
“It is remarkable that water survived the entire process of stellar birth,” Cleeves told Astrobio, “to then be incorporated into the planetary bodies in the solar system,”
As Alexander sees it, so much water surviving the birth of our Sun improves the odds of life evolving elsewhere in the Universe.
“Since the Solar System seems fairly typical, this indicates that some interstellar water and organic matter would survive the formation of most solar systems,” Alexander told Astrobio.
“Many people have speculated that the organic matter in comets and asteroids helped kick start life. If that organic material had an interstellar heritage, then most forming planetary systems who have had that same ‘soup’ of organic material available.”
This research was supported by the NSF, the Rackham Predoctoral Fellowship, NASA Astrobiology, NASA Cosmochemistry and NASA.