The mysterious source of Earth’s water has intrigued generations of scientists. Learning how this liquid—the cornerstone of life as we know it—made its way to our planet has sweeping implications, for the possibility of alien biospheres not only elsewhere in the solar system but also on worlds orbiting other stars. But understanding how water arrived on Earth has proven surprisingly difficult.

After the sun formed from a cloud of dust and gas, the remaining protoplanetary disk of material was probably rich in water’s raw ingredients, hydrogen and oxygen. But conventional wisdom holds that the newborn star’s radiance boiled away much of those volatile gases from the inner solar system, leaving mostly dry material from which to build Earth and the other rocky planets. The majority of Earth’s moisture must have arrived later, by some other means.

For decades, scientists considered icy comets of the outer solar system as the most likely suspects, until observations revealed that most comets’ compositions did not quite match that of Earth’s oceans. And so consensus shifted toward asteroids as the source of Earth’s seas, since these rocky bodies also contain nontrivial amounts of water and are conveniently located close by, where they could have easily rained down on the young Earth. Now, however, an investigation of comet 46P/Wirtanen suggests that the bulk of Earth’s water may have come from comets after all, even though asteroids likely still played an important role.

Using NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA), an airplane-mounted telescope that can fly above much of Earth’s atmosphere, a team of researchers measured the proportion of heavy water, or deuterium, to normal water in comet 46P. Whereas the hydrogen nucleus of regular water contains a solitary neutron, deuterium’s nucleus contains both a proton and neutron, making it twice as heavy—and, more importantly, making it evaporate more slowly than normal water. This means the deuterium-to-hydrogen ratio (D/H) of any given object would be expected to vary depending on the distance at which it formed and lingered around the young sun, allowing the ratio to serve as a fingerprint for tracing water’s origins. Find a comet or asteroid with a D/H ratio identical to Earthly seawater, and you have perhaps found a chunk of undelivered ocean; obtaining D/H ratios for multiple objects may yield patterns that reveal the migration of water around the early solar system. Out of a handful of comets whose D/H ratio have been studied, comet 46P is the third known to have a D/H ratio similar to Earth’s.

“It’s fantastic that they’ve got another D/H ratio,” says cometary scientist Karen Meech, of the University of Hawaii’s Institute for Astronomy. Meech was not part of the new research. “It’s very important for trying to understand what’s going on.”

D/H may trace water through the young planetary disk, but it turns out to be a tricky process. Some models suggest the abundance of deuterium grows linearly moving away from the sun; others suggest the abundance shrinks under those same circumstances. Several that seek to replicate the chaotic, turbulent mixing of material in the early solar system predict deuterium abundances varying wildly at different points for no discernible reason. And observations have shown indeed that comets—even those apparently born in close proximity to each other—can have dramatically different D/H ratios. “Until now, we had a dozen measurements that looked kind of random,” says team leader Dariusz Lis, an astrophysicist at the California Institute of Technology. But 46P revealed a surprising new relationship that makes at least some of the measurements appear a little less random. Along with 46P, the other two comets known to have D/H ratios similar to Earth’s oceans, comets 103P/Hartley and 45P/Honda-Mrkos-Pajdušáková, are “hyperactive” objects, meaning they spew off more water than would be predicted based on their surface area alone. “Now, for the first time, we see a correlation between the D/H ratio … and the active fraction,” Lis says.

The results may have implications for all comets. The excess activity in hyperactive comets comes from water brought up from their interior. If, as Lis and his co-authors suggest, water from hyperactive comets’ nuclei has a more Earth-like D/H fingerprint, this may mean Earth-like water could be hidden deep inside other, nonhyperactive comets as well, putting the spotlight back on comets as an early water source.

Soon to be published in the journal Astronomy & Astrophysics, the result could not only bolster the case for comets as deliverers of Earth’s water, but also tweak the initial conditions that led to life’s origins. “If you knew comets were raining down on Earth during the early stages of formation, that would have profound implications for what material was available for the very beginning stages of life,” says Maria Womack, a comet researcher at the University of South Florida who was not part of the new study.

When comets draw close to the sun, their icy surface warms, jumping from solid to gas through a process called sublimation. Hyperactive comets such as comet 46P, however, do something more, somehow spewing off large chunks of ice into their coma, the nebulous cloud that surrounds the cometary nucleus. The tumbling ice chunks remain solid, sublimating in the coma rather than on the surface and providing the “hyper” in hyperactivity.

Those solid chunks could explain the near-Earthly D/H ratio in comets like 46P. Lis and his colleagues suggest that, even if a comet’s surface material is heated and altered by the sun, its inner nucleus could remain relatively pristine for eons. On the surface, solar heat and radiation could evaporate some of the regular water, changing the ratio of normal and heavy water. Deep inside, however, those ratios may remain unchanged from their initial fingerprint (one that could match Earth’s oceans) set billions of years ago during the solar system’s formation. Heat-induced pressures in the comet trigger the release of volatile gases such as carbon dioxide or carbon monoxide, which are buried deep down in the nucleus. As the heated volatiles rise, they may push material from the nucleus to the surface, where it is blasted off to sublimate in the coma, revealing a fingerprint strikingly similar to Earth’s. If that is the case, the researchers suggest that all comets may carry water in their nucleus with a D/H ratio more like our planet’s.

Meech is not yet convinced. In 2005, NASA’s Deep Impact mission blasted a crater in comet Tempel 1. Meech, who was part of that mission, says it showed that fresh material was only a few centimeters beneath the surface rather than hidden deep inside the nucleus. Thus, material blown from the heart of a comet should be similar to what is sublimating from the near-surface. Other missions to comets seem to support that finding. “Based on what was seen with the Deep Impact, EPOXI and Rosetta missions, I, don’t see any reason why the stuff [a hyperactive comet is] ejecting would be any more or less primitive than any other comet,” she says.

Others, such as comet researcher David Jewitt at the University of California, Los Angeles, are more concerned with simply getting that water to Earth. In addition to D/H ratios, celestial mechanics make a solid argument for asteroids as a dominant source of Earth’s water. Asteroids from the asteroid belt can crash into Earth much more readily than even the closest comets in the outer solar system, and research has revealed that many asteroids contain water with Earth-like fingerprints locked up inside of minerals. And, given the relative ease with which asteroids can pummel the inner planets, it is straightforward to envision them bombarding Earth in necessary numbers to fill the oceans—something that cannot be readily said for comets. According to Jewitt, all of the water in Earth’s oceans would make a single ball about 600 kilometers across or about a billion one-kilometer sized comets roughly the size of 46P. (The average comet is less than 10 kilometers across.)

The idea that all comets carry Earth-like water in their nucleus remains “a very provocative idea,” says Sean Raymond, a researcher at the Laboratoire d’Astrophysique de Bordeaux in France who models early solar system evolution. “It’s definitely one worth testing.” More in-depth laboratory tests could help reveal whether a comet hiding Earth-like water could be giving off a different D/H ratio, Jewitt says, and that could provide insights into water in the early solar system. But alone, it’s not enough.

Right now, with only three hyperactive comets and a handful of regular comets having measured D/H ratios, the connection between the two remains nebulous. Fundamentally, the most important way to test whether all comets harbor Earth-like water in their nuclei is to find and study many more. “We’ve got to go out and get more of these and see if that prediction holds true,” says Edwin Bergin, a researcher at the University of Michigan who hunts for water in the protoplanetary disks around other stars. Bergin was not part of the new research.

Improving technology should continue to make it easier to measure the D/H ratio of more comets from the ground, while future missions could make even more detailed observations from space. “We need more measurements,” Lis says. “We have gathered a little more than a dozen measurements in the past 25 years. That’s not enough to make a statistical study.”

Quelle: SCIENTIFIC AMERICAN