Space & Innovation

Earth's Water Didn't Come from Comets Like Rosetta's

The European mission has discovered that the water locked in Comet 67P/Churyumov-Gerasimenko doesn't match the water on Earth.

Europe's Rosetta spacecraft has found that water shedding from its host comet has a different chemical fingerprint than water on Earth, raising serious doubts that Kuiper Belt cousin comets were the source of baby Earth's oceans.

Scientists have long wondered how Earth got its water. Any indigenous water molecules likely were lost to space during the planet's hot and violent formation.

The leading theory is that the water came later, during a period roughly 800 million years after the solar system's formation when the inner planets were being blitzed by asteroids, comets and other small bodies. It was during this time, known as the "late heavy bombardment" that a Mars-sized object is believed to have crashed into Earth, sending clouds of debris into space that later consolidated into the moon.

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With their icy bodies, comets were an early first choice to be Earth's water-bearers, an idea buttressed by studies three years ago when Europe's Herschel spacecraft made chemical measurements of water in another Kuiper Belt transplant, Comet Hartley 2. Its water matched Earth's perfectly.

Comet 67P/Churyumov-Gerasimenko, the subject of the ongoing Rosetta studies, tells a different story. Its water has three times more of the hydrogen isotope deuterium than Earth's water, one of the highest concentrations ever measured in a solar system body, said University of Bern's Kathrin Altwegg, lead researcher for one of Rosetta's science instruments.

"That now probably rules out Kuiper Belt comets as bringing the water on the Earth," Altwegg said.

Even a mixture of water from Hartley 2-type comets and those chemically similar to 67P would have too much deuterium to match what exists on Earth today.

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The ratio of deuterium to regular hydrogen in 67P is so high "you need only a small fraction of these comets to (spoil) the Hartley 2 results if you mix them up to make terrestrial water," Altwegg told Discovery News.

Additional measurements from other comets may shed more light on the mystery. The new findings, the first reported since Rosetta arrived at Comet 67P in August, increase the odds that asteroids were the source of Earth's water.

"Today asteroids have very limited water, that's clear. But that was probably not always the case. The late heavy bombardment was 3.8 billion years ago and at that time asteroids could well have had much more water than they have today. They have just lived in the vicinity of the sun for 4.6 billion years" and lost water due to heating, Altwegg said.

Earth's Water Didn't Come from Comets Like Rosetta's (Page 2)

Related studies show the line between asteroids and comets is blurring. Astronomers have discovered comet-like jetting asteroids, tail-less comets that look like asteroids and asteroids with comet-like tails.

"Comets also are amazingly asteroid-like ... in their non-ice components," added astronomer Donald Brownlee, with the University of Washington and lead researcher for NASA's Stardust mission, which in 2006 returned samples from a comet.

The Rosetta results also show that comets hailing from the Kuiper Belt, a region of icy bodies beyond Neptune's orbit, formed at different distances from the sun and under different conditions.

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That idea is consistent with results from the Stardust mission, which found large-scale mixing of materials formed in the inner and outer solar system.

Rosetta scientists and engineers meanwhile continue the hunt for the Philae lander, which last month touched down on the comet's surface, bounced twice and finally came to rest at a still-to-be-determined location, most likely in a crater.

"One side of the lander appears to be in a hole. We see the legs sticking up in the air. We can see a bit of the sky. We also see an overhanging cliff-like structure, so it gives us an idea of what the local terrain is like," said Rosetta project scientist Matt Taylor with the European Space Agency.

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The verdict is still out as to whether Philae was able to successfully deliver samples drilled out from the comet's body into ovens to chemical analysis.

"It looks like maybe -- well, I would say possibly not -- but we're waiting for some more conclusions," Taylor told Discovery News.

The Rosetta team's research is published in this week's Science.

This four-image mosaic comprises images taken by Rosetta’s navigation camera on Dec. 7, 2014, from a distance of 19.7 km from the centre of Comet 67P/Churyumov-Gerasimenko.

On Aug. 6, 2014, the European Space Agency's Rosetta spacecraft completed its decade-long journey to reach Comet 67P/Churyumov-Gerasimenko, becoming the first spacecraft to ever orbit a comet. The mission will reach its epic climax when it releases a small robotic lander, called Philae, onto the cometary surface in November. The lander will drill into the surface while Rosetta tags along with the comet's orbit as Churyumov-Gerasimenko makes close approach of the sun. Although Rosetta is unprecedented in that no other mission has achieved orbital insertion around a comet, it's certainly not the first robotic probe to make an intimate cometary encounter. So here's a rundown of 7 encounter of 6 comets by 5 spacecraft since the first close encounter with Halley's Comet in 1986.

Unquestionably the most famous comet in history, Halley's Comet was a prime target for space agencies in 1986 during its 75- to 76-year orbit through the inner solar system. Comet science is still a developing field, but in 1986, very little was known about the composition of these interplanetary vagabonds. In October of that year, the 15-kilometer-long Halley's Comet was visited by the European Space Agency's Giotto mission. The half-ton probe came within 600 kilometers (373 miles) of the comet's nucleus, taking the first photographs of the outgassing vapor from discrete areas of the surface producing its tail and coma (the gas surrounding the nucleus). It was this mission that confirmed the "dirty snowball" theory of cometary composition: a mix of volatile ices and dust. However, Giotto was only able to get so close to the famous comet with the help of the "Halley Armada," a number of international spacecraft all tasked with observing this rare event. Giotto captured the closest imagery, but two Russia/France probes (Vega 1 and 2) and two Japanese craft (Suisei and Sakigake) observed from afar.

At roughly half the size of Halley's comet, Comet Borrelly was found to have similar attributes to its famous cousin. The nucleus was also potato-shaped and blackened. Outgassing vapor was also observed coming from cracks in the nucleus crust where volatiles were exposed to sunlight, sublimating ices into space. NASA's Deep Space 1 probe flew past the comet with a close approach of 3,417 kilometers on Sept. 22, 2001.

Comet Wild 2 -- pronounced "Vilt" after its Swiss discoverer Paul Wild who spotted it in 1978 -- underwent a dramatic alteration in 1974. It is calculated that due to a close pass of Jupiter in 1974, the 5 kilometer-wide comet now orbits the sun every 6 years as opposed to its leisurely 43 years before the gas giant bullied it. The orbital modification meant that Wild 2 was an ideal target for NASA's Stardust mission to lock onto. On Jan. 4, 2004, the Stardust probe gave chase, getting so close to the comet that it was able to collect particles from Wild 2's coma. This image was taken at a distance of less than 240 kilometers (149 miles). The Stardust sample return canister came back to Earth safely, landing in Utah on Jan. 15, 2006. The microscopic particles captured from the comet continue to provide a valuable insight into the organic compounds comets contain. Interestingly, the Stardust spacecraft was granted a mission extension (dubbed New Exploration of Tempel 1 -- NExT). In 2011 it rendezvoused with its second comet, Tempel 1 -- the scene of NASA's 2005 Deep Impact mission -- to analyze the crater that Deep Impact's impactor left behind on the cometary surface.

NASA's Deep Impact mission reached the eight-kilometer-wide (five-mile-wide) comet Tempel 1 in 2005. On July 4, the probe deliberately smashed its impactor into the comet's nucleus, producing a cloud of fine material. A crater -- 100 meters wide (328 feet) by 30 meters (98 feet) deep -- was left behind. A treasure trove of compounds were spotted by the Deep Impact spacecraft and the explosion could be observed from Earth. In 2011, the recycled Stardust-NExT mission visited comet Tempel 1 for the second time.

The fifth space probe encounter with a comet happened on Nov. 4, 2010. NASA's recycled Deep Impact probe -- now the EPOXI mission -- visited comet Hartley 2, examining its strange-shaped nucleus. Described as a "peanut" or "chicken drumstick," this comet is an oddity. During its close approach of under 700 kilometers (435 miles), EPOXI photographed the comet's irregular topography: two rough lobes connected by a smooth center. Jets of gas could be seen being ejected from discrete locations. During the Hartley 2 flyby press conference at NASA's Jet Propulsion Laboratory (JPL), mission scientists expressed their surprise that these jets of vapor are being emitted from sun-facing and shaded regions on the comet surface. Needless to say, analysis of the Hartley 2 flyby data will keep scientists busy for some time to come. "This is an exploration moment," remarked Ed Weiler, NASA's Associate Administrator for the Science Mission Directorate, during the conference.

On Feb. 14, 2011, the veteran Stardust-NExT (New Exploration of Tempel) mission made history by visiting a comet for the second time. Comet Tempel 1 was first encountered by NASA's Deep Impact mission in 2005 after smashing the cometary nucleus with an impactor. This second encounter provided scientists with an unprecedented opportunity to study the same comet after six years of orbiting the sun. Preliminary findings suggested Tempel 1 has undergone some erosion during those six years in deep space. Also, the impact crater left behind by Deep Impact was imaged during the Stardust-NExT flyby and it appeared to match the size and shape predicted after the 2005 impact. However, the crater appeared smoother than expected, so work is ongoing to analyze the 72 photographs taken by the flyby to understand the processes shaping the comet's nucleus.

At 5:29 a.m. EDT (9:29 a.m. GMT) on Aug. 6, 2014, the European Rosetta spacecraft completed a 6.5 minute-long engine burn to insert itself into orbit around Comet 67P/Churyumov-Gerasimenko. Once under the influence of the comet's weak gravity, the spacecraft began to carry out a series of triangular loops, taking several days to complete. The long-duration mission is the first of its kind, where the spacecraft will study the comet from orbit, watching for surface changes as it approaches the sun, making perihelion (the point of closest solar approach). In November, a small lander called Philae will touch down on the surface to drill into the comet's material, revealing its small-scale composition. This photograph from Rosetta was captured on Aug. 3 when the probe was fast approaching the comet at a distance of less than 300 kilometers.

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