Did Earth's oceans come from comets?

Comet Hartley 2 observed by ESA’s Herschel

This illustration shows the orbit of comet Hartley 2 in relation to those of the five innermost planets of the Solar System. The comet made its latest close pass of Earth on 20 October, coming to 19.45 million km. On this occasion, Herschel observed the comet. The inset on the right side shows the image obtained with Herschel’s PACS instrument. The two lines are the water data from HIFI instrument. Credits: ESA/AOES Medialab; Herschel/HssO Consortium. HI-RES JPEG (Size: 168 kb)

Comet Hartley 2’s orbit in context

The left panel shows Comet Hartley 2’s orbit. The central panel shows a larger portion of the Solar System, including the Kuiper Belt. The Kuiper Belt is one of the two main reservoirs of comets in the Solar System. Comets like Hartley 2 are believed to have formed here and to have migrated inwards. The right panel shows the Oort Cloud, the other main reservoir of comets located well beyond the outer Solar System. Credits: ESA/AOES Medialab. HI-RES JPEG (Size: 458 kb)

The Heterodyne Instrument for the Far Infrared (HIFI) is a high-resolution heterodyne spectrometer. It works by mixing the incoming signal with a stable monochromatic signal, generated by a local oscillator, and extracting the frequency difference for further processing in a spectrometer. HIFI will have seven separate local oscillators covering two bands from 480-1250 gigaHertz and 1410–1910 gigaHertz. HIFI was developed by a consortium led by SRON (Groningen, The Netherlands). Credits: ESA (image by C. Carreau). HI-RES JPEG (Size: 1020 kb)

ESA's Herschel infrared space observatory has found water in a comet with almost exactly the same composition as Earth's oceans. The discovery revives the idea that our planet's seas could once have been giant icebergs floating through space.

The origin of Earth's water is hotly debated. Our planet formed at such high temperatures that any original water must have evaporated. Yet today, two-thirds of the surface is covered in water and this must have been delivered from space after Earth cooled down.

Comets seem a natural explanation: they are giant icebergs travelling through space with orbits that take them across the paths of the planets, making collisions possible. The impact of comet Shoemaker-Levy 9 on Jupiter in 1994 was one such event. But in the early Solar System, when there were larger numbers of comets around, collisions would have been much more common.

However, until now, astronomers' observations have failed to back up the idea that comets provided Earth's water. The key measurement they make is the level of deuterium – a heavier form of hydrogen – found in water.

All the deuterium and hydrogen in the Universe was made just after the Big Bang, about 13.7 billion years ago, fixing the overall ratio between the two kinds of atoms. However, the ratio seen in water can vary from location to location. The chemical reactions involved in making ice in space lead to a higher or lower chance of a deuterium atom replacing one of the two hydrogen atoms in a water molecule, depending on the particular environmental conditions.

Thus, by comparing the deuterium to hydrogen ratio found in the water in Earth's oceans with that in extraterrestrial objects, astronomers can aim to identify the origin of our water.

All comets previously studied have shown deuterium levels around twice that of Earth's oceans. If comets of this kind had collided with Earth, they could not have contributed more than a few percent of Earth's water. In fact, astronomers had begun to think that meteorites had to be responsible, even though their water content is much lower.

Now, however, Herschel has studied comet Hartley 2 using HIFI, the most sensitive instrument so far for detecting water in space, and has shown that at least this one comet does have ocean-like water.

"Comet Hartley's deuterium-to-hydrogen ratio is almost exactly the same as the water in Earth's oceans," says Paul Hartogh, Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany, who led the international team of astronomers in this work.

The key to why comet Hartley 2 is different may be because of where it was born: far beyond Pluto, in a frigid region of the Solar System known as the Kuiper Belt.

The other comets previously studied by astronomers are all thought to have formed near to Jupiter and Saturn before being thrown out by the gravity of those giant planets, only to return much later from great distances.

Thus the new observations suggest that perhaps Earth's oceans came from comets after all – but only a specific family of them, born in the outer Solar System. Out there in the deep cold, the deuterium to hydrogen ratio imprinted into water ice might have been quite different from that which arose in the warmer inner Solar System.

Herschel is now looking at other comets to see whether this picture can be backed up.

"Thanks to this detection made possible by Herschel, an old, very interesting discussion will be revived and invigorated," says Göran Pilbratt, ESA Herschel Project Scientist.

"It will be exciting to see where this discovery will take us."

Contact for further information:

Markus Bauer ESA Science and Robotic Exploration Communication Officer Email: markus.bauer@esa.int
Tel: +31 71 565 6799 Mob: +31 61 594 3 954

Paul Hartogh
Max-Planck-Institut für Sonnensystemforschung
Tel: +49 5556 979 342
Email: hartogh@mps.mpg.de

Göran Pilbratt
ESA Herschel Project Scientist
Tel: +31 71 565 3621
Email: gpilbratt@rssd.esa.int

Notes to editors

Ocean-like water in the Jupiter-family comet 103P/Hartley 2 by Paul Hartogh et al. is published online today DOI: 10.1038/nature10519 - http://dx.doi.org/10.1038/nature10519.

Herschel studied comet Hartley on 17 November 2010 using the Heterodyne Instrument for the Far Infrared (HIFI).

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Science Paper Details NASA Epoxi Flyby of Hyper Comet

This image of comet Hartley 2 was taken as NASA’s EPOXI mission flew by around 6:59 a.m. PDT (9:59 a.m. EDT), from a distance of about 435 miles (700 kilometers). credit: NASA/JPL-Caltech/UMD .

A large, diffuse cloud of CN gas surrounds the nucleus of Hartley 2 in this image from NASA's EPOXI mission. The gas forms a cloud of more than 200,000 kilometers (about 124,000 miles) in radius, compared to the comet's size of about 2 kilometers (1.24 miles). Credit: NASA/JPL-Caltech/UMD.

PASADENA, Calif. – Comet Hartley 2's hyperactive state, as studied by NASA's EPOXI mission, is detailed in a new paper published in this week's issue of the journal Science.

After visiting a comet and imaging distant stars for hints of extrasolar planets, you could say the spacecraft used for EPOXI had seen its fair share of celestial wonders. But after about 3.2 billion miles (5.1 billion kilometers) of deep space travel, one final wonder awaited the mission's project and science teams. On Nov. 4, 2010, the EPOXI mission spacecraft flew past a weird little comet called Hartley 2.

"From all the imaging we took during approach, we knew the comet was a little skittish even before flyby," said EPOXI Project Manager Tim Larson of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "It was moving around the sky like a knuckleball and gave my navigators fits, and these new results show this little comet is downright hyperactive."

The EPOXI mission found that the strong activity in water release and carbon dioxide-powered jets did not occur equally in the different regions of the comet. During the spacecraft's flyby of the comet – with closest approach of 431 miles (694 kilometers) – carbon-dioxide-driven jets were seen at the ends of the comet, with most occurring at the small end. In the middle region, or waist of the comet, water was released as vapor with very little carbon dioxide or ice. The latter findings indicate that material in the waist likely came off the ends of the comet and was redeposited.

"Hartley 2 is a hyperactive little comet, spewing out more water than most other comets its size," said Mike A'Hearn, principal investigator of EPOXI from the University of Maryland, College Park. "When warmed by the sun, dry ice -- frozen carbon dioxide -- deep in the comet's body turns to gas jetting off the comet and dragging water ice with it."

Although Hartley 2 is the only such hyperactive comet visited by a spacecraft, scientists know of at least a dozen other comets that also are relatively high in activity for their size and which are probably driven by carbon dioxide or carbon monoxide.

"These could represent a separate class of hyperactive comets," said A'Hearn. "Or they could be a continuum in comet activity extending from Hartley 2-like comets all the way to the much less active, 'normal' comets that we are more used to seeing."

The study provides several new twists in the unfolding story of this small cometary dynamo, including: (1) Hartley 2 has an 'excited state of rotation' because it spins around one axis, but also tumbles around a different axis; and (2) on its larger, rougher ends, the comet's surface is dotted with glittering blocks that can reach approximately 165 feet (50 meters) high and 260 feet (80 meters) wide. The block-like, shiny objects, some as big as one block long and 16 stories tall, appear to be two to three times more reflective than the surface average.

EPOXI was an extended mission that utilized the already "in-flight" Deep Impact spacecraft to explore distinct celestial targets of opportunity. The name EPOXI itself is a combination of the names for the two extended mission components: the extrasolar planet observations, called Extrasolar Planet Observations and Characterization (EPOCh), and the flyby of comet Hartley 2, called the Deep Impact Extended Investigation (DIXI). The spacecraft retained the name "Deep Impact." During its approach, encounter and departure from comet Hartley 2, the spacecraft beamed back more than 117,000 images and spectra.

JPL managed the EPOXI and Deep Impact missions for NASA's Science Mission Directorate, Washington. The EPOXI mission was part of the Discovery Program managed at NASA's Marshall Space Flight Center in Huntsville, Ala. The University of Maryland, College Park, is home to Michael A'Hearn, principal investigator for EPOXI. Drake Deming of NASA's Goddard Space Flight Center, Greenbelt, Md., is the science lead for the EPOXI mission's extrasolar planet observations. The spacecraft was built for NASA by Ball Aerospace & Technologies Corp., Boulder, Colo.

DC Agle 818-393-9011
Jet Propulsion Laboratory, Pasadena, Calif
agle@jpl.nasa.gov

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