Herschel spots comet massacre around nearby star

Herschel image of the star Fomalhaut and its debris disc.
Credit: ESA/Herschel/PACS/Bram Acke, KU Leuven, Belgium

HI-RES JPEG (Size: 185 kb)

ESA’s Herschel Space Observatory has studied the dusty belt around the nearby star Fomalhaut. The dust appears to be coming from collisions that destroy up to thousands of icy comets every day.

Fomalhaut is a young star, just a few hundred million years old, and twice as massive as the Sun. Its dust belt was discovered in the 1980s by the IRAS satellite, but Herschel’s new images of the belt show it in much more detail at far-infrared wavelengths than ever before.

Bram Acke, at the University of Leuven in Belgium, and colleagues analysed the Herschel observations and found the dust temperatures in the belt to be between –230 and –170ºC. However, because Fomalhaut is slightly off-centre and closer to the southern side of the belt, the southern side is warmer and brighter than the northern side.

Both the narrowness and asymmetry of the belt are thought to be due to the gravity of a possible planet in orbit around the star, as suggested by earlier Hubble Space Telescope images.

The Herschel data show that the dust in the belt has the thermal properties of small solid particles, with sizes of only a few millionths of a metre across.

But this created a paradox because the Hubble Space Telescope observations suggested solid grains more than ten times larger.

Those observations collected starlight scattering off the grains in the belt and showed it to be very faint at Hubble’s visible wavelengths, suggesting that the dust particles are relatively large. But that appears to be incompatible with the temperature of the belt as measured by Herschel in the far-infrared.

To resolve the paradox, Dr Acke and colleagues suggest that the dust grains must be large fluffy aggregates, similar to dust particles released from comets in our own Solar System.

These would have both the correct thermal and scattering properties. However, this leads to another problem.

The bright starlight from Fomalhaut should blow small dust particles out of the belt very rapidly, yet such grains appear to remain abundant there.

The only way to overcome this contradiction is to resupply the belt through continuous collisions between larger objects in orbit around Fomalhaut, creating new dust.

To sustain the belt, the rate of collisions must be impressive: each day, the equivalent of either two 10 km-sized comets or 2000 1 km-sized comets must be completely crushed into small fluffy, dust particles.

“I was really surprised,” says Dr Acke, “To me this was an extremely large number.”

To keep the collision rate so high, there must be between 260 billion and 83 trillion comets in the belt, depending on their size. Our own Solar System has a similar number of comets in its Oort Cloud, which formed from objects scattered from a disc surrounding the Sun when it was as young as Fomalhaut.

“These beautiful Herschel images have provided the crucial information needed to model the nature of the dust belt around Fomalhaut,” says Göran Pilbratt, ESA Herschel Project Scientist.

Further information

Notes for Editors:

“Herschel Images of Fomalhaut – An extrasolar Kuiper Belt at the height of its dynamical activity,” by B. Acke et al, Astronomy & Astrophysics 540, A125, published online on 11 April 2012. DOI: 10.1051/0004-6361/201118581

For further information, please contact:

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

Bram Acke
University of Leuven
Tel: +32 16 327 939
Email: bram@ster.kuleuven.be

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

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Comet Corpses in the Solar Wind

A paper published in today's issue of Science raises an intriguing new possibility for astronomers: unearthing comet corpses in the solar wind. The new research is based on dramatic images of a comet disintegrating in the sun's atmosphere last July.

Comet Lovejoy grabbed headlines in Dec. 2011 when it plunged into the sun's atmosphere and emerged again relatively intact. But it was not the first comet to graze the sun. Last summer a smaller comet took the same trip with sharply different results. Comet C/2011 N3 (SOHO) was completely destroyed on July 6, 2011, when it swooped 100,000 km above the stellar surface. NASA's Solar Dynamics Observatory (SDO) recorded the disintegration.

Comet C/2011 N3 fragments as it passes through the sun's atmosphere on July 6, 2011. Credit: Solar Dynamics Observatory/K. Schrijver et al [larger image]

An extreme ultraviolet movie recorded by SDO shows comet Comet C/2011 N3 flying through the sun's atmosphere. [Quicktime video]

"For the first time, we saw a comet move across the face of the sun and disappear," says Dean Pesnell, a co-author of the Science paper and Project Scientist for SDO at the Goddard Space Flight Center. "It was unprecedented."

In Jan. 20th issue of Science, the research team reported their analysis of the SDO images.

A key finding was the amount of material deposited into the sun's atmosphere. "The comet dissolved into more than a million tons of electrically charged gas," says Pesnell. "We believe these vapors eventually mixed with the solar wind and blew back into the solar system."

Pesnell says it might be possible to detect such "comet corpses" as they waft past Earth. Comets are rich in ice (frozen H₂O), so when they dissolve in the hot solar atmosphere, the gaseous remains contain plenty of oxygen and hydrogen. A solar wind stream containing extra oxygen could be a telltale sign of a disintegrated comet. Other elements abundant in comets would provide similar markers.

Comet corpses are probably plentiful. There's a busy family of comets known as "Kreutz sungrazers," thought to be fragments of a giant comet that broke apart hundreds of years ago. Every day or so, SOHO sees one plunge into the sun and vanish. Each disintegration event creates a puff of comet vapor that might be detectable by spacecraft sampling the solar wind.

Why bother? Researchers are beginning to think of sungrazers as 'test particles' for studying the sun's atmosphere--kind of like tossing rocks into a pond. A lot can be learned about the pond by studying the ripples.

Indeed, SDO observed some extraordinary interactions between the sun and the doomed comet. As C/2011 N3 (SOHO) moved through the hot corona, cold gas lifted off the comet's nucleus and rapidly (within minutes) warmed to more than 500,000K, hot enough to shine brightly in SDO's extreme ultraviolet telescopes.

"The evaporating comet gas was glowing as brightly as the sun behind it," marvels Pesnell.

The gas was also rapidly ionized by a process called "charge exchange," which made the gas responsive to the sun's magnetic field. Caught in the grip of magnetic loops which thread the solar corona, the comet's ionized tail wagged back and forth wildly in the moments before final disintegration.

Watching this kind of sun-comet interaction could reveal new things about the thermal and magnetic structure of the solar atmosphere. Likewise, measuring how long it takes for "comet corpses" to reach Earth, and then sampling the gases when they arrive, could be very informative.

Watching this kind of sun-comet interaction could reveal new things about the thermal and magnetic structure of the solar atmosphere. Likewise, measuring how long it takes for "comet corpses" to reach Earth, and then sampling the gases when they arrive, could be very informative.

"Before SDO, no one dreamed we could observe a comet disintegrate inside the sun's atmosphere," says Pesnell who confesses that even he was a skeptic. But now, "I'm a believer."

The original research described in this story may be found in the Jan. 20th edition of Science: Destruction of Sun-grazing comet C/2011 N3 (SOHO) by C. J. Schrijver, J. C. Brown, K. Battams, P. Saint-Hilaire, W. Liu, H. Hudson, and W. D. Pesnell.

Author: Dr. Tony Phillips
Production editor: Dr. Tony Phillips
Credit: Science@NASA

More Information

Comet Lovejoy Plunges into the Sun and Survives -- Science@NASA

Comet's Demise Observed for the First Time -- videos from SDO

Some Comets Like it Hot -- Science@NASA feature story

Sungrazing Comet -- ScienceCast video

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NASA's Spitzer Detects Comet Storm in Nearby Solar System

This artist's conception illustrates a storm of comets around a star near our own, called Eta Corvi. Evidence for this barrage comes from NASA's Spitzer Space Telescope, whose infrared detectors picked up indications that one or more comets was recently torn to shreds after colliding with a rocky body. Image credit: NASA/JPL-Caltech . Full image and caption

PASADENA, Calif. -- NASA's Spitzer Space Telescope has detected signs of icy bodies raining down in an alien solar system. The downpour resembles our own solar system several billion years ago during a period known as the "Late Heavy Bombardment," which may have brought water and other life-forming ingredients to Earth.

During this epoch, comets and other frosty objects that were flung from the outer solar system pummeled the inner planets. The barrage scarred our moon and produced large amounts of dust.

Now Spitzer has spotted a band of dust around a nearby bright star in the northern sky called Eta Corvi that strongly matches the contents of an obliterated giant comet. This dust is located close enough to Eta Corvi that Earth-like worlds could exist, suggesting a collision took place between a planet and one or more comets. The Eta Corvi system is approximately one billion years old, which researchers think is about the right age for such a hailstorm.

"We believe we have direct evidence for an ongoing Late Heavy Bombardment in the nearby star system Eta Corvi, occurring about the same time as in our solar system," said Carey Lisse, senior research scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., and lead author of a paper detailing the findings. The findings will be published in the Astrophysical Journal. Lisse presented the results at the Signposts of Planets meeting at NASA's Goddard Space Flight Center in Greenbelt, Md., today, Oct. 19.

Astronomers used Spitzer's infrared detectors to analyze the light coming from the dust around Eta Corvi. Certain chemical fingerprints were observed, including water ice, organics and rock, which indicate a giant comet source.

The light signature emitted by the dust around Eta Corvi also resembles the Almahata Sitta meteorite, which fell to Earth in fragments across Sudan in 2008. The similarities between the meteorite and the object obliterated in Eta Corvi imply a common birthplace in their respective solar systems.

A second, more massive ring of colder dust located at the far edge of the Eta Corvi system seems like the proper environment for a reservoir of cometary bodies. This bright ring, discovered in 2005, looms at about 150 times the distance from Eta Corvi as the Earth is from the sun. Our solar system has a similar region, known as the Kuiper Belt, where icy and rocky leftovers from planet formation linger. The new Spitzer data suggest that the Almahata Sitta meteorite may have originated in our own Kuiper Belt.

The Kuiper Belt was home to a vastly greater number of these frozen bodies, collectively dubbed Kuiper Belt objects. About 4 billion years ago, some 600 million years after our solar system formed, scientists think the Kuiper Belt was disturbed by a migration of the gas-giant planets Jupiter and Saturn. This jarring shift in the solar system's gravitational balance scattered the icy bodies in the Kuiper Belt, flinging the vast majority into interstellar space and producing cold dust in the belt. Some Kuiper Belt objects, however, were set on paths that crossed the orbits of the inner planets.

The resulting bombardment of comets lasted until 3.8 billion years ago. After comets impacted the side of the moon that faces Earth, magma seeped out of the lunar crust, eventually cooling into dark "seas," or maria. When viewed against the lighter surrounding areas of the lunar surface, those seas form the distinctive "Man in the Moon" visage. Comets also struck Earth or incinerated in the atmosphere, and are thought to have deposited water and carbon on our planet. This period of impacts might have helped life form by delivering its crucial ingredients.

"We think the Eta Corvi system should be studied in detail to learn more about the rain of impacting comets and other objects that may have started life on our own planet," Lisse said.

NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages the Spitzer mission for the agency's Science Mission Directorate in Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA.

For more information about Spitzer, visit: http://spitzer.caltech.edu/ and http://www.nasa.gov/spitzer

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov

Trent J. Perrotto 202-358-0321
Headquarters, Washington
trent.j.perrotto@nasa.gov

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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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