Cometary Impact on Neptune Two Centuries Ago

July 16, 2010
Two centuries ago a comet may have hit Neptune, the outer-most planet in our solar system. Image: NASA

( -- ( -- A comet may have hit the planet Neptune about two centuries ago. This is indicated by the distribution of carbon monoxide in the atmosphere of the gas giant that researchers - among them scientists from the French obser-vatory LESIA in Paris, from the Max Planck Institute for Solar System Re-search (MPS) in Katlenburg-Lindau (Germany) and from the Max Planck Insti-tute for Extraterrestrial Physics in Garching (Germany) - have now studied. The scientists analyzed data taken by the research satellite Herschel, that has been orbiting the Sun in a distance of approximately 1.5 million kilome-ters since May 2009. (Astronomy & Astrophysics, published online on July 16th, 2010)

When the Shoemaker-Levy 9 hit Jupiter sixteen years ago, scientists all over the world were prepared: instruments on board the space probes Voyager 2, Galileo and Ulysses documented every detail of this rare incident. Today, this data helps scientists detect cometary impacts that happened many, many years ago. The "dusty snowballs" leave traces in the atmosphere of the gas giants: water, carbon dioxide, carbon monoxide, hydrocyanic acid, and carbon sulfide. These molecules can be detected in the radiation the planet radiates into space.

In February 2010 scientists from Max Planck Institute for Solar System Research discovered strong evidence for a cometary impact on Saturn about 230 years ago (see Astronomy and Astrophysics, Volume 510, February 2010). Now new measurements performed by the instrument PACS (Photodetector Array Camera and Spectrometer) on board the Herschel space observatory indicate that experienced a similar event. For the first time, PACS allows researchers to analyze the long-wave infrared radiation of Neptune.

The atmosphere of the outer-most planet of our solar system mainly consists of hydrogen and helium with traces of water, carbon dioxide and carbon monoxide. Now, the scientists detected an unusual distribution of carbon monoxide: In the upper layer of the atmosphere, the socalled stratosphere, they found a higher concentration than in the layer beneath, the troposphere. "The higher concentration of carbon monoxide in the stratosphere can only be explained by an external origin", says MPS-scientist Paul Hartogh, principle investigator of the Herschel science program "Water and related chemistry in the ". "Normally, the concentrations of carbon monoxide in troposphere and stratosphere should be the same or decrease with increasing height", he adds.

The only explanation for these results is a cometary impact. Such a collision forces the comet to fall apart while the carbon monoxide trapped in the comet’s ice is released and over the years distributed throughout the stratosphere. "From the distribution of carbon monoxide we can therefore derive the approximate time, when the impact took place", explains Thibault Cavalié from MPS. The earlier assumption that a comet hit Neptune two hundred years ago could thus be confirmed. A different theory according to which a constant flux of tiny dust particles from space introduces into Neptune’s atmosphere, however, does not agree with the measurements.

In Neptune’s stratosphere the scientists also found a higher concentration of methane than expected. On Neptune, methane plays the same role as water vapor on Earth: the temperature of the socalled tropopause - a barrier of colder air separating troposphere and stratosphere - determines how much water vapor can rise into the stratosphere. If this barrier is a little bit warmer, more gas can pass through. But while on Earth the temperature of the tropopause never falls beneath minus 80 degrees Celsius, on Neptune the tropopause's mean temperature is minus 219 degrees.

Therefore, a gap in the barrier of the tropopause seems to be responsible for the elevated concentration of methane on Neptune. With minus 213 degrees Celsius, at Neptune’s southern Pole this air layer is six degrees warmer than everywhere else allowing gas to pass more easily from troposphere to stratosphere. The methane, which scientists believe originates from the planet itself, can therefore spread throughout the stratosphere.

The instrument PACS was developed at the Max Planck Institute for . It analyzes the long-wave infrared radiation, also known as heat radiation, that the cold bodies in space such as Neptune emit. In addition, the research satellite Herschel carries the largest telescope ever to have been operated in space.

Explore further: Tibet Pathway for Chemicals To Reach Stratosphere

More information:
-- E. Lelouch et al. First results of Herschel /PACS observations of Neptune, Astronomy & Astrophysics, Vol. 518, L152 (2010), DOI:10.1051/0004-6361/201014600
-- T. Cavalié et. al. A cometary origin for CO in the stratosphere of Saturn? Astronomy & Astrophysics, Vol. 510, February 2010, DOI:10.1051/0004-6361/200912909

Related Stories

Tibet Pathway for Chemicals To Reach Stratosphere

May 10, 2006

NASA and Georgia Tech researchers have found that thunderstorms over Tibet provide a main pathway for water vapor and chemicals to travel from the lower atmosphere, where human activity directly affects atmospheric composition, ...

'This Planet Tastes Funny,' According to Spitzer

April 21, 2010

( -- NASA's Spitzer Space Telescope has discovered something odd about a distant planet -- it lacks methane, an ingredient common to many of the planets in our solar system.

Transcontinental wildfire emissions monitored from space

May 8, 2007

Using data from the SCIAMACHY instrument aboard ESA’s environmental satellite Envisat, scientists have determined that the carbon monoxide hovering over Australia during the wildfire season largely originated from South ...

Triton's summer sky of methane and carbon monoxide

April 7, 2010

According to the first ever infrared analysis of the atmosphere of Neptune's moon Triton, summer is in full swing in its southern hemisphere. The European observing team used ESO's Very Large Telescope and discovered carbon ...

The lower atmosphere of Pluto revealed

March 2, 2009

( -- "With lots of methane in the atmosphere, it becomes clear why Pluto's atmosphere is so warm," says Emmanuel Lellouch, lead author of the paper reporting the results.

Where comets emit dust

April 26, 2010

Studying comets can be quite dangerous - especially from close up. Because the tiny particles of dust emitted into space from the so-called active regions on a comet's surface can damage space probes.

Recommended for you

Astronomers detect dozens of new quasars and galaxies

April 25, 2017

(—A team of astronomers led by Yoshiki Matsuoka of the National Astronomical Observatory of Japan (NAOJ) has detected a treasure trove of new high-redshift quasars (or quasi-stellar objects) and luminous galaxies. ...

New survey hints at exotic origin for the Cold Spot

April 25, 2017

A supervoid is unlikely to explain a 'Cold Spot' in the cosmic microwave background, according to the results of a new survey, leaving room for exotic explanations like a collision between universes. The researchers, led ...

Team discovers lull in Mars' giant impact history

April 25, 2017

From the earliest days of our solar system's history, collisions between astronomical objects have shaped the planets and changed the course of their evolution. Studying the early bombardment history of Mars, scientists at ...

Preliminary results of Breakthrough Listen project released

April 25, 2017

(—The team of researchers working on the Breakthrough Listen project (affiliated with SETI) has released preliminary findings after sifting through several petabytes of data obtained from three telescopes involved ...


Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Jul 17, 2010
"... research satellite Herschel, that has been orbiting the Sun in a distance of approximately 1.5 million kilometers since May 2009."
This statement is confusing at best. Herschel is in orbit around L2 which means it is also orbiting the sun, however, the 1.5 million kilometers refers to its average distance from Earth. See for details.
not rated yet Jul 17, 2010
This statement is confusing at best.

Good catch. There has been a lot of this sort of inaccuracy on physorg lately.

It seems that the current batch of authors are particularly weak when it comes to distances (and whether or not they are reasonable)

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.