Comet Smacked Neptune 200 Years Ago, Data Suggests

New measurements of Neptune's atmosphere by a European space telescope suggest that a comet may have crashed into the gas giant about 200 years ago.

Scientists analyzed the composition of Neptune's atmosphere using data from the Herschel space observatory. They found a peculiar distribution of carbon monoxide in the gas giant's atmosphere, which could be an indication of an earlier comet impact.

The research is detailed in the July 16 online issue of the journal Astronomy & Astrophysics.

Using past knowledge

Other similar collisions between comets (or asteroids) and planets helped the astronomers detect the telltale signs of cometary impacts. [Photos: Jupiter Hit By Asteroid or Comet]

When pieces of the comet Shoemaker-Levy 9 slammed into Jupiter in 1994, scientists were able to examine the trajectory and debris to better understand planetary impacts. Instruments aboard the space probes Voyager 2, Galileo and Ulysses also documented every detail of the rare incident.

The data now helps scientists detect the telltale signs of cometary impacts that happened many years ago. Comets, which are sometimes described as "dusty snowballs," leave their mark in the atmospheres of gas giants like Jupiter and Neptune in the form of water, carbon dioxide, carbon monoxide, hydrocyanic acid and carbon sulfide. Trace molecules of these compounds can be detected in the radiation emitted by the planets into space.

In February, scientists from the Max Planck Institute for Solar System Research (MPS) presented strong evidence for a comet impact on Saturn about 230 years ago. That study was published in the February 2010 issue of Astronomy & Astrophysics.

Neptune as well?

New measurements performed by the Photodetector Array Camera and Spectrometer (PACS) instrument aboard Herschel allowed astronomers to analyze the long-wave infrared radiation of Neptune. These observations indicate that Neptune may have experienced a similar cosmic collision.

The scientists examined the atmosphere of Neptune, which mainly consists of hydrogen and helium with traces of water, carbon dioxide and carbon monoxide. They detected an unusual distribution of carbon monoxide in Neptune's atmosphere, with much higher concentrations in the upper layer, called the stratosphere, compared to the troposphere layer beneath.

"The higher concentration of carbon monoxide in the stratosphere can only be explained by an external origin," said Paul Hartogh, a scientist at MPS and principle investigator of the Herschel science program that examines water and related chemistry in the solar system. "Normally, the concentrations of carbon monoxide in troposphere and stratosphere should be the same or decrease with increasing height."

A cometary impact would explain such curious results. The collision forces the comet to fall apart, while the carbon monoxide trapped in the comet's ice is released and distributed throughout the stratosphere over the years.

"From the distribution of carbon monoxide we can therefore derive the approximate time, when the impact took place," said Thibault Cavali?, also from MPS. This would help scientists confirm that a comet hit Neptune around 200 years ago, the researchers added.

The new study also appears to disprove a previous theory that tried to explain the strange distribution of carbon monoxide in Neptune's atmosphere. The older theory suggested that a constant flux of tiny dust particles from space introduces carbon monoxide into the gas giant's atmosphere.

But, in Neptune's stratosphere, the scientists also found a higher concentration of methane than expected.

What's methane got to do with it?

On Neptune, methane operates in similar ways to water vapor on Earth: the temperature of the tropopause ? a barrier of colder air that separates the troposphere and stratosphere ? determines how much water vapor can rise into the stratosphere.

If this barrier becomes warmer, more gas is able to pass through. On Earth, the temperature of the tropopause never falls beneath minus 176 degrees Fahrenheit (minus 80 degrees Celsius). On Neptune, however, the tropopause's mean temperature is minus 426 degrees Fahrenheit (minus 219 degrees Celsius).

This means that a gap in the barrier of the tropopause is the likely culprit for the elevated concentration of methane on Neptune.

At minus 415 degrees Fahrenheit (minus 213 degrees Celsius), the atmosphere at Neptune's southern pole is six degrees warmer than everywhere else, which allows gas to pass more easily from troposphere to stratosphere. The methane, which scientists think originates from the planet itself, can therefore spread throughout the stratosphere.

The European Space Agency (ESA) launched the Herschel infrared space telescope in May 2009. The observatory is the largest, most powerful infrared telescope in orbit today.

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