Carbon monoxide in Pluto's atmosphere

Artist's impression of Pluto's huge atmosphere of carbon monoxide. The source of this gas is erratic evaporation from the mottled icy surface of the dwarf planet. The Sun appears at the top, as seen in the ultra-violet radiation that is thought to force some of the dramatic atmospheric changes. Pluto's largest moon, Charon, is seen to the lower right. Credit: P.A.S. Cruickshank

By Royal Astronomical Society, United Kingdom
Published: April 19, 2011

Pluto was discovered in 1930 and considered the Sun’s smallest and most distant planet. Since 2006, astronomers have regarded it as a “dwarf planet,” one of a handful of such bodies that orbit in the distant reaches of the solar system, out beyond Neptune. Pluto is the only dwarf planet known to have an atmosphere.A British-based team of astronomers has discovered carbon monoxide gas in the atmosphere of Pluto, after a worldwide search lasting for nearly 2 decades.
The new results, obtained at the 15-meter James Clerk Maxwell Telescope in Hawaii, show a strong signal of carbon monoxide gas. Previously, the atmosphere was known to be more than 60 miles (100 kilometers) thick, but the new data raise this height to about 2,000 miles (3,000 km), which is a quarter of the way to Pluto’s largest moon, Charon. The gas is extremely cold, about -365 Fahrenheit (-220° Celsius). A big surprise for the team was that the signal is more than twice as strong as an upper limit obtained by another group, who used the IRAM 30-meter telescope in Spain in 2000.
Fig:The discovery data. The spectrum of carbon monoxide around Pluto is shaded in red. The surrounding signals are random "noise". The brightness of the signal (on the vertical axis) is given in units of degrees that are equivalent to a temperature of the source. This is much less than the 50° above absolute zero (50 Kelvin) estimated for the atmosphere, because Pluto fills only a tiny fraction of the sky-area seen by the telescope. J.S. Greaves/Joint Astronomy Center


“It was thrilling to see the signal gradually emerge as we added in many nights of data,” said Jane Greaves from the University of St. Andrews. “The change in brightness over the last decade is startling. We think the atmosphere may have grown in size, or the carbon monoxide abundance may have been boosted.” Such changes have been seen before but only in the lower atmosphere, where methane, the only other gas ever positively identified, has also been seen to vary.In 1989, Pluto made its closest approach to the Sun, a comparatively recent event given that it takes 248 years to complete each orbit. The gases are probably the result of solar heating of surface ice, which evaporates as a consequence of the slightly higher temperatures during this period. The resulting atmosphere is probably the most fragile in the solar system, with the top layers blowing away into space.“The height to which we see the carbon monoxide agrees well with models of how the solar wind strips Pluto’s atmosphere,” said Christiane Helling from the University of St. Andrews.

Unlike the greenhouse gas carbon dioxide, carbon monoxide acts as a coolant, while methane absorbs sunlight and thus produces heating. The balance between the two gases, which are just trace elements in what is thought to be a nitrogen-dominated atmosphere, is critical for its fate during the many decades-long seasons. The newly discovered carbon monoxide may hold the key to slowing the loss of the atmosphere. But if the chilling effect is too great, it could result in nitrogen snowfalls and all the gases freezing out onto the ground. “Seeing such an example of extraterrestrial climate change is fascinating,” said Greaves. “This cold, simple atmosphere that is strongly driven by the heat from the Sun could give us important clues to how some of basic physics works, and act as a contrasting test bed to help us better understand the Earth’s atmosphere.”

The lower atmosphere of Pluto revealed

Using ESO's Very Large Telescope, astronomers have gained valuable new insights about the atmosphere of the dwarf planet Pluto. The scientists found unexpectedly large amounts of methane in the atmosphere, and also discovered that the atmosphere is hotter than the surface by about 40 degrees, although it still only reaches a frigid minus 180 degrees Celsius. These properties of Pluto's atmosphere may be due to the presence of pure methane patches or of a methane-rich layer covering the dwarf planet's surface.

Tuesday, March 03, 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.

Pluto, which is about a fifth the size of Earth, is composed primarily of rock and ice. As it is about 40 times further from the Sun than the Earth on average, it is a very cold world with a surface temperature of about minus 220 degrees Celsius!

It has been known since the 1980s that Pluto also has a tenuous atmosphere [1], which consists of a thin envelope of mostly nitrogen, with traces of methane and probably carbon monoxide. As Pluto moves away from the Sun, during its 248 year-long orbit, its atmosphere gradually freezes and falls to the ground. In periods when it is closer to the Sun — as it is now — the temperature of Pluto's solid surface increases, causing the ice to sublimate into gas.

Until recently, only the upper parts of the atmosphere of Pluto could be studied. By observing stellar occultations, a phenomenon that occurs when a Solar System body blocks the light from a background star, astronomers were able to demonstrate that Pluto's upper atmosphere was some 50 degrees warmer than the surface, or minus 170 degrees Celsius. These observations couldn't shed any light on the atmospheric temperature and pressure near Pluto's surface. But unique, new observations made with the CRyogenic InfraRed Echelle Spectrograph (CRIRES), attached to ESO's Very Large Telescope, have now revealed that the atmosphere as a whole, not just the upper atmosphere, has a mean temperature of minus 180 degrees Celsius, and so it is indeed "much hotter" than the surface.

In contrast to the Earth's atmosphere [2], most, if not all, of Pluto's atmosphere is thus undergoing a temperature inversion: the temperature is higher, the higher in the atmosphere you look. The change is about 3 to 15 degrees per kilometre. On Earth, under normal circumstances, the temperature decreases through the atmosphere by about 6 degrees per kilometre.

"It is fascinating to think that with CRIRES we are able to precisely measure traces of a gas in an atmosphere 100 000 times more tenuous than the Earth's, on an object five times smaller than our planet and located at the edge of the Solar System," says co-author Hans-Ulrich Käufl. "The combination of CRIRES and the VLT is almost like having an advanced atmospheric research satellite orbiting Pluto."

The reason why Pluto's surface is so cold is linked to the existence of Pluto's atmosphere, and is due to the sublimation of the surface ice; much like sweat cools the body as it evaporates from the surface of the skin, this sublimation has a cooling effect on the surface of Pluto. In this respect, Pluto shares some properties with comets, whose coma and tails arise from sublimating ice as they approach the Sun.

The CRIRES observations also indicate that methane is the second most common gas in Pluto's atmosphere, representing half a percent of the molecules. "We were able to show that these quantities of methane play a crucial role in the heating processes in the atmosphere and can explain the elevated atmospheric temperature," says Lellouch.

Two different models can explain the properties of Pluto's atmosphere. In the first, the astronomers assume that Pluto's surface is covered with a thin layer of methane, which will inhibit the sublimation of the nitrogen frost. The second scenario invokes the existence of pure methane patches on the surface.

"Discriminating between the two will require further study of Pluto as it moves away from the Sun," says Lellouch. "And of course, NASA's New Horizons space probe will also provide us with more clues when it reaches the dwarf planet in 2015."

Notes:

[1] The atmospheric pressure on Pluto is only about one hundred thousandth of that on Earth, or about 0.015 millibars.

[2] Usually, air near the surface of the Earth is warmer than the air above it, largely because the atmosphere is heated from below as solar radiation warms the Earth's surface, which, in turn, warms the layer of the atmosphere directly above it. Under certain conditions, this situation is inverted so that the air is colder near the surface of the Earth. Meteorologists call this an inversion layer, and it can cause smog build-up.

Pluto is not so alone

photo: Pluto with its moons (Charon,Nix,Hydra)

A pair of small moons that NASA's Hubble Space Telescope discovered orbiting Pluto now have official names: Nix and Hydra. Photographed by Hubble in 2005, Nix and Hydra are roughly 5,000 times fainter than Pluto and are about two to three times farther from Pluto than its large moon, Charon, which was discovered in 1978.