Pluto has a CO glow

Artist's impression of Pluto and Charon

An artist's impression of Pluto and its largest moon, Charon. Astronomers have detected thin traces of carbon monoxide gas in the dwarf planet's atmosphere.

ASTRONOMERS HAVE DISCOVERED carbon monoxide in the atmosphere of Pluto, capping off nearly two decades of work to detect the gas in the ‘air’ of the distant, icy world.

Pluto, discovered in 1930, was long considered the Sun’s smallest and most distant planet. Since 2006, though, it has been regarded by many astronomers as a ‘dwarf planet’…one of a handful of such bodies with sizes of hundreds of kilometres that orbit in the distant reaches of the Solar System, out beyond Neptune.

Pluto is the only dwarf planet known to have an atmosphere. The thin layer of gases was detected in 1988 when it dimmed the light of a distant star as Pluto passed in front of it.

The new results, obtained using the 15-metre James Clerk Maxwell Telescope in Hawaii, show a strong signal of carbon monoxide gas.

Team leader Dr Jane Greaves of the University of St Andrews will present the new discovery today at the UK National Astronomy Meeting in Wales.

Fragile atmosphere

Previously, Pluto’s atmosphere was known to be over a hundred kilometres thick, but the new data raise this height to more than 3,000 kilometres—a quarter of the way out to Pluto’s largest moon, Charon.

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 probably result from solar heating of surface ice, which sublimates (goes directly from ice to gas) 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,” commented team member Dr Christiane Helling, also of the University of St Andrews.

Artist's impression of the view from the surface of Pluto

Artist's impression of the view from the surface of Pluto, showing a thin, hazy atmosphere.

Deep space cold snap

The gas is extremely cold, about -220 degrees Celsius. A big surprise for the team was that the CO measurement was more than twice as strong as an upper limit obtained by another group, who used the IRAM 30-metre telescope in Spain in 2000.

“It was thrilling to see the signal gradually emerge as we added in many nights of data”, said Dr Jane Greaves, the team leader from the University of St Andrews.

“The change in brightness over the last decade is startling,” she added. “We think the atmosphere may have grown in size, or the carbon monoxide abundance may have been boosted.”

Such changes have been seen with Pluto before, but only in the lower atmosphere, where methane—the only other gas ever positively identified—has also been seen to vary.

Critical balance

Unlike the greenhouse gas carbon dioxide, carbon monoxide acts as a coolant, while methane absorbs sunlight and so 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 Pluto’s atmosphere. But if the chilling effect is too great, it could result in nitrogen snowfalls and all the gases freezing back onto the ground.

“Seeing such an example of extra-terrestrial climate-change is fascinating”, says Dr Greaves. “This cold, simple atmosphere that is strongly driven by the heat from the Sun could give us important clues to how some of the basic physics works, and act as a contrasting test-bed to help us better understand the Earth’s atmosphere.”

The JCMT is operated jointly by the UK, Canada and the Netherlands and is approaching its twenty-fifth anniversary.

The team has another Pluto observing run scheduled at the JCMT for the end of April, and in the long-term, they hope to continue tracking the changes in the atmosphere at least up to the fly-by of NASA’s New Horizons space probe in 2015.

Adapted from information issued by RAS. Images courtesy ESO / L. Calcada.

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