Pluto's Atmosphere Is Changing

Still so little is known about Pluto, as evidenced in this Hubble Space Telescope image.

Recent observations show Pluto's atmosphere has changed significantly over the past 14 years.  A recent occultation of a star by Pluto confirms suspicions that the faraway planet's atmosphere changes during its 248-year journey about the sun.   The new data also suggests that the changes are even more drastic than imagined.

Similar to the moon passing in front of the sun during a solar eclipse, a solar system object passes in front of a star during an occultation.  If the object has an atmosphere, astronomers can scrutinize the star's light just before and just after the object begins to blot out the starlight.  Careful observations can reveal details about the atmosphere's density, temperature, and pressure.

On July 19, 2007, Marc Buie of Arizona's Lowell Observatory set up a portable 14-inch telescope in northern Chile to watch Pluto occult a star called P126A.  Pluto's stellar occultations are rare and this was the first one observed successfully since 1988.  After the latest event, James Eliott, a planetary scientist at the Massachusetts Institute of Technology, compared the results of the two occultations.

Predicted Path of 19 July 2002 Pluto Occultation
This map shows the predicted path of Pluto's shadow during its occultation of P126A on

July 19, 2002.

"In the last 19 years, one or more changes have occurred," Buie reports.  "Pluto's atmosphere is undergoing global cooling, while other data indicate that the surface seems to be getting slightly warmer.  Some change is inevitable as Pluto moves away from the sun, but what we're seeing is more complex than expected."  From the earlier occultation and other observations, planetary scientists learned that Pluto has a delicate atmosphere with nitrogen and some methane and carbon dioxide.  In 1988, the occulted star's light dipped slightly at first before plunging sharply.  The unexpected behavior suggested Pluto's atmosphere could have a smog layer or a sudden drop in temperature near the planet's surface.


In contrast, the July occultation provided a much smoother light curve, suggesting the cause of the abrupt drop witnessed in 1988 is no longer present.  Additionally, the recent results show that Pluto's atmosphere has cooled between 20 and 55 degrees Fahrenheit.  "A 1997 Triton occultation revealed that the surface of Triton, Neptune's largest moon, had warmed since the Voyager spacecraft first explored the moon in 1989," Elliot says.  "But the changes observed in Pluto's atmosphere are much more severe.  We cannot fully explain what has caused these dramatic changes to Pluto's atmosphere," Buie adds. "The Pluto-Kuiper Belt mission is our best hope for putting all the puzzle pieces together."

New Horizons
New Horizons will become the first spacecraft to visit Pluto.

For years, NASA has included a spacecraft to Pluto in its plans, but budget problems have repeatedly scrapped or delayed the mission.  Finally, the New Horizons mission to Pluto launched on January 19, 2006.  It will reach Pluto around July, 2015. 

Astronomers around the world tried to catch a glimpse of the July 19 occultation, but the path of the object's shadow is difficult to predict because it is so small.  By some good fortune, astronomers will have another opportunity to study Pluto as it occults another star, this one called P131.1, on August 20, 2012.

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