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CAMBRIDGE, Mass. -- We're not the only ones experiencing global warming. A Massachusetts Institute of Technology researcher has reported that observations obtained by NASA's Hubble Space Telescope and ground-based instruments reveal that Neptune's largest moon, Triton, seems to have heated up significantly since the Voyager space probe visited it in 1989. The warming trend is causing part of Triton's surface of frozen nitrogen to turn into gas, thus making its thin atmosphere denser.
While no one is likely to plan a summer vacation on Triton, this report in the June 25 issue of the journal Nature by MIT astronomer James L. Elliot and his colleagues from MIT, Lowell Observatory and Williams College says that the moon is approaching an unusually warm summer season that only happens once every few hundred years. Elliot and his colleagues believe that Triton's warming trend could be driven by seasonal changes in the absorption of solar energy by its polar ice caps.
"At least since 1989, Triton has been undergoing a period of global warming. Percentage-wise, it's a very large increase," said Elliot, professor of Earth, Atmospheric and Planetary Sciences and director of the Wallace Astrophysical Observatory. The 5 percent increase on the absolute temperature scale from about minus-392 degrees Fahrenheit to about minus-389 degrees Fahrenheit would be like the Earth experiencing a jump of about 22 degrees Fahrenheit.
Triton is a simpler subject than Earth for studying the causes and effects of global warming. "It's generally true around the solar system that when we try to understand a problem as complex as global warming -- one in which we can't control the variables -- the more extreme cases we have to study, the more we can become sure of certain factors," Elliot said. "With Triton, we can clearly see the changes because of its simple, thin atmosphere."
The moon is approaching an extreme southern summer, a season that occurs every few hundred years. During this special time, the moon's southern hemisphere receives more direct sunlight. The equivalent on Earth would be having the sun directly overhead at noon north of Lake Superior during a northern summer.
Elliot and his colleagues believe that Triton's temperature has increased because of indications that the pressure of the atmosphere has increased. Because of the unusually strong correlation between Triton's surface ice temperature and its atmospheric pressure, Elliot said scientists can infer a temperature increase of 3 degrees Fahrenheit over nine years based on its recent increase in surface vapor pressure. Any ice on Triton that warms up a little results in a big increase in atmospheric pressure as the vaporized gas joins the atmosphere.
Scientists used one of the Hubble telescope's three Fine Guidance Sensors in November 1997 to measure Triton's atmospheric pressure when the moon passed in front of a star. Two of Hubble's guidance sensors are normally used to keep the telescope pointed at a celestial target by monitoring the brightness of guide stars. The third can serve as a scientific instrument.
In this case, the guidance sensor measured a star's gradual decrease in brightness as Triton passed in front of it. The starlight got dimmer as it traveled through Triton's thicker atmosphere and then got cut off completely by the moon's total occultation of the star. This filtering of starlight through an atmosphere is similar to what happens during a sunset. As the sun dips toward the horizon, its light dims because it is traveling through denser air and because the sun's disk gets "squashed."
By detecting that Triton's atmosphere had thickened, astronomers were able to deduce that the temperature of the ice on Triton's surface has increased. "This pressure increase implies a temperature increase," Elliot wrote. "At this rate, the atmosphere has at least doubled in bulk since the time of the Voyager encounter." Like the Earth, Triton's atmosphere is composed mostly of molecular nitrogen, but its surface pressure is much less than that of the Earth--about the same as that 45 miles high in the Earth's atmosphere.
In their Nature paper, Elliot and his colleagues list two other possible explanations for Triton's warmer weather. Because the frost pattern on Triton's surface may have changed over the years, it may be absorbing a little more of the sun's warmth. Or changes in reflectivity of Triton's ice may have caused it to absorb more heat. "When you're so cold, global warming is a welcome trend," said Elliot.
About the same size and density as Pluto, Triton--one of Neptune's eight moons--is 30 times as far from the sun as the Earth. It is very cold and windy, with winds close to the speed of sound, and has a mixed terrain of icy regions and bare spots. Triton is a bit smaller than our moon, but its gravity is able to keep an atmosphere from completely escaping because it is so cold. Its composition is believed to be similar to a comet's, although it is much larger than a comet. Triton was captured into a reverse orbit by Neptune's strong gravitational pull.
Other astronomers who participated in this investigation are MIT research assistant Heidi B. Hammel and technical assistants Michael J. Person and Stephen W. McDonald of MIT; Otto G. Franz, Lawrence H. Wasserman, John A. Stansberry, John R. Spencer, Edward W. Dunham, Catherine B. Olkin and Mark W. Buie of Lowell Observatory; Jay M. Pasachoff, Bryce A. Babcock and Timothy H. McConnochie of Williams College.
This work is supported in part by NASA, the National Science Foundation and the National Geographic Society.