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What do you get if you mix together one part helium and five-and-a-half parts hydrogen, add a dash of methane, heat to 60 Kelvins (-351 degrees F) and let sit for over four billion years? If you made a large enough batch of these mixed gases, you might get the planet Neptune.

One of a broad range of objectives of the European Space Agency's Infrared Space Observatory was to observe Neptune from 1995 to 1998, to find out the ratio of hydrogen to helium, two gases known to make up almost all of the planet's thick atmosphere. Dr. Glenn Orton, senior research scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif., analyzed data from the spaceborne observatory's long wavelength spectrometer, which measures how elements reflect or absorb long wavelengths of light, and from the short wavelength spectrometer, which does the same for short wavelengths of light.

Neptune seems to be generating some of its own heat, Orton said. Data from the two spectrometers imply that Neptune is radiating heat at about 60 Kelvins (-351 degrees Fahrenheit.) Scientists would expect Neptune to be only about 46.6 Kelvins (-375 degrees Fahrenheit), if its only source of heat were sunlight. According to Orton, it can be deduced from this that Neptune must have an internal heat source.

The long wavelength spectrometer also looked at how much hydrogen and how much helium make up Neptune. It discovered that the planet is made up of about 83 percent hydrogen, 15 percent helium and two percent methane. This is the latest in a series of evaluations of composition data from the gas planets, Jupiter, Saturn, Uranus and Neptune, which have increasingly shown that their makeup is very close to the amounts of hydrogen and helium in the sun.

The first spacecraft to determine the main composition of any of the outer planets was Pioneer, which studied Jupiter and Saturn, and more substantial data were taken by the twin Voyager spacecraft. Recent revision of the Jupiter and Saturn data has led scientists to reexamine the data from all the outer planets and has revealed that the gas giant planets' main compositions are almost identical to that of the Sun. This means that helium hasn't sunk to the center of the planet, as some models had suggested, but has stayed mixed in the atmosphere.

Orton was joined in his studies by Dr. Martin Burgdorf, ISO Data Centre, Astrophysics Division, European Space Agency, Villafranca, Spain; Dr. Gary Davis, University of Saskatchewan, Saskatoon, Canada; Dr. Bruce Swinyard, Rutherford Appleton Laboratory, Chilton, United Kingdom; Dr. Sunil Sidher and Dr. Matt Griffin, Queen Mary and Westfield College, London, United Kingdom; and Dr. Helmut Feuchtgruber, MPI Extraterrestrische Physik, Garching, Germany. The Infrared Space Observatory was operational between 1995 and 1998.