Paula Cleggett-Haleim
Headquarters, Washington, D.C.                                                      
August 6, 1993
(Phone:  202/358-0883)                                               
Embargoed until Noon, EDT

Diane Farrar 
Ames Research Center, Mountain View, Calif. 
(Phone:  415/604-3934)

RELEASE:  93-142

NITROGEN ICE DISCOVERED ON PLUTO

        The distant planet Pluto is covered with surface ices that 
are 98 percent nitrogen, University of Hawaii, NASA and other 
scientists say.  With such abundant nitrogen surface ice, Pluto's 
thin atmosphere must be primarily gaseous nitrogen, they conclude.

        This is the first clear detection of nitrogen on Pluto and 
the first clear indication that the atmosphere is mostly nitrogen 
gas rather than methane, as previously believed.  Carbon monoxide 
also was detected for the first time.

        "Rather than methane as previously thought, it appears 
that frozen nitrogen dominates the surface," said Dr. Ted Roush.  
Roush, employed by San Francisco State University, works at NASA's 
Ames Research Center, Mountain View, Calif.

        Methane was detected on Pluto's surface in 1976.  
Extremely small amounts of methane are easy to detect because it 
strongly absorbs specific wavelenths of sunlight.

        "The small amounts (1.5 percent) of methane ice are 
'dissolved,' or mixed at a molecular level, in the frozen 
nitrogen," Roush said.

        The abundant nitrogen recently found on Pluto is a poor 
absorber of sunlight and produces very weak features in the light 
reflected from the planet, so it previously had not been 
identified.

        The observations were made in Hawaii with a new instrument 
on the United Kingdom Infrared Telescope in May 1992.  The results 
are published in the current issue of Science magazine, along with 
similar observations of Neptune's moon, Triton.

        Pluto resembles Triton in size and in surface and 
atmospheric composition.  Both have nitrogen, methane and carbon 
monoxide ices on their surface.
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        Because their surfaces are made of similar materials, 
scientists think Pluto and Triton may have formed in a similar 
location in the solar nebula, Roush said.

        Pluto, almost 3 billion miles from Earth, is the only 
planet not yet explored by a spacecraft.  Pluto is unusual in 
several respects.  Although classified as a planet, it is smaller 
than Earth's moon.  Pluto's only moon, Charon, is at least half as 
big as the planet itself.  It also is the only planet in the solar 
system with an orbit highly inclined out of the plane of the solar 
system.

        The authors, with Roush, include first author Dr. Tobias 
Owen, University of Hawaii; Dr. Dale Cruikshank of Ames; and Drs. 
J. L. Elliot and L. A. Young, Massachusetts Institute of 
Technology.

        Additional authors are C. de Bergh, Observatoire de 
Paris-Meudon, France; B. Schmitt, St. Martin d'Heres, France; T. 
R. Geballe, Joint Astronomy Center, Hilo, Hawaii; R. H. Brown, Jet 
Propulsion Laboratory, Pasadena; and M. J. Bartholomew, Ames.

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