INSTITUTE FOR ASTRONOMY
University of Hawaii

Information Bulletin 29

Pluto and Triton: The Coldest Worlds We Know

Tobias C. Owen
Institute for Astronomy, University of Hawaii

Way out at the edge of the system of planets that orbits our sun, so far away that it takes sunlight reflected from their surfaces 4 hours to reach us (at a speed of 186,000 miles per second!), there are two small worlds made mostly of ice. One of these is Pluto, the most distant of all the planets. The other is Triton, the largest moon of the planet Neptune. These objects are both about the same size--roughly two-thirds as big as our own moon. Because they are so terribly far from the sun, their surfaces are extremely cold, just 37 degrees above absolute zero (-393 degrees on the familiar Fahrenheit scale).

This view, the clearest yet of Pluto and Charon, was taken by NASA's Hubble Space Telescope in February 1994, when the planet was 2.6 billion miles (4.4 billion kilometers) from Earth. (Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility; NASA)

Their great distance and small size make Pluto and Triton very faint, so we can only study them well with our largest telescopes. Until very recently, we had no idea what substances were present on the surfaces of these icy worlds. Were there rocky landscapes, like those on our moon, or were these surfaces covered with water ice, like the moons of Saturn? In 1993, a team led by University of Hawaii astronomers using the 3.8-meter (150-inch) United Kingdom Infrared Telescope on Mauna Kea discovered that the surfaces of both Pluto and Triton are covered with frozen nitrogen ice that is mixed with small amounts of frozen carbon monoxide, methane, and on Triton, carbon dioxide. Both bodies have extremely thin atmospheres composed mainly of nitrogen, the same gas that dominates our atmosphere here on Earth.

It seems that the surface temperatures of Pluto and Triton are so low that common compounds we know as gases freeze into ices. In fact, we are familiar with solid carbon dioxide, since this is the substance we call "dry ice." On Earth, this ice forms at a temperature of -100 degrees Fahrenheit. Now imagine the surface of Pluto, at a temperature of -393 degrees Fahrenheit! If Earth suddenly moved to Pluto's distance from the sun, not only would the oceans freeze through, from top to bottom, our atmosphere would freeze out on the surface of our planet. The top layer of this coating would be nitrogen ice, just like that on Pluto.

The Voyager 2 spacecraft obtained this high-resolution image of Triton during its close flyby in August 1989. There is a large south polar cap at the bottom. (Courtesy of NASA/JPL)

Many mysteries about these tiny, frigid worlds remain unsolved. For example, the Voyager 2 spacecraft revealed the presence of plumes of dark material moving up into Triton's atmosphere from its surface, and we don't yet know how this could happen on such a cold world. NASA is considering plans for a spacecraft to flyby Pluto in the year 2006, which would give us a close look at the last unexplored planet in the solar system. We can only guess what new marvels would be revealed by this mission.

Prepared by the Institute for Astronomy, University of Hawaii

For more information or additional copies, write to the
Assistant to the Director
UH Institute for Astronomy
2680 Woodlawn Drive
Honolulu, HI 96822

Acknowledgments: Louise H. Good, Wendy F. Nakano, and Karen M. Rehbock.

© 1996 University of Hawaii Institute for Astronomy
Latest revision January 1996

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