Pluto: The Surface and Atmosphere

     Nearly 4.3 billion kilometers to Earth at its nearest approach, Pluto is the only planet never visited by a space probe. Consequently, quantitative measurements can only be approximated. Much of the information gathered on Pluto and its only moon Charon come from the period between 1985 and 1990. During this time, Earth's alignment with the orbit of the Pluto-Charon pair (both are tidally-locked to each other) allowed Earth-based astronomers to study Charon's eclipse of Pluto. Each eclipse lasted up to four hours, and it happened every Plutonian day. Although not highly accurate, this data allowed scientists to approximate several fundamental characteristics of Pluto and Charon, including their masses and diameters. Due to Pluto and Charon's relative similarity as compared with other planet-moon pairs, the two are sometimes referred to as a double planet. So related are these that many measurements taken are in terms of both Pluto and Charon. For example, researchers know the sum of the masses of the two celestial bodies fairly well but must rely on educated guesses to determine individual masses. The best guess for Pluto is 1.27 x 10²² kg. Its diameter is a meager 2,274 km and its density 2,050 kg/m³. Its surface gravity is calculated to be 0.4 m/s² with an atmospheric pressure 100,000 times weaker than the Earth's.
     A second major source of information is the Hubble Space Telescope, which imaged 85% of Pluto's surface in 1994, revealing bright and dark areas. Astronomers believe bright areas may be shifting fields of nitrogen ice, while darker areas house methane ice colored by interaction with sunlight. Some dark spots may simply be fresh impact craters or valleys. In any case, scientists agree that the surface is probably 50 to 75% rock, the rest ice. This is consistent with the theory that the planet formed under low pressure and temperature. Extensive ice caps are also known to form at the poles. The South Pole is very reflective and bright, while the North Pole is quite dim. The average geometric albedo is 0.49 to 0.66.
     Pluto's atmosphere is very thin and is only existent when Pluto nears perihelion. The atmosphere appears yellowish according to one source and reddish according to another. This thin layer is 98% nitrogen, the remaining being carbon monoxide and mostly methane. Temperatures vary from -223 to -233 degrees Celsius as the planet revolves about the Sun.

Copyright © 2000 by Gary Chan and Matthew McDermott. All rights reserved.