Uwe Fink investigated specific infrared absorption bands of Pluto and detected significant amounts of methane gas believed to be unusual for such a small body.
Although Pluto was discovered in 1930, not much was known about this body until photometry made possible detailed spectroscopic observations in the 1970’s and 1980’s. Its orbit, for example, is the most eccentric in the solar system, varying in distance from 4.4 to 7.4 billion kilometers. This very elliptical orbit results in significant temperature variations during its orbital year.
In 1976, methane ice was discovered on Pluto’s surface. Previously, astronomers assumed the surface was dark, which would have explained its dimness; methane ice, however, reflects a high percentage of sunlight, and researchers came to believe that Pluto is faint not because it is dark but because it is small. Photoelectric observations made of Pluto have revealed considerable brightness variations. Analysis of these variations indicates that they are caused by irregularities in surface reflection.
Previous investigations using absorption spectra had suggested the existence of a methane atmosphere, but imaging instruments at that time were not sensitive enough in the stronger absorption bands to be conclusive. Response was generally weak in wavelengths below 900 nanometers, where absorption bands may be present but are difficult to detect.
In May, 1980, a team led by Uwe Fink of the University of Arizona completed observations of Pluto using the 155-centimeter Catalina telescope. The team constructed a low-resolution spectrograph to which was attached a silicon charge-coupled device (CCD). A CCD typically contains a quarter million microscopic, light-sensitive diodes in a space the size of a postage stamp. CCDs are capable of detecting both bright and faint objects and are much more sensitive than photographic plates. The CCD was manufactured by Texas Instruments and arranged in an array containing twenty-five hundred elements in a 500-by-500 matrix only 15.2 microns square. To enhance the detection capability, the array was cooled to -120 degrees Celsius. Both the electronics and the detector were built at the California Institute of Technology for use by the investigation group of the Space Telescope Wide Field Planetary Camera.
A spectrograph was assembled at the Lunar and Planetary Laboratory of the University of Arizona and designed for use with the CCD array. The spectrograph utilized a transmission diffraction grating, with a total spectral range of 570-1,100 nanometers and a resolution capability of 1.1 nanometers for first-order spectra. The longer-wavelength threshold of 1,100 nanometers was determined by the CCD sensitivity, and the shorter wavelength limit was set by the response of a special filter built to eliminate overlapping spectral orders.
The Fink team observed Pluto, and its light, which was allowed to fall upon the spectrograph’s entrance slit, was imaged directly on the CCD array. The width of the slit, although very small at 18 millimeters, allowed adequate coverage of the background sky. When contrasted to Pluto, the sky background was distinct but only one-tenth as strong as Pluto.
In the data reduction, the signals from Pluto were added at each wavelength from eleven tiny detector units called pixels. Next, the background sky counts and direct current offsets caused by the CCD array were subtracted from the five pixels on either side of Pluto. Data were also taken from standard stars and reduced using the same method. Dividing Pluto’s average brightness by the stars’ average brightness effectively removed sensitivity variations that occurred across the detector array. This technique of ratio comparison allowed the astronomers to locate regions of no absorption in the atmosphere. When the data were compared to the sky-only spectrum, the researchers noted no perceptible data fluctuations in the position of Pluto’s spectrum.
The ratio spectra indicated definite methane bands at 620, 720, 790, 840, 860, 890, and 1,000 nanometers; the strongest of these bands was located at 890 nanometers. The observed band saturation was definitive evidence for gaseous methane absorption, as opposed to liquid or solid-state absorption. Methane, in both the liquid and solid states, exhibits abundant linear absorption with no visible saturation. One would not expect a contribution of more than a few percentage points from either the liquid or the solid state of methane. The absorption spectra of Pluto, therefore, clearly showed the presence of a methane atmosphere.
The researchers made an estimate for the total atmospheric pressure on Pluto at 0.05 that of Earth. This value was based on calculations of the partial pressure of methane alone in Pluto’s atmosphere, along with variables such as the mass of the methane molecule, estimates of Pluto’s gravitational acceleration, and the abundance of methane molecules in a given column of atmosphere. Fink’s research team measured significant amounts of gaseous methane and believed that, although methane surface frost still might be present, it would not be an important factor in explaining the absorption spectra. They also did not rule out the possibility of a heavier gas being mixed in with the methane, which might be necessary for Pluto to hold onto its atmosphere if its mass were quite small.
Astronomers, in general, believe water—in addition to methane—is a major component of Pluto because of its low density as well as the general abundance of water elsewhere in the solar system. The presence of water on Pluto has not been confirmed, because the spectrum of methane completely overcomes the spectral bands of water.
The atmosphere of Pluto is believed to be caused by the sublimation of methane from ice to gas on its surface. The effect will be more pronounced when it is closer to the Sun at perihelion, resulting in more intense surface heating. Some of the methane that is vaporizing will drift toward the poles and freeze at the surface. This effect will be enhanced as Pluto travels farther out from the Sun, resulting in the sublimation of less methane. It is believed that Pluto’s poles remain covered with methane throughout its 248-year period of revolution.
Investigations were undertaken as early as 1944 to search for an atmosphere on Pluto. All results proved negative until 1980, when Fink’s team obtained their high-resolution spectral measurements, revealing methane gas. What was particularly surprising to the astronomers was that such a small body as Pluto could have an atmosphere.
The presence of methane in Pluto’s atmosphere presented a problem regarding its mass and its radius. Assuming a smaller mass than that accepted at the time, calculations would have placed the radius at a maximum of 950 kilometers, which would predict a stable atmosphere; however, this calculation would have led to a surface reflectivity (albedo) of greater than 100 percent, which was unrealistic. On the other hand, at a slightly larger radius, 1,100 kilometers, the albedo drops to a reasonable 67 percent. A mass increase of 50 percent over the latter model led to the prediction of a radius of about 1,400 kilometers, which fell within the limits determined by the technique of speckle-interferometry, a photographic method designed to “freeze out” interfering motions of the earth’s atmosphere. Also, if the mass of Pluto used in the determination were too small, then the addition of a heavier gas would be required to maintain atmospheric stability over a long period of time. The calculations of the total atmospheric pressure were not refined enough either to confirm or refute the additional heavier gas. It is now known that Pluto’s radius is just over 1,100 kilometers—1,151 kilometers, to be more precise—and its mass is 1.3 1022 kilograms (or 0.0022 that of Earth). The thin atmosphere is now understood to consist of nitrogen, methane, and carbon monoxide that have sublimated from its surface ices. This atmosphere is greatest at perihelion (when Pluto is closest to the Sun), and it solidifies and falls to the surface at aphelion (when Pluto is farthest from the Sun).
Fink’s discovery team realized that Pluto’s small size, high inclination of orbit, and large distance from the Sun made Pluto a very unusual type of body. The discovery of its moons and a fuller understanding of its atmosphere would enhance its image and established Pluto as a more typical planetary member of the solar system. On August 24, 2006, however, the debate surrounding Pluto’s celestial status seemed to receive a final pronouncement when the International Astronomical Union (IAU) deemed Pluto to be a “dwarf planet.” Pluto’s astronomical demotion occurred in the wake of a new definition of the term “planet,” which required any such body to meet three criteria: It must orbit the Sun, must have a sufficient mass to have rounded itself into a ball through the force of gravity, and must have moved other bodies away from its orbital neighborhood. Pluto fails to meet the final criterion. To a large degree such a definition was necessary in order to eliminate such smaller bodies as Eris (found in the Kuiper Belt) and Ceres (an asteroid) from also being considered “planets.” The line between planets and smaller bodies orbiting the Sun had to be drawn somewhere, and bodies such as Pluto, Eris, and Ceres fell into the intermediary category that comprises the dwarf planets.
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Brown, Robert H., and Dale P. Cruikshank. “The Moons of Uranus, Neptune, and Pluto.” Scientific American 253 (July, 1985): 38-47. Presents a nontechnical discussion of ground-based studies of the outer solar system prior to the Voyager 2 flyby. Charts indicate that Pluto’s absorption bands are characteristic of methane frost, and a diagram shows why eclipses between Pluto and its moon Charon take place during only two brief periods. -
Croswell, Ken. “Pluto: Enigma on the Edge of the Solar System.” Astronomy 14 (July, 1986): 6-22. Covers the observational history of Pluto up to the 1978 discovery of Pluto’s moon Charon. Discusses the eclipse cycle of Pluto and its moon as well as spectroscopic studies of Pluto’s surface and atmosphere. Includes a table of Pluto’s physical and orbital characteristics. -
Davies, John Keith. Beyond Pluto: Exploring the Outer Limits of the Solar System. New York: Cambridge University Press, 2001. A history of the theories, scientists, and discoveries that led to the discovery of the Kuiper Belt, a band of planet-like bodies that orbit the Sun just beyond Pluto’s path. Pluto’s status was redefined by these objects. -
Fink, Uwe, et al. “Detection of a Methane Atmosphere on Pluto.” Icarus 44 (October, 1980): 62-71. Technical article describes the actual observations, instrumentation used, and data reduction that led to the discovery of methane gas on Pluto. Thoroughly discusses prior photometric and spectroscopic research. Includes comprehensive reference list. -
Freedman, David H. “When Is a Planet Not a Planet? Arguments for and Against Demoting Pluto.” Atlantic Monthly, February, 1998. Reviews the debate that arose in the mid-1990’s regarding the status of Pluto as a major planet, given new discoveries. -
Littmann, Mark. “The Smallest Planet.” In Planets Beyond: Discovering the Outer Solar System. 1989. Reprint. Mineola, N.Y.: Dover, 2004. Comprehensive observational history of Pluto when it was still considered a planet, accompanied by detailed photographs and diagrams. -
Minard, Anne, and Carolyn Shoemaker. Pluto and Beyond: A Story of Discovery, Adversity, and Ongoing Exploration. Flagstaff, Ariz.: Northland Publishing, 2007. A history not only of Pluto discoveries but also of the Lowell Observatory, where Pluto was first observed in 1930. -
Weintraub, David A. Is Pluto a Planet? A Historical Journey Through the Solar System. Princeton, N.J.: Princeton University Press, 2007. A Vanderbilt astronomy professor answers his question by relating the history of the concept of “planet,” from Aristotle through Johannes Kepler and Sir Isaac Newton to the present.
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