Gerard Peter Kuiper discovered evidence of a methane atmosphere in the spectrum of Titan, the largest satellite of Saturn.
Surprising progress occurred in observational astronomy during World War II, despite the calling of many astronomers for service in the war effort. In the midst of assignments from the military Office of Scientific Research and Development and consulting for the Eighth Air Force in England, Gerard Peter Kuiper found time for “pure” research. The January 29, 1944, Science News Letter published a summary of the evidence that led him to believe that Titan had an atmosphere: methane (marsh gas) and possibly ammonia had been discovered in its spectrum. Thus, Titan became the first satellite demonstrated to have an atmosphere, a primacy that lasted thirty years, until space probes gave information about the thin atmospheres of Ganymede, Callisto, and Io, Jupiter’s large satellites.
After brief stays at Lick Observatory and Harvard University, Kuiper was appointed to the faculty of the University of Chicago in 1935, becoming a full professor in 1943. The long-term relationship of Kuiper with the University of Chicago was fruitful for both, as Kuiper became the leading solar system astronomer during the 1950’s and 1960’s. Kuiper served as the director of both the Yerkes Observatory at Williams Bay, Wisconsin, and the McDonald Observatory at Fort Davis, Texas. In 1960, he moved to the University of Arizona where he founded the Lunar and Planetary Laboratory. Following his discovery of Titan’s atmosphere, Kuiper conducted extensive studies of the atmosphere of Mars with an eye toward the popularly held hope that life existed there, discovered satellites of Uranus and Neptune, revitalized a nebular hypothesis of planetary formation, studied Pluto extensively, and was one of the leading analysts of the first Lunar surveys.
Kuiper’s doctoral dissertation was a study of the spectra of binary stars.
Titan was discovered in 1655 by Christiaan Huygens while testing a new lens he had configured and polished. Four additional satellites of Saturn were discovered during the 1600’s, but since Titan was the brightest, it was assumed to be the largest, a position confirmed as observations advanced and the system of defining magnitudes of light sources developed. Since Titan was magnitude 8.3 and the next brightest was magnitude 10 (the higher the magnitude, the dimmer the object), there seemed no doubt that it was the largest satellite.
In the decades prior to Kuiper’s discovery, additional physical information was established with difficulty. However, astronomers found that Titan revolved around Saturn in a little less than sixteen days, at a mean distance slightly more than 1.2 million kilometers from the center of the planet. Although Titan’s observable disk was too small to measure accurately, estimates of size and mass were made on the basis of changes Saturn’s satellites caused in one another’s orbits. Estimates of a mass were about twice that of the Moon, with a density approximately two thirds of the Moon. By the 1940’s, estimates based on brightness and distance led to a value of 4,200 kilometers for the diameter of Titan, thus projecting Titan as a Mercury-sized object, much larger than Earth’s moon (3,476 kilometers in diameter).
By the early 1970’s, estimates of Titan’s size would grow to 4,880 kilometers. Voyager 2 in the early 1980’s would establish an upward revision to about 5,150 kilometers in diameter, second only to Ganymede, which has a diameter of 5,262 kilometers. Voyager 2 would also verify the moon’s mass by establishing that Titan is about 52 percent rock and about 48 percent ice.
Measurements of light prior to Kuiper’s discovery provided estimates of the albedo (reflectivity) of Titan. Some astronomers suggested that it was an ice ball, possibly ices of the same gases that had been found on the surface of the mother planet. That suggestion was uncertain because Titan was beyond the reach of the spectroscopes available before the early 1940’s.
Despite the failure of spectroscopic evidence, the suspicion existed still that Titan had an atmosphere. As early as 1908, the Spanish astronomer José Comas Sola of Barcelona observed a darkening around the periphery of the disk of Titan, an indication that it could have an atmosphere. In 1916, Sir James Jeans
Titan, photographed by the Cassini spacecraft in April, 2005.
In the beginning of the twentieth century, Titan was estimated to have only 14 percent the gravity of Earth, but unlike the Moon, Titan receives only one hundredth of the light from the Sun, and thus its temperature is much colder. The gases move more slowly and are more easily retained by the attenuated gravity. Jeans reasoned that Jupiter’s satellites were too warm to retain atmospheres, but Titan and Neptune’s large moon Triton were candidates to have atmospheres.
Kuiper was able to gather superior equipment (the 208-centimeter telescope at McDonald Observatory and infrared spectroscopes that worked well with the cool gases), experience in the observation of dim spectra from his studies of binary stars, and good observational technique to obtain the first useful spectrum of a satellite. He determined that methane (consisting of one atom of carbon and four of hydrogen) was present and that ammonia (consisting of one atom of nitrogen and three of hydrogen) probably was present. This fit the expectations, for if Jeans’s estimate of temperature was accurate, all but methane would be frozen on the surface.
The presence of ammonia was uncertain, although it was theoretically possible for droplets of ammonia to be suspended in the methane. The possible lines of ammonia in the spectrum were mere traces of the element and at the very limits of the equipment. Kuiper thus verified that at least one of the constituent elements of the atmosphere of Saturn itself was present on Titan, although the methane bands for Titan were much weaker than for the planet, indicating a more attenuated atmosphere. This finding supported the earlier expectations. Kuiper noted also that the color of Titan was orange, which he presumed was the result of action of the atmosphere with the surface. He reasoned analogously with speculations about Mars, whose reddish hue was presumed to result from oxidation. Dissociation of methane by the Sun would leave hydrogen and an orange-colored organic precipitate, as laboratory experiments had shown; it was presumed to be either in the atmosphere or on the surface of the planet.
Kuiper enjoyed the astronomical acclaim that went with establishing the first atmosphere on a planetary satellite. For nearly three decades, Titan remained the only satellite with an observed atmosphere; finally, Pioneer 10 found a thin atmosphere around Io, a moon of Jupiter, on December 4, 1973, a clear indication of the quality of Kuiper’s work. By the early 1970’s, observations of the occultation of a star indicated that Ganymede probably had a very thin atmosphere, because the light of the star was gradually extinguished as it passed behind the satellite. These discoveries verified the earlier calculations that had presumed that the other large satellites contained too little mass to retain a dense atmosphere of the light gases at their temperatures, which were warmer than Titan’s. It was clear that had Titan been significantly closer to the sun and its surface temperature higher, the methane would have long since been driven into space beyond the reach of Titan’s limited gravity.
During the 1960’s, various measurements of Titan’s brightness at different wavelengths gave inconsistent measurements of the surface temperature, but all the theories led to higher temperatures than expected. In 1973, Carl Sagan, a former student of Kuiper, and Joseph Veverka
The length of time required before Kuiper’s work could be superseded—and that it was superseded only by spacecraft passing in relatively near proximity to the satellite—implies the care exercised in the research and the quality of the equipment that was used in this particular astronomical effort. Kuiper’s first notable accomplishment was an enduring one.
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Atreya, Sushil K. Atmospheres and Ionospheres of the Outer Planets and Their Satellites. New York: Springer-Verlag, 1986. Section 4 of chapter 7 relates to Titan and focuses exclusively on the atmosphere. While technical, the opening and concluding paragraphs are helpful. -
Coustenis, Athena, and Fred Taylor. Titan: The Earth-Like Moon. River Edge, N.J.: World Scientific, 1999. Extensive study of Titan, comparing it to Earth. Bibliographic references. -
Federer, Charles A., Jr. “Titan.” Science News 99 (January 28, 1944): 10. A brief five-paragraph first announcement of Kuiper’s discovery. The significance of Titan’s having the first verified atmosphere was noted. The importance of the atmosphere’s resemblance to that of Saturn for cosmological theory was also prominent. -
_______. “Titan’s Atmosphere.” Science News Letter 45 (January 29, 1944): 67. A more extensive early summary of the significance of Kuiper’s discovery. Focuses on the importance of verification of the first satellite atmosphere and the possible implications for cosmology. -
Lorenz, Ralph, and Jacqueline Mitton. Lifting Titan’s Veil: Exploring the Giant Moon of Saturn. New York: Cambridge University Press, 2002. Detailed study of the satellite, including sixteen pages of photographic plates. Bibliographic references and index.
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