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Summary Article: Sagan, Carl Edward (1934-1996) from The Hutchinson Dictionary of Scientific Biography

Place: United States of America

Subject: biography, astronomy

US astronomer and popularizer of astronomy whose main research was on planetary atmospheres, including that of the primordial Earth. His most remarkable achievement was to provide valuable insights into the origin of life on our planet.

Sagan was born on 19 November 1934 in New York City. Completing his education at the University of Chicago, he obtained his bachelor's degree in 1955 and his doctorate in 1960. Then, for two years, he was a research fellow at the University of California in Berkeley, before he transferred to the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, lecturing also at Harvard, where he became assistant professor. Finally, in 1968 Sagan moved to Cornell University, in Ithaca, New York, and took up a position as director of the Laboratory for Planetary Studies; in 1970 he became professor of astronomy and space science there. He died on 20 December 1996.

The editor of the astronomical journal Icarus, Sagan wrote a number of popular books including Broca's Brain: Reflections on the Romance of Science (1979); Cosmos (1980), based on his television series of that name; and the science fiction novel Contact.

In the early 1960s Sagan's first major research was into the surface and atmosphere of Venus. At the time, although intense emission of radiation had shown that the dark-side temperature of Venus was nearly 600K (327°C/621°F), it was thought that the surface itself remained relatively cool - leaving open the possibility that there was some form of life on the planet. Various hypotheses were put forward to account for the strong emission actually observed: perhaps it was due to interactions between charged particles in Venus' dense upper atmosphere; perhaps it was glow discharge between positive and negative charges in the atmosphere; or perhaps emission was due to a particular radiation from charged particles trapped in the Venusian equivalent of a Van Allen belt. Sagan showed that each of these hypotheses was incompatible with other observed characteristics or with implications of these characteristics. The positive part of Sagan's proposal was to show that all the observed characteristics were compatible with the straightforward hypothesis that the surface of Venus was very hot. On the basis of radar and optical observations the distance between surface and clouds was calculated to be between 44 km/27 mi and 65 km/40 mi; given the cloud-top temperature and Sagan's expectation of a ‘greenhouse effect’ in the atmosphere, surface temperature on Venus was computed to be between 500K (227°C/440°F) and 800 K (527°C/980°F) - the range that would also be expected on the basis of emission rate.

Sagan then turned his attention to the early planetary atmosphere of the Earth, with regard to the origins of life. One way of understanding how life began is to try to form the compounds essential to life in conditions analogous to those of the primeval atmosphere. Before Sagan, Stanley Miller and Harold Urey had used a mixture of methane, ammonia, water vapour, and hydrogen, sparked by a corona discharge that simulated the effect of lightning, to produce amino and hydroxy acids of the sort found in life forms. Later experiments used ultraviolet light or heat as sources of energy, and even these had less energy than would have been available in Earth's primordial state. Sagan followed a similar method and, by irradiating a mixture of methane, ammonia, water, and hydrogen sulphide, was able to produce amino acids - and, in addition, glucose, fructose, and nucleic acids. Sugars can be made from formaldehyde (methanal) under alkaline conditions and in the presence of inorganic catalysts. These sugars include five-carbon sugars, which are essential to the formation of nucleic acids, and the six-carbon sugars glucose and fructose - all common metabolites found as constituents of present-day life forms. Sagan's simulated primordial atmosphere not only showed the presence of those metabolites, it also contained traces of adenosine triphosphate (ATP) - the foremost agent used by living cells to store energy.

In 1966, in work done jointly with Pollack and Goldstein, Sagan was able to provide evidence supporting a hypothesis about Mars put forward by Wells, who observed that in regions on Mars where there were both dark and light areas, the clouds formed over the lighter areas aligned with boundaries of adjacent dark areas. Wells suggested that they were lee clouds formed by the Martian wind as it crossed dark areas. The implication, that dark areas mark the presence of ridges, was given support by Sagan's finding that dark areas had a high radar reflectivity that was slightly displaced in longitude. Sagan concluded that these dark areas were elevated areas with ridges of about 10 km/6 mi and low slopes extending over long distances.

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