Audouin Charles Dollfus (12 November 1924 – 1 October 2010) was a French astronomer and aeronaut, specialist in studies of the Solar System and discoverer of Janus, a moon of Saturn. Life and career Dollfus was born in Paris to aeronaut Charles Dollfus. Dollfus studied at the University of Paris, obtaining a doctorate in physical sciences in 1955. Beginning in 1946, Dollfus worked as an astronomer at the Meudon Observatory, following his advisor and mentor Bernard Lyot. In particular, he directed the Laboratory of Solar System Physics there. Until his death, he was an honorary astronomer at the Paris Observatory. Most of his work was carried out based on observations from the Pic du Midi Observatory, and his preferred research method is the use of polarized light as a diagnostic of the properties of Solar System objects. Through patient and persistent research and the development of new observational techniques, he was able to obtain many remarkable results. Dollfus published more than 300 scientific publications, relating primarily to astrophysics of the Solar System. Before the Viking spacecraft landed on Mars, the composition of the Martian surface was the subject of many debates. Dollfus tried to determine the composition of the Martian desert, through comparison with the appearance in polarized light of several hundred terrestrial minerals. He found that only pulverized limonite (FeO(OH)) corresponded with the appearance of Mars, and concluded that the Martian surface could be composed of iron oxide. Astronomer Gerard P. Kuiper of the University of Chicago disagreed with this conclusion, believing that fine-grained igneous rocks were a better fit to the data, but subsequent observations proved Dollfus correct. By using the polarization of light, it is possible to detect an atmosphere around a planet or natural satellite. In 1950, most scientists thought that Mercury, because of its small size, had probably lost its atmosphere due to molecular escape into space. Dollfus announced that he had detected a very small atmosphere, again using polarization measurements made at the Pic du Midi Observatory in the French Pyrenees. His discovery contradicted the previous theoretical predictions based on the kinetic theory of gases. Dollfus estimated that the atmospheric pressure at the surface of Mercury was approximately 1 mm of mercury. The nature of gas composing this atmosphere was unknown but thought to be a dense, heavy gas. It was however certain that the atmosphere of Mercury must be less than 1/300th that