Audouin Dollfus

Audouin Dollfus
Audouin Dollfus
	Versailles, France
Nationality	French
Alma mater	University of Paris
Known for	Discoverer of Janus
Awards	Prix Jules Janssen (1993)
	Scientific career
Fields	astronomy
Institutions	Paris Observatory
Doctoral advisor	Bernard Lyot

Audouin Charles Dollfus (12 November 1924 – 1 October 2010

aeronaut, specialist in studies of the Solar System and discoverer of Janus, a moon of Saturn

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Life and career

Dollfus was born in Paris to aeronaut Charles Dollfus.

Dollfus studied at 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.^{[citation needed}

] Dollfus published more than 300 scientific publications, relating primarily to astrophysics of the Solar System.

Before the

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.^{[citation needed}

]

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 of the Earth. Currently, it is known that the atmosphere of Mercury is very thin indeed: only 10⁻¹⁵ bar, with the total mass of the atmosphere not exceeding 1000 kg.

Mercury has dark zones which contrast with a brighter bottom; this was observed first by Giovanni Schiaparelli in 1889. By using the refracting telescope of the Pic du Midi Observatory, Dollfus was able in 1959 to clearly resolve surface features as small as 300 km.

Dollfus also studied the possible presence of an atmosphere around the Moon. The rate of dissipation into space of any gases on the Moon (except for certain rare heavy elements) is so high that no substantial atmosphere is possible. The presence of any atmosphere should be detectable by the polarization of light; Bernard Lyot and later Dollfus showed that there was no detectable polarization, thereby confirming the theoretical prediction that the Moon lacks an atmosphere.

Direct visual observation became rare in astronomy. By 1965 Robert S. Richardson called Dollfus one of two great living experienced visual observers as talented as Percival Lowell or Giovanni Schiaparelli.^[3] In 1966, Dollfus discovered Janus, a small inner moon of Saturn. He made this discovery by observing at a time when the rings, very close to Janus, were nearly edge-on to the Earth and thus practically invisible. At this time he probably also observed Epimetheus, a smaller moon which shares the same orbit as Janus, but he did not realize these were two separate objects and it is Richard Walker that holds credit for this discovery.

In 1981, Dollfus became a founding member of the World Cultural Council.^[4]

Aerospace and Ballooning Pioneer

With his father, the aeronautical pioneer Charles Dollfus, he holds several world records in ballooning, including the first stratospheric flight in France. He was the first to carry out astronomical observations from a stratospheric balloon, in particular to study Mars in a detailed way.