Audouin Dollfus

Infobox Scientist
name = Audouin Dollfus
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caption = Audouin Dollfus
birth_date = November 12 1924
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nationality = French
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field = astronomy
work_institutions = Paris Observatory
alma_mater = University of Paris
doctoral_advisor = Bernard Lyot
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known_for = Janus
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Audouin Charles Dollfus (November 12 1924 – ) is a French astronomer and aeronaut,specialist in studies of the solar system and discoverer of Janus, a moon of Saturn.

Astronomical Career and Research

He studied at the University of Paris, obtaining a doctorate in physicalsciences in 1955. Beginning in 1946, Dollfus worked as an astronomer at the MeudonObservatory, following his advisor and mentor Bernard Lyot. In particular, hedirected the Laboratory of Solar System Physics there. Today he iscurrently an honorary astronomer at the Paris Observatory. Most of his work wascarried out based on observations from the Pic du Midi Observatory, and his preferred research methodis 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 has published more than 300 scientificpublications, relating primarily to astrophysics of the solar system.

Before the Viking spacecraft landed on Mars, the composition of theMartian surface was the subject of many debates.Dollfus tried to determine the composition of the Martian desert, throughcomparison with the appearancein polarized light ofseveral hundred terrestrial minerals.He found that only pulverized limonite(Fe2O3) corresponded with the appearance of Mars, and concluded that the Martian surface could becomposed of iron oxide. However, another astronomer, Gerald 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 provedDollfus correct.

By using the polarization of light, it is possible to detect an atmospherearound a planet or natural satellite. In 1950, most scientists thought that
Mercury, because of its small size, had probably lost its atmosphere due tomolecular escape into space. Dollfus announced that he had detected a verysmall atmosphere, again using polarization measurements made at the Pic du MidiObservatory in the French Pyrenees. His discovery contradicted the previoustheoretical predictions based on the kinetic theory of gases. Dollfus estimatedthat the atmospheric pressure at the surface of Mercury was approximately 1 mmof mercury. The nature of gas composing this atmosphere was unknown but thoughtto be a dense, heavy gas. It was however certain that the atmosphere of Mercurymust be less than 1/300th that of the Earth. Currently, it is known that theatmosphere of Mercury is very thin indeed: only 10-15 bar, with the total massof the atmosphere not exceeding 1000 kg.

Mercury has dark zones which contrast with a brighter bottom; this was observedfirst by Giovanni Schiaparelli in 1889. By using the refractingtelescope 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 certainrare heavy elements) is so high that no substantial atmosphere ispossible. The presence of any atmosphere should be detectable by thepolarization of light; Bernard Lyot and later Dollfus showed that there wasno detectable polarization, thereby confirming the theoretical prediction thatthe Moon lacks an atmosphere.

In 1966, Dollfus discovered Janus, a small inner moon of
Saturn. He made this discovery by observing at a time whenthe rings, very close to Janus, were nearly edge-on to the Earth and thuspractically invisible. At this time he probably also observed Epimetheus,a smaller moon which shares the same orbit as Janus, but he did not realizethese were two separate objects and it is Richard Walker that holdscredit for this discovery.

Aerospace and Ballooning Pioneer

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

Honors and Awards

The asteroid 2451 Dollfus was named in his honor.