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Audouin Charles Dollfus (12 November 1924 – 1 October 2010) was a French
astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...
and
aeronaut Aeronautics is the science or art involved with the study, design process, design, and manufacturing of air flight-capable machines, and the techniques of operating aircraft and rockets within the atmosphere. While the term originally referred ...
, specialist in studies of the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
and discoverer of
Janus In ancient Roman religion and myth, Janus ( ; ) is the god of beginnings, gates, transitions, time, duality, doorways, passages, frames, and endings. He is usually depicted as having two faces. The month of January is named for Janus (''Ianu ...
, a moon of
Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...
.


Life and career

Dollfus was born in Paris to aeronaut Charles Dollfus. Dollfus studied at the
University of Paris The University of Paris (), known Metonymy, metonymically as the Sorbonne (), was the leading university in Paris, France, from 1150 to 1970, except for 1793–1806 during the French Revolution. Emerging around 1150 as a corporation associated wit ...
, 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 Bernard Ferdinand Lyot (2 7 February 1897 in Paris – 2 April 1952 in Cairo) was a French astronomer. Biography An avid reader of the works of Camille Flammarion, he became a member of the Société Astronomique de France in 1915 and made ...
. In particular, he directed the Laboratory of Solar System Physics there. Until his death, he was an honorary astronomer at the
Paris Observatory The Paris Observatory (, ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centres in the world. Its historic building is on the Left Ban ...
. 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 Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
, 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 In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): M ...
. He found that only pulverized
limonite Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as , although this is not entirely accurate as the ratio of oxide to hydroxide can vary qu ...
(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 The University of Chicago (UChicago, Chicago, or UChi) is a Private university, private research university in Chicago, Illinois, United States. Its main campus is in the Hyde Park, Chicago, Hyde Park neighborhood on Chicago's South Side, Chic ...
disagreed with this conclusion, believing that fine-grained
igneous rock Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...
s 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 An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
around a planet or
natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are colloquially referred to as moons, a deriv ...
. 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 The Pyrenees are a mountain range straddling the border of France and Spain. They extend nearly from their union with the Cantabrian Mountains to Cap de Creus on the Mediterranean coast, reaching a maximum elevation of at the peak of Aneto. ...
. 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 Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
. 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 Giovanni Virginio Schiaparelli ( , , ; 14 March 1835 – 4 July 1910) was an Italian astronomer and science historian. Biography He studied at the University of Turin, graduating in 1854, and later did research at Berlin Observatory, unde ...
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 Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
. 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 Percival Lowell (; March 13, 1855 – November 12, 1916) was an American businessman, author, mathematician, and astronomer who fueled speculation that there were canals on Mars, and furthered theories of a ninth planet within the Solar System ...
or
Giovanni Schiaparelli Giovanni Virginio Schiaparelli ( , , ; 14 March 1835 – 4 July 1910) was an Italian astronomer and science historian. Biography He studied at the University of Turin, graduating in 1854, and later did research at Berlin Observatory, unde ...
. In 1966, Dollfus discovered
Janus In ancient Roman religion and myth, Janus ( ; ) is the god of beginnings, gates, transitions, time, duality, doorways, passages, frames, and endings. He is usually depicted as having two faces. The month of January is named for Janus (''Ianu ...
, a small inner moon of
Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...
. 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 In Greek mythology, Epimetheus (; ) is the brother of Prometheus, the pair serving "as representatives of mankind". Both sons of the Titan Iapetus, while Prometheus ("foresight") is ingeniously clever, Epimetheus ("hindsight") is inept and fool ...
, 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 The World Cultural Council is an international organization whose goals are to promote cultural values, goodwill and philanthropy among individuals. The organization founded in 1982 and based in Mexico, has held a yearly award ceremony since 198 ...
.


Aerospace and Ballooning Pioneer

With his father, the aeronautical pioneer Charles Dollfus, he holds several world records in
ballooning Ballooning may refer to: * Hot air ballooning * Balloon (aeronautics) * Ballooning (spider) * Ballooning degeneration, a disease * Memory ballooning In computing, memory ballooning is a technique that is used to eliminate the need to overcommit ...
, 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.


Honours and awards

The
asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
2451 Dollfus is named in his honour. One of the largest craters on Mars was also named for him in 2013. Dollfus was the President of the Société astronomique de France (SAF), the French popular astronomy society, from 1979 to 1981. The Société astronomique de France awarded him its
Prix Jules Janssen The Prix Jules Janssen is the highest award of the Société astronomique de France (SAF), the French astronomical society. This annual prize is given to a professional French astronomer or to an astronomer of another nationality in recognition ...
in 1993.


References

{{DEFAULTSORT:Dollfus, Audoin 20th-century French astronomers 1924 births 2010 deaths Discoverers of moons Janus (moon) Founding members of the World Cultural Council University of Paris alumni