Audouin Dollfus
French astronomer who discovered Saturn's moon Janus.
Audouin Charles Dollfus (12 November 1924 – 1 October 2010) was a French astronomer and balloonist who specialized in the Solar System and discovered Saturn’s moon Janus. He was born in Paris to Charles Dollfus, an aeronautical pioneer. After studying at the University of Paris, he earned a doctorate in physical sciences in 1955. From 1946 onward, he worked as an astronomer at the Meudon Observatory under Bernard Lyot, eventually directing its Laboratory of Solar System Physics. He remained an honorary astronomer at the Paris Observatory until his death. Most of his research relied on observations from the Pic du Midi Observatory, and his preferred method was using polarized light to study Solar System objects. Through patient work and new techniques, he achieved many notable results and published over 300 scientific papers, mostly on Solar System astrophysics.
Before the Viking landers reached Mars, scientists debated the planet’s surface composition. Dollfus compared the polarized light of hundreds of terrestrial minerals with that of the Martian desert. Only pulverized limonite matched, leading him to conclude the surface contained iron oxide. Astronomer Gerard P. Kuiper disagreed, favoring fine-grained igneous rocks, but later observations confirmed Dollfus was correct. Polarization also allowed detection of thin atmospheres. In 1950, most scientists thought Mercury had lost its atmosphere due to its small size. Dollfus announced he had detected a very thin atmosphere using polarization measurements at Pic du Midi, contradicting kinetic theory predictions. He estimated the surface pressure at about 1 mm of mercury, likely a dense heavy gas, but less than 1/300th of Earth’s. Today we know Mercury’s atmosphere is extremely tenuous—only 10⁻¹⁵ bar, with a total mass under 1000 kg.
Mercury’s dark zones against brighter terrain were first noted by Giovanni Schiaparelli in 1889. In 1959, using the Pic du Midi refractor, Dollfus resolved surface features as small as 300 km. He also investigated whether the Moon had an atmosphere. Theory predicted no substantial atmosphere because gases dissipate quickly, except for rare heavy elements. Bernard Lyot and later Dollfus showed no detectable polarization, confirming the Moon lacks an atmosphere. By 1965, Robert S.
Quick Facts
- Birth Date
- 1924-11-12
- Birth Place
- Paris, France
- Death Date
- 2010-10-01
- Death Place
- Versailles, France
- Field
- astronomy
- Workplaces
- Paris Observatory
- Education
- University of Paris
- Doctoral Advisor
- Bernard Lyot
- Known For
- Discoverer of Janus
- Awards
- Prix Jules Janssen (1993)
Facts from the source article.
Lore & Background
Dollfus studied at the University of Paris, obtaining a doctorate in physical sciences in 1955. Beginning in 1946, he worked as an astronomer at the Meudon Observatory, following his advisor and mentor Bernard Lyot, and directed the Laboratory of Solar System Physics there. Most of his work was carried out based on observations from the Pic du Midi Observatory, and his preferred research method was the use of polarized light as a diagnostic of the properties of Solar System objects.
Before the Viking spacecraft landed on Mars, 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 corresponded with the appearance of Mars, concluding the Martian surface could be composed of iron oxide. Astronomer Gerard P. Kuiper disagreed, but subsequent observations proved Dollfus correct. In 1950, Dollfus announced he had detected a very small atmosphere on Mercury using polarization measurements, contradicting theoretical predictions; he estimated the atmospheric pressure at the surface of Mercury was approximately 1 mm of mercury.
In 1966, Dollfus discovered Janus, a small inner moon of Saturn, by observing at a time when the rings were nearly edge-on to the Earth and thus practically invisible. At this time he probably also observed Epimetheus, a smaller moon sharing the same orbit, but did not realize these were two separate objects; Richard Walker holds credit for that discovery.
Reader's Guide
Dollfus's significance lies in his persistent use of polarized light to study Solar System objects, yielding results that were often controversial but later validated. His determination that the Martian surface is composed of iron oxide (limonite) was initially disputed by Gerard P. Kuiper but confirmed by subsequent observations. His detection of a thin atmosphere on Mercury contradicted kinetic theory predictions and was a notable observational achievement. The discovery of Janus in 1966 added a new moon to Saturn's system, made possible by his skill as a visual observer at a time when direct visual observation was becoming rare. By 1965, Robert S. Richardson called Dollfus one of two great living experienced visual observers as talented as Percival Lowell or Giovanni Schiaparelli. Dollfus also studied the Moon's atmosphere, confirming with Bernard Lyot that no detectable polarization existed, supporting the theoretical prediction that the Moon lacks an atmosphere. His legacy includes more than 300 scientific publications, the asteroid 2451 Dollfus named in his honor, and a large crater on Mars named for him in 2013.
Did You Know?
- Dollfus discovered Janus, a moon of Saturn, in 1966.
- He used polarized light to determine that the Martian surface could be composed of iron oxide (limonite).
- He detected a very small atmosphere on Mercury in 1950, contradicting theoretical predictions.
- Dollfus was one of two great living visual observers as talented as Percival Lowell or Giovanni Schiaparelli, according to Robert S. Richardson in 1965.
Frequently Asked Questions
Who is Audouin Dollfus?
Audouin Charles Dollfus (1924–2010) was a French astronomer and balloonist from Paris who spent his career studying the Solar System. He is best remembered for identifying Saturn's moon Janus.
What moon of Saturn did Audouin Dollfus discover?
He discovered Janus, one of Saturn's small inner moons. The satellite was later officially named in his honor.
Where did Audouin Dollfus conduct most of his astronomical work?
Beginning in 1946, he worked at the Meudon Observatory in France under Bernard Lyot and eventually took the helm of its Laboratory of Solar System Physics. He also held an honorary astronomer position at the Paris Observatory until his death in 2010.
What was Audouin Dollfus's academic training?
He studied at the University of Paris and completed a doctorate in physical sciences in 1955. His father, Charles Dollfus, was an aeronautical pioneer, which likely influenced his later interest in ballooning alongside his astronomy.
Why is Audouin Dollfus significant in Saturn-moon research?
His identification of Janus expanded the known inventory of Saturn's satellites and gave the planetary community a new object to study. His decades of observational work at Meudon also made him a foundational figure in Solar System astronomy in France.
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