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. 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. 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. Honours and awards The asteroid 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 in 1993. References 20th-century French astronomers 1924 births 2010 deaths Discoverers of moons Janus (moon) Founding members of the World Cultural Council