French physicist and astronomer whose preferred method of research is to use polarization of light. He was the first to detect a faint atmosphere round the planet Mercury, in 1950, and established that the Moon does not have one. In pursuit of his detailed investigations into Mars, Dollfus made the first French ascent in a stratospheric balloon.
Dollfus was born and educated in Paris. In 1946 he became an astronomer at the Meudon Observatory there.
Hoping to establish the mineral composition of the Martian deserts, Dollfus checked the polarization of light by several hundreds of different terrestrial minerals to try to find one for which the light matched that polarized by the bright Martian desert areas. He found only one, and that was pulverized limonite.
In 1966 Dollfus discovered Janus, the innermost moon of Saturn, at a time when the rings - to which it is very close - were seen from Earth edgeways on and practically invisible.