Astronomy Bio...Audouin Dollfus

Jay Bitterman

Audouin Charles Dollfus, a French physicist and astronomer was born on November 12, 1924. He studied at the Lycee Janson-de-Sadly and at the Faculty of Sciences in Paris, where he gained his doctorate in mathematical sciences. Since 1946, he has been Astronomer of the Astrophysical Section of Meudon Observatory, in Paris. Dollfus made the first ascent in a stratospheric balloon in France in order to pursue detailed investigations of Mars. Dollfus has achieved remarkable results as a practical astrophysicist, through patient and persistent research. His preferred method of research is to use polarization of light.

Before Viking landed on Mars, the mineral composition of the Martian deserts was the subject of considerable dispute. By determining the polarization of light of several hundreds of different terrestrial minerals, Dollfus tried matching their light to that of the polarized light of the bright Martian desert areas. He found that only pulverized limonite (Fe203) matched. He concluded that Mars could be composed of oxidized cosmic iron. Another astronomer, Gerard Kuiper of the University of Chicago, however, did not agree with Dollfus' findings. In his work, iron oxides gave poor results and he determined that the closest match was with brownish fine-grained igneous rocks.

By utilizing polarization of light it is possible to detect an atmosphere around a planet or satellite. In 1950, it was thought the planet Mercury, because of its small size, had probably lost its atmosphere through the escape of the molecules into space. Dollfus announced that he had detected a very faint atmosphere from polarization measurements carried out at the Pic-du-Midi Observatory in the French Pyrenees. This was also in contrast to theoretical expectations based on the kinetic theory of gases. Dollfus estimated that the atmospheric pressure at ground level was about 1 mm of mercury. The nature of the gas making up this atmosphere was unknown, but it must be a dense, heavy gas. It is certain that the atmosphere on Mercury is not more than 1/300 that on Earth.

Mercury shows faint shady markings, set against a dull whitish background that was first observed by Giovanni Schiaparelli in 1889. Using the 60cm refractor at the Pic-du-Midi Observatory in 1959, Dollfus was able to clearly resolve spots about 300 km apart.

Dollfus has also investigated the possibility of an atmosphere around the Moon. The rate of thermal dissipation into space of all but the heavier gases (which are cosmically very scarce) from the Moon is so high that an atmosphere cannot be expected. The most telling evidence is the complete absence of the twilight phenomena on the Moon. Any elongation of the points (or the cusps) of the Moon beyond 90 caused by scattered sunlight should be detectable by polarization. But Bernard Lyot, and later Dollfus, proved that there was no detectable polarization.

In 1966 Dollfus discovered Janus, the innermost moon of Saturn, at a time when the rings to which it is very close were seen edge on from Earth (and practically invisible).

Published in the November 2000 issue of the NightTimes