THE ATMOSPHERES OF THE 

 PLANETS. 



BY 



Dr. H. SPENCER JONES, F.R.S. 



Being the Fourteenth Annual Norman Lockyer Lecture, delivered 

 on December 6, 1938, in the Hall of the Goldsmiths' Company, 

 London. 



During the last few decades the main interests of astronomical research 

 have shifted rapidly from the solar system outwards. The application of 

 the spectroscope to the study of the stars and nebulse, the use of photo- 

 graphy, facilitating the study of faint objects, shortening the time of 

 observation at the telescope, and providing permanent records, and the 

 construction of larger and larger telescopes have made it possible for the 

 astronomer to study objects at greater and greater distances. Whole new 

 fields of research have been opened up, and the exploration of these has 

 proved so attractive and has been so productive of results that the planets 

 of the solar system have received much less attention than was formerly 

 given to them. Nevertheless, the planets have not been entirely neglected 

 in recent research. The great light-gathering power of large modern 

 telescopes has enabled spectrographs of very high dispersion to be used 

 for the more detailed study of the spectra of the planets, and the great 

 advances in the manufacture of plates sensitive to the infra-red region of 

 the spectrum have made possible the investigation of a region of the spec- 

 trum whose importance arises from the fact that the selective absorptions 

 by planetary atmospheres lie mainly in this region. It is my purpose 

 this afternoon to summarise some of the conclusions about the physical 

 conditions on the planets derived from the investigations of recent years. 



From theoretical considerations it is possible to decide whether or not 

 any planet may be expected to possess an atmosphere. The natural 

 tendency of an atmosphere is to diffuse away into space. The molecules 

 of the atmosphere are flying about in all directions at high speeds, con- 

 tinually colliding with one another and rebounding. In the upper layers 

 of the atmosphere, the preponderant tendency is for them to be pushed 

 outwards. They are prevented from escaping only by the gravitational 

 pull of the planet. 



In order to overcome this pull and to fly away into space, any particle, 

 whether large or small, must acquire a velocity greater than a certain 

 minimum value, determined by the mass and radius of the planet. If 

 the radial component of the outward velocity is greater than this minimum 

 value, the particle will escape from the planet, provided its motion is not 

 impeded by collision with another particle. 



