The conclusions reached are of great interest. Measurements of the heat radiations from Mercury give values which support the conclusion that the planet's periods of rotation and orbital revolu tion are the same, and that its atmosphere is negligible or nearly so. In the case of Venus a considerable amount of heat is appar ently emitted even by the unilluminated surface, which suggests a quick rotation. Observations of Mars at the Lowell observatory in 1922, 1924 and 1926, which are in fair agreement with those made at Mount Wilson, show that despite the planet's distance from the sun the temperature during the day rises well above the freezing point, and exceeds that value even at the polar regions during the summer ; that the bright regions are cooler than the dark regions ; that the summer hemisphere is warmer than the winter hemisphere ; that afternoon temperatures are higher than those of the morning, and that low temperatures are recorded from what appear to be cloud surfaces which check the escape of heat from below. These radiometric results for Mars are consistent with telescopic observations of the surface features—particularly with the melting of the polar caps—and with terrestrial exper iences of seasonal and diurnal temperature changes.
As regards Jupiter and Saturn the results obtained have given occasion for some surprise. Physical observations of the sur face features of these planets, particularly the former, com bined with the known low density, suggest very strong analogies with the sun, and apparently give evidence of considerable stores of heat. Yet the radiometric researches of Coblentz, Lampland and Menzel indicate a temperature of about 1 oo° C for the visible surface of Jupiter and a value slightly higher for that of Saturn. Dr. H. Jeffreys, too, has concluded on theoretical grounds that the temperature of these planets is low. Observers, however, find it difficult to accept the position that the very large and violent dis turbances and rapid changes exhibited by Jupiter can have their origin merely in solar radiation, and incline to the view that what ever the superficial temperature there must be a considerable dif ference between that at the centre and that of the external parts of the planet's mass. The difficulty might be removed if it can
be supposed that the cloudy surfaces of Jupiter and Saturn are composed of substances which have a very low boiling point.
Apart from the question of temperature their low density rules out the large planets of the outer group as possible habitations for anything at all resembling human life. It is on the planets of the inner group with their solid surfaces and their proximity to the sun that the requisite conditions must be looked for. Of these planets Mercury, whose periods of axial rotation and orbital revo lution are identical, with its scanty atmosphere, its extremely high temperature on the sunlit side and the intense cold of that which is in perpetual night cannot be habitable unless it be in the re stricted region near the boundary of the light and dark hemi spheres. Of Venus almost nothing is known apart from its re semblance to the earth in volume and mass and its possession of a seemingly cloud-laden atmosphere. About Mars much informa tion has been gathered. Its transparent atmosphere has permitted a study in considerable detail of its surface markings and physical condition with the result that the planet is known despite its small dimensions to exhibit important resemblances to the earth. It may, accordingly, well support some form of animate life, though certain important differences in the conditions prevailing on its surface would seem to preclude its habitation by beings quite like man. More than this can hardly be said, but it may be held probable that on a planet where the required conditions are found life of some sort exists, and that at some period in such a planet's life evolutionary processes may bring its inhabitants to the status of intelligent beings.
See ASTRONOMY and articles under planetary names.