Planet

The interpretation of the spectra of the outer planets, apart from the mere fact of considerable absorption, is puzzling. From the accompanying plate and Table III. it will be seen that there are one or two rather faint but broad bands in the spectrum of Jupiter which are seen to be much stronger in the spectra of Saturn, Uranus and Neptune. The two latter also contain a number of additional bands. The origin of the bands was still unknown in 1927.

Temperatures of the Planets.

A matter of special interest in regard to the planets is that of the temperature of their sur faces. Speaking generally it may be said that the surface tempera ture of a planet which is without appreciable heat of its own and is warmed only by the solar radiation is determined by a number of factors of which the following may be specially mentioned : (I) The distance of the planet from the sun, the intensity of solar radiation obeying the well-known inverse-square law. (2) The degree in which the planet's surface approaches that of a perfect radiator. (3) The presence or absence of an atmosphere and the nature of such atmosphere. (4) The planet's rotation. It can be shown that a black sphere without an atmosphere rotating with sufficient rapidity to be heated equally all round would have a mean surface temperature given by the formula T=277VVR where T is the temperature in degrees centigrade reckoned from the absolute zero and R is the distance of the body from the sun, taking the earth's distance from the sun as unity. But should the periods of axial rotation and orbital revolution be equal, so that the body turns always the same face to the sun, the formula be comes for the point immediately under the sun T=392°/VR. This means, e.g., that on a perfectly radiating planet without an atmosphere at the distance of Mercury the point on the surface where the sun is overhead would have a temperature of 631° C (absolute). With a quick rotation the mean surface temperature would be about 445° C (absolute) or 342° F. The corresponding absolute temperatures at the distance of Mars come out 318° C (I13° F) and 224° C F), and at that of Neptune only 71° C (-332° F) and 51° C (-368° F). The actual tempera tures will, of course, be affected to a large extent by the presence of an atmosphere which, while directly reflecting a large proportion of the solar rays, checks the escape by radiation of such as pene trate it and reach the planet's surface.

Until comparatively lately, however, the only available methods of estimating the surface temperatures of the planets were those based on general physical principles, and in view of the great uncertainty as to the conditions prevailing on the planets, and par ticularly as regards the nature of their atmospheres, the conclusions arrived at were necessarily of a highly speculative character. But in recent years great strides have been made in the development of radiometric apparatus, and by the use of thermo-couples in vacuum cells with extremely small receivers and sensitive gal vanometers it has been found practicable to measure with consid erable accuracy the radiations received from the planetary sur faces, and even, in some cases, from particular parts of those sur faces. In these measurements transmission screens of such mate rials as water, quartz, glass, fluorite, are interposed in the path of the rays which absorb the radiations in certain parts of the spec trum, and thus enable the total radiation to be resolved into a number of spectral components. For example, when a water cell is employed, the infra-red heat radiations are cut out, and the ratio of the transmitted to the total radiation of the planet provides data for determining the temperature of the planet's surface. It will be clear that if the ratio of transmission through the water cell is large the heat radiated by the planet is small, but that if the transmission ratio is small the proportion of heat rays is com paratively great. The matter is, of course, complicated somewhat by the presence of water vapour in the terrestrial and planetary atmospheres, but the necessary corrections can be estimated, and it is found that the reduction of the observations by various methods such as those based on (I) the distribution of energy in the spectrum, (2) the fourth power law and the measured water cell transmission, as worked out by Menzel and (3) a comparison with observations of the moon, all lead to satisfactorily accordant results.