Astronomical Discoveries relating to the Planet Jupiter in 1927
THE LATEST ASTRONOMICAL DISCOVERIES and opinions regarding the great planet Jupiter are briefly gathered in a leaflet issued by the Astronomical Society of the Pacific (San Francisco). In it, E. C. Slipher, of the Lowell Observatory, Flagstaff, Arizona, tells us that Jupiter's claim on our interest is not so much because of his bodily form or great size, or because he forms the chief ornament of our night sky, but rather by reason of the conspicuous part he has played in the historical development of astronomy and the interesting example he presents in planetary evolution—a chaotic something between sun and world. Mr. Slipher writes:
Long ago it was learned that he was the giant of the planets, standing fifth in order of distance from the sun; that his size is greater than that of all the other planets combined; that his diameter is so great that even at his tremendous distance he shows the largest disk of any of the planets, and shines with a brilliancy equaling that of any of the other planets except Venus; that his great circumference of 271,750 miles combined with his rapid rotation once every 9 hours and 50 minutes, causes particles on his equator to travel at the speed of 26,000 miles per hour, almost equaling his orbital speed of 28,800 miles per hour; that his density is only one-quarter that of the earth, but that his mass is such that objects on his surface weigh two and five-eighths times as much as they would on the earth.
"His satellite system of nine members has played a unique part in astronomical discovery. First, because the four largest ones were the first objects revealed by Galileo's crude telescope in 1610; second, because it was from the variability in the time of occurrence of their eclipses that Roemer discovered, in 1675, the finite velocity of light; third, because it was from a study of their motions that an early determination of the mass of Jupiter was deduced.
"It was not until 1892 that his tiny fifth satellite was discovered by Barnard at the Lick Observatory, while the still fainter sixth, seventh, eighth and ninth satellites were more recently detected only by the means of photography, three at the Lick Observatory, one at Greenwich.
"Jupiter, through the influence of his great mass, has become possest of a large family of comets whose orbits have been shaped by his influence. More than thirty comets circle round the sun with their aphelia closely hugging Jupiter's orbit. Some time in the past these comets must have passed close to him in such a manner that they were greatly disturbed by the influence of the planet and their previous orbits radically reduced to the present forms. They are then said to be captured and to belong to his family of comets, altho still revolving around the sun.
"When we look upon the planet in the telescope, we find that his disk is striated with belts of various tones and tints, conforming almost exactly to the parallels of latitude; while the disk darkens from center to edge. These cloud-like belts are long-lived, lasting generally for weeks, months, or even years. They do disappear at times, but usually reform again after an indefinite interval, assuming somewhat the same general form and position. The sun's action appears wholly inadequate to produce the wonderful changes observed on him. The high brilliancy of his disk indicates that his visible surface is one of dense clouds, and the fact that the planet does not rotate as a whole, but that different parts go round at different rates seems clearly to evidence this. From the character and behavior of the belts it appears that they must be clouds raised by Jovian energy, not by solar energy.
"Photographs of Jupiter taken in light of different colors, in recent years, show that his dark belts are recorded about as well in ultra-violet light as in yellow and red lights, whereas the limb is darkest in the red images and brightest in the violet ones. Further, the diameter of the ultra-violet images is much greater than that of the red. These facts indicate that the atmosphere of Jupiter is extensive and dense, and that the dark and bright belts lie relatively high in this atmosphere and at about the same level.
"Radiometric measures at Flagstaff and Mt. Wilson indicate that the temperature of Jupiter is much higher than can be explained on the basis of heating by solar radiation and is probably attributable to a warm interior.
"With these facts before us, it is still not easy to explain the colored belts on his surface. The colors seem to be the same at Jovian morning and evening as at his noonday. Such constancy is not what we should expect of ordinary water-vapor clouds, and the low temperature makes it even more difficult to explain them as such. It may be necessary to find some other substance to explain satisfactorily his variegated belts.
"We shall probably not be far from the truth if we infer that Jupiter is still in an early stage in his evolution, rather than far advanced like the Earth and Mars; that he possesses an enormous gaseous atmosphere which is in rapid circulation, and that possibly the energy of his internal fires give rise to violent motions which are reflected by changes in his belts. He appears as a body somewhat more than half-way in life's journey from sun to world. No life, as we know it, can now exist there."
Source: The Literary Digest for November 5, 1927