Part 17

Geologists tell us that more oxygen is to be found in the first six feet of the earth's crust than in all of the atmosphere above. Does oxygen not exist in the surface rocks of the moon as well?

Volcanic action, we are told, is primarily an escape of gases from the interior, chiefly hydrogen, nitrogen, hydrocarbons, sulphur, and various compounds, as well as vast quantities of steam. Beneath the surface chemical change is continually taking place which results in the release of an enormous amount of heat. Some of the gases mentioned above combine with the oxygen in the surface rocks and heat is evolved. It is a known fact that there is great inherent heat in the earth's surface crust. Why not in the moon's surface crust as well?

The water that would be expelled in the form of steam from volcanic vents on the moon would be transformed immediately into hoar-frost, snow and ice and would settle down upon the flanks of the crater-cones or vents.

It should be borne in mind that only volcanic activity on an enormous scale would be plainly visible to us even with the powerful telescopes at our command. Ordinary eruptions such as occur on our own planet would be very difficult to detect. Since the escaping vapors would rapidly pass into the solid state and settle down upon the flanks of the crater-cones or vents, we would observe in general little if any change in an object unless we chanced to be looking at it at the time of the eruption, when it might appear to be temporarily obscured by a veil of vapors. What are the chances that we would be carefully observing at the precise time of an eruption, a minute marking, two or three miles in diameter, on a surface as large as all of North America, a surface that is covered with some 30,000 charted craters, numberless crater-pits, streaks, rays, spots, clefts and rills in intricate systems, mountain chains and valleys and a mass of intricate detail?

If we were looking at the earth from the moon with the aid of a powerful telescope would we be apt to notice an eruption of Vesuvius or Katmai or Mauna Loa? Objects four or five miles in diameter would appear as hazy spots with nothing distinctive or remarkable in their appearance. Yet vapor and steam arising from terrestrial volcanoes would be carried by our atmosphere over an area of many square miles, while there is no atmosphere on the moon to spread the vapors that may arise from similar volcanic vents. It would have to be a cataclysmic change indeed to be accepted as indisputable evidence that change is taking place on the moon, and the days of gigantic upheavals are probably over on our satellite as well as on the earth. If volcanic activity is still taking place on the moon it is probably in a mild form such as a comparatively quiet emission of gases from volcanic vents and fumaroles. Such forms of activity would not be plainly visible at this distance, even with the aid of powerful telescopes. The problem of detecting changes on the moon is complicated by the fact that a change of illumination greatly alters the appearance of all lunar markings. Such a change is continually taking place in the course of the month. A marking that stands out in bold relief at lunar sunrise or sunset will change entirely in appearance a few days later under a high sun or even disappear from view entirely. These changes in phase or illumination have to be taken account of in the search for evidence of actual change. To decide whether or not change has actually taken place the object must be viewed under similar conditions, so far as they can be obtained. Even when special care is taken in this respect the suspected evidence of change is usually "explained away" as due to differences in illumination or seeing, by those who have not observed the object themselves and are not in sympathy with the view that the moon is anything but a dead world.

As regards the question of life on the moon, it is interesting to consider the facts brought out by investigations made by scientists connected with the Geophysical Laboratory of the Carnegie Institute in the Valley of Ten Thousand Smokes. The volcanic activity there takes the form of eruptions from numerous small vents or fumaroles and ninety-nine per cent. of the emanations are water vapor. It was observed that blue-green algae were living at the edge of active vents emitting ammonia compounds at a temperature of 212° F. They were not found, however, near vents from which ammonia compounds were not being emitted. If life exists under such conditions it is conceivable that suitable conditions for the support of certain forms of life, animal as well as vegetable, may be found in low-lying valleys and crevices and upon the floors of craters, where certain gases essential to the support of life might be evolved from many small volcanic vents and fumaroles.

Many theories have been advanced to explain the origin of the lunar craters which have no counterpart on our own planet. They are saucer-like depressions in the surface of the moon, frequently of such great size that an observer standing in the center would not be able to see either side of the crater owing to the curvature of the moon's surface. Craters fifty, sixty or one hundred miles in diameter are by no means uncommon, while there are thousands between five and fifty miles in diameter. A characteristic feature of many craters is a central peak, and the surrounding walls are often a mile or more high and in some instances are symmetrically terraced. New craters have been formed on the sides or floors of old craters, and these are always more clear-cut and sharper in outlines than the old formations, and generally much smaller. A number of craters are surrounded by a system of light streaks or rays of unknown origin that extend in some instances to enormous distances on all sides of the crater. The most conspicuous system is the one surrounding the lunar crater Tycho near the south pole of the moon. The rays originating in this crater extend in all directions for hundreds of miles without turning aside for any obstructions, passing over mountains, craters and plains in their course in practically straight lines like spokes in a wheel. This ray system of Tycho is the most noticeable marking on the moon's surface at the time of full moon. As these streaks cast no shadow they are apparently cracks in the surface that have filled up with some light-colored material from below. Their origin has never been satisfactorily explained.

As to the origin of the lunar craters, some believe that they were produced in past ages by a bombardment of the lunar surface by huge meteoric masses; but there are many objections to this theory that we will not take up here. It is more generally believed that the lunar craters are a result of volcanic activity on an enormous scale which took place on the moon many ages ago and which has now practically ceased, its only manifestations now taking the form of a quiet emission of gases from small volcanic vents or fumaroles which exist all over the lunar surface but which are to be found in greatest numbers on the floors and sides of craters.

XXII

COMETS

The orbits of comets are inclined at all angles to each other and to the orbits of the planets which, on the other hand, lie very nearly in the same plane.

The larger members of the sun's family, the planets and their satellites, revolve from west to east around the sun. Comets on the contrary frequently retrograde or back around the sun in the opposite direction--from east to west.

The paths that these erratic visitors follow in their journeys around the sun bear not the slightest resemblance to the paths of the planets, which are almost perfect circles. The orbits of comets are ellipses that are greatly elongated or parabolas. If the orbit is a parabola the comet makes one and only one visit to the sun, coming from interstellar space and returning thereto after a brief sojourn within our solar system.

Donati's comet of 1858, one of the greatest comets of the nineteenth century, had a period of more than two thousand years and its aphelion (the point in its orbit farthest away from the sun) was five times more distant than the orbit of Neptune.

There is, however, a class of comets known as periodic comets that have extremely short periods of revolution around the sun. To this class belongs Halley's comet whose period of seventy-five years exceeds that of any other short period comet. Encke's comet, on the other hand, has a period of three and a third years which is the shortest cometary period known. Most of the periodic comets are inconspicuous and only visible telescopically even when comparatively near to the earth. Halley's comet is the only one of this class that lays any pretensions to remarkable size or brilliancy and it also is showing the effects of disintegration resulting from too frequent visits to the sun.

Comets are bodies of great bulk or volume and small total mass. Their tails, which only develop in the vicinity of the sun, are formed of the rarest gases, and the best vacuum that man can produce would not be in as tenuous a state as the material existing in the tails of comets. There are many proofs of the extreme tenuity of comets. The earth has on a number of occasions passed directly through the tails of comets without experiencing the slightest visible effects. Stars shine undimmed in luster even through the heads of comets. If the earth should encounter a comet "head on" it is doubtful if it would experience anything more serious than a shower of meteors which would be consumed by friction with the earth's atmosphere, or a fall of meteorites over a small area of a few square miles. It is possible, however, that matter in the nucleus, the star-like condensation in the head of a comet, may consist of individual particles weighing in some instances a number of tons, surrounded by a gaseous envelope and held together by the loose bonds of their mutual attraction. If the earth should encounter the nucleus of a comet considerable damage might be done over a portion of the earth's surface, but the chances of such an occurrence are less than one in a million.

Since the total mass of a comet is so small, a close approach to one of the planets, especially Jupiter, produces great changes in the form of the comet's orbit, though the motion of the planet is not disturbed in the slightest degree by the encounter.

The majority of all the short-period comets have been "captured" by Jupiter, that is, the original orbits have been so changed by the perturbations produced by close approaches to the giant planet that their aphelia, or the points in their orbits farthest from the sun, lie in the vicinity of Jupiter's orbit. Several of the other planets have also "captured" comets in this sense, and the fact that the aphelia of a number of comets are grouped at certain definite intervals beyond the orbit of Neptune has been considered by some astronomers to be an indication that there are two or more additional planets in the solar system revolving around the sun at these distances.

The most interesting feature of a comet is its characteristic tail which develops and increases in size and brilliancy as the comet approaches the sun. As the tail is always turned away from the sun it follows the comet as it draws near the sun and precedes it as it departs. Its origin is due, it is believed, both to electrical repulsion and light-pressure acting upon minute particles of matter in the coma or head of the comet.

The curvature of the tail depends upon the nature of the gases of which it is composed. Long, straight tails consist chiefly of hydrogen, it has been found, curved tails of hydrocarbons and short, bushy tails of mixtures of iron, sodium and other metallic vapors. At times the same comet will have two or more tails of different types.

Since the material driven off from the nucleus or head of a comet by electrical repulsion and light-pressure is never recovered, it is evident that comets are continually disintegrating. Also, comets that have passed close to the sun at perihelion have frequently been so disrupted by tidal forces that one nucleus has separated into several parts and the newly formed nuclei have pursued paths parallel to the original orbit, each nucleus developing a tail of its own.

Many periodic comets, it is now known, have gradually been broken up and dissipated into periodic swarms of meteors as a result of the disruptive effect produced by too frequent returns to the vicinity of the sun.

These swarms of meteors continue to travel around the sun in the orbits of the former comets. The earth encounters a number of such swarms every year at certain definite times.

The largest and best known of these swarms or showers are the Leonids, which appear about November 15; the Andromedas (or Bielids), which appear later in the same month and the Perseids, which appear early in August. These swarms are named for the constellations in which their "radiant" lies, that is, the point in the heavens from which they appear to radiate. The position of the radiant depends upon the direction from which the swarm is coming. It is simply a matter of perspective that the individual particles appear to radiate from the one point, for they are actually travelling in parallel lines.

The luminosity of these meteoric particles is caused by the friction produced by their passage through the atmosphere. They always appear noiselessly because they are mere particles of meteoric dust weighing at the most scarcely a grain. They differ greatly in this respect from their large and noisy relatives, the meteorites, bolides and fireballs.

Numberless small meteoric particles are entrapped by the earth's atmosphere every day. They are referred to as "shooting" stars or "falling" stars though, of course, they are not in any sense stars. It is only when these meteoric particles travel in well-defined cometary orbits and appear at certain definite times every year that they are referred to as swarms or showers of meteors.

The luminosity of comets is due not only to reflected sunlight, but to certain unknown causes that produce sudden and erratic increases or decreases of brilliancy. The causes of these sudden changes in luminosity are unknown; possibly electrical discharges or chance collisions between fragments of considerable size may account for some of them.

The peculiar behavior of the tails of comets at certain times has frequently been noted and suggests the existence of quantities of finely-divided meteoric or gaseous matter within the solar system that has no appreciable effect upon the huge planetary masses, but offers sensible resistance to the passage of the tenuous gases of which the tails of comets are composed. The fact that the earth daily encounters meteoric dust, meteorites and fireballs also indicates that meteoric matter exists in considerable quantities within our solar system. Tails of comets appear at times to be twisted or brushed aside as if they had encountered some unknown force or some resisting medium.