though they appear more indistinct than those of Jupiter on account
of the greater distance of Saturn. Saturn also resembles Jupiter
in its physical composition which is largely, if not entirely,
gaseous, and in the extremely short period of rotation on its axis
which is approximately ten hours. In more ways than one Saturn is
a very unusual planet. In addition to possessing an enormous ring
system it is the lightest of all the planets, its density being only
sixty-three hundredths that of water, and it is the most oblate,
its flattening at the poles amounting nearly to one-tenth of its
diameter. Its equator is more highly inclined to its orbit than is
the case with any other planet, not even excepting the earth and
Mars. For this reason its seasonal changes are very great, in marked
contrast to Jupiter whose equator lies very nearly in the plane of
its orbit. Since Saturn is so far away from the sun that it receives
only one ninetieth as much light and heat per unit area as the earth,
its outer gaseous surface must be extremely cold unless considerable
heat is conveyed to the surface from within its hot interior.
The late Prof. Lowell concluded from certain observations made at
Flagstaff, Ariz., that Saturn is composed of layers of different
densities and that the inner layers are more flattened at the poles
and rotate faster than the outer layers. Marked variations in the
color and brightness of the ball of the planet have been noted from
time to time. In 1916 observers of Saturn described the planet as
pinkish-brown and conspicuously darker than the brighter portions of
the rings.
It is believed that these very noticeable changes in the color and
brightness of Saturn are due to slight, irregular changes in the
intensity of the radiations of the sun which set up certain secondary
effects in the atmosphere of the planets. Similar changes in color
and brightness have been observed also in the case of Jupiter.
XXI
IS THE MOON A DEAD WORLD?
It has been a generally accepted belief among astronomers for years
that the moon is a dead world devoid of air and water and so,
necessarily, lifeless. It is certain that the moon has no extensive
atmosphere such as envelops our own planet. There is abundant proof
of this fact. The edge of the lunar disk is clear-cut. Whenever,
as happens frequently, the moon passes between us and a star the
disappearance of the star is instantaneous. There is no gradual
dimming or refraction of the star's light by atmospheric vapors.
Moreover, lunar shadows are harsh and black. There is no evidence of
diffusion of light on the moon by atmospheric gases.
The absence of water or water vapor on the visible surface of the
moon, at least in any appreciable quantity, is plainly evident to
anyone who observes the moon through the telescope. Even with small
telescopes, objects five miles or so in diameter can be readily
detected and clouds drifting over the surface could not possibly
escape our observation if they existed.
Bodies of water, great or small, would be plainly visible and would
besides give rise to water vapor and clouds, which we would not fail
to detect.
Since the surface of the moon is unscreened by air and water vapor to
absorb the incoming rays from the sun, and the outgoing radiations
from the surface, the extremes of temperature between day and night
are very great, and are augmented by the fact that the lunar day
equals the lunar month in length, so that fourteen days of untempered
heat are followed by fourteen days of frigid darkness. Observations
of the rate of radiation from the moon's surface during total
eclipses of the moon indicate that the moon's radiation is very
rapid, and that its temperature during the height of the lunar day
probably approaches 200° F., while at the lunar midnight it may have
fallen to 100° below zero F., or even lower.
With air and water both lacking and such extremes of temperature
existing why should we seriously consider the question of life on the
moon?
This is the point of view of the majority of astronomers and it seems
well taken. Yet many astronomers who have made a special study of
the lunar surface for years under all conditions of illumination and
phase, and have most carefully observed and mapped and photographed
its characteristic markings, are agreed that there are evidences
that changes are taking place on the moon, and recently Prof. W. H.
Pickering has expressed the belief, substantiated by drawings, that
there is a progressive change of color or darkening within certain
lunar craters with the advance of the lunar day, indicating, in his
opinion, a rapid vegetational growth that springs up in the height
of the lunar day and dies out as the lunar night approaches.
Some years ago certain selenographers suggested that there might
exist in the numberless crater-pits and craters, in the deep-lying
_maria_ or "seas," and in the clefts and rills and cracks that form
intricate systems all over the lunar surface, certain exhalations
from the surface and heavy vapors including possibly carbon dioxide
and water vapor to temper the extremes of the long lunar days and
night and furnish the necessary medium for the support of certain
forms of animal and vegetable life.
Many astronomers, including a number who are not in sympathy with
the above view, believe that snow and ice exist on the moon, even
though water in the form of liquid and vapor is not observable. All
the extremely brilliant portions of the surface, according to some
astronomers, are covered with snow and ice. Certainly, some portions
of the moon's surface reflect sunlight as brilliantly as if they
were covered with freshly fallen snow, while other portions appear
to be black by contrast. There also appears to be evidence that
certain small markings, described as crater-cones and resembling our
terrestrial volcanoes more than any other lunar feature, are at times
temporarily obscured from view by a veil of vapors. Many observers
believe that these crater-cones are active volcanic vents, and that
there is considerable volcanic activity still taking place upon the
moon.
These small crater-cones resemble, we are told, parasitic cones found
on the sides of terrestrial volcanoes, and they are frequently seen
on the floors of craters closely associated with light streaks.
These crater-cones appear under a high sun as minute white spots
and can be studied to advantage only with powerful instruments.
The Italian astronomer, Maggini, observing the floor of the lunar
crater, Plato, in 1916 noted that one of the small crater-cones that
exist there in great numbers, was temporarily obscured from view by
a cloud of reddish vapors, and Prof. W. H. Pickering, at Arequipa,
Peru, observing the same region some years ago, believed that he
saw evidence of change in some of these small markings. The crater,
Plato, has probably been more carefully studied than any other
portion of the lunar surface. It is sixty miles in diameter and may
be seen even without a telescope as a dark "eye" not far from the
northern edge of the moon. Its floor is one of the darkest objects in
the moon--a dark steel-grey in color--and there is no doubt that for
some unknown reason its dark hue deepens from the time the sun has an
altitude of twenty degrees until after full moon. It has a brilliant
white wall rising from 3,000 to 4,000 feet above its floor, crowned
with several lofty peaks and intersected by a number of valleys and
passes. The spots and faint light markings on the floor have been the
object of much study with small or moderate sized instruments, and
at least six of them are known to be crater-cones. Since they can
only be studied to advantage with powerful instruments and as such
instruments are rarely used for a systematic study of lunar markings,
it is difficult to settle the controversy as to whether they have
changed in appearance or have been at any time obscured by vapors.
Most lunar observing is done--necessarily--with smaller instruments
because the majority of astronomers appear to have accepted the
view that the moon is a dead world, and those who are engaged in
astronomical work with our greatest telescopes seem to feel that
other fields of research will prove more fruitful. Possibly it is
for this reason that we know so little about our nearest neighbor in
space! There are at least as many unsolved problems confronting us on
the moon as there are among the distant stars.
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.
Up to the present time several hundred comets have been discovered.
Nearly three-fourths of this number travel in orbits that appear to
be parabolas. Of the remaining number there are about forty that
have been "captured" by the major planets, Jupiter, Saturn, Uranus
and Neptune, though Jupiter possesses the lion's share of these
captured comets. Scarcely a year passes by that several comets are
not discovered. Most of these are telescopic, however, even when they
are near the sun and at their greatest brilliancy. Naked-eye comets
of great splendor and brilliancy are comparatively rare and there
has been a particular dearth of such unusual comets during the past
thirty years or so.
The last spectacular comet, unless we make an exception of Halley's
periodic comet, which made its return according to prediction in
1910, was the great comet of 1882 which was visible in broad
daylight close to the sun and at its perihelion passage swept through
the solar corona with a velocity that exceeded two hundred and fifty
miles a second and carried it through one hundred and eighty degrees
of its orbit in less than three hours.