MARS.
Mars, though considerably smaller than the Earth, is commonly regarded
as the planet which, taken all in all, bears most resemblance to the
Earth, though only one-fourth its size. Under circumstances which have
already been briefly alluded to in Chapter I., Mars exhibits from time
to time a slight phase, but nothing approaching in amount the phases
presented by the two inferior planets, Mercury and Venus. When in
opposition to the Sun, that is to say when on the meridian at midnight,
it has a truly circular disc; but between opposition and its two
positions of quadrature it is gibbous. At the minimum phase, which is at
each quadrature, E. or W. as the case may be, the planet resembles the
Moon 3 days from its “full.” These phases are an indication that Mars
shines by the reflected light of the Sun. It is a remarkable tribute to
Galileo’s powers of observation that with his trumpery telescope, only a
few inches long, he should have been able to suspect the existence of a
Martial phase. Writing to a friend in 1610 he says:—“I dare not affirm
that I can observe the phases of Mars; however, if I mistake not, I
think I already perceive that he is not perfectly round.”
[Illustration: Fig. 12.—Four views of Mars differing 90° in longitude
(Barnard).]
The period in which Mars performs its journey round the Sun (called the
sidereal period) is about 687 days; but owing to the Earth’s motion we
are more concerned with what is called the planet’s synodical period of
780 days than with its sidereal period of 687 days. The synodical period
is the interval between two successive conjunctions or oppositions of
the planet as regards the Earth, and 780 days being twice 365 and 50
days over, it follows that we have an opportunity of seeing the planet
at its best about every 2 years; and this is one of the reasons why Mars
has been so much and so thoroughly studied as regards its physical
appearance. Of course Mars is not equally well seen every 2 years,
because it may so happen at a given opposition that it may be at its
nearest to the Sun (perihelion), and the Earth at its farthest from the
Sun (aphelion), in which case the actual distance between the two bodies
will be the greatest possible. What is therefore wanted is for the
planet to be nearest to the Sun and nearest to the Earth at the same
time, under which circumstances it shines with a brilliancy rivalling
Jupiter. This favourable combination occurs once in 7 synodical
revolutions, or about every 15 years. The most favourable oppositions
occur at the end of August, and the least favourable at the end of
February. The next very favourable opposition will not occur until 1909.
Mars may approach to within about 35 millions of miles from the Earth at
a favourable opposition, whilst under extreme circumstances the other
way it may be no nearer than 61 millions of miles at opposition.
Mars in opposition is a very conspicuous object in the Heavens, shining
with a fiery red light which has always been regarded as a peculiar
attribute of the planet, so much so that its name, or epithet, in many
languages conveys the idea of “fiery” or “blazing.” It is recorded that
in August 1719 its brilliancy was such as to cause a panic amongst the
public.
Telescopically examined, Mars is always found to exhibit patches of
shade of various sizes and shapes, and, on the whole, fairly permanent
from year to year. During the last few years in particular these
markings have been subjected to very careful scrutiny and measurement at
the hands of numerous observers of skill and experience, and armed in
many cases with very powerful telescopes. The conjoint effect of the
observations obtained has been largely to augment our knowledge of the
planet’s geography, or (to use the proper term) “areography.” Before
describing the minutest details recorded and pencilled by the best
observers, it will be best to speak of the leading general features
which are within the grasp of comparatively small telescopes—say,
refractors of 6 inches and reflectors of 12 inches in aperture. The
first thing which presents itself as very obvious on the disc of Mars,
is the fact that certain portions are ruddy, whilst others are greenish
in hue. It is generally assumed that the red areas represent land and
the green areas water. On this subject Sir John Herschel’s remarks,
penned about half a century ago, may be said still to stand good. He
ascribes the ruddy colour to “an ochrey tinge in the general soil, like
what the red sandstone districts on the Earth may possibly offer to the
inhabitants of Mars, only more decided.” The propriety of this thought
will be best appreciated by a reader who has travelled through parts of
North Gloucestershire, and seen a succession of ploughed fields in that
locality. The deep red colour of the soil is in many places very
conspicuous. It has often been remarked that the redness of Mars is much
more noticeable with the naked eye than with a telescope; and Arago
carried this idea one step further in suggesting that the higher the
optical power the less the colour. This, however, might naturally be
expected.
The most prominent surface marking on Mars is that known as the “Kaiser
Sea,” sometimes called the “V-mark” from its resemblance to that letter,
though a leg of mutton would be quite as good a simile. East of the
Kaiser Sea and a little north of the planet’s equator is a well-defined
dark streak known as “Herschel II. Strait”; whilst on the west side is
another shaded area which has been called “Flammarion Sea.” These three
features are so very conspicuous, that, provided the hemisphere in which
they are situated is fairly in front of the observer, his telescope, if
it will show anything on Mars, will show these. The white patches seen
on certain occasions _at_ Mars’s N. pole and _close to_ its S. pole form
another special feature of interest connected with this planet. It
admits of no doubt whatever that these are immense masses of snow and
ice which undergo at stated intervals changes analogous to the changes
which we know happen in the great fields of ice situated in the regions
of the Earth surrounding the Earth’s two poles. Not only do these white
patches look like snow, but if attention is paid to the changes they
undergo and the epochs at which the changes take place there will be
found abundant confirmation of this theory, for these patches decrease
in size when brought under the Sun’s influence on the approach of summer
and increase again in size when the summer is over and winter draws
near. In the second half of 1892 the Southern Pole was in full view, and
during especially July and August the diminution of the snow area from
week to week was very evident. Schiaparelli, who observed it with great
attention during that season, noted at the commencement of the season
that the snow reached at the first as far as latitude 70° and formed a
polar cap some 1200 miles in diameter. Its subsequent decrease, however,
was so marked that two or three months later the diameter of the snow
patch had dwindled to no more than 180 miles, and became indeed still
smaller at a later period. The summer solstice on Mars occurred on
October 13, 1892, which was therefore the epoch of midsummer for Mars’s
southern hemisphere. Whilst these changes were taking place in the
southern hemisphere, no doubt changes of the reverse character were
going on in the northern hemisphere, but they were not visible from the
Earth because the North Pole was situated in that hemisphere of Mars
which was turned away from the Earth. In previous years, however, the
North Pole being turned towards the Earth its snow was also seen to
undergo the same sort of change; in other words, was seen to melt. This
happened, and was seen in 1882, 1884, and 1886. These observations of
the alternate increase and decrease of the polar snow on Mars may be
viewed with telescopes of moderate power, but of course it is more
interesting and profitable to watch them with a large telescope. The
fact (for it is an undoubted fact) that the north polar snow is
concentric with the planet’s axis whilst the southern polar patch is
eccentric to the extent of about 180 miles from the southern pole is one
which has not yet received a satisfactory explanation. If both patches
were eccentric so as to be exactly opposite to one another an
explanation would be much more easy for we might say that the poles of
rotation lay in one direction and the poles of cold in another.
I have spoken on a previous page of three specially conspicuous shadings
of Mars, and other similar shadings to the number perhaps of a couple of
dozen were generally recognised by astronomers (having been mapped and
named) down to about the year 1877. In that year the astronomical world
was startled by the announcement that Schiaparelli of Milan, an able and
competent observer, had discovered that those shaded areas which all
previous astronomers had regarded as continents or vast tracts of land,
were in reality islands, that is to say, so far, that the continents in
question were cut up by innumerable channels intersecting one another at
various angles. When this discovery was announced, and older
observations and drawings came to be examined, it was found, or at
any-rate thought, that these so-called canals might be traced in
drawings of earlier dates by Dawes, Secchi, and Holden. So much for
1877. In December, 1881, the planet was again in opposition, but farther
off in distance, and therefore smaller in size than in 1877. It was,
however, higher up in the Heavens as seen at Milan and the weather
appears to have been more favourable. In these altered circumstances
Schiaparelli again saw his canals, but this time they were in at least
as many as twenty instances seen in duplicate; that is to say, a twin
canal was seen to run parallel to the original one at a distance of from
200 to 400 miles, as the case might be. The existence of not only single
canals but of twin canals seems an established fact, for Schiaparelli’s
drawings and descriptions have been confirmed by competent testimony;
but explanation is nowhere; especially in view of Schiaparelli’s own
idea that the duplication of his canals is perhaps not a permanent
feature but a periodical phenomenon depending on, or connected in some
way with, Mars’s seasons.
[Illustration: Fig. 13.—Mars, August 27, 1892 (Guiot).]
Several points stand out clearly established by the observations of Mars
during the opposition of 1894, especially the correctness of
Schiaparelli’s discoveries and maps. Most of the canals originally seen
by him were again seen, and thus their existence was confirmed, whilst
new ones were also noticed. Many of these canals were double. The great
extent of the S. Polar cap and its rapid disappearance as Mars’s summer
approached was also a special feature of the observations of 1894. It
dwindled until it became almost invisible, or at best showed as a tiny
speck. It is thought by some observers that as the Polar cap melts, the
water collects round the Pole, and thence flows over the planet’s
surface, giving rise to the phenomenon of canals, and that this is the
way the planet’s surface is irrigated. It may here be remarked that the
word “canal,” which has been given to these dark streaks crossing and
cutting up the large areas of land in Mars, is an unfortunate one,
suggesting as it does artificial agency. But these Martial canals are
probably, especially the largest, a great many miles in width and
hundreds of miles in length, though some are smaller; and they are
probably nature’s method of distributing over the continents and lands
of Mars the water which collects round the Pole during the rapid melting
of the Polar snows.
The idea of the presence of cloud or mist on Mars also received strong
confirmation in 1894. Large portions of the planet’s disc were found to
be hidden from view. “Herschel I. Continent” and the “Maraldi Sea” (both
well-known markings on Mars, readily visible with small telescopes) were
at times quite obscured by cloud. Indeed, the Maraldi Sea was
occasionally quite blotted out: other well-known markings were also
either blotted out or only faintly seen. These facts seem almost to
prove conclusively the existence of cloud and vapour in Mars, especially
as some of these markings subsequently again assumed their ordinary form
and colour. Bright projections too were seen at times on the terminator
of Mars, giving rise to the belief that there are high mountains on the
planet, though some observers regarded these projections as high clouds
powerfully reflecting the Sun’s light.
Mars rotates on its axis in 24h. 37m. 22s., a period so nearly
coincident with the period of the Earth’s rotation as greatly to
facilitate the mapping of Mars’s features by work continued from day to
day by observers who have the necessary instrumental means and artistic
skill in handling the pencil.
Mars has an atmosphere which may be said to be no more than moderately
dense; that is to say much less dense than the Earth’s atmosphere. Of
course the existence of snow, which has been taken for granted on a
previous page, carries with it the existence of water and aqueous
vapour—a fact capable of independent spectroscopic proof.
The inclination of Mars’s axis to the ecliptic has not been ascertained
with all desirable certainty, but if Sir W. Herschel’s estimate that the
obliquity on Mars is 28¾° (the Earth’s obliquity being 23½°) is correct,
it is evident that there must be a very close similarity between the
seasons of the Earth and the seasons of Mars, thereby furnishing another
link of proof to support the statement made at the commencement of this
chapter that, taken all in all, Mars is the planet which bears most
resemblance to the Earth.
The apparent absence of satellites in the case of Mars was long a matter
of regret to astronomers; they seemed to think that such a planet ought
to have at least one companion. At last, in 1887, two were found by Hall
at Washington, U. S., using a very fine refractor of 26 inches aperture.
These satellites, which have been named Phobos and Deimos, are, however,
very small, for Phobos at its best only resembles a star of mag. 11½,
whilst Deimos is no brighter than a star of mag. 13½; from this it will
be understood that only very large telescopes will show either of them.
Phobos revolves round Mars in 7½ hours at a distance of about 6000
miles, whilst Deimos revolves in 30 hours at a distance of about 15,000
miles. It has been thought that neither of them can be more than about 6
or 7 miles in diameter, and therefore that they can not afford much
light to their primary.
Mars revolves round the Sun in 686d. 23h. 30m., at a mean distance of
141 million of miles, which the eccentricity of its orbit may increase
to 154 millions or diminish to 128 millions. The planet’s apparent
diameter varies between 4″ in conjunction and 30″ in opposition. Owing
to the great eccentricity of the orbit the planet’s apparent diameter as
seen from the Earth varies very much at different oppositions. The real
diameter is rather more than 4000 miles.