VENUS, MERCURY AND THE ASTEROIDS
Of all the planets, Venus appears, to the unassisted eye, by far the
loveliest. When seen in the early morning before sunrise--its "western
elongation"--or after sundown in the evening--its "eastern
elongation"--and still more as it attains its greatest brilliancy, it has
attracted attention everywhere and in all ages. It then shines with
brilliance ten times as great as Jupiter in opposition, and the brightest
members of the heavenly host look pale and dim beside it. It is
emphatically the morning or the evening star, Lucifer, or Vesper, herald
or follower of the Sun; it can even assert itself in the presence of the
Lord of Day, for it has often been seen at noonday by watchers who knew
where to look; sometimes by the general crowd.
But in the telescope Venus appears less satisfying. It is a pretty
spectacle indeed to watch the phases of the gleaming little globe of
silver, for, like the Moon under varying illumination from the Sun, it
undergoes change of apparent shape. But the surface of the planet yields
little detail, and that little is illusive and ill-defined. The clear-cut
outlines and black shadows of the Moon have no place here, nor do the
ruddy plains and blue-grey "seas" of Mars find any analogues. All that can
be observed beyond the changes of phase are a few faint, ill-defined
patches, where the molten silver of the general surface is slightly dimmed
and tarnished, and perhaps one or two spots, not less evasive and
difficult to fix, that exceed the rest of the surface in brightness.
This very difficulty in making out the markings on Venus is hopeful for
our search; it points to a veiling over the planet, a veiling by an
atmosphere. And the statistics of the Table show that Venus closely
resembles our Earth in size and mass, and therefore probably in
atmospheric equipment. If we assume that the atmosphere of any planet is
in direct proportion to its mass--and as Venus is so nearly the twin of
the Earth there is no reason to expect any great difference between the
two in this respect--the atmosphere of Venus would have a pressure of
about 11.2 lb. on the square inch, and the level of half pressure would be
nearly four miles above the surface. In other words the atmosphere would
be both thinner and deeper than that of the Earth, but the difference
would not be important in amount.
But Venus is nearer to the Sun than the Earth, and receives nearly double
the light and heat. Its theoretical equatorial temperature is 368 deg.abs., or
95 deg.C, and its corresponding mean temperature is 69 deg. C. But water under a
pressure of 11.2 lb. will boil at 93 deg. C, so that at the equator of Venus
the upper limit for water as a liquid is just passed, but, for the planet
in general, a fairly safe margin is maintained. Here then is sufficient
explanation why the topography of Venus is concealed. The atmosphere will
always be abundantly charged with water-vapour, and an almost unbroken
screen of clouds be spread throughout its upper regions. Such a screen
will greatly protect the planet from the full scorching of the Sun, and
tend to equalize the temperature of day and night, of summer and winter,
of equator and poles. The temperature range will be slight, and there will
be no wide expanses of polar ice. Water that flows will be abundant
everywhere.
So far all the facts connected with Venus are favourable for life, even
though the picture called up to the mind may not seem inviting to us. For
views of the heavens must be rare; the Sun must seldom pierce through the
cloud veil; there is no moon and the stars must be almost always hidden.
The Earth with its Moon might form a beautiful ornament at times in the
midnight sky if the cloud-shell should occasionally open, but on the
whole, the planet is shut up to itself in a perpetual vapour-bath, and its
condition will approach that of some of the most humid countries in the
terrestrial tropics during the height of their rainy seasons.
But it would seem that life both of plants and animals, under such
conditions, might flourish and be abundant. The mean temperature would
not, in general, be high enough to drive off the water as steam, nor low
enough to congeal it into ice; it would remain water--water that flows.
But there is still a possible hindrance to life on Venus, a hindrance that
actually exists in the case of Mercury.
Mercury, the "Twinkler," is not an easy object in our Northern latitudes,
but, in countries near the tropics, is often quite conspicuous, a little
scintillating gem of light in the bright sky, before sunrise or after
sunset. In the telescope it is not so attractive as Venus, partly because
it is smaller, partly because, though it receives more than three times as
much light from the Sun, it is duller in hue. Yet it is not quite so
secretive as its neighbour, and a certain number of markings have been
detected upon its disc, markings which, like those of the Moon, appear to
be permanent.
A glance at the Table will show that this was to be expected. In size,
Mercury comes between the Moon and Mars, and the atmospheric veil ought
therefore to be, as it evidently is, very slight and transparent; offering
little or no hindrance to an observer scanning it from another world. The
other necessary consequences of small size and mass will follow; the
feeble force of gravitation, the languid atmospheric circulation, the
extreme range of temperatures, the low temperature at which water will
boil.
But the heat to which Mercury is exposed far transcends our terrestrial
experience. In the mean it receives nearly seven times as much heat from
the Sun as the Earth does, but this supply is not maintained uniformly,
for Mercury moves round the Sun in a very eccentric orbit, so that when in
aphelion it receives, surface for surface, only about four times as much
heat as the Earth, but some six weeks later when in perihelion it receives
more than eleven times. The great range of temperature due to the thinness
of the atmosphere must therefore be further increased by the varying
distance of the planet from the Sun.
A reference to Prof. Poynting's figures shows that the mean temperature of
Mercury must approximate to 194 deg. C., while water will boil at 40 deg. C. or
even lower. Here, then, is a condition the exact reverse of Mars. Water as
a liquid will be rare on Mercury, not because it is congealed, but because
it is evaporated; on the dark side of the planet it may, indeed, pass into
ice, but on the side exposed to the Sun it must exist normally as a
constituent of the atmosphere. Water in a liquid state, water that flows,
must be almost unknown.
But we have good reason to believe that that which is the dark side of
Mercury at one time is always dark; that which is exposed to the Sun is
always exposed to it.
Since Mercury wears no concealing veil of atmosphere, and displays
markings that can be identified and followed, a surprising circumstance
has come to light. In 1889, Schiaparelli discovered that Mercury, instead
of rotating on its axis in about 24 hours like the Earth and Mars, rotates
in 88 days; that is to say, it always turns the same face towards the Sun,
just as the Moon turns the same face towards the Earth. This fact,
confirmed theoretically by Prof. G. H. Darwin in his development of the
theory of tidal friction, puts the condition of Mercury in quite a new
light. No alternation of day or night refreshes and restores the little
world; one hemisphere is for ever exposed to the blasting heat of the Sun,
seven times hotter for it than for the Earth; the other hemisphere is for
ever exposed to the darkness and cold of outer space, a range from
something like 390 deg. C. above freezing-point, to 270 deg. C. below. It is true
that between the two hemispheres there is a "debatable land," for, owing
to the ellipticity of the orbit, the face turned to the Sun is not exactly
the same at all times, and a region about 47 deg. in width on each side of the
planet, that is to say, rather more than a quarter of its entire surface,
has one day and one night in each period of 88 days, but these more
favoured sections can scarcely be considered more habitable than the rest.
The conditions of Mercury are so unfavourable for life that, even if this
remarkable relation of rotation period to revolution did not hold good,
it would still be impossible to regard it as a world for habitation. But
its case shows that a further condition of habitability has to be
satisfied by a planet. Size and distance from the Sun afford the first two
conditions; a suitable rotation period is now seen to be a third.
And it is possible that in this very particular Venus fails to qualify.
Schiaparelli, the first observer of his time, assisted by the clear
Italian sky, believed that he had demonstrated that Venus, like Mercury,
rotates once in her year; her day being thus equal in length to 225 of
ours, and the face that she turns to the Sun being always the same.
And in her case this statement requires practically no qualification, for,
her orbit being nearly circular, there is hardly any libration; a place
that has the Sun in its zenith has it so for ever; one on the night side
of Venus can never have a sunrise, or gladden in the daylight. The side
exposed to the Sun will wither in a temperature of about 227 deg. C., in which
all moisture will be evaporated; the side remote from it will be bound in
eternal ice. In neither hemisphere will water exist in the liquid state;
in neither hemisphere will life be possible.
But as yet the evidence is not conclusive that Venus has this long
rotation period. Several observers of high rank believe that our neighbour
rotates in nearly the same time as the Earth, but its markings are so
faint and elusive that the problem is a difficult one. The spectroscopic
method of determining the speed of rotation has been equally indecisive.
Until, therefore, the rotation period has been decided, the habitability
of Venus must remain in question. If it always turns the same face to the
Sun, there can be no more life upon it than upon Mercury; if on the
contrary it rotates in much the same time as the Earth, then, so far as we
know, it may well be a habitable world. Whether it is actually inhabited
is a matter at present entirely beyond our knowledge.
A page or two back we touched lightly on the eccentricity of the orbit of
Mercury--lightly, because it was not the chief factor in disabling the
planet for habitation. But the condition introduced by this eccentricity
is one which of itself would be sufficient to put it out of court. In the
six weeks in which Mercury moves from aphelion to perihelion, it
approaches the Sun by fourteen millions of miles, and the heat received by
it is increased 2-1/2 times. Then, in the next six weeks, it recedes as
far, and there is a like diminution. In other words, six weeks makes a
greater proportional change in this one planet's condition than we should
experience if our Earth were transported from its own orbit to that of
Mars.
But there are other members of the solar system whose orbits are so
elongated that that of Mercury seems in comparison almost circular. These
are the comets, some of which all but graze the surface of the Sun at
perihelion, and then recede from him for periods that it takes even
thousands of years to complete. But without dwelling on such extreme
cases, two of the best known of the periodic comets may be taken as
examples of the rest. Encke's is the comet of shortest period, returning
in about 3.3 years. At perihelion it is 31 millions of miles from the Sun;
one-third the distance of the Earth. It receives, therefore, at this part
of its orbit, 9 times as much light and heat as the Earth. But at aphelion
it retreats deep into the region of the asteroids, and is much more than
four times the mean distance of the Earth. At this part of its orbit it
receives but 1/17th as much heat as the Earth. By far the most famous of
all the comets is that known by the name of Halley, and its mean period is
76 years. At perihelion it comes within the orbit of Venus; indeed, nearly
halfway between that and the orbit of Mercury. At aphelion it recedes to
thirty-five times the distance of the Earth, far beyond the orbit of
Neptune. The range in its light and heat from the Sun is from 3 times that
of the Earth to less than 1/1200th; or, in other words, the supply of heat
at one time is nearly 4000 times that at another, and of the 76 years of
its period, only 80 days are spent within the orbit of the Earth.
Comets cannot be homes of life; they are not sufficiently condensed;
indeed, they are probably but loose congeries of small stones. But even if
comets were of planetary size it is clear that life could not be supported
on them; water could not remain in the liquid state on a world that rushed
from one such extreme of temperature to another.
Between the orbits of Mars and Jupiter there are scattered an untold
number of little planets commonly known as asteroids or minor planets.
Minor planets indeed they are, for the one first discovered--Ceres--
probably outweighs all the rest, known and unknown, put together,
though something like 700 have already been detected, and the list
grows at the rate of about one a week.
As the Table shows, Ceres is so small that the Earth exceeds it in volume
5000 times; even the Moon is 90 times as large. The mass of Ceres is not
known; being so small, its density is probably less than that of the Moon,
so that the Earth may easily outweigh it 10,000 times. The unfavourable
conditions resulting from smallness of size that the Moon presents are
therefore exaggerated exceedingly in the case of Ceres; its atmosphere
must approach in tenuity what we should regard as a vacuum in a
terrestrial laboratory, and water as a liquid be entirely unknown. Its
distance from the Sun is another hostile factor; for in consequence it
receives per unit of surface only 13 per cent of the light and heat that
falls on the Earth; its maximum temperature under a zenith Sun will fall
far below freezing-point, the minimum on the dark side will approach the
absolute zero.
With Ceres the whole of the asteroidal family can be dismissed as possible
abodes of life. No astronomer can regard them as such. Yet they have their
lesson to teach. Life can exist on the Earth only on the upper face of its
crust, and in a very thin film of air and water; but the enormous solid
bulk within, inert though it be, that supports the stage on which the
great drama of life is played, is as really essential as air and water
themselves. If that bulk were much smaller and less massive life could
find no place upon its surface.