There is no doubt that Venus is in much better plight than Mercury, the
other inferior planet, in regard to atmosphere. Until recently no one
has questioned the belief that her atmosphere is very extensive--twice
as heavy, perhaps, as that of the earth, dense, and full of clouds.
The luminous ring about her, shown when she is making a transit across
the face of the sun, points to a heavy atmosphere; and no less certain
indications of it are given in the faint light which stretches beyond
the termination of the horns when she is in the crescent phase, near
inferior conjunction. Her very high reflecting power is also indicative
of an atmosphere laden with clouds. White clouds form one of the most
highly reflecting surfaces known, and the peculiar brilliancy of Venus
is thought to be in great part due to the presence of large masses
of clouds in her atmosphere. By the spectroscope, and in other ways,
the water necessary to form clouds is shown to be abundant in her
atmosphere. Even those astronomers who doubt the long-current belief
in the large extent of her atmosphere concede an atmosphere of more or
less density, though by one authority it is characterized as somewhat
gauzy.
There is one vital point concerning the development of Venus upon which
we have as yet no positive knowledge: the length of time in which she
rotates on her axis. This is unfortunate, because until her time of
rotation is known we cannot know much about her physical condition.
Her rotation, we know, determines the length of her day and night, or
whether, indeed, she has any. The time of it has been calculated to be
anywhere from a little less than one of our days to two hundred and
twenty-five, the latter being also the time of her revolution about
the sun. Astronomers of equal reputation have come to exactly opposite
results in their investigations. To one, the spectroscope has indicated
the short day and night; to another it has shown no day and night,
but a planet with one face forever toward the sun, like Mercury. What
appeared to be stable surface markings have been observed, but have
indicated under the eyes of different observers both the short day and
no day at all. The disc has been measured during a transit, and shows
so little flattening as to indicate a slow rotation and the long day.
On the other hand, the best authorities think it unlikely that at the
distance of Venus the sun could so retard the planet’s rotation as to
make it coincide with its time of revolution. Thus the question is
still an open one.
The truth may be that, owing to the density of her atmosphere, the
surface of Venus has never been seen at all, and that the apparently
stable markings are but clouds more or less lacking in stability. The
difficulty of observing Venus will probably make it impossible to
determine this point by visual observation. It may some day be settled
beyond a doubt by the spectroscope. In some way it will surely be
settled. Astronomers have too often made possible what seemed to be
impossible for us to doubt that some one will find a way to discover
this secret of Venus. With them a failure to prove a conclusion does
not mean to abandon the subject, but to try some other means of getting
at the truth.
* * * * *
The sun viewed from Venus would appear considerably larger than it does
to us. Its apparent diameter to us is a little more than thirty-two
minutes, while on Venus it would be something more than thirty-eight
minutes; that is, it would appear about one-fifth larger on Venus than
it does to us. This is enough to make a material difference between
the two planets in the amount of heat and light they receive. Venus
receives nearly twice (1.9) as much heat and light from the sun as we
receive, but less than one-third as much as Mercury. If she had no
atmospheric protection, there is no question but that she would have a
climate disastrously warm for a race of beings constituted as we are.
The normal temperature of an unprotected body at the distance of Venus
is about 158° Fahrenheit (70° Centigrade).
If Venus is finally proved to have no alternations of day and night,
she is still better off than Mercury, who has practically no atmosphere
to protect him from the intense heat of the sun. How much protection
she has depends altogether on the extent of her atmosphere. It is
probably not enough to make the hot side comfortable from our point of
view; and Venus, being undoubtedly a solid body with no internal heat,
the cold side must be cold beyond anything we have any conception of.
But there may be a very considerable part on each side that, owing
to the refraction of light by the atmosphere, is more or less well
lighted, and is also more or less protected by this same beneficent
atmosphere from deadly extremes of heat and cold. In this situation
there would undoubtedly be lively currents of air from the heated side
to the cooler; but even these may in some way carry with them some
tempering effects on the climate, as we know such currents do here on
the earth.
If it should prove that the length of the day and night on Venus is
something near that of the earth’s (and this seems not unlikely), she
would then be indeed more like a twin sister to us. Being next to each
other in our distances from the sun, and of nearly the same size,
differing but little in density, mass, volume, and force of gravity,
with her greater normal heat probably reduced by her heavier atmosphere
to a temperature producing climatic conditions not very unlike ours,
and with not very different alternations of day and night, we might
well be considered more nearly related than any of the other members of
the solar family.
The seasons, however, on Venus and the earth would not have much
resemblance to each other. The axis of the earth is inclined to the
ecliptic nearly twenty-three and one-half degrees, so that we receive
the sun’s rays with varying degrees of obliquity during our yearly
journeying around it, which is the cause of our agreeable change of
seasons. Venus travels with her axis so slightly inclined to her orbit
(a little more than three degrees) that each particular parallel of
latitude receives practically the same amount of sunlight every day
in the year, though at different parallels the sun’s rays strike with
varying degrees of obliquity. However delightful or disagreeable the
climate may be, there are no changes of seasons to speak of, and one
could find variety only by going from place to place on the planet. She
receives no compensation for this monotony by alternately receding from
and approaching the sun as Mercury does, or by librations, such as he
has. Her orbit being, as we have seen, so nearly circular as to permit
of only small variations in her distance from the sun, and her axis so
nearly perpendicular to her orbit, it follows that she has nothing to
mark the year; and, whether she turns on her axis many times or only
once during a revolution, life on Venus would be very monotonous to
any one accustomed to our delightful variety of climate and seasons.
Still, there is nothing in this monotony to prevent Venus from being a
fairly comfortable habitation in some parts for such beings as inhabit
the earth. The only real obstacle to habitability on Venus would be her
lack of rotation and all that it involves.
Since we are not sure that we can see the surface of Venus, we cannot
say what that surface is. Nevertheless, there is some reason to
suspect that we would find there mountains of vast height. Certain
irregularities have been observed at times, of a kind to indicate
mountains covered with snow, extending beyond the clouds. They have
been estimated to be many miles higher than any mountains we have on
earth, their height depending somewhat upon the temperament of the
observer. But inasmuch as these same high mountains have sometimes been
thought to be only masses of clouds, it seems hardly safe to pronounce
definitely upon them.