If the lands of Mars are framed as are those of our own earth, there should be ridges of mountains constituting what we may term the backbones of the continent. As yet such have not been discerned, which may be due to the fact that they have not been carefully looked for. The only peculiar physical features which have as yet been discerned on the lands of Mars are certain long, straight, rather narrow crevicelike openings, which have received the name of "canals." These features are very indistinct, and are just on the limit of visibility. As yet they have been carefully observed by but few students, so that their features are not yet well recorded; as far as we know them, these fissures have no likeness in the existing conditions of our earth. It is difficult to understand how they are formed or preserved on a surface which is evidently subjected to rainfalls.
It will require much more efficient telescopes than we now have before it will be possible to begin any satisfactory study on the geography of this marvellous planet. We can not hope as yet to obtain any indications as to the details of its structure; we can not see closely enough to determine whether rivers exist, or whether there is a coating which we may interpret as vegetation, changing its hues in the different seasons of the year. An advance in our instruments of research during the coming century, if made with the same speed as during the last, will perhaps enable us to interpret the nature of this neighbour, and thereby to extend the conception of planetary histories which we derive from our own earth.
Fig. 5.—Comparative Sizes of the Planets (Chambers).
Beyond Mars we find one of the most singular features of our solar system in a group of small planetary bodies, the number of which now known amounts to some two hundred, and the total may be far greater. These bodies are evidently all small; it is doubtful if the largest is three hundred and the smaller more than twenty miles in diameter. So far as it has been determined by the effect of their aggregate mass in attracting the other spheres, they would, if put together, make a sphere far less in diameter than our earth, perhaps not more than five hundred miles through. The forms of these asteroids is as yet unknown; we therefore can not determine whether their shapes are spheroidal, as are those of the other planets, or whether they are angular bits like the meteorites. We are thus not in a position to conjecture whether their independence began when the nebulous matter of the ring to which they belonged was in process of consolidation, or whether, after the aggregation of the sphere was accomplished, and the matter solidified, the mass was broken into bits in some way which we can not yet conceive. It has been conjectured that such a solid sphere might have been driven asunder by a collision with some wandering celestial body; but all we can conceive of such actions leads us to suppose that a blow of this nature would tend to melt or convert materials subjected to it into the state of vapour, rather than to drive them asunder in the manner of an explosion.
The four planets which lie beyond the asteroids give us relatively little information concerning their physical condition, though they afford a wide field for the philosophic imagination. From this point of view the reader is advised to consult the writings of the late R.A. Proctor, who has brought to the task of interpreting the planetary conditions the skill of a well-trained astronomer and a remarkable constructive imagination.
The planet Jupiter, by far the largest of the children of the sun, appears to be still in a state where its internal heat has not so far escaped that the surface has cooled down in the manner of our earth. What appear to be good observations show that the equatorial part of its area, at least, still glows from its own heat. The sphere is cloud-wrapped, but it is doubtful whether the envelope be of watery vapour; it is, indeed, quite possible that besides such vapour it may contain some part of the many substances which occupy the atmosphere of the sun. If the Jovian sphere were no larger than the earth, it would, on account of its greater age, long ago have parted with its heat; but on account of its great size it has been able, notwithstanding its antiquity, to retain a measure of temperature which has long since passed away from our earth.
In the case of Saturn, the cloud bands are somewhat less visible than on Jupiter, but there is reason to suppose in this, as in the last-named planet, that we do not behold the more solid surface of the sphere, but see only a cloud wrap, which is probably due rather to the heat of the sphere itself than to that which comes to it from the sun. At the distance of Saturn from the centre of the solar system a given area of surface receives less than one ninetieth of the sun's heat as compared with the earth; therefore we can not conceive that any density of the atmosphere whatever would suffice to hold in enough temperature to produce ordinary clouds. Moreover, from time to time bright spots appear on the surface of the planet, which must be due to some form of eruptions from its interior.
Beyond Saturn the two planets Uranus and Neptune, which occupy the outer part of the solar system, are so remote that even our best telescopes discern little more than their presence, and the fact that they have attendant moons.
From the point of view of astronomical science, the outermost planet Neptune, of peculiar interest for the reason that it was, as we may say, discovered by computation. Astronomers had for many years remarked the fact that the next inner planetary sphere exhibited peculiarities in its orbit which could only be accounted for on the supposition that it was subjected to the attraction of another wandering body which had escaped observation. By skilful computation the place in the heavens in which this disturbing element lay was so accurately determined that when the telescope was turned to the given field a brief study revealed the planet. Nothing else in the history of the science of astronomy, unless it be the computation of eclipses, so clearly and popularly shows the accuracy of the methods by which the work of that science may be done.