The most peculiar formations on the Moon are the so-called “bright streaks” which as a rule issue in almost straight lines from some of the larger craters, particularly Tycho and Copernicus. Those around Tycho (see [Fig. 28]) do not seem to be either raised above nor depressed below the surroundings to any degree worth mentioning. For this reason they are not visible under oblique illumination as in [Fig. 25]. They proceed in straight lines independent of elevations. This quality is in striking conformity with the characteristics of fissures on earth as for instance those that traverse the Tyrrhenian Sea and the Calabrian mountains. They also resemble the canals on Mars in this respect. Nasmyth and Carpenter caused a glass ball, containing water under pressure, to break at one point and obtained a system of beams radiating from this point and vividly reminding of the streaks around the lunar craters. The same effect appears if a homogeneous plate, of glass for instance, is broken by a blow in one point. No doubt these streak centres were once centres of collapse although they sometimes now are found at a considerable elevation, like Tycho. This may be the result of a later secular lifting of the rocky substrata like the slow rise of the Scandinavian peninsula. The streaks around Copernicus (see [Fig. 29]) are very different from those around Tycho. They are not rectilinear and consist next to the crater of distinct mountain chains plainly visible under oblique illumination. They penetrate into Mare Imbrium ([Fig. 29] below) crossing the mighty “Carpathian” mountains. Frequently, they are provided with minor volcanoes, as in the streak directed almost straight downward on the figure, i. e., to the north. They are obviously volcanic fissures like those on the Earth.
Fig. 28. Tycho in full illumination with surrounding magnificent system of streaks. In the lower right corner Copernicus with a less regular system appears. Between them Mare Nubium, in the upper right Mare Humonum, with the great crater Gassendi below. The moon diameter corresponds to 16.7 cm. Photo by Yerkes Observatory. Compare [Figs. 25] and [28], showing parts of the same territory under side light.
The streaks, in many cases, would not be visible at all were it not for their different colour, which is considerably lighter than that of the surroundings. The only explanation offered for this fact is the assumption that the original cracks were filled by some light matter forced out from the interior of the Moon, that is by the lunar magma. This magma was not very viscous, as it has spread out considerably beyond the edges of the cracks proper. These presumably, like those on Earth, were of a rather moderate width, not enough to be distinguishable at the Moon’s distance. Similar light-flowing emanations from long fissures are known also on our planet, for instance, from the Laki eruption on Iceland in 1783. The colour may be light simply by comparison with the previously solidified crust, which, optically, as regards reflexion of light, has proved very similar to obsidian or, even more like another volcanic mineral product, vitrophyre. It is also possible, however, that gas bubbles were liberated as the lava solidified and gave the surface a milkwhite appearance—gravity on the Moon is only one sixth of that on the Earth so that the bubbles would rise and evaporate extremely slowly from the magma. Due to the very low atmospheric pressure on the Moon, the bubbles would also occupy a larger volume than in a corresponding case on Earth and become more conspicuous in proportion. They probably partly remained on the surface of the outpoured lava as a thin scum, which hardened in that state. Since then, it has suffered no more change than all other formations on the Moon, while on the Earth it would soon have been scoured away by sand and water.
Fig. 29. The great lunar crater Copernicus surrounded with streaks. Below the Carpathian mountain range and at bottom part of Mare Imbrium. The moon diameter corresponds to 55 cm. Photo by Yerkes Observatory.
Before we leave the Moon it may be well to say a few words about its colour. Mädler states in agreement with several other observers that Mare Serenitatis, a “sea” on the Moon’s north side (25° latitude) just to the right of the centre meridian (see [Fig. 27]) is remarkable for its beautiful pure green colour, while Mare Crisium about 16° Lat. N. near the right edge of the Moon is of a dark grey-green hue. In Mare Humorum (about 22° Lat. S., not far from the edge of the Moon, see [Fig. 28]) grey and dark green shades alternate and in Mare Frigoris, just inside the lunar north pole, the colour is a dingy yellowish-green. In other words, the characteristic colour of the great lava seas is apparently green. This agrees closely with conditions on the Earth where similar formations are coloured green by silicates of ferrous protoxides, certain species of which are called green-stones. Franz, however, questions the observations of Mädler and professes the belief that very light craters appear bluish and assumes this to be a contrast effect to the general yellow hue of the Moon. Langley investigated the lunar radiation with the spectroscope and found that the ratio of blue to yellow was smaller in the moonlight than in the sunlight, for which reason the general colour of the moon resembles that of yellow sandstone.
A very interesting observation was made at the Lowell observatory when investigating the spectrum of the sparse light reflected from the Earth to the portions of the Moon not exposed to the sunlight. It proved to be of a far more blue tinge than sunlight reflected from the Moon. Our conclusion must be that the Earth shines with a blue lustre. This is perfectly natural, as the diffused light which reaches us after having been scattered by particles suspended in the air (and by gas molecules as well) is a deep blue and there exists no reason why that part of the light which is thrown outward into space should be of a different colour. The Earth, therefore, is blue in contradistinction to Mars which is red, on account of its desert surface, and Venus which is bright white. The cloudy portions around the equator and the poles should appear light blue from without and should be separated by dark blue bands over the so-called horse latitudes, under which the cloudless desert regions are located on either side of the equator. (Compare the title page illustration.)
Compared to Mars the Moon offers a scene of far greater desolation. On Mars, we observe at least some considerable changes such as the disappearance of the white pole-caps at midsummer when at the same time a dark ring appears to surround them; then the “lakes” and the “canals” come into view, beginning close to the ring mentioned, later on nearer the equator, and finally on its other side, while the opposite pole-cap puts on its winter hue. Again, we have the sudden appearance, and equally hasty disappearance, of white spots, particularly in the neighbourhood of the lakes, and the sand storms which hide the surface of Mars and often fill its canals. The abruptness of the changes indicate that they are confined to a very thin surface layer. The formation, on the other hand, of canals, for many years unobserved, must be ascribed to a volcanic activity which, while feeble, yet must be seated in the deeper portions of the planet. In addition, a stunted vegetation of low forms is not unthinkable in the polar regions.
As against this, the Moon is undoubtedly a stellar body entirely insusceptible of surface change. Near its centre, it is probably not completely solidified and an extremely slow growth of the firm crust is therefore likely. Gases are no doubt set free during this process, but they are unable to penetrate the enclosing thick armour and remain therefore as bubbles in the hardening magma.