Page:Scientific Monthly, volume 14.djvu/587



N 1902, Dr. Lowell published an extremely interesting article on the rifts in the north polar cap of Mars (Popular Astronomy for March, 1902). It is on account of this article that the present paper has its being.

Realizing that the oppositions of 1916, 1918 and 1920 had occurred at about the same Martian season as that viewed by Lowell—a study was made of drawings of the planet at these times. It is interesting to note that the observations not only confirm those of Lowell but add their weight to his conclusions.

It is purposed to chronicle the observations of the three last oppositions with deductions and show in what manner they agree with the conclusions arrived at by Dr. Lowell.

In 1916, the two drawings showing the region termed Aeria near the center of the disk and the north polar cap—those of March 4 and 9, in the cut—depict a rift in the cap, which seemingly the continuation of the canal Cadmus. That of March 9 also shows another rift to the north and west of the Arethusa Lucus. The season on Mars corresponded at that time to our May 24. The cap was in process of melting and the rifts were seemingly due to some underlying cause such as would hasten the melting of the polar snows at those points.

A similar rift to that in the drawing of March 9, 1916, was observed on February 15, 1918, with a period of slightly over a Martian year between the two observations. The effect of the later seasonal date can be observed in the diminished size of the polar cap. The positions of this second rift on the two dates do not correspond exactly, but this can be accounted for by the difficulty of positioning small detail or by the fact that there are many nearly parallel canals in this region, any one of which may have produced the effect observed.

In 1920, the north polar cap was at about its smallest on May 24. and the canal Cadmus was easily seen to extend northward to the cap along the position of the rift seen in the drawings of 1916. In the large drawing of the polar cap of May 27, 1920 (Martian date August 13) the Cadmus can be seen very well and there is another rift in the small remaining cap.