It was at Lowell that the only consistent observations of canals, originally termed canale by Secchi (1858) and Schiaparelli (1877), were made. The Encyclopedia Brittanica 11th Edition (1910) doubts their being waterways, noting that their breadth of many miles made it absurd to to call them canals, but believed that the oft-noted seasonal changes were evidence of blue-green vegetation. Lowell maintained that the linear features were indeed of artificial origin. Here is the account:
"Professor Lowell's theory is supported by so much evidence of different kinds that his own exposition should be read in extenso in Mars and its canals (1906) and Mars as the abode of life (1909). In order, however, that his views may be adequately presented here, he has kindly supplied the following summary in his own words:
"Owing to inadequate atmospheric advantages generally, much misapprehension exists as to the definiteness with which the surface of Mars is seen under good conditions. In steady air the canals are perfectly distinct lines, not unlike the Fraunhofer ones of the Spectrum, pencil lines or gossamer filaments acording to size. All the observers at Flagstaff concur in this. The photographs of them taken there also confirm it up to the limit of their ability. Careful experiments by the same observers on artificial lines show that if the canals had breaks amounting to 16 mi. across, such breaks would be visible. None are; while the lines themselves are thousands of miles long and perfectly straight (Astrophys. J., Sept. 1907). Between expert observers representing the planet at the same epoch the accordance is striking; differences in drawings are differences of time and are due to seasonal and secular changes in the planet itself. These seasonal changes have been carefully followed at Flagstaff, and the law governing them detected. They are found to depend upon the melting of the polar caps. After the melting is under way the canals next to the cap proceed to darken, and the darkening thence progresses regularly down the latitudes. Twice this happens every Martian year, first from one cap and then six Martian months later from the other. The action reminds one of the quickening of the Nile valley after the melting of the snows in Abyssinia; only with planet-wide rhythm. Some of the canals are paired. The phenomenon is peculiar to certain canals, for only about one-tenth of the whole number, 56 out of 585, ever show double and these do so regularly. Each double has its special width; this width between the pair being 400 mi. in some cases, only 75 in others. Careful plotting has disclosed the fact that the doubles cluster round the planet's equator, rarely pass 40° Lat., and never occur at the poles, though the planet's axial tilt reveals all its latitudes to us in turn. They are thus features of those latitudes where the surface is greatest compared with the area of the polar cap, which is suggestive. Space precludes mention of many other equally striking peculiarities of the canals' positioning and development. At the junctions of the canals are small, dark round spots, which also wax and wane with the seasons. These facts and a host of others of like significance have led Lowell to the conclusion that the whole canal system is of artificial origin, first because of each appearance and secondly because of the laws governing its development. Every opposition has added to the assurance that the canals are artificial; both by disclosing their peculiarities better and better and by removing generic doubts as to the planet's habitability. The warmer temperature disclosed from Lowell's investigation on the subject, and the spectrographic detection by Slipher of water-vapour in the Martian air, are among the latest of these confirmations."
Unfortunately, Lowell's descriptions were flawed. Slipher's methods were not sensitive enough to detect atmospheric water vapor. Contemporary astronomers Antoniadi and Hale disputed the geometrical patterns that he termed canals, and few believed them to be waterways. MOLA will provide invaluable data on surface slopes and drainage patterns. Such patterns, revealed by the Mariner and Viking spacecraft, are unlike anything described by Lowell. While we do not expect to find "canals", we are confident that MOLA will be able to resolve linear features on Mars wider than 130 m and deeper than 1 m.