154 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 9 



planet rotates in a day that is some 40 minutes longer than our day. 

 Careful observation shows that these markings undergo changes, both 

 of form and of coloration, which are in part seasonal in character. 

 Around whichever pole is visible there is seen a bright white cap ; the 

 two polar caps show regular seasonal changes in size, growing in the 

 winter and shrinking in the summer. Their appearance suggests some- 

 thing similar to the regions of ice and snow around the poles of the 

 Earth, though from the rate at which they melt as the summer ad- 

 vances it can be inferred that the Martian polar caps are not more 

 than a few inches in thickness. 



There has been much controversy over the existence of the so-called 

 canals on Mars, a geometrical network of perfectly straight, narrow, 

 and sharply defined lines connecting up the dark markings, which 

 some observers claim to have seen. The American astronomer, Percival 

 Lowell, built up a romantic theory about these canals. He regarded 

 them as irrigation channels, carrying water to the dark regions or 

 oases from the melting polar caps. His conclusion was that the canals 

 were artificial structures, made by a race of intelligent Martian be- 

 ings, who were engaged in a desperate struggle for existence on a 

 semiarid planet. Lowell's geometrical network of sharply defined 

 canals has not been confirmed, however, by the most keen-sighted ob- 

 servers, with the largest telescopes, observing under the most favor- 

 able conditions. It seems that they are an illusion of vision, arising 

 from a psychological tendency for the eye, when looking at something 

 that is almost at the limit of vision, to connect up detail by narrow 

 lines to form a geometrical pattern. We must abandon Lowell's 

 theory. What is certain is that we can see markings on the surface 

 of Mars, and that these undergo seasonal changes, which seem to 

 indicate the seasonal growth of vegetation. 



PROOFS OF AN ATMOSPHERE 



The low velocity of escape from Mars suggests that, if Mars has 

 an atmosphere, it must be very tenuous. We obtain direct proof that 

 it has an atmosphere by taking photographs in infrared and ultra- 

 violet light. The infrared photographs show the surface markings 

 clearly; the ultraviolet photographs do not show them at all. The 

 scattering of the ultraviolet light by the atmosphere is enough to pre- 

 vent its penetrating to the surface and out again. The images in 

 ultraviolet light are larger than those in infrared light, and a compar- 

 ison of them shows that the atmosphere has a considerable depth — not 

 less than 60 miles. Comparison with terrestrial photographs taken 

 under favorable conditions suggests, nevertheless, that the total atmos- 

 pheric pressure on Mars does not amount to more than a few percent 

 of that at the surface of the Earth. 



