The sudden appearance of a large dark spot on the surface of the planet Mars was announced yesterday by the National Geographic Society in Washington. It was also claimed that this discovery supported the conclusion "that Mars is not a dead world, that the darkening is due to the growth of plant life." The observation has been made by Dr E. C. Slipher, the director of the Lowell Observatory, in South Africa.
Dr Slipher has spent a large part of his long life (he is nearly 80) photographing the surface of Mars.
This event is certainly remarkable, but it is unlikely that scientists will accept the suggestion that the appearance of this spot is evidence of vegetable life on the planet. Professor Znedik Kopal of Manchester University said last night that the claim "must be taken with a pinch of salt." The difficulty is that there are at least two ways of accounting for the appearance of dark spots on the surface of the planet and simple photography of them cannot decide between the two theories.
It is, however, clear that the dark spots must be produced by some active mechanism. Most of the surface of the planet is covered with a thick layer of sandy dust (it is a desert), and it is known that winds with speeds of several miles a second blow in the thin Martian atmosphere. Any dark material on the surface would rapidly become covered with a thin layer of obscuring dust if there were not some way in which it was regenerated.
The assumption that the dark patches are areas of vegetation has been common, for several decades. The difficulty is to see how any vegetation could grow in the planet's strange atmosphere.
The most favoured of the alternative theories is that the dark spots are caused by volcanic activity. Each black area would be brought about by pumice dust from an active volcano falling on the sandy desert and concealing it from view. This supposes that the volcanoes remain active for a considerable length of time, but there is no direct way of proving or disproving this.
To distinguish between these theories experiments are now being carried out in several laboratories throughout the world. In the end the question will be decided by the colour of the light which the spots reflect. When accurate measurements of the proportions of red, blue, and yellow light from the spots are available, these will be compared with laboratory measurements of the light reflected from pumice, lava, and vegetation. The prize will go to that material which best simulates the behaviour of the spots.