MARS, THE RED PLANET

Press, Volume CVI, Issue 31333, 1 April 1967, Page 16

MARS, THE RED PLANET

Mars is in opposition to the sun on April 16. It is at such times that the planet presents the largest disc, which will occur a few days after opposition when Mars is closest to the earth on April 22. This year the opposition is not particularly good because Mars will be roughly 56 million miles from the earth. As a result its apparent diameter will be less than 16 seconds of arc, equivalent to less than one-hundredth the size of the moon. At the most favourable oppositions Mars can approach the earth to within 35 million miles. The last close approach was in 1956 and the next will be in 1971. The reason for the large difference in the distance of Mars at oppositions is due to the eccentricity of Mars’ orbit.

First View Disappointing A first view of Mars in a telescope is apt to disappoint the layman mainly because one has heard or read so much about this planet and therefore expects to* see a great deal more than* is possible. In appearance most of the surface is yellowish orange or reddish in colour. White areas, known as polar caps, can be seen at one or both of the plant’s poles. Large dark areas, of various hues, can generally be seen. Quite often the surface features are veiled by a haze and on occasions clouds have been detected over Mars, moving across the surface at about 20 miles an hour. In addition to these coarser features a spidery network of fine lines has been seen by many observers. These were first observed in 1877 by an Italian astronomer. Schiaparelli, who called them canali or channels. This term immediately suggested to many people that they represented artificial constructions. It was not long before suggestions were made that they represented the efforts of an intelligent race to bring water from the polar regions to irrigate the arid deserts of the planet. From this has grown a mass of literature depicting the Martians and tbeir way of life. Mariner’s Evidence Unfortunately for these flights of fancy, modern research has shown that conditions are entirely different from those depicted in such far-fetched theories. Photographs taken by Mariner IV show that Mars has about as many craters, area for area, as the moon. This is in accordance with a prediction made by E. J. Opik, an Irish astronomer, in 1951. Subse-

quent investigations have shown that Mars should experience roughly 20 times as many impacts from asteroids, area for area, as the moon. Several research teams have also investigated the probable age of the Martian craters and are In general agreement that these are roughly 300 million years old, compared with the estimate of 5 billion years as the age of the lunar craters.

It was formerly thought that the dark areas were depressions on the Martian surface and the bright reddish regions were the highlands. This opinion was formed because the receding polar caps appeared to linger longer on the bright areas. It was assumed that, like the earth, any elevated regions would be the coldest More recent researches tend to show that actually the reverse is the case because on Mars differences in elevation would not be accompanied by differences in temperature. Instead frost would have a tendency to persist mainly in the lowest regions, which would have the greatest protection from any winds. Thus it is now thought that the bright regions are the depressions and the dark areas the elevations. It is even thought that the polar caps may not be due to water vapour but consist of solid carbon-dioxide. Such deposits would form in depressed areas when the temperature fell below a certain limit However there is still much argument amongst astronomers as to whether the observed effects are due to carbon-dioxide or water vapour that has frozen. Unsolved Problems Whilst the surface features of Mars still pose a large number of problems for astronomers to solve, there remain others connected with this planet that are just as interesting. Mars is accompanied by two satellites, called Phobos and Deimos, the companions of the war god in Greek mythology. Phobos is only 5800 miles from the centre of Mars and revolves around the planet in 7 hours 39 minutes. Deimos needs 30 hours 18 minutes to revolve at a distance of 14.600 miles from the planet Both are extremely small, Phobos having a diameter of about eight miles and Deimos of approximately five miles. j Phobos’s orbital motion appears to be steadily accelerating. It is now well known that the effect of atmospheric resistance is to speed up a satellite and thus force it into

a lower obit. It is known that Mars has an atmosphere from the way tn which it scatters sunlight It is not very dense, probably less than one-tenth of the sea-level pressure of the earth’s atmosphere. The presence of haze and scattered clouds over Mars also shows that it has an atmosphere. By assuming Phobos has a diameter of eight miles it is possible to calculate, for different densities of the satellite, just bow much atmosphere Mars must have to cause the observed acceleration to the orbital motion of this satellite. The result, giving a surface pressure on Mars of from 83 to 89 millibars, has at least removed one very bizarre theory. This proposed that Phobos was really a hollow shell, probably of artificial origin. However, like so much of our knowledge of Mars, a great many more observations are needed before we can be sure of exactly how this strange satellite will behave in the future.