March 20th 1963 ~ New Studies of Mars and Venus

In December 1962 the US probe Mariner II passed within 'striking range' of Venus. The results obtained were released in early 1963, and proved to be most surprising. In particular, the surface temperature of the planet was given as -\-800 degrees Fahrenheit, making Venus far too hot to support any life built upon the terrestrial pattern.

Doubts have been cast on the accuracy of the information; it has been pointed out that the Mariner instruments were designed to function at less than 10,000 miles from Venus, whereas in fact they had to operate from over 20,000 miles. However, most authorities consider that the results are at any rate of the right order. Further researches with unmanned vehicles will pre­sumably provide confirmation or denial. Until this is done, there is little more that can be added.

Taking part in the programme was Howard Miles, of the Lanchester College of Technology at Coventry, who is Director of the Artificial Satellite Section of the British Astronomical Association.

The first months of 1963 have been most eventful in both astronomy and space research. There has, for instance, been the comparatively bright Nova Herculis, a 'new' or temporary star not far from Vega, discovered by the Swedish amateur Dahlgren; it has been clearly visible to the naked eye, and is fading only very slowly. There has also been a total eclipse of the Sun, unfortunately invisible from Britain. In the field of space-research, the surprising results obtained by Mariner II have been announced; and the first flights of the telescope-carrying balloon Stratoscope II seem to be en­couraging.

Whereas Mariner II was concerned with Venus, the Stratoscope observations deal with Mars. Already one important fact has emerged. There seems every likelihood that the Moon will be reached within the foreseeable future, and there has been much discussion as to the next space-target. Venus and Mars are the only alternatives, and there has been an increasing tendency to give Venus preference as being potentially a less hostile world than Mars. Yet if the Mariner results are valid, or are even of the right order, Venus is hopelessly unfriendly, and it will be extremely difficult to land even an unpiloted vehicle there; any talk of a full-scale expedition may be disregarded. The result will pre­sumably be to swing the main interest back once more to Mars.

On December 14, 1962, Mariner II passed by Venus at a dis­tance of roughly 21,000 miles; Venus would have looked about thirty-five times larger than the Moon appears from the surface of the Earth. About six and a half hours before closest approach the instruments were brought into operation. One of the most impor­tant was the microwave radiometer, which is really a small radio telescope twenty inches in diameter. The radiometer was adjusted so that it was able to scan the whole planet, and three scans were in fact obtained - one on the dark side of Venus, one down the terminator {i.e., the boundary between the day and night hemi­spheres), and one up the sunlit side.

The objects of the microwave experiments were to measure the surface temperature of Venus, and to analyse the planet's atmo­sphere. Two wavelengths were used, but the channel which is expected to give information about the presence - or absence - of atmospheric water vapour has not yet been analysed. It is the temperature measures which have caused so much discussion.

First, it seems that the surface temperatures for the day and night sides are more or less the same, and this argues against a captured rotation - that is to say, a rotation period of 224-7 days, which is the same as the period taken by Venus to complete one journey round the Sun, and would therefore mean that the planet would keep the same hemisphere turned permanently sunward. Mercury behaves in such a fashion, so that one area of the surface is fiercely hot while the region of permanent night is intensely cold. The fact that similar conditions do not obtain on Venus indicates that all parts of the surface must have sunlight at some time or another.

By noting the temperature along each scan, it was found that there was strong 'limb darkening'. This showed that the source of the radiation must be the actual surface of the planet. The diagram will make the position clear, and there is a useful analogy with the appearance of the Sun as seen from Earth; when the Sun is low in the sky, its light is coming to us through a greater thickness of our atmosphere, so that it appears reddened and dimmed. A limb brightening would have indicated that the radiation came not from the surface of Venus but from a high level in the planet's atmosphere. The calculated result is that the surface has a tem­perature of about +800 degrees F. This is much hotter than had been anticipated; and if it proves to be correct, our ideas about Venus as a world will have to be drastically revised.

Astronomers have been divided into two main camps. Some have maintained that Venus is a desert world, without a trace of surface moisture and utterly devoid of life. However, there has been growing support for a completely different picture, due to F. L. Whipple and D. H. Menzel of the United States, according to which the plane! is covered mainly or wholly with water, and that the clouds consist of HaO. Following up this idea, I had suggested that Venus might well be a Cambrian-type world, and that primi­tive marine life-forms might exist in the oceans there.

A temperature of 800 degrees F at once disposes of the marine theory, since Venus will be much hotter than the temperature of boiling water. We must return to the desert theory, but in accen­tuated form. Yet it would be premature to conclude that the whole matter is settled, and further researches are needed. It would not be wise to pin complete faith on a single experiment, and it must also be remembered that the instruments in Mariner were designed to function at a distance of less than 10,000 miles from Venus; actually, because of launching and guidance errors, they had to manage from more than 20,000 miles, which means that there was inevitable loss of precision and added difficulty in interpretation of the results.

Russian radio astronomers have also been measuring the tem­perature of Venus, and have just announced a value of only +150 degrees F; they believe that the high Mariner result is, after all, due to atmospheric effects in the planet's atmosphere. At the moment there is no point in discussing the matter further, though it is clear that the balance of evidence is in favour of great surface heat.

The second relevant experiment related to the amount of carbon dioxide in Venus' atmosphere. It involved two bands in the infra-red, one of which would be absorbed by carbon dioxide, while the other would not. The apparatus was fixed to the side of the microwave radiometer, and the planet was scanned in the same way as before. Again there was strong limb darkening, but for a rather different reason. Looking directly 'down' on to Venus means that one can penetrate farther into the clouds than is possible by looking toward the limb. Since temperature usually decreases with height, what we are actually recording is the lower temperature of the top part of the clouds; the value derived for the upper surface of the cloud was —30 degrees F, though there was one local area which appeared to be a full 20 degrees colder.

Much has been said about the 'cold spot', and there have even been suggestions of a colossal mountain rising to forty miles or so. This appears to be most improbable, and if Venus is a fiercely hot world, with violent winds, erosion would surely have reduced any lofty elevations. It is best to concede that at the moment we have no idea what the 'cold spot' may be.

There have also been developments with regard to the rotation period of Venus - not only by way of Mariner but by measurements made from Earth. Previously, a period of about thirty days had been regarded as likely, though Audouin Dollfus and other French astronomers had supported a captured rotation of 224-7 days, and value of only about ten days had been announced by the Russians.

The American results are now said to indicate that, relative to the Sun, the rotation is extremely slow, and retrograde - that is to say, in the opposite sense to that of the Earth, so that an observer on Venus would see the Sun rise in the west and set in the east no days later. There have been suggestions that there is no appreciable rotation relative to the stars, which would, therefore, appear fixed in the planet's sky (assuming that they could be seen at all through the cloudy mantle). If confirmed, this will indeed be a most peculiar situation, unique in our experience, but for the moment it is wisest not to come to any definite conclusions.

Let us sum up our picture of Venus, assuming that the Mariner findings are at least approximately correct. The surface is fiercely hot, perpetually darkened by clouds, although during the daytime the Sun may appear as a vague patch of light in the sky. The stars will never be visible; they will be hidden by the dense cloud layer, which is estimated to be fifteen miles thick and to consist of con­densed hydrocarbons, lying some forty-five miles above the ground. The planet is a wild, unfriendly desert, without liquid water - which would at once boil - and, presumably, without life of any type known to us; neither is there an appreciable magnetic field. Yet it must again be stressed that these results are preliminary only, and may require extensive modification in the future. * Meanwhile, we must simply wait and see, though it is starting to look as though our future efforts in space research, following successful landings on the Moon, must be concentrated upon Mars rather than Venus.

Mars, which has been - and is - a prominent object in the evening sky lately, has also been under study from America, but by very different methods. What has been done is to send up a 36-inch reflecting telescope by means of a balloon, lifting it to 80,000 feet in order to take photographs of Mars in the infra-red. This is a logical development of earlier experiments made in 1957 and 1959, when a 12-inch telescope was lifted in the balloon Stratoscope I to an altitude of over fifteen miles, and obtained magnificent photographs of the surface of the Sun.

The reflector carried by the new balloon is extremely powerful, and is capable of a photographic resolution of o-i seconds of arc; that is to say, it is capable of distinguishing two objects thirty inches apart at a distance of 1,000 miles. It has a total weight of over two and a half tons. The balloon is actually a compound structure in two parts. Only the smaller balloon is inflated during the launch; as it rises, the helium gas in the small balloon gradually leaks into the main one. The recent flight was extremely successful; even though the landing was much more violent than had been antici­pated, the telescope was not damaged, and all the scientific information was recovered intact.

One of the main objects of the flight was to obtain more infor­mation about the composition of the Martian atmosphere. It is known that the atmospheric density is relatively low - no greater, probably, than that at a height of 55,000 feet above sea-level on Earth, so that no terrestrial creature could possibly breathe it. Moreover, there is not much oxygen or water-vapour, and it is generally thought that the bulk of the atmosphere consists of nitrogen together with a certain amount of carbon dioxide. How­ever, spectral analysis has indicated that the famous polar caps of Mars consist of an icy or frosty deposit, and so presumably there must be a little moisture in the atmosphere.

Shortly after the flight there were some sensational reports, widely publicized. It was said that Mars proved to be 'mainly desert, and capable of supporting only an extraordinary form of life'. Evidently these reports are unfounded. It is true that much of Mars is desert, using the word in its broadest sense; but there is strong evidence that the dark areas are due to living organisms of lowly type, and there is no reason to suppose that these organisms are basically different from forms found on our own world, though they may differ markedly in detail. In fact, the only results released by the authorities at Princeton University, who are responsible for the project, concern water vapour in the Martian atmosphere. Five complete scans of the planet were made in the infra-red, and traces of water vapour have apparently been detected. This comes as no surprise; indeed, any other result would have been rather unexpected. At 80,000 feet the Stratoscope was above 95 per cent of the Earth's atmosphere, so that the effects of water-vapour in our own mantie were naturally very much reduced.

It may seem strange to turn from rocket probes, millions of miles from Earth, to humble balloons a few miles above the ground; but there is no doubt that the balloon technique is highly promising, and much may be expected from it. The Stratoscope will give more information about Mars, apparently, than the Russian rocket probe Mars I, which is still on its way to the Red Planet, but whose signal strength is fading.* It now seems improbable that the hopes of obtaining short-range television pictures of Mars will be realized, at least for the moment; and since Mars will not again be favourably placed until 1965, no spectacular developments in this direction are to be expected in the immediate future.

Much remains to be learned, and we cannot yet claim to have anything like a satisfactory knowledge of either Mars or Venus, but the new methods are at least encouraging, and will certainly lead us on to a much better understanding of our neighbour worlds.

* Contact with the Mars Probe was lost shortly after this, and was never regained.