Venus

September 02, 2020

Venus

VENUS

Venus, the second planet in order of distance from the Sun, is as different from Mercury as it could possibly be. It is far brighter than any other star or planet, and can cast strong shadows; very keen-sighted people can see the phase with the naked eye during the crescent stage, and binoculars show it easily.

Yet telescopically Venus is a disappointment. Little can be seen, and generally the disk appears blank. We are looking not at a solid surface, but at the top of a layer of cloud which never clears. Before the Space Age, we knew very little about Venus as a world.

We knew the size and mass; Venus is only very slightly inferior to the Earth, so that the two are near-twins. The orbital period is 224.7 days, and the path round the Sun is almost circular. Estimates of the rotation period ranged from less than 24 hours up to many months, but the favored value was about a month. The vague shadings sometimes visible on the disk were much too indefinite to give any reliable results.

There was also the Ashen Light, or dim visibility of the ‘night’ side, when Venus was in the crescent phase. It seemed to be real, but few people agreed with the 19th-century astronomer Franz von Paula Gruithuisen that it might be due to illuminations on the planet’s surface lit by the local inhabitants to celebrate the accession of a new emperor!

It was suggested that Venus might be in the condition of the Earth during the Coal Forest period, with swamps and luxuriant vegetation of the fern and horse-tail variety; as recently as the early 1960s many astronomers were confident that the surface was mainly covered with water, though it was also thought possible that the surface temperature was high enough to turn Venus into a raging dust-desert.

Certainly it had been established that the upper part of the atmosphere, at least, was made up mainly of carbon dioxide, which tends to shut in the Sun’s heat.

The first positive information came in December 1962, when the American spacecraft Mariner 2 passed by Venus at a range of less than 35,000 kilometres (21,800 miles) and sent back data which at once disposed of the attractive ocean’ theory.

In 1970 the Russians managed to make a controlled landing with Venera 7, which transmitted for 23 minutes before being put out of action, and on 21 October 1975 another Russian probe, Venera 9, sent back the first picture direct from the surface.

It showed a forbidding, rock-strewn landscape, and although the rocks are grey they appear orange by reflection from the clouds above. The atmospheric pressure was found to be around 90 times that of the Earth’s air at sea level, and the temperature is over 480°C.

Radar measurements have shown that the rotation period is 243.2 days – longer than Venus’ ‘year’; moreover, the planet rotates from east to west, in a sense opposite to that of the Earth.

If it were possible to see the Sun from the surface of Venus, it would rise in the west and set in the east 118 Earth-days later, so that in its way the calendar of Venus is every bit as strange as that of Mercury.

The reason for this retrograde rotation is not known. According to one theory, Venus was hit by a massive body early in its evolution and literally knocked over. This does not sound very plausible, but it is not easy to think of anything better. It has been found that the top of the atmosphere lies around 400 kilometres (250 miles) above the surface, and that the upper clouds have a rotation period of only 4 days.

The upper clouds lie at an altitude of 70 kilometres (44 miles), and there are several definite cloud-layers, though below 30 kilometres (19 miles) the atmosphere is relatively clear and calm. The atmosphere’s main constituent is indeed carbon dioxide, accounting for over 96 per cent of the whole; most of the rest is nitrogen.

The clouds are rich in sulphuric acid; at some levels there must be sulphuric acid ‘rain’ which evaporates before reaching ground level.

MAPPING VENUS

Because we can never see the surface of Venus, the only way to map it is by radar. It has been found that Venus is a world of plains, highlands and lowlands; a huge rolling plain covers 65 per cent of the surface, with lowlands accounting for 27 per cent and highlands for only 8 per cent.

The higher regions tend to be rougher than the lowlands, and this means that in radar they are brighter (in a radar image, brightness means roughness). There are two main upland areas, Ishtar Terra and Aphrodite Terra.

Ishtar, in the northern hemisphere, is 2900 kilometres (1800 miles) in diameter; the western part, Lakshmi Planum, is a high, smooth, lava-covered plateau. At its eastern end are the Maxwell Mountains, the highest peaks on Venus, which rise to 11 kilometres (nearly 7 miles) above the mean radius and 8.2 kilometres (5 miles) above the adjoining plateau.

Aphrodite straddles the equator; it measures 9700 3200 kilometres (6000 * 2000 miles), and is made up of several volcanic massifs, separated by fractures. Diana Chasma, the deepest point on Venus, adjoins Aphrodite. (En passant, it has been decreed that all names of features in Venus must be female.

The only exception is that of the Maxwell Mountains. The Scottish mathematician James Clerk Maxwell had been placed on Venus before the official edict was passed!) A smaller highland area, Beta Regio, includes the shield volcano, Rhea Mons and the rifted mountain Theia Mons. Beta, which is cut by a huge rift valley rather like the Earth’s East African Rift, is of great interest.

It is likely that Rhea is still active, and there can be no doubt that the whole surface of Venus is dominated by vulcanism. Venus’ thick crust will not slide over the mantle in the same way as that of the Earth, so that plate tectonics do no apply; when a volcano forms over a hot spot it will remain there for a very long period. Lava flows are found over the whole of the surface.

Craters are plentiful, some of them irregular in shape while others are basically circular. The largest, Mead, has a diameter of 280 kilometres (175 miles), though small craters are less common than on Mercury, Mars or the Moon.

There are circular lowland areas, such as Atalanta Planitia, east of Ishtar; there are systems of faults, and there are regions now called tesserae – high, rugged tracts extending for thousands of square kilometres and characterized by intersecting ridges and grooves.

Tesserae used to be called ‘parquet terrain’, but although the term was graphic it was abandoned as being insufficiently scientific.

Venus has been contacted by fly-by probes, radarcarrying orbiters and soft-landers; in 1985 the two Russian probes en route for Halley’s Comet even dispatched two balloons into the upper atmosphere of the planet, so that information could be sent back from various levels as the balloons drifted around. The latest probe, Magellan, has confirmed and extended the earlier findings that Venus is overwhelmingly hostile.

Source: Atlas of the Universe - Sir Patrick Moore