Philips Atlas of the Universe

V enus

ATLAS OF THE UNIVERSE

 Four photographs of

Venustaken on the same

scale. As the phase shrinks,

the apparent diameter

increases.

▲ Venusdrawn with my

31-cm (12^1 / 2 -inch) reflector.

All that could be seen were

very vague, cloudy shadings

which are necessarily rather

exaggerated in the sketch,

together with slightly

brighter areas near the

cusps. The terminator

appeared essentially smooth.

The internal structure of

Venus may not be too unlike

that of the Earth, but with a

thicker crust and an iron-rich

core which is smaller both

relatively and absolutely.

There is no detectable

magnetic field and, like

Mercury, Venus has no

satellite.

V

enus, 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

favoured 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 tem-

perature is over 480°C.

Radar measurements have shown that the rotation

period is 243.2 days – longer than Venus’ ‘year’; more-

over, 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 evol-

ution 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.

PLANETARY DATA – VENUS

Sidereal period 224.701 days

Rotation period 243.16 days

Mean orbital velocity 35.02 km/s (21.76 miles/s)

Orbital inclination 3°23’ 39”.8

Orbital eccentricity 0.007

Apparent diameter max. 65”.2

min. 9”.5

mean 37”.3

Reciprocal mass, Sun = 1 408,520

Density, water = 1 5.25

Mass, Earth = 1 0.815

Volume, Earth = 1 0.86

Escape velocity 10.36 km/s (6.43 miles/s)

Surface gravity, Earth = 1 0.903

Mean surface temperature cloud-tops 33°C

surface 
480°C

Oblateness 0

Albedo 0.76

Maximum magnitude 4.4

Diameter 12,104 km (7523 miles)

Earth

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