When I greet a friend, I am seeing her in reflected visible light, generated by the Sun, say, or by an incandescent lamp. The light rays bounce off my friend and into my eye. But the ancients, including no less a figure than Euclid, believed that we see by virtue of rays somehow emitted by the eye and tangibly, actively contacting the object observed. This is a natural notion and can still be encountered, although it does not account for the invisibility of objects in a darkened room. Today we combine a laser and a photocell, or a radar transmitter and a radio telescope, and in this way make active contact by light with distant objects. In radar astronomy, radio waves are transmitted by a telescope on Earth, strike, say, that hemisphere of Venus that happens to be facing the Earth, and bounce back. At many wavelengths the clouds and atmosphere of Venus are entirely transparent to radio waves. Some places on the surface will absorb them or, if they are very rough, will scatter them sideways and so will appear dark to radio waves. By following the surface features moving with Venus as it rotates, it was possible for the first time to determine reliably the length of its day – how long it takes Venus to spin once on its axis. It turns out that, with respect to the stars, Venus turns once every 243 Earth days, but backwards, in the opposite direction from all other planets in the inner solar system. As a result, the Sun rises in the west and sets in the east, taking 118 Earth days from sunrise to sunrise. What is more, it presents almost exactly the same face to the Earth each time it is closest to our planet. However the Earth’s gravity has managed to nudge Venus into this Earth-locked rotation rate, it cannot have happened rapidly. Venus could not be a mere few thousand years old but, rather, it must be as old as all the other objects in the inner solar system.
Radar pictures of Venus have been obtained, some from ground-based radar telescopes, some from the Pioneer Venus vehicle in orbit around the planet. They show provocative evidence of impact craters. There are just as many craters that are not too big or too small on Venus as there are in the lunar highlands, so many that Venus is again telling us that it is very old. But the craters of Venus are remarkably shallow, almost as if the high surface temperatures have produced a kind of rock that flows over long periods of time, like taffy or putty, gradually softening the relief. There are great mesas here, twice as high as the Tibetan plateau, an immense rift valley, possibly giant volcanoes and a mountain as high as Everest. We now see before us a world previously hidden entirely by clouds – its features first explored by radar and by space vehicles.
The surface temperatures on Venus, as deduced from radio astronomy and confirmed by direct spacecraft measurements are around 480°C or 900°F, hotter than the hottest household oven. The corresponding surface pressure is 90 atmospheres, 90 times the pressure we feel from the Earth’s atmosphere, the equivalent of the weight of water 1 kilometer below the surface of the oceans. To survive for long on Venus, a space vehicle would have to be refrigerated as well as built like a deep submersible.
Something like a dozen space vehicles from the Soviet Union and United States have entered the dense Venus atmosphere, and penetrated the clouds; a few of them have actually survived for an hour or so on the surface.* Two spacecraft in the Soviet Venera series have taken pictures down there. Let us follow in the footsteps of these pioneering missions, and visit another world.
* Pioneer Venus was a successful US mission in 1978-79, combining an orbiter and four atmospheric entry probes, two of which briefly survived the inclemencies of the Venus surface. There are many unexpected developments in mustering spacecraft to explore the planets. This is one of them: Among the instruments aboard one of the Pioneer Venus entry probes was a net flux radiometer, designed to measure simultaneously the amount of infrared energy flowing upwards and downwards at each position in the Venus atmosphere. The instrument required a sturdy window that was also transparent to infrared radiation. A 13.5-karat diamond was imported and milled into the desired window. However, the contractor was required to pay a $12,000 import duty. Eventually, the US Customs service decided that after the diamond was launched to Venus it was unavailable for trade on Earth and refunded the money to the manufacturer.
In ordinary visible light, the faintly yellowish clouds of Venus can be made out, but they show, as Galileo first noted, virtually no features at all. If the cameras look in the ultraviolet, however, we see a graceful, complex swirling weather system in the high atmosphere, where the winds are around 100 meters per second, some 220 miles per hour. The atmosphere of Venus is composed of 96 percent carbon dioxide. There are small traces of nitrogen, water vapor, argon, carbon monoxide and other gases, but the only hydrocarbons or carbohydrates present are there in less than 0.1 parts per million. The clouds of Venus turn out to be chiefly a concentrated solution of sulfuric acid. Small quantities of hydrochloric acid and hydrofluoric acid are also present. Even at its high, cool clouds, Venus turns out to be a thoroughly nasty place.
High above the visible cloud deck, at about 70 kilometers altitude, there is a continuous haze of small particles. At 60 kilometers, we plunge into the clouds, and find ourselves surrounded by droplets of concentrated sulfuric acid. As we go deeper, the cloud particles tend to get bigger. The pungent gas, sulfur dioxide, SO2, is present in trace amounts in the lower atmosphere. It is circulated up above the clouds, broken down by ultraviolet light from the Sun and recombined with water there to form sulfuric acid – which condenses into droplets, settles, and at lower altitudes is broken down by heat into SO2 and water again, completing the cycle. It is always raining sulfuric acid on Venus, all over the planet, and not a drop ever reaches the surface.
The sulfur-colored mist extends downwards to some 45 kilometers above the surface of Venus, where we emerge into a dense but crystal-clear atmosphere. The atmospheric pressure is so high, however, that we cannot see the surface. Sunlight is bounced about by atmospheric molecules until we lose all images from the surface. There is no dust here, no clouds, just an atmosphere getting palpably denser. Plenty of sunlight is transmitted by the overlying clouds, about as much as on an overcast day on the Earth.
With searing heat, crushing pressures, noxious gases and everything suffused in an eerie, reddish glow, Venus seems less the goddess of love than the incarnation of hell. As nearly as we can make out, at least some places on the surface are strewn fields of jumbled, softened irregular rocks, a hostile, barren landscape relieved only here and there by the eroded remnants of a derelict spacecraft from a distant planet, utterly invisible through the thick, cloudy, poisonous atmosphere.*
* In this stifling landscape, there is not likely to be anything alive, even creatures very different from us. Organic and other conceivable biological molecules would simply fall to pieces. But, as an indulgence, let us imagine that intelligent life once evolved on such a planet. Would it then invent science? The development of science on Earth was spurred fundamentally by observations of the regularities of the stars and planets. But Venus is completely cloud-covered. The night is pleasingly long – about 59 Earth days long – but nothing of the astronomical universe would be visible if you looked up into the night sky of Venus. Even the Sun would be invisible in the daytime; its light would be scattered and diffused over the whole sky – just as scuba divers see only a uniform enveloping radiance beneath the sea. If a radio telescope were built on Venus, it could detect the Sun, the Earth and other distant objects. If astrophysics developed, the existence of stars could eventually be deduced from the principles of physics, but they would be theoretical constructs only. I sometimes wonder what their reaction would be if intelligent beings on Venus one day learned to fly, to sail in the dense air, to penetrate the mysterious cloud veil 45 kilometers above them and eventually to emerge out the top of the clouds, to look up and for the first time witness that glorious universe of Sun and planets and stars.
Venus is a kind of planet-wide catastrophe. It now seems reasonably clear that the high surface temperature comes about through a massive greenhouse effect. Sunlight passes through the atmosphere and clouds of Venus, which are semi-transparent to visible light, and reaches the surface. The surface being heated endeavors to radiate back into space. But because Venus is much cooler than the Sun, it emits radiation chiefly in the infrared rather than the visible region of the spectrum. However, the carbon dioxide and water vapor* in the Venus atmosphere are almost perfectly opaque to infrared radiation, the heat of the Sun is efficiently trapped, and the surface temperature rises – until the little amount of infrared radiation that trickles out of this massive atmosphere just balances the sunlight absorbed in the lower atmosphere and surface.