We, People of Venus…

We, People of Venus…

It now seems reasonably clear that the high surface temperature on Venus 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.

In the stifling landscape of Venus, 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. Image: © Megan Jorgensen (Elena)

At the present time there is still a little uncertainty about the abundance of water vapor on Venus. The gas chromatograph on the Pioneer Venus entry probes gave an abundance of water in the lower atmosphere of a few tenths of a percent. On the other hand, infrared measurements by the Soviet entry vehicles, Venera 11 and 12, gave an abundance of about a hundredth of a percent. If the former value applies, then carbon dioxide and water vapor alone are adequate to seal inn almost all the heat radiation from the surface and keep the Venus ground temperature at about 480C. If the latter number applies – and my guess is that it is the more reliable estimate – then carbon dioxide and water vapor alone are adequate to keep the surface temperature only at about 380C, and some other atmospheric constituent is necessary to close the remaining infrared frequency windows in the atmospheric greenhouse. However, the small quantities of SO2, CO and HCl, all of which have been detected in the Venus atmosphere, seem adequate for this purpose. Thus recent American and Soviet missions to Venus seem to have provided verification that the greenhouse effect is indeed the reason for the high surface temperature.