Pale Blue Dot - Carl Sagan [63]
You peer through the eyepiece of a telescope—even a big telescope, even the largest optical telescope on Earth—and you can make out no detail at all. Over the months, you see a featureless disk methodically going through phases, like the Moon: crescent Venus, full Venus, gibbous Venus, new Venus. There is not a hint of continents or oceans.
Some of the first astronomers to see Venus through the telescope immediately recognized that they were examining a world enshrouded by clouds. The clouds, we now know, are droplets of concentrated sulfuric acid, stained yellow by a little elemental sulfur. They lie high above the ground. In ordinary visible light there’s no hint of what this planet’s surface, some 50 kilometers below the cloud tops, is like, and for centuries the best we had were wild guesses.
You might conjecture that if we could take a much finer look there might be breaks in the clouds, revealing day by day, in bits and pieces, the mysterious surface ordinarily hidden from our view. Then the time of guesses would be over. The Earth is on average half cloud-covered. In the early days of Venus exploration, we saw no reason that Venus should be 100 percent overcast. If instead it was only 90 percent, or even 99 percent, cloud-covered, the transient patches of clearing might tell us much.
In 1960 and 1961, Mariners 1 and 2, the first American spacecraft designed to visit Venus, were being prepared. There were those, like me, who thought the ships should carry video cameras so they could radio pictures back to Earth. The same technology would be used a few years later when Rangers 7, 8, and 9 would photograph the Moon on the way to their crash landings—the last making a bull’s-eye in the crater Alphonsus. But time was short for the Venus mission, and cameras were heavy. There were those who maintained that cameras weren’t really scientific instruments, but rather catch-as-catch-can, razzle-dazzle, pandering to the public, and unable to answer a single straightforward, well-posed scientific question. I thought myself that whether there are breaks in the clouds was one such question. I argued that cameras could also answer questions that we were too dumb even to pose. I argued that pictures were the only way to show the public—who were, after all, footing the bill—the excitement of robotic missions. At any rate, no camera was flown, and subsequent missions have, for this particular world, at least partly vindicated that judgment: Even at high resolution from close flybys, in visible light it turns out there are no breaks in the clouds of Venus, any more than in the clouds of Titan.* These worlds are permanently overcast.
In the ultraviolet there is detail, but due to transient patches of high-altitude overcast, far above the main cloud deck. The high clouds race around the planet much faster than the planet itself turns: super-rotation. We have an even smaller chance of seeing the surface in the ultraviolet.
When it became clear that the atmosphere of Venus was much thicker than the air on Earth—as we now know, the pressure at the surface is ninety times what it is here—it immediately followed that in ordinary visible light we could not possibly see the surface, even if there were breaks in the clouds. What little sunlight is able to make its tortuous way through the dense atmosphere to the surface would be reflected back, all right; but the photons would be so jumbled by repeated scattering off molecules in the lower air that no image of surface features could be retained. It would be like a “whiteout” in a polar snowstorm. However, this effect, intense Rayleigh scattering, declines rapidly with increasing wavelength; in the near-infrared, it was easy to calculate, you could see the surface if there were breaks in the clouds—or if the clouds were transparent there.
So in 1970 Jim Pollack, Dave Morrison,