James' World 2

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On the day that I was born—winter solstice, 1959—a headline in Life magazine proclaimed “Target Venus: There May be Life There!” It told of how scientists rode a balloon to an altitude of 80,000 feet to make telescope observations of Venus’s atmosphere, and how their discovery of water raised hopes that there could be living things there. As a kid, I thrilled to tales of undersea adventure with telepathic Venusian frogs in Isaac Asimov’s juvenile science-fiction novel Lucky Starr and the Oceans of Venus. In 1975, when I was 15, a family friend—a planetary scientist—gave me a picture of the first-ever photograph taken from the surface of another planet: Venus. The Soviet Venera 9 probe had sent back a black-and-white image of a landscape with angular rocks and fine-grained dirt. A bright patch of sky made it seem much less unearthly than the Apollo moon shots I had obsessed over, and more like a strange, overcast desert land that you might hope to visit someday.

For many of my peers, though, Venus quickly lost its romance. The very first thing that scientists discovered with a mission to another planet was that Venus was not at all the Earthly paradise that fiction and speculative science had portrayed. It is nearly identical to our own planet in bulk properties such as mass, density, and size. But its surface has been cooked and desiccated by a suffocating ocean of carbon dioxide. Trapped in the scorching death-grip of a runaway greenhouse effect, Venus has long been held up as a cautionary tale for everything that could go wrong on a planet like Earth. As a possible home for alien life, it has been voted the planet least likely to succeed.

But I have refused to give up on Venus, and over the years my stubborn loyalty has been vindicated. The rocky vistas glimpsed by Venera 9 and other Russian landers suggested a tortured volcanic history. That was confirmed in the early 1990s by the American Magellan orbiter, which used radar to peer through the planet’s thick clouds and map out a rich, varied, and dynamic surface. To judge from the paucity of impact craters, the surface formed mostly in the last billion years, which makes it fresher and more recently active than any rocky planet other than Earth. Russian and American spacecraft also found hints that the primordial climate might have been wetter, cooler, and possibly even friendly to life. Measurements of density and composition imply that Venus originally formed out of basically the same stuff as Earth. That presumably included much more water than the trace equivalent to one thousandth of 1 percent of Earth’s oceans that we find wafting in the thick air today.

Confirmation comes from the measured ratio of deuterium (heavy hydrogen) to normal hydrogen. Venus’s atmosphere has proportionately much more deuterium than Earth’s does. This skew is a sign that the planet’s hydrogen has gradually escaped to space; deuterium, being heavier, leaks out more slowly than its lighter cousin, so that a difference builds up over time. Winding the clock backward, Venus must once have had much more hydrogen, which is to say, much more water.

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Venus

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