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though permanently shaded—be covered by a thin layer of rocky
debris to prevent it from sublimating into space.
125
Mercury has never supported water oceans. Its proximity to the
sun's solar wind, its extreme temperatures, and its low gravity means
that gases in its thin atmosphere easily escape to space. To find traces
of oceans on other worlds in our solar system we must visit our other
rocky neighbours, Venus and Mars, and delve into their distant past.
The Bright Queen of Hell
The earliest satellites aimed at Venus had little luck. The Russian
Ve n -
era
spacecraft probably got close in 1961, within 100,000 kilometres
or so—but radio contact had been lost soon after it left Earth. The
American
Mariner I
did not even get that far, blowing up on the launch
pad, but in 1962
Mariner 2
got close enough for its microwave and
infrared detectors to pierce the cloud cover to measure the surface
temperature as higher than 400 degrees Celsius. At that moment, the
dream of a habitable sister planet died in human minds—as did the
(respectable) supply of at least one subgenre of science fiction. Such
temperatures are far beyond the limits of tolerance of even the most
heat-loving bacteria on Earth.
Venera 3
crash-landed on Venus in 1966, the first human-made
object to fall upon that planet, but its communication system failed
before it could return any information to Earth. The next three
Ve n e ra
spacecraft got a little farther, and revealed the reason for the extreme
surface heat levels: the extraordinarily high pressure of the carbon
dioxide atmosphere, equivalent to 90 Earth atmospheres. The atmos-
phere of Venus is laced with sulphuric acid for good measure—it is
droplets of this corrosive substance that make up the thick cloud
cover around the planet. The
Ve n e ra
craft were sturdily built; to one
visitor to the Soviet workshops, they brought Henry VIII's armour to
mind. Yet they were crushed by that atmosphere before they reached
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