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chemistry of Venus's atmosphere. Of the small amount of hydrogen
bound to the tiny content of water in the atmosphere, there is a strik-
ingly high abundance of the heavy isotope deuterium: about one in
every 10,000 hydrogen atoms (which means that one in every 5,000
water molecules contains a deuterium atom and is thus 'heavy water').
This ratio may seem small, but it is nevertheless 100 times greater
than on Earth.
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The discovery of the deuterium enrichment was quirky in the
extreme. When the NASA
Pioneer
mission parachuted down in 1978 it
had a mass spectrometer on board to analyse the chemistry of the
Venusian atmosphere. A drop of sulphuric acid from the clouds con-
densed in the inlet to this instrument, seriously compromising virtu-
ally all of the data. However, the spectrometer then analysed the acid
droplet as it evaporated—and discovered the very high deuterium
values.
127
Assuming that the original ratio of deuterium to hydrogen was
similar on Earth and Venus—and this is a reasonable assumption, as
their waters probably came from much the same sources—Venus has
lost at least 99.5 per cent of its original water.
128
This has simply leaked
into outer space and been carried away by the solar wind (with more
of the deuterium having been retained because, being heavier, it is less
easily lost from Venus's atmosphere). This may have been the greatest
planetary tragedy (to date) in the history of the solar system. For,
while the fate of Mars would have been inevitable in any case—as
Arrhenius realized—perhaps Venus, under slightly different circum-
stances, might have clung on to its seas (and life?) for longer. How
then did Venus dehydrate and die?
When Venus was young the Sun was fainter than it is today—faint
enough for Venus to retain something resembling an Earth-like sup-
ply of water. If so, seas or oceans must have existed on its surface.
Perhaps, too, if stores of that planetary lubricant, water, were plentiful
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