THERE is no liquid water on Venus. The water vapour in its atmosphere would, if condensed, form a layer only 20 cm deep which means, in contrast to the 3-km-deep oceans that cover its sister planet Earth, that Venus is very dry indeed. It is not known with certainty whether Venus has lacked water since its formation, or if water once present has been lost during its lifetime; the question is of special interest as water is generally thought to be a necessary ingredient for the development of life. The abundance of deuterium in the atmosphere of Venus is an important clue to the planet's history, because ordinary and deuterated water escape at different rates. Using the high-resolution mode of the Inter-national Ultraviolet Explorer (IUE), we measured hydrogen Lyman-α-emission but found only an upper limit on deuterium Lyman-α-emission, from which we inferred a D/H ratio of less than 2-5 x 10-3. This is smaller by a factor of 3-8 than the D/H ratio derived from measurements by the Pioneer Venus Large Probe, and may indicate either a stratification of D/H ratio with altitude or a smaller overall ratio than previously thought.
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