The picture was computer generated from the ultraviolet spectrometer measurements made by the Pioneer Venus Orbiter spacecraft. It shows the distribution of atomic hydrogen and atomic oxygen in the Venusian atmosphere. The oxygen is trapped in the upper atmosphere (crescent) while the lighter hydrogen escapes slowly into space (horizontal bars). There may have been extensive water on Venus in the past, but as the greenhouse effect caused by the planet's mainly carbon dioxide atmosphere accelerated, it would have boiled away. Pioneer Venus was launched on 20th May 1978, and went into orbit around Venus in December 1978. It radar mapped 93% of the planet's surface and returned over 1000 images of the Venusian clouds.
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