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Venus, although a twin planet of the Earth in size, has surprisingly little water in its carbon dioxide atmosphere, an equivalent of a 0.1 m deep layer. However, no water exists on its basaltic surface which, with a temperature of 650 K and clouds of sulphuric acid overhead, resembles a classical vision of hell. The question of water on Venus is dilemmatic and by no means answered completely (Donahue et.al.,1982, Greenspoon, 1987, Kasting, 1988). If Venus once had water, where is it now? And if Venus never had oceans, why? It is likely that Venus had an amount of water comparable to that of the Earth; however, with the runaway greenhouse process, the oceans evaporated and the water dissociated by solar radiation: in this case the oxygen was absorbed by the rocks and the free hydrogen escaped into space. The other alternative is that perhaps Venus formed so close to the Sun that water from the solar nebula never condensed on the planet. In this case Venus should, however, contain some water of later cometary origin. To make things more difficult, no trace of free oxygen has been found in the Venusian atmosphere. The recent Magellan mission will search by radar for evidence of ancient ocean terraces, river beds and deltas or other features which would point towards the past existence of running water on its surface.
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Measurements made by the Pioneer Venus Orbiter during the latter stages of its fourteen-year exploration of Venus (which ended in late 1992) suggest
that the planet very well could have been relatively
wet in the distant past. This spacecraft found that the ratio of deuterium (hydrogen with an added neutron) to plain hydrogen is some 150 times greater than on Earth. This excess is likely due to the escape of the lighter isotope (plain hydrogen) over the ages. If this is the case, then Venus may have had 150 times as much water as it does today.
Current models assume that Venus initially had
an Earth-like deuterium to hydrogen ratio (D/H) which has risen due to atmospheric evolution. With the results of these D/H tests in hand, two predominant theories exist as to the reasoning for this alarming ratio. The first is the "primordial-ocean model", which interprets the high D/H ratio as the result of the fractionation loss from a large initial ocean-like body of water. This theory argues that Venus once had reservoirs of water (or even oceans), but lost it as the water vapor in its upper atmosphere was broken into its components, and the lighter hydrogen escaped into the solar system. Supporters of this model believe that Venus started off wet because it could not avoid receiving some of the same volatile-rich material that formed on Earth (because of its similar size and proximity). The abundance of deuterium in comparison with hydrogen represents the notion of this theory's followers that the water's ordinary hydrogen escaped, while the much heavier form of hydrogen (deuterium) stayed behind because of its weight. Supporters of this theory are enthusiastic about the possibility that Venus once had large quantities of water.
The second major model is referred
to as the "steady-state model." According to this theory, the ratio results from a balance between sources of water and fractionation losses, with the total abundance of water remaining constant over time. This theory basically states that Venus' total amount of water has remained constant through the ages and the source of the D/H ratio is terrestrial-like (likely volcanic).
If
Venus did in fact have oceans at some point in time, then the implications of this could be remarkable. Although these oceans would have likely existed at near boiling temperatures for much of the time, the possibility still exists that life could have arisen in these bodies of water.
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