An ocean on Venus ? New revelations from scientists

© Image generated by AI DALL-E for Presse-citron

The climatic history of Venus is one of the most enduring puzzles in modern planetary science. For decades, scientists have vacillated between two radically opposed visions of its past: that of a planet that harbored oceans of liquid water for billions of years, like the early Earth, and that of a world condemned from its formation to perpetual aridity.

A new study published in Nature Astronomyprovides some answers to this fundamental question. By studying the rates of destruction and renewal of different chemical species in the current atmosphere of Venus, researchers have been able to go back in time to reconstruct the planet's water history.

The Venusian interior: primordial aridity

The study reveals a specific characteristic of the internal structure of Venus: extreme dryness of its mantle, the intermediate layer between the crust, the outermost part, and the core, the innermost part. Analyses of current volcanic emanations show a water content not exceeding 6% of the molecules emitted, a rate drastically lower than that of terrestrial magmas in similar conditions.

This particular composition cannot be explained by a simple gradual evolution: it testifies to an early and profound desiccation, which occurred during the primitive phase of the planet's magma ocean. Venus may have experienced a sudden cataclysmic event that occurred very early in its history, while the planet was still forming. This event caused a massive and irreversible loss of water, giving Venus its current desert appearance. Solar winds, runaway greenhouse effect or giant impact with a massive celestial body, several hypotheses are conceivable.

The deuterium-to-hydrogen ratio of the Venusian atmosphere, 150 times that of Earth, had long been interpreted as irrefutable evidence of vanished ancient oceans.

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However, new calculations demonstrate that such a ratio can be explained by a steady-state supply balance, where atmospheric losses are offset by continuous volcanic outgassing from an inherently dry mantle. This fundamental discovery completely calls into question all our hypotheses on the primitive evolution of Venus.

Venus: a furnace from the beginning

As explained above, the scientific community was until then divided between two evolutionary paradigms. The first, largely influenced by our understanding of Earth's history, envisaged a primitive temperate Venus, with liquid oceans for several billion years. This vision was based in particular on general circulation models (simplified mathematical representation of the climate system of a planet) demonstrating the theoretical possibility of such a state, maintained by a complex system of clouds forming on the diurnal face of the planet.

The second, more radical scenario, postulated a perpetually torrid planet, whose initial conditions would have prevented any condensation of water on the surface. The new geochemical data therefore strongly corroborate this second hypothesis. The composition of current volcanic gases suggests that the planet has preserved the scars of a particularly hot primitive phase, during which the water vapor atmosphere would have persisted long enough to allow complete dehydration of the planetary mantle.

The data collected now allow us to definitively rule out the hypothesis of primitive oceans on Venus. The arid nature of the current Venusian mantle can only be explained by a particular primitive history: that of a planet that has never experienced the condensation of liquid water on its surface. Future space missions to Venus, such as VERITAS in 2031 and DAVINCI+ in the 2030s, should help us better understand the geological and climatic processes that have shaped this hellish planet.

• Venus has never had liquid oceans, its mantle has been dry since its beginnings.
• Water losses were compensated by volcanic degassing, thus confirming an early drought.
• Venus' initial conditions prevented any condensation of water, making the planet torrid from its formation.

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