Oxidizing Venus’ Surface: Consequences for Volatile Inventory

Venus shares some striking similarities with Earth; at the same time, it exhibits characteristics that are widely different from that of our own planet. Indeed, it is an example of an active planet that may have followed a radically different evolutionary pathway despite the similar mechanisms at work and probably comparable initial conditions. The evolution of Venus is still poorly constrained, partly due to a lack of relevant measurements. As a result, there is currently no consensus on the history of Earth’s sister’s surface conditions. It has, however, been suggested that water could have been stable for long periods of time at the surface of Venus, depending on the specific composition of the atmosphere.

Venus observation has shown the D/H ratio in its atmosphere is consistent with water loss, possibly amounting to 100 times its atmosphere present content. Fractionation of hydrogen however depends on the mechanisms at work and the conditions of loss, meaning this estimate is still very crude and qualitative. Material on Venus has also been shown to be consistent with surface oxidation, but an oxidized small layer of 10 μm depth can explain the observed spectra. We investigate how Venus’ atmosphere, mantle and surface could have evolved in the past in light of the multiple mechanisms affecting volatile exchanges. We have developed a self-consistent coupled numerical simulation of the evolution of Venus, striving to identify and model mechanisms that are important to the behavior of the planet and its surface conditions.

Loss mechanisms are of special interest, since they provide a way to quantify how much water could have been lost over time and thus could potentially put an upper limit to the amount of water in Venus’ past. The current simulations include modeling of mantle dynamics, volcanism, atmospheric escape (both hydrodynamic and non-thermal), evolution of atmosphere composition, surface oxidation and evolution of surface conditions (greenhouse effect) and the coupling between interior and atmosphere of the planet.

Volatile fluxes between the different layers of the planet seem critical to estimate how Venus changed over time. This is especially important as we have highlighted the strong role played by mantle/atmosphere coupling in regulating both mantle dynamics and surface conditions through surface temperature evolution. It is also seemingly a major factor governing, in turn, volatile outgassing and outgassed species composition.

In recent evolution, volatile exchanges seem very limited, with low release of water in the atmosphere, especially. Loss mechanisms also appear to be able to remove very low amounts of water and oxygen, from the surface/atmosphere (4 mbar to a few bar), making it quite difficult to accommodate large bodies of water, especially during Venus’ recent past. Trapping oxygen on the surface through oxidation of newly emplaced volcanic material is more uncertain. It can certainly explain the loss of a few more bars. The process is likely to remains inefficient, but it cannot be ruled out that larger volumes of oxidized material exist on Venus and could contain oxygen from past liquid water layers of a few meters to tens of meters deep.