It’s Getting Hot in Here, So Take Off All Your H2O

Title: A dry Venusian interior constrained by atmospheric chemistry

Authors: Tereza Constantinou, Oliver Shorttle, and Paul B. Rimmer

First Author’s Institution: Institute of Astronomy, University of Cambridge, Cambridge, UK

Status: Published in Nature, Open Access

She’s hot, she’s temperamental, and she’s a bit of a mystery. Her name is Venus. Venus is widely regarded as Earth’s less friendly twin, since they are of similar size and mass. However, while Earth is covered in vast oceans and lush forests, Venus’s surface is inhospitable. Under layers of toxic clouds of sulfuric acid exists a dry terrain with mountains, valleys, and thousands of volcanoes. Despite being similar in size, composition, and distance to the Sun as the Earth, Venus’s blistering surface is not conducive to life as we know it, which requires the presence of liquid water. But was she always this way? The authors of today’s paper investigate the history of our nearest neighbor, exploring the question: was Venus always a formidable hellscape, or did it once have a temperate climate with liquid water on its surface?

A Tale of Two Venuses

When considering the history of Venus, two paradigms emerge: one “temperate and wet Venus” and one “dry Venus”. In both scenarios, Venus forms with a magma ocean on its surface. The authors suggest that if Venus’s magma ocean phase was short, that is, its magma ocean cooled and solidified quickly, then it could have held on to its liquid water supply. In this scenario, clouds would form on its dayside, reducing radiation absorption, and disassociate on its nightside, allowing for the dayside heat to efficiently radiate back into space. This would keep it cool enough to maintain liquid water on its surface for several billion years until the “great climate transition” on Venus – an era where it’s believed that high volcanic activity gave rise to substantial outgassing of CO2 and SO2 – left behind the Venusian surface that exists today. 

On the other hand, if Venus’s magma oceans solidified slowly, it would have dried out much earlier in its evolution. In this scenario, clouds would continue to exist on Venus’s nightside, trapping in the hot air from the dayside radiation and heating up the planet. Any liquid water initially present would be ejected as steam. In order to put constraints on these two models, the authors investigate whether present-day Venus has a water-rich interior.

It’s What’s Inside That Counts

The authors suggest that if Venus had retained its water oceans after its magma ocean phase, then water should still be present in its interior. In the temperate and wet Venus scenario, the water that existed on its surface would eventually get trapped in its interior, and we would expect to observe a hydrogen-rich interior. Conversely, in the dry Venus scenario where surface water quickly evaporates out of its atmosphere, we would expect to see a hydrogen-poor interior. These two pathways are illustrated in Figure 1. To study what lies beneath the surface, the authors use the present-day composition of the volcanic gases on Venus to inform them on the composition of its interior. 

Volcanic eruptions release gases trapped in the planet’s deeper layers, allowing us to probe regions where we would expect H2O to be present in the wet and temperate scenario. The authors calculate the relative abundances of molecules in Venus’s volcanic gas and compare them to that of Earth’s volcanic gas, which is 96% H2O because of our water-rich interior. They find that the maximum content of water in the volcanic gases on Venus is 6%, suggesting that its interior is very dry. This result is consistent with a scenario in which Venus’s magma ocean solidified slowly, never allowing for a sustained water ocean. If so, Venus has always been an uninhabitable world.

Though Venusians may just be characters of science fiction, this result is interesting because it tells us which planets are more likely to host liquid water, and therefore potential life. When searching for future habitable planets with JSWT or future missions, we may want to steer clear of exoplanets within the Venus zone and focus on those that are more likely to be able to sustain life.

Astrobite edited by Kat Lee

Featured image credit: NASA/JPL