Title: Has Venus ever been habitable? Constraints of an interior-atmosphere-redox couple Evolution model
Authors: Joshua Krissansen-Totton, Jonathan J. Fortney, Francis Nimmo
Institution of the first author: University of California, Santa Cruz
Status: Published in the Planetary Science Journal [open access]
Where did the water go? (And was that there to start?)
Although sometimes referred to as “Earth’s twin,” Venus is not very similar to Earth beyond its size and composition. With an overwhelmingly toxic atmosphere filled with CO2 and a surface full of volcanoes, it’s definitely more like the Earth wrong double. Even a spaceship can only survive on its surface for no more than 2 hours before succumbing to the high pressure and temperature of a planet in the grip of the uncontrollable greenhouse effect.
But has Venus always been such a hellish place? For a long time, we hypothesized that Venus had an ocean of liquid water on its surface several billion years ago, but being closer to the Sun, an uncontrollable greenhouse effect set in: as as the Sun grew brighter over time, the more sunlight the radiation hit the surface of the planet and led to more water vapor in its atmosphere. This led to an increase in the evaporation of surface water. As the presence of water vapor warmed the planet even further, the intense radiation from the Sun split the molecules, causing hydrogen to leak into space. This left room for carbon escaping from the planet’s surface to combine with some of the remaining free oxygen and accumulate CO.2 in the atmosphere, trapping even more heat and causing an uncontrollable greenhouse effect.
Different models of Venus’ climate change have led to conflicting stories about its past. Some models that incorporate the effects of clouds responding to a warming or cooling of a planet have found that habitable conditions may have existed on the planet only 0.7 Gyr ago. In addition, unlike the Moon or the Earth, whose craters are altered or degraded, most of Venus’ craters are in perfect condition and also randomly distributed over its surface. From this, we believe that most of Venus’ geological history has been erased due to resurfacing events such as volcanic explosions and lava flows that have occurred very recently. This means that the surface we can see is very young (2). But if the water vapor in the atmosphere was broken down by radiation and most of the hydrogen escaped into space, that would mean there should be some the remains of oxygen in the atmosphere. So what happened to all the oxygen?
Let PACMAN eliminate all our doubts …
The authors of today’s article attempt to reconcile all the clues we have about Venus by using a coupled atmosphere-interior model called PACMAN (Planetary Atmosphere, Crust, and MANtle) to reproduce its climatic conditions over time in order to to see if the planet could ever have supported liquid water on its surface. All of this means that they keep track of conditions in both atmosphere and his interior while taking into account any effect one system has on the other. People have used these kinds of models to study Venus before, but none of them ever considered the possibility of having water on its surface.
The model is divided into two phases. Initially, Venus had an ocean of magma on its surface created from impacts with other pieces of space rock that were abundant during the formation of the planet. It was a giant layer of molten, sparkling rock that you definitely wouldn’t want to dip your toes into. As this ocean cooled and released gases into the atmosphere, the temperature dropped to a point where this ocean âfrozeâ and became a solid mantle, initiating phase two of the model. The authors calculate quantities such as the surface temperature, the amount of radiation emitted and absorbed by the planet, the amount of water vapor in the atmosphere, and the amount of water at the surface during both phases. They also keep track of the abundance of various molecules containing carbon, hydrogen and oxygen (carbon dioxide, water, O2, etc.) and calculate their flux between the atmosphere and the interior (i.e. how many of these molecules enter or exit over time). In addition, they also calculate the accumulation of 40Ar and 4It in the atmosphere which tells us about total magmatic activity and more recent magmatic activity, respectively. Together, these allow us to better determine whether a habitable or uninhabitable past is better able to predict the current atmosphere of Venus.
There are a lot of unknown parameters and initial conditions in the model such as CO2 pressure and planetary albedo (reflectivity), so they run their model 10,000,000 times to sample all 24 of these unknown parameters. Of all these passes, only 10% of them successfully ended in a state that reflects modern Venus atmospheric and surface conditions and chemical abundances. What’s interesting about these successful models is that they predict two different stories: some models tell us that Venus was NEVER habitable in its past, while others predict that Venus was temporarily habitable, which means that it could have contained an ocean up to ~ 100 meters deep on its surface between 0.04 and 3.5 Gyrs before succumbing to the uncontrollable greenhouse effect. This latter scenario should have left deposits of salt or minerals on the surface after all the water evaporated, leaving them potentially accessible to future remote sensing observations!
And the winner isâ¦
So which model is correct? Unfortunately, there is no definitive answer since the authors found that one or the other model is favored under different conditions. CO2 tends to make it difficult for hydrogen to escape if the water concentration is too low. Therefore, in habitable scenarios where no surface water is present, H2The O in the atmosphere has difficulty escaping because the CO2 continually dominates the atmosphere instead of being locked in the surface. This means that these scenarios cannot recover the modern Venus without water and oxygen that we see today. But if the CO2 is allowed to radiatively cool the upper atmosphere, then water can condense on the surface and CO2 is removed from the atmosphere and stored inside the planet, giving Venus a chance to have a period of increased water loss which can then initiate the uncontrollable greenhouse effect before CO2 is degassed into the atmosphere.
On the other hand, most modern models assume that when the ocean phase of the magma ends, virtually all of the carbon and water in the magma (so called birds) live in the atmosphere. But he is possible that some of these birds are trapped in the resulting solid mantle instead. If this is allowed, then far fewer models allow Venus to have been habitable. This is because it would take longer for water to then be released into the atmosphere, making it difficult to explain Venus’ current almost non-existent abundance of water.
The bottom line here is that either of these two scenarios is possible and consistent with modern observations. The winning scenario depends on our assumptions and the parameters of the model. While it may seem a bit anticlimactic, understanding and constraining the evolution of Venus is important for interpreting the atmospheres and stories of other exoplanets that may have undergone similar processes. JWST is (fingers crossed!) Going to launch in a little over a month, and it might have the ability to constrain what are the atmospheres of other so-called exo-Venuses, such as some of the planets in the TRAPPIST system. -1. Hopefully our studies of Venus and exo-Venus can symbiotically help shed light on planetary evolution!
Astrobite edited by Ishan mishra
Featured Image Credit: NASA / JPL-Caltech
About Katya Gozman
Salvation! I am a second year doctoral student at the University of Michigan. I am originally from the northwest suburbs of Chicago and did my undergraduate studies at the University of Chicago. There, my research mainly focused on gravitational lenses and galaxies, while also focusing on machine learning and neural networks. Today I am working on galaxy mergers and stellar halos, currently studying the spiral galaxy M94. I love doing astronomy outreach and often volunteer with a STEAM education nonprofit in Wisconsin called Geneva Lake Astrophysics and STEAM.