Title: Carbon Cycle Imbalances on Arid Terrestrial Planets with Implications for Venus
Authors: Haskelle T. White-Gianella and Joshua Krissansen-Totton
First Author’s Institution: Department of Earth and Space Sciences/Astrobiology Program, University of Washington, Seattle, WA 98195, USA
Status: Published in The Planetary Science Journal [open access]
The Carbon Cycle Thermostat
When discussing the habitability of planets, we usually focus on the habitable zone, the region around a star where a planet can host a temperature suitable for supporting liquid water on its surface. Yet, being in the right place doesn’t necessarily guarantee that a planet will be habitable.
A planet also needs the right atmospheric composition to sustain the necessary temperatures for hosting liquid water. A key mechanism found on Earth for maintaining this composition is the geologic carbon cycle, acting as a built-in climate control system. This cycle begins with volcanic eruptions releasing carbon dioxide (\(\mathrm{CO_2}\)) into the atmosphere, which then dissolves in rainwater and forms a weak acid that weathers rocks on continents. The weathering products wash into oceans, where they form carbonate rocks, locking carbon away. Eventually, plate tectonics recycles some of that carbon back to volcanoes. The weathering process works faster when the planet is warmer, thus regulating the amount of \(\mathrm{CO_2}\) and stabilizing the climate over long periods of time (but it should be noted that it can take up to a few hundred thousand years to rebalance this slow carbon cycle through the weathering process).
An important caveat is that the weathering process requires sufficient liquid water on its surface, so what happens in the case for planets with shallower oceans? Low-mass M dwarfs, the most common type of star found in the galaxy, are expected to host less massive disks and therefore lower water inventories for forming planets. If more “dry” planets are indeed a more likely outcome of planet formation, we’d want to know how their arid conditions influence their long-term evolution and habitability.
Creating Carbon-Tracking Models
The authors built a model that tracks how water and \(\mathrm{CO_2}\) move between a planet’s interior, oceans, and atmosphere over 4.5 billion years. The model notably includes:
- A sophisticated weathering model (MAC) which accounts for how runoff can limit rock weathering, improving upon previous models that only included a temperature dependence on weathering
- Wind-driven evaporation limits that influence the amount of water evaporation in addition to sunlight-driven evaporation
- Multiple deep water cycle parameterizations which explore how water moves between the interior and the surface
The model tests for four initial surface water inventories (0.1%, 1%, 10%, and 100% of Earth’s oceans), and then simulates how the surface water, atmosphere, and climate evolve over time.
A Critical Water Threshold
The models reveal that planets require at least 20-50% of Earth’s ocean mass to maintain a balanced carbon cycle. Below this threshold, the carbon cycle becomes unbalanced, leading to devastating consequences for habitability.
For the more arid planets, models reveal a concerning runaway process. Limited surface water reduces precipitation and slows rock weathering. Weathering cannot keep up with volcanic \(\mathrm{CO_2}\) release, causing \(\mathrm{CO_2}\) buildup and warming the atmosphere. The cycle repeats, driving runaway warming until all water evaporates.
Important Implications (Within and Outside Our Solar System)
Looking within the Solar System, the results offer an explanation for how a potentially habitable Venus prior in history could have transitioned into its current, inhospitable state. If Venus had formed with an initial water inventory below this critical 20-50% Earth ocean mass threshold, then its carbon cycle would have become unbalanced, creating the \(\mathrm{CO_2}\) inferno seen on Venus today.
In future exoplanet studies, telescopes like the Habitable Worlds Observatory might detect ocean glint (specular reflection from liquid water) or measure land fraction from light curves. Finding a planet in the habitable zone, but with limited ocean coverage would likely be bad news, as it could be quietly losing its grip on habitability. Astronomers may want to be more careful when looking at seemingly-promising planets within their habitable zones.
Astrobite edited by Natalie Price
Featured image credit: NASA
