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Title: Carbon Cycle Imbalances on Arid Terrestrial Planets with Implications for Venus
Authors: Haskelle T. White-Gianella and Joshua Krissansen-Totton
Authors’ Institution: University of Washington
Status: Published in PSJ

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, which acts as a built-in climate control system. This cycle begins with volcanic eruptions releasing carbon dioxide (CO₂₎, 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 CO₂ 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 the planet’s 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 CO₂ move between a planet’s interior, oceans, and atmosphere over 4.5 billion years. The model notably includes the following:

• A sophisticated weathering model that 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 that 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 (Figure 1).

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 CO₂ release, causing CO₂ buildup and warming of 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 could have transitioned into its current inhospitable state. If Venus had formed with an initial water inventory below this critical threshold of 20–50% of Earth’s ocean mass, then its carbon cycle would have become unbalanced, creating the CO₂ 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 the planet 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.

Original astrobite edited by Natalie Price.

About the author, Jared Bull:

I am a 2nd-year PhD student at Johns Hopkins University. I study brown-dwarf variability and am interested in using time-series observations to uncover dynamic processes within their atmospheres. In my free time I like to read, cook, and do astrophotography.

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NASA is once again sending scientific missions to the Moon. But under the new commercial framework for getting there, researchers need to invent a new method for how to pick where to land.

A New Process for Picking Sites

NASA’s Commercial Lunar Payload Services program is a bold innovation in organizing robotic missions to the Moon. Under this model, rather than keeping the mission process entirely in-house, the agency selects the scientific mission they want to support then simply buys a ride to the lunar surface from a commercial vendor. But, how do you pick a landing site before you know exactly how you’re getting there?

The Lunar Vulkan Imaging and Spectroscopy Explorer (Lunar-VISE) mission, set to launch in mid-2028, was one of the first programs to go through this new landing site selection process. After being officially selected in 2022, the team had just nine months to nail down exactly where their mission should touch down. A recent article led by Jean-Pierre Williams (University of California, Los Angeles) detailed exactly how the team accomplished this task and presented some lessons they learned along the way.

No Parking Lots on the Moon

Picking a landing site is no small task: for context, selecting the landing sites for each of the most recent Mars rovers took five years and multiple community workshops. The Lunar-VISE team therefore took a streamlined and logic-driven approach. They started by explicitly laying out their science objectives: Lunar-VISE’s purpose is to investigate some strangely silicate-rich volcanic formations called the Gruithuisen domes, so they had to land either on or near those. They also had to have access to boulders that were excavated from deep within the domes during a crater formation event, so they had to pick a spot with boulders relatively nearby.

In addition to objectives, they also laid out a slew of constraints. Although they needed boulders nearby, they couldn’t have too many boulders, or any particularly large boulders, since those could pose hazards for landing. There also couldn’t be any steep slopes in the field. Most of the area had to maintain a line of sight with a supporting orbiter for communications, so they couldn’t set down near any ridges. Also, they had to consider how the terrain would cast shadows as the lunar day progressed, since they didn’t want their mission cut short by freezing temperatures and a lack of solar power. As the team quipped, “many interesting, high-science value locations on the Moon may not be in proverbial ‘parking lots.'”

By combing through archival data and working with the team that operates the Lunar Reconnaissance Orbiter Camera, a key asset for making detailed maps of the Moon, the researchers found the perfect spot: alongside a 190-meter-diameter crater named Mareta. This feature is named after Mareta West, one of the first female planetary geologists at the United States Geological Survey and the scientist who selected the landing site for Apollo 11’s historic touchdown.

In addition to presenting their landing site, the researchers offered a handful of lessons to others who may someday be in the fortunate and thrilling position of needing to pick a landing spot on the Moon. For one, they urged teams to coordinate with the Lunar Reconnaissance Orbiter specialists as early as possible, since processing the imaging data for lander-quality maps is a specialized and intensive task. For another, they recommended that all of the software tools used be made publicly available so that future missions don’t need to duplicate their work.

As this article laid out, the Commercial Lunar Payload Services program and Lunar-VISE take us one step closer to making missions to the Moon routine — but we’re still figuring out exactly how to do that.

Citation

“Lunar-VISE Landing Site Selection and Characterization at Mons Gruithuisen Gamma,” Jean-Pierre Williams et al 2026 Planet. Sci. J. 7 161. doi:10.3847/PSJ/ae7d2c

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