[Paper Review] Mantle Degassing Lifetimes through Galactic Time and the Maximum Age Stagnant-lid Rocky Exoplanets can Support Temperate Climates

Cayman T. Unterborn, Bradford J. Foley+5 more|arXiv (Cornell University)|Apr 8, 2022

High-pressure geophysics and materials126 references32 citations

TL;DR

This paper models the lifetime of mantle degassing in stagnant-lid rocky exoplanets by combining Galactic chemical evolution with Monte Carlo thermal evolution simulations, estimating how long planets can sustain a temperate climate via radiogenic heat from 40K, 232Th, 235U, and 238U. It finds that only 7 of 17 observed rocky exoplanets likely remain actively degassing today under pessimistic assumptions, with degassing lifetimes strongly dependent on initial heat-producing element (HPE) abundances and planetary mass.

ABSTRACT

The ideal exoplanets to search for life are those within a star's habitable zone. However, even within the habitable zone planets can still develop uninhabitable climate states. Sustaining a temperate climate over geologic ($\sim$Gyr) timescales requires a planet contain sufficient internal energy to power a planetary-scale carbon cycle. A major component of a rocky planet's energy budget is the heat produced by the decay of radioactive elements, especially $^{40}$K, $^{232}$Th, $^{235}$U and $^{238}$U. As the planet ages and these elements decay, this radiogenic energy source dwindles. Here we estimate the probability distribution of the amount of these heat producing elements (HPEs) that enter into rocky exoplanets through Galactic history, by combining the system-to-system variation seen in stellar abundance data with the results from Galactic chemical evolution models. Using these distributions, we perform Monte-Carlo thermal evolution models that maximize the mantle cooling rate. This allows us to create a pessimistic estimate of lifetime a rocky, stagnant-lid exoplanet can support a global carbon cycle and temperate climate as a function of its mass and when it in Galactic history. We apply this framework to a sample of 17 likely rocky exoplanets with measured ages, 7 of which we predict are likely to be actively degassing today despite our pessimistic assumptions. For the remaining planets, including those orbiting TRAPPIST-1, we cannot confidently assume they currently contain sufficient internal heat to support mantle degassing at a rate sufficient to sustain a global carbon cycle or temperate climate without additional tidal heating or undergoing plate tectonics.

Motivation & Objective

• To determine the maximum age a stagnant-lid rocky exoplanet can sustain a temperate climate via mantle degassing.
• To quantify the impact of galactic chemical evolution on the initial abundance distribution of heat-producing elements (HPEs) in rocky exoplanets.
• To assess the habitability of observed exoplanets by estimating whether they retain sufficient internal heat to power a global carbon cycle.
• To evaluate the role of mantle water content and tidal heating in extending degassing lifetimes.
• To provide a pessimistic but observationally constrained estimate of degassing lifetime based on stellar HPE abundance data.

Proposed method

• Combines stellar abundance data from the Hypatia catalog with Galactic chemical evolution models to estimate initial HPE distributions in exoplanets.
• Uses Monte Carlo sampling of HPE abundances (40K, 232Th, 238U/Eu) based on observed system-to-system variations.
• Applies updated thermal evolution models to simulate mantle cooling and degassing rates under maximum cooling assumptions.
• Defines degassing lifetime as the time when degassing rate drops below 10% of Earth’s current rate, scaled by surface area.
• Varying mantle water content and solidus temperature to assess their impact on degassing duration.
• Scales results to 17 observed rocky exoplanets with measured ages to assess current habitability potential.

Experimental results

Research questions

• RQ1 What is the maximum lifetime a stagnant-lid rocky exoplanet can sustain mantle degassing and temperate climate, given radiogenic heat decay?
• RQ2 How does the initial abundance of heat-producing elements (HPEs) vary across Galactic history and affect planetary thermal evolution?
• RQ3 Which observed exoplanets are likely to be actively degassing today, given pessimistic assumptions about internal heat and HPE content?
• RQ4 How does mantle water content influence degassing lifetime and the potential for long-term climate stability?
• RQ5 To what extent do tidal heating or plate tectonics need to be present to sustain habitable conditions on older exoplanets?

Key findings

• Only 7 out of 17 observed rocky exoplanets are predicted to be actively degassing today under pessimistic assumptions, indicating they likely still support a global carbon cycle.
• Degassing lifetimes are strongly dependent on initial HPE abundances, with planets forming in early Galactic history (high HPE) having longer potential habitable lifetimes.
• For Earth-mass planets, degassing lifetime is maximized when initial 40K abundance is at Earth-like levels; doubling or halving 40K shifts Agemax by ±2 Gyr.
• Mantle water content has a complex effect: while it can lower solidus temperature and promote melting, it may also suppress degassing at high concentrations (>0.2 wt%) due to increased pressure.
• Planets with high initial HPE content (e.g., 2x Earth’s 40K) can sustain degassing for up to ~10 Gyr, while low-HPE planets may cease degassing within 3–5 Gyr.
• TRAPPIST-1 planets and other older systems cannot be confidently assumed to sustain degassing or temperate climates without additional heat sources like tidal heating or plate tectonics.