Rock weathering alters a planet’s atmospheric chemistry

Swiss scientists are investigating how the weathering of rocks influences the climate and atmospheric chemistry of planets, altering their habitability.

The conditions on Earth are perfect to support life because its temperatures are neither too hot nor too cold and maintain liquid water. These factors depend on the atmospheric chemistry of the planet. Too little or too much of certain gases, such as carbon dioxide, and Earth could become uninhabitable. Therefore, when scientists look for potentially habitable planets, a key component is their atmosphere.

On terrestrial planets like Mars, Venus, or Earth, primitive atmospheres are lost. Instead, their remaining atmospheres are strongly influenced by surface geochemistry. Processes like the weathering of rocks alter the composition the atmosphere and thereby influence the habitability of the planet.

How rocks change our atmosphere

As published in The Planetary Science Journal, a team of scientists, led by Kaustubh Hakim of the Centre for Space and Habitability (CSH) at the University of Bern and PlanetS, is investigating the role of atmospheric chemistry in increasing a planet’s habitability.

Hakim explains: “We want to understand how the chemical reactions between the atmosphere and the surface of planets change the composition of the atmosphere. On Earth, this process – the weathering of silicate rocks assisted by water – helps to maintain a temperate climate over long periods of time.

“When the concentration of CO2 increases, temperatures also rise because of its greenhouse effect. Higher temperatures lead to more intense rainfall. Silicate weathering rates increase, which in turn reduce the CO2 concentration and subsequently lower the temperature.”

Using computer simulations, the team tested how different conditions affect the weathering process. For example, they found that even in very arid climates, weathering can be more intense than on Earth if the chemical reactions occur sufficiently quickly. According to Hakim, rock types also influence the process and can lead to very different weathering rates. The team also found that at temperatures of around 70°C, contrary to popular theory, silicate weathering rates can decrease with rising temperatures.

Implications for detecting habitable planets

If astronomers ever find a habitable planet, it will likely be in what they call the ‘Goldilocks Zone’. This zone is the area around a star, where the dose of radiation would allow water to be liquid.

Professor of astronomy and planetary sciences at the University of Bern and member of PlanetS, Kevin Heng, said: “Geochemistry has a profound impact on the habitability of planets in the habitable zone.” As the team’s results indicate, increasing temperatures could reduce weathering and its balancing effect on other planets. What would potentially be a habitable world could turn out to be a hellish greenhouse instead.

As Heng further explains, understanding geochemical processes under different conditions is not only important to estimate the potential for life, but also for its detection. Heng added: “Unless we have some idea of the results of geochemical processes under varying conditions, we will not be able to tell whether bio-signatures – possible hints of life like the Phosphine that was found on Venus last year – indeed come from biological activity.”

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