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 by Susanna Kohler for AAS Nova Washington DC (SPX) Nov 16, 2020
A critical component of a habitable planet is its ability to stabilize its climate over long timescales. In a new study, scientists explore whether a world covered in water can keep its climate as stable as an Earth-like, continental world. Over the span of millions of years, a planet's host star might gradually dim or brighten, or the planet's volcanic outgassing patterns might slowly shift. If evolution like this also caused dramatic changes in the overall climate of a planet, this would spell bad news for habitability: the planet might not be able to retain liquid water over timescales long enough for life to form and evolve. So how do you keep a climate stable against these slow shifts? One crucial factor is having a carbonate-silicate cycle. This cycle dictates how carbon is moved around a planet, sometimes burying it deep below the planet's surface, sometimes releasing it out into the atmosphere. On Earth, a simplified description of the carbonate-silicate cycle is: + Atmospheric carbon dioxide dissolves in rainwater, forming carbonic acid, which falls to the ground. + Over long timescales, weathering from this weak acid dissolves silicate rocks, and the dissolved products are carried to the oceans, where they accumulate. + Subduction of the seafloor carries the products to great depths, where they reform into silicates and gaseous carbon dioxide. + The carbon dioxide is restored to the atmosphere by volcanism.
When Negative Feedback Is a Good Thing This cycle only stabilizes the climate against very slow external changes, like a star's gradual dimming - so this isn't the solution to our current global warming crisis caused by fossil fuel emissions. Nonetheless, it's an important component when considering the general habitability of other worlds. In a new study, scientists Benjamin Hayworth and Bradford Foley (Pennsylvania State University) consider how this cycle might be affected by the geography of a planet. Will worlds covered in water do a better or worse job of keeping their climates stable?
Stability from the Sea By accounting for these different dependencies in climate and weathering models, the authors show that water worlds - which are dominated by seafloor weathering - are actually better than their continental counterparts at stabilizing planet-wide temperatures against gradual changes in host star luminosity. This means that temperate climates can exist over a wider range of stellar luminosities for water worlds than for continental planets, and they can stay stable for longer - indicating that these worlds may be worthwhile targets in the search for life.
Research Report: "Waterworlds May Have Better Climate Buffering Capacities than Their Continental Counterparts"
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