Airless Exoplanet's Mantle Could Flow in Halves - Eos

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Forty-five light-years from Earth sits an exoplanet with no air: LHS 3844b. It orbits so close to its star that gravity keeps one half of the planet in daylight and casts the other side in permanent night. Last year, a team of astronomers found that the planet, slightly larger than Earth and probably rocky, also likely lacks an atmosphere.

With no atmosphere in the way, the team determined that crust reaches a scorching 770°C on the dayside and near absolute zero on the nightside. Now, a team of geodynamics researchers is trying to explore beneath the planet’s surface. The team used the measured surface temperatures as observational constraints to mantle convection models and found that LHS 3844b’s lithosphere could flow in a half-and-half pattern: Material in one hemisphere flows mostly upward from the core to the crust and mostly downward on the opposite side of the planet.

“We are interested in whether this large difference in temperature from the dayside to the nightside may also lead to a dichotomy in the interior metal flow,” Tobias Meier, lead researcher on the project, noted in a presentation he recorded for AGU’s Fall Meeting 2020. According to the team’s models, “the surface temperature dichotomy can lead to this hemispheric tectonic regime, which is a tectonic regime that is absent in our own solar system at present day.” Meier, a geodynamics doctoral student at the University of Bern in Switzerland, will discuss this research on 8 December.

Half Up, Half Down

Meier and his team used simplified geodynamics models to test four mantle convection scenarios. Two tested weak lithospheric material that flows easily, and the other two used stronger but still mixable material, like wet sand or concrete. “We also vary the heating mode,” Meier said, “where in one set of models we have basal heating, which means that all the heat that we generate comes from within the core. And in a second set of models we also add internal heating to that, which means that we generate heat within the mantle that could come, for example, from the decay of radioactive elements.”

“A hemispheric tectonic regime is reasonable and is an exciting extension of known tectonic regimes.”

The team found that only one of the combinations (with strong material and internal heat) produced mantle convection like Earth’s, with upwelling and downwelling material on both sides of the planet. The other three scenarios produced mantle convection that favored upwelling on one side of the planet and downwelling on the other—which side was which differed among cases.

“The hypothesis that under the extreme conditions of LHS 3844b, similar exoplanets will develop a hemispheric tectonic regime is reasonable and is an exciting extension of known tectonic regimes,” said Rene Gassmoeller, a computational geodynamicist at the University of Florida in Gainesville who was not involved with this research.

LHS 3844b is so far the only exoplanet suspected to lack an atmosphere. “As the composition and thickness of the lithosphere of LHS 3844b [are] unknown,” Gassmoeller said, “it is appropriate to start with the simplest reasonable model and test two end-member cases, like a weak or strong lithosphere,” which the team of researchers did.

A Novel Tectonic Regime

LHS 3844b’s extreme environment—one side always facing its sun, a large temperature change between day and night, and no atmosphere—is key for developing half-and-half mantle convection, Gassmoeller said. “Since these conditions are very strict, it is unlikely any planets in our solar system fulfilled them at any point in the history of the solar system. Mars is too far out. Earth and Venus have atmospheres and are not tidally locked. Mercury is tidally locked, but not in a synchronous rotation, so all surface locations still experience day and night.”

“Constraining rheology is crucial but very challenging.”

One way to figure out whether LHS 3844b has hemispheric tectonics, Meier said, is with a higher-resolution map of surface temperature. “Future observations may be able to distinguish these different regimes if, for example, we can link upwellings on one side to regions of enhanced outgassing or volcanism.” If the high dayside mantle temperature doesn’t melt the surface too much, Gassmoeller said, volcanic activity or outgassing could show up as hot spots.

As scientists learn more about LHS 3844b, especially what it’s made of, geodynamics models of its subsurface can become more refined. “The biggest unknown is rheology,” said planetary scientist Miki Nakajima, referring to material properties that govern how matter flows and deforms. “Grain size and volatiles can also affect rheology and mantle convection patterns.” Nakajima works at the University of Rochester in New York and was not involved with this study.

“Constraining rheology is crucial but very challenging. For this reason, we still do not know when Earth’s plate tectonics started. By understanding Earth, we can refine our model predictions for tectonic regimes of exoplanets.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

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