Researchers announced at the Rocky Worlds conference, and in a preprint posted to arXiv, that immense tidal waves of lava might rage on hot alien worlds. And these molten rock surges could distort the measurements of faraway planet atmospheres.
The Sun and Moon cause tides on Earth, but these tidal bulges would be raised by the powerful gravitational forces acting on planets in tight orbits around their stars. For instance, lava tides on the scorching exoplanet 55 Cancri e—a rocky world that circles its star every 18 hours—could rise several hundred meters high and travel at a runner’s pace, says Mohammad Farhat, a planetary scientist at the University of California, Berkeley, who presented the modeling results.
Lava tides are not merely a curious detail. The waves may rapidly spread warmth across the planets, causing temperature swings of hundreds or even thousands of degrees, which might complicate the data astronomers frequently use to infer the presence of atmospheres on exoplanets. “This is precisely what is interesting, new, and potentially testable,” says planetary astronomer Laura Kreidberg of the Max Planck Institute for Astronomy, who was not involved in this research. “I held a naive view that lava flows moved too sluggishly to have an observable effect, but this new work suggests otherwise.”
When planet scientists investigate tides on lava worlds, they typically treat lava as a fluid solid. Under tidal flexing, solids release heat generated by friction, whereas fluid, less viscous materials emit less. But lava is not a fluid solid, Farhat states. It is a viscous liquid that flows and churns in a way that releases much more warmth under tidal stress. Consequently, scientists have underestimated the amount of heat tides can pump into a lava planet, Farhat explains.
In computer simulations, Farhat and his UC Berkeley colleague Eugene Chang examined lava waves driven by slightly eccentric orbits—deviations of a few percent from a perfect circle. This doesn’t seem incredible; Mercury’s eccentricity exceeds 20%. Under these conditions, Farhat and Chang found that the ebb and flow of lava waves release enough energy to melt deep layers of the planets. On Earth-sized worlds, this could potentially liquefy everything between the surface and the core. Farhat says this is surprising because researchers usually picture lava worlds as possessing only shallow puddles of magma on their star-facing hemispheres. (Worlds close to Earth are expected to be “tidally locked,” with hemispheres experiencing perpetual day and night.)
Where the lava waves overlap, Farhat and Chang discovered they can merge and rise into scorching blobs of liquid rock, hundreds of degrees hotter than the surrounding material. This extra warmth might be visible to NASA’s JWST orbiting observatory as erratic heat flashes and wandering hot spots, meaning lava tides could be detectable. But this could also complicate the search for alien atmospheres.
Scientists often search for alien atmospheres by measuring the temperatures of the planets’ daysides. The presence of an atmosphere would distribute heat to the night side, making the dayside appear cooler than expected for bare rock. But if lava waves can melt deep magma oceans and create roving pockets of hot material, it raises questions about whether this “will mimic an atmospheric signal,” says planetary scientist Joanna Teske of the Carnegie Earth and Planetary Sciences Laboratory, who was not part of the study.
Lava tides may have already misled scientists. 55 Cancri e exhibits dramatic 1000° temperature variations that have remained unexplained for about a decade. Researchers have considered various explanations, ranging from lava rain to reflective rocky clouds, but have found no consensus. However, if 55 Cancri e’s orbit is sufficiently eccentric—and scientists don’t know if it is—lava tides could account for the planet’s fluctuating temperatures, states Jayesh Patel, a planetary scientist at the Technical University of Denmark who uses the JWST to observe 55 Cancri e. “If true, this could explain our observations.”
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