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Can a huge impact explain Jupiter’s quirky core?

28 August 2019

Simulations of the gas giant colliding head on with another planet during the early days of the solar system reproduce the present-day Jovian density profile.

As planetary cores go, Jupiter’s is hard to explain. In most planetary-formation models of gas giants, dust grains made of elements heavier than hydrogen and helium collide, clump, and grow, eventually squeezing into a compact core that is later surrounded by a vast envelope of H and He gas. However, the latest observations from NASA’s Juno space probe suggest that heavy elements in Jupiter extend beyond its core and can be found at almost half of the planet’s radius. One hypothesis contends that a giant impact early in Jupiter’s history may have dispersed the heavy elements. Shang-Fei Liu of Sun Yat-sen University in China and his colleagues analyzed the impact hypothesis more carefully by simulating a collision between young Jupiter and another still-forming planet during the early solar system about 4.5 billion years ago, when such events were more common. The model successfully matches Jupiter’s present-day diluted core structure.

Computer model of density before, during, and after an impact

In the researchers’ simulation, an 8 Earth mass (M) silicate–ice rock hit Jupiter’s compact 10 M silicate–ice core head on. The series of images above shows how, in Jupiter’s primordial core, the collision induces turbulent mixing, which smears the heavy elements into the surrounding gaseous envelope a mere 10 hours after the collision. A second set of simulations computes the thermal evolution of the post-impact planet over its 4.56-billion-year life span. To produce a density profile consistent with Jupiter’s diluted core, the authors assume a central temperature of 30 000 K after the head-on impact. Determining whether that starting temperature is reasonable will take more research.

Other mechanisms that have been proposed to explain Jupiter’s core include the mixing of H, He, and heavy elements during planetary formation and the erosion of a dense core by convective flows post-formation. Whereas the new result provides compelling evidence for a collision-induced dilute core in Jupiter, it doesn’t rule out the other hypotheses just yet. (S.-F. Liu et al., Nature 572, 355, 2019; thumbnail photo courtesy of NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill.)

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