In recent years, the Juno mission revealed that Jupiter's core was much more diffuse than astronomers had expected.

One theory is that "within a few million years" of its formation, Jupiter experienced a head-on collision with a planetesimal of about $10M_{⊕}$, adding a lot more mass to its core from the silicate planetesimal, but also causing the core's contents to be broken up and mixed with the inner envelope.

The models used in this theory placed Jupiter at a distance of 5.2 AU from the Sun, which is approximately the same as its semi-major axis in the present day. The planetesimal in question would be at the higher end of the valid mass range for a Super-Earth.

Now, according to the Grand Tack theory, Jupiter originally formed at a distance of 3.5 AU and migrated inwards toward the Sun, before gravitational interactions with Saturn caused the two planets to move outward and brought Jupiter to its present-day orbit. This would all have occurred in a 700,000 year time period, "approximately 2-3 million years" after Jupiter's formation.

So far, the two models look quite compatible with each other, with the collision occuring after Jupiter settles into its 5.2 AU orbit. But there is one detail I'm not sure about. Here comes the question:

  • Descriptions of the Grand Tack theory describe Jupiter as scattering early planetesimals as its gravity disturbed their orbits. Some collided with each other, some were propelled into the Sun... Is it more likely that one of these collided with Jupiter itself at the head-on angle needed for the core-warping collision? Which would mean the impact occurred before Jupiter reached its final orbit.

In addition...

  • The few million years" timescale is quite vague. Does anyone know any additional detail which might suggest the impact occurred prior to the Grand Tack's timeframe?

Reference (paywalled, so I can only link to the abstract):

Liu, S. F., Hori, Y., Müller, S., Zheng, X., Helled, R., Lin, D., & Isella, A. (2019). The formation of Jupiter’s diluted core by a giant impact. Nature, 572(7769), 355-357.

Reference (not paywalled):

Guillot, T. (2019). Signs that Jupiter was mixed by a giant impact.


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