http://www.sciencemag.org/news/2019/08/cataclysmic-collision-could-explain-jupiter-s-fuzzy-core

Jupiter harbors a deep mystery: Rather than the distinct core scientists expected, it has a fuzzy center, according to recent observations by NASA’s Juno spacecraft, which extends for up to half the planet’s radius.

One possible explanation is a cataclysmic head-on collision some 4.5 billion years ago between a young Jupiter and one of the many large protoplanets that likely populated the early solar system, planetary scientists report today in Nature. Such an event would have seen Jupiter absorb the protoplanet, which would have been 10 times Earth’s mass, causing their two dense cores to combine and diffuse after only 10 hours, as seen in the simulation above. Over time, cooling and winds could sweep some of the fuzzy core upward, explaining Jupiter’s puzzling enrichment in elements like carbon and nitrogen close to its surface.