Jupiter is up to 9% rock and metal meaning it ate a lot of planets in its youth

Jupiter is made up almost entirely of hydrogen and helium. The amounts of each closely match the theoretical quantities in the original solar nebula. But it also contains other heavier elements, which astronomers call metals. Although metals are a small part of Jupiter, their presence and distribution tell astronomers a lot.

Jupiter’s metal content and distribution means the planet ate plenty of rocky planetesimals in its youth, according to a new study.

Since NASA’s Juno spacecraft reached Jupiter in July 2016 and began collecting detailed data, it has changed our understanding of Jupiter’s formation and evolution. One of the features of the mission is the Gravity Science instrument. It sends radio signals back and forth between Juno and the Deep Space Network on Earth. The process measures Jupiter’s gravitational field and tells researchers more about the planet’s composition.

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When Jupiter formed, it started accreting rocky material. Then followed a period of rapid gas accretion from the solar nebula, and after many millions of years Jupiter became the behemoth it is today. But there is an important question regarding the initial period of rocky accretion. Has it grown larger masses of rock such as planetesimals? Or has the pebble-sized material accumulated? Depending on the answer, Jupiter emerged on different time scales.

NASA’s Juno spacecraft captured this image of Jupiter during the mission’s 40th close pass past the giant planet on Feb. 25, 2022. The large, dark shadow to the left of the image was cast by Jupiter’s moon Ganymede. Image data: NASA/JPL-Caltech/SwRI/MSSS Image processing by Thomas Thomopoulos

A new study sought to answer that question. It’s called “Jupiter’s Inhomogeneous Envelope” and it’s published in the journal Astronomy and Astrophysics. The lead author is Yamila Miguel, an assistant professor of astrophysics at the Leiden Observatory and the Netherlands Institute for Space Research.

Thanks to the Juno spacecraft’s JunoCam, we are getting used to beautiful images of Jupiter. But what we see is only skin-deep. All those mesmerizing images of the clouds and storms are just the thin 50 km (31 mi) outer layer of the planet’s atmosphere. The key to Jupiter’s formation and evolution is buried deep in the planet’s atmosphere, which is tens of thousands of miles deep.

The Juno mission helps us gain a better understanding of Jupiter's mysterious interior.  Image: by Kelvinsong - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=31764016
The Juno mission helps us gain a better understanding of Jupiter’s mysterious interior. Image: by Kelvinsong – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=31764016

It is generally accepted that Jupiter is the oldest planet in the solar system. But scientists want to know how long it took to form. The authors of the paper wanted to investigate the metals in the planet’s atmosphere using Juno’s Gravity Science experiment. The presence and distribution of pebbles in the planet’s atmosphere play a central role in understanding Jupiter’s formation, and the Gravity Science experiment has measured the distribution of pebbles through the atmosphere. Before Juno and his Gravity Science experiment, there was no precise data on Jupiter’s gravitational harmonics.

The researchers found that Jupiter’s atmosphere is not as homogeneous as previously thought. There are more metals near the center of the planet than in the other layers. In total, the metals add up to between 11 and 30 Earth masses.

With data in hand, the team constructed models of Jupiter’s internal dynamics. “In this paper, we collect the most comprehensive and diverse collection of Jupiter interior models to date and use them to study the distribution of heavy elements in the planet’s shell,” they write.

The team has created two sets of models. The first set is three-layer models and the second is thin-core models.

The researchers created two contrasting types of models of Jupiter. The three-layer models contain more distinct regions, with an inner core of metals, a middle region dominated by metallic hydrogen, and an outer layer dominated by molecular hydrogen (H2.). In the thin-core models, the metals of the inner core are blended to the midrange resulting in a thinned core.

“There are two mechanisms for a gas giant like Jupiter to acquire metals during its formation: through the accretion of small pebbles or larger planetesimals,” said lead author Miguel. “We know that once a baby planet is big enough, it starts pushing out pebbles. The richness of metals in Jupiter that we see now is impossible to achieve for that. So we can rule out the scenario with only pebbles as solids during the formation. from Jupiter. Planetesimals are too big to be blocked, so they must have played a role.”

The abundance of metals in Jupiter’s interior decreases with distance from the center. That means a lack of convection in the planet’s deep atmosphere, which scientists thought was present. “Previously, we thought Jupiter has convection, like boiling water, which makes it completely mixed,” Miguel said. “But our finding shows otherwise.”

“We strongly demonstrate that the abundance of heavy elements is not homogeneous in Jupiter’s envelope,” the authors write in their paper. “Our results imply that Jupiter continued to accrete heavy elements in large quantities as its hydrogen-helium shell grew, contrary to predictions based on the siliceous isolation mass in its simplest incarnation, preferring instead planetesimal-based or more complex hybrid models.”

Artistic representation of a protoplanet forming in the accretion disk of a protostar Credit: ESO/L.  Calcada http://www.eso.org/public/images/eso1310a/
Artistic representation of a protoplanet forming in the accretion disk of a protostar Credit: ESO/L. Calcada http://www.eso.org/public/images/eso1310a/

The authors also conclude that Jupiter did not mix by convection after it formed, even when it was young and hot.

The team’s results also extend to the study of gaseous exoplanets and efforts to determine their metallicity. “Our result … provides a basic example for exoplanets: a non-homogeneous shell implies that the observed metallicity is a lower bound for the planet’s bulk metallicity.”

In the case of Jupiter, there was no way to determine the metallicity from a distance. It wasn’t until Juno arrived that scientists were able to measure metallicity indirectly. “Therefore, metallicities derived from external atmospheric observations in exoplanets may not represent the planet’s bulk metallicity.”

When the James Webb Space Telescope begins scientific operations, one of its tasks is to measure the atmospheres of exoplanets and determine their composition. As this work shows, the data Webb provides may not capture what’s happening in the deeper layers of giant gas planets.

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