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

Jupiter is composed almost entirely of hydrogen and helium. The amounts of each closely match the theoretical amounts in the primordial solar nebula. But it also contains other heavier elements, which astronomers call metals. Even though metals are a small component of Jupiter, their presence and distribution tell astronomers a lot.

Jupiter’s metal content and distribution means the planet ate a lot 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 transformed our understanding of Jupiter’s formation and evolution. One of the characteristics 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 composition of the planet.

When Jupiter formed, it began by accrete rocky material. A period of rapid accretion of solar nebula gas followed, and after several million years Jupiter became the juggernaut it is today. But there is an important question regarding the initial period of rock accretion. Did it accrete larger rock masses like planetesimals? Or has he accumulated materials the size of a pebble? According to the answer, Jupiter formed on different time scales.

NASA’s Juno spacecraft captured this view of Jupiter during the giant planet’s 40th close pass on February 25, 2022. The large dark shadow on the left side 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 attempted to answer that question. It is called “Jupiter’s inhomogeneous envelope», and it is published in the journal Astronomy and Astrophysics. The lead author is Yamila Miguel, assistant professor of astrophysics at the Leiden Observatory & The Netherlands Institute for Space Research.

We’re getting more and more used to gorgeous images of Jupiter thanks to the Juno spacecraft JunoCam. But what we see is only superficial. All of these haunting images of clouds and storms are just the outermost thin 50 km (31 miles) layer of the planet’s atmosphere. The key to Jupiter’s formation and evolution is buried deep in the planet’s atmosphere, which lies tens of thousands of miles deep.

The Juno mission is helping us better understand 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 is helping us better understand 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 widely believed 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 probe metals in the planet’s atmosphere using Juno’s Gravity Science experiment. The presence and distribution of pebbles in the planet’s atmosphere plays a central role in understanding Jupiter’s formation, and the Gravity Science experiment measured the dispersion of pebbles throughout the atmosphere. Prior to Juno and its Gravity Science experiment, there was no precise data on Jupiter’s gravity harmonics.

Researchers have discovered 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 total between 11 and 30 earth masses.

With the data in hand, the team built models of Jupiter’s internal dynamics. “In this paper, we bring together the most comprehensive and diverse collection of interior models of Jupiter to date and use it to study the distribution of heavy elements in the planetary envelope,” they write.

The team created two sets of models. The first set consists of 3-layer models and the second of dilute-core models.

The researchers created two types of contrasting models of Jupiter. 3-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 dilute-core models, the metals of the inner core are mixed in the middle region, resulting in a dilute core.

“There are two mechanisms by which a gas giant like Jupiter acquires metals during its formation: through the accretion of small pebbles or larger planetesimals,” said lead author Miguel. “We know that once a small planet gets big enough, it starts pushing pebbles. The metal richness inside Jupiter that we see now is impossible to achieve before then. with only pebbles as solids when Jupiter formed.Planetsimals are too big to block, so they must have played a role.

The abundance of metals inside Jupiter decreases as one moves away from the center. This means a lack of convection in the planet’s deep atmosphere, which scientists thought was present. “Previously, we thought Jupiter had convection, like boiling water, which made it completely mixed,” Miguel said. “But our discovery shows differently.”

“We strongly demonstrate that the abundance of heavy elements is not homogeneous in the envelope of Jupiter”, write the authors in their article. “Our results imply that Jupiter continued to accrete heavy elements in large quantities as its hydrogen-helium envelope expanded, contrary to predictions based on pebble isolation mass in its simplest incarnation, favoring instead hybrid models based on planetesimals or more complex.”

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

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

The team’s findings also extend to the study of gaseous exoplanets and efforts to determine their metallicity. “Our result…provides a basic example for exoplanets: an inhomogeneous envelope implies that the observed metallicity is a lower bound on the planet’s overall metallicity.”

In the case of Jupiter, there was no way to determine its metallicity from a distance. It was not until the arrival of Juno that scientists were able to indirectly measure metallicity. “Therefore, metallicities inferred from remote atmospheric observations on exoplanets might not represent the overall metallicity of the planet.”

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


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