Jupiter is composed almost entirely of hydrogen and helium. The amounts of each agree with the theoretical amounts in the primordial solar nebula. But it also contains other heavier elements, which astronomers call metals. Although metals are a small component of Jupiter, their presence and distribution say a lot to astronomers.
According to a new study, Jupiter’s metal content and distribution means the planet ate a lot of rocky planetesimals in its youth.
Since NASA’s Juno spacecraft arrived at Jupiter in July 2016 and began collecting detailed data, it’s been transforming our understanding of Jupiter’s formation and evolution. One of the mission’s features is the Gravity Science instrument. It sends radio signals 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 by accumulating rocky material. A period of rapid accretion of gas from the solar nebula followed, and after many millions of years, Jupiter became the giant it is today. But there is a significant issue regarding the initial period of rock accretion. Did it aggregate larger masses of rocks like planetesimals? Or aggregated material the size of pebbles? Depending on the answer, Jupiter formed on different timescales.
A new study set out to answer this question. It is titled “Jupiter’s Nonhomogeneous Envelope” and was published in the journal Astronomy and Astrophysics. The lead author is Yamila Miguel, assistant professor of astrophysics at the Leiden Observatory and the Dutch Institute for Space Research.
We are getting used to beautiful images of Jupiter thanks to JunoCam from the Juno spacecraft. But what we see is only superficial. All these fascinating images of clouds and storms are just the thin 50 km (31 mi) 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 kilometers deep.
It is widely accepted that Jupiter is the oldest planet in the Solar System. But scientists want to know how long it took to form. The paper’s authors wanted to probe 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 the formation of Jupiter, and the Gravity Science experiment measured the scattering of pebbles throughout the atmosphere. Before Juno and her Gravity Science experiment, there was no accurate data on Jupiter’s gravitational harmonics.
The researchers found that Jupiter’s atmosphere is not as homogeneous as previously thought. More metals are near the center of the planet than in the other layers. Altogether, the metals add up to between 11 and 30 Earth masses.
With the data in hand, the team built models of Jupiter’s internal dynamics. “In this paper, we’ve gathered the most comprehensive and diverse collection of models of Jupiter’s interior to date and used it to study the distribution of heavy elements in the planet’s envelope,” they write.
The team created two sets of models. The first set is 3-tier models and the second set is diluted core models.
“There are two mechanisms for a gas giant like Jupiter to acquire metals during its formation: through the addition of small pebbles or larger planetesimals,” said lead author Miguel. “We know that once a baby planet is big enough, it starts pushing pebbles. The wealth of metals within Jupiter that we see now is impossible to achieve before then. Thus, we can exclude the scenario with only pebbles as solids during the formation of Jupiter. Planetesimals are too big to block, so they must have played a role.”
The abundance of metals in Jupiter’s interior decreases with distance from the center. This means a lack of convection in the planet’s deep atmosphere, which scientists thought was present. “Before, we thought that Jupiter had convection, like boiling water, making it completely mixed,” said Miguel. “But our discovery shows it differently.”
“We have robustly demonstrated 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 accumulate heavy elements in large quantities while its hydrogen-helium envelope was growing, contrary to predictions based on the isolation mass of pebbles in its simplest incarnation, favoring more complex or hybrid-based models. into planetesimals.”
The authors also conclude that Jupiter did not mix by convection after it formed, even when it was still 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 inhomogeneous envelope implies that the observed metallicity is a lower bound for the planet’s metallicity.”
In the case of Jupiter, there was no way to determine its metallicity from a distance. Only when Juno arrived were scientists able to measure metallicity indirectly. “Therefore, metallicities inferred from remote atmospheric observations on exoplanets may not represent the planet’s mass metallicity.”
When the James Webb Space Telescope begins scientific operations, one of its tasks is to measure exoplanet atmospheres and determine their composition. As this work shows, the data provided by Webb may not capture what is happening in the deeper layers of the gas giant planets.