Jupiter is composed almost entirely of hydrogen and helium. The amounts of each closely conform to the theoretical quantities 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.
According to a new study, Jupiter’s metal content and
distribution mean that the planet ate a lot of rocky planetesimals in its
youth.
Ever since NASA’s Juno spacecraft reached Jupiter in
July 2016 and started gathering 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 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.
When Jupiter formed, it began by accreting rocky
material. A period of rapid gas accretion from the solar nebula followed that,
and after many millions of years, Jupiter became the behemoth it is today. But
there’s a significant question regarding the initial period of rocky accretion.
Did it accrete larger masses of rocks like planetesimals? Or did it accrete
pebble-sized material? Depending on the answer, Jupiter formed on different
time scales.
A new study set out to answer that question. It’s
titled “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 & The Netherlands
Institute for Space Research.
We’re growing accustomed to gorgeous images of Jupiter
thanks to the Juno spacecraft’s JunoCam. But what we see is only skin deep. All
those spellbinding images of the clouds and storms are only the thin 50 km (31
miles) outermost layer of the planet’s atmosphere. The key to Jupiter’s
formation and evolution is deeply buried in the planet’s atmosphere, which is
tens of thousands of kilometres deep.
The Juno mission is helping us piece together a better
understanding of Jupiter’s mysterious interior. Image: By Kelvinsong |
It’s 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 Jupiter’s
formation, and the Gravity Science experiment measured pebble dispersion
throughout the atmosphere. Before Juno and its Gravity Science experiment, there
was no precise data on Jupiter’s gravity harmonics.
The researchers found that Jupiter’s atmosphere isn’t
as homogenous as previously thought. More metals are near the planet’s center
than in the other layers. Altogether, 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 assemble the most comprehensive
and diverse collection of Jupiter interior models to date and use 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-layer models and the second is dilute core models.
“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
inside Jupiter that we see now is impossible to achieve before that. So we can
exclude the scenario with only pebbles as solids during Jupiter’s formation.
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 signifies a lack of convection in
the planet’s deep atmosphere, which scientists thought was present. “Earlier,
we thought that Jupiter has convection, like boiling water, making it
completely mixed,” said Miguel. “But our finding shows differently.”
“We robustly demonstrate that the heavy element
abundance 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 amounts while its hydrogen-helium envelope was growing, contrary to
predictions based on the pebble-isolation mass in its simplest incarnation,
favouring instead planetesimal-based or more complex hybrid models.”
Artistic rendition of a protoplanet forming within the
accretion disk of a protostar Credit: ESO/L. Calçada |
The authors also conclude that Jupiter didn’t 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
base example for exoplanets: a non-homogeneous envelope implies that the
metallicity observed is a lower limit to the planet bulk metallicity.”
In Jupiter’s case, there was no way of determining its
metallicity from a distance. Only when Juno arrived could scientists measure
the metallicity indirectly. “Therefore, metallicities inferred from remote
atmospheric observations in exoplanets might not represent the bulk metallicity
of the planet.”
When the James Webb Space Telescope starts science
operations, one of its tasks is measuring exoplanet atmospheres 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.
Reference: Press Release, Peer research
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