The atmosphere of Jupiter is enriched with heavy elements by a factor of about 3 compared to a protosolar composition. The origin of this enrichment and whether it represents the composition of the planetary atmosphere are unknown.
Hubble’s photo of Jupiter displays the ever-changing landscape of its turbulent atmosphere. Image credit: NASA / ESA / Hubble / Amy Simon, NASA’s Goddard Space Flight Center / Michael H. Wong, University of California, Berkeley / Joseph DePasquale, STScI.
NASA’s Galileo probe measured the elemental abundances in Jupiter’s atmosphere and found that several heavy elements (elements heavier than helium) are enriched by a factor of 3 compared to a protosolar composition.
Also, the recent measurement of Jupiter’s water abundance by NASA’s Juno orbiter implies that oxygen is enriched by a factor of a few.
The origin of this enrichment remains unknown, and several ideas have been suggested to explain it.
Internal structure models of Jupiter suggest that its atmosphere is separated from the deep interior and that the planet is not fully mixed.
This implies that the Jovian atmosphere was enriched with heavy elements just before the end of its formation. Such enrichment can be a result of planetesimal accretion.
However, in situ Jupiter formation models suggest a decreasing accretion rate with increasing planetary mass, which cannot explain Jupiter’s atmospheric enrichment.
“Since we now know that the interior of Jupiter is not fully mixed, we would expect heavy elements to be in a giant gas planet’s deep interior as heavy elements are mostly accreted during the early stages of the planetary formation,” said Professor Ravit Helled, a researcher at the University of Zurich.
“Only in later stages, when the growing planet is sufficiently massive, can it effectively attract large amounts of light element gases like hydrogen and helium.”
“Finding a formation scenario of Jupiter which is consistent with the predicted interior structure as well as with the measured atmospheric enrichment is therefore challenging yet critical for our understanding of giant planets.”
“Our idea was that Jupiter had collected these heavy elements in the late stages of its formation by migrating,” she added.
“In doing so, it would have moved through regions filled with so-called planetesimals — small planetary building blocks that are composed of heavy element materials — and accumulated them in its atmosphere,” added Dr. Sho Shibata, a postdoctoral researcher at the University of Zurich.
“Yet, migration by itself is no guarantee for accreting the necessary material.”
“Because of complex dynamical interactions, the migrating planet does not necessarily accrete the planetesimals in its path.”
“In many cases, the planet actually scatters them instead — not unlike a shepherding dog scattering sheep.”
Sketch for planetesimal accretion and the forming composition gradient in Jupiter’s atmosphere. The above panel shows the case when proto-Jupiter migrates from 7 to 5 AU. Infalling gas covers the envelope enriched via planetesimal accretion and only the inner region of the Jupiter’s atmosphere is enriched with heavy elements. The bottom panel shows the case when proto-Jupiter migrates from 20 to 5 AU. If the protoplanet reaches the sweet spot for accretion just before the end of gas accretion, planetesimals are deposited into the outer envelope. A several-times-enriched Jupiter atmosphere can be formed if the size of the outermost convective layer is as small as 0.2 Jupiter masses. Image credit: Shibata & Helled, doi: 10.3847/2041-8213/ac54b1.
In their study, the authors simulated Jupiter’s formation, including planetary migration, and investigated the accretion rate of planetesimals.
They considered two formation pathways: proto-Jupiter migrates from 7 AU to its current location, and it migrates from 20 AU.
They found the migration of proto-Jupiter from 20 AU to its current location can lead to late planetesimal accretion and atmospheric enrichment.
“What we found was that a sufficient number of planetesimals could be captured if Jupiter formed in the outer regions of the Solar System — about four times further away from the Sun than where it is located now — and then migrated to its current position,” Dr. Shibata said.
“In this scenario, it moved through a region where the conditions favored material accretion — an accretion sweet spot, as we call it.”
“This shows how complex giant gas planets are and how difficult it is to realistically reproduce their characteristics,” Professor Helled said.
“It took us a long time in planetary science to get to a stage where we can finally explore these details with updated theoretical models and numerical simulations.”
“This helps us close gaps in our understanding not only of Jupiter and our Solar System, but also of the many observed giant planets orbiting far away stars.”
The team’s results were published in the Astrophysical Journal Letters.
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Sho Shibata & Ravit Helled. 2022. Enrichment of Jupiter’s Atmosphere by Late Planetesimal Bombardment. ApJL 926, L37; doi: 10.3847/2041-8213/ac54b1
