Astrogeology

Evolution of Gas Envelopes and Outgassed Atmospheres of Rocky Planets Formed via Pebble Accretion

By Keith Cowing

Status Report

astro-ph.EP

September 18, 2024

Filed under astro-ph.EP, astrochemistry, astrogeology, atmosphere, Earth, exoplanet, magma ocean, Mantle, oceans, pebble accretion, rocky planet, self-oxidation, Water

Evolution of Gas Envelopes and Outgassed Atmospheres of Rocky Planets Formed via Pebble Accretion — Upper panels: The peak hydrogen envelope mass fraction at the end of accretion, before any mass loss, shown in colour for the slow, medium and fast host star rotators. The time when 90% of the envelope mass is lost due to solar XUV radiation is shown in white contour lines in Myr. Lower panels: The atomic composition of the outgassed atmospheres of the slow, medium and fast rotator simulations at 300 Myr. Values of R/O < 1 (blue) indicate an atmosphere with free oxygen, a value of R/O=1 (green) imply sufficient oxygen to oxidise H to H2O and C to CO2, and values of R/O > 1 (red) indicate reducing conditions (see definition of R/O in equation 8). The white contour lines in the lower panels show a value of R/O = 1.3. The patch of R/O=0 present for the slow rotator is due to the perseverance of the envelope for those high planetary masses even after 300 Myr, with the magma ocean still providing a significant pressure of free oxygen. — astro-ph.EP

We present here results of numerical simulations of the formation and early evolution of rocky planets through pebble accretion, with an with an emphasis on hydrogen envelope longevity and the composition of the outgassed atmosphere.

We model planets with a range in mass from 0.1 to 5 Earth masses that orbit between 0.7 and 1.7 AU. The composition of the outgassed atmosphere is calculated with the partial pressure of free oxygen fit to geophysical models of magma ocean self-oxidation. XUV radiation powered photoevaporation is considered as the main driver of atmospheric escape.

We model planets that remain below the pebble isolation mass and hence accrete tenuous envelopes only. We consider slow, medium or fast initial stellar rotation for the temporal evolution of the XUV flux. The loss of the envelope is a key event that allows the magma ocean to crystallise and outgas its bulk volatiles. The atmospheric composition of the majority of our simulated planets is dominated by CO2. Our planets accrete a total of 11.6 Earth oceans of water, the majority of which enters the core.

The hydrospheres of planets lighter than the Earth reach several times the mass of the Earth’s modern oceans, while the hydrospheres of planets ranging from 1 to 3.5 Earth masses are comparable to those of our planet. However, planets of 4-5 Earth masses have smaller hydrospheres due to trapping of volatiles in their massive mantles.

Overall, our simulations demonstrate that hydrogen envelopes are easily lost from rocky planets and that this envelope loss triggers the most primordial partitioning of volatiles between the solid mantle and the atmosphere.

Piia Maria Tomberg (University of Copenhagen, Globe Institute), Anders Johansen (University of Copenhagen, Globe Institute)

Comments: Accepted for Astronomy & Astrophysics
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2409.11005 [astro-ph.EP] (or arXiv:2409.11005v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2409.11005
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Submission history
From: Piia Maria Tomberg Ms
[v1] Tue, 17 Sep 2024 09:11:32 UTC (12,438 KB)
https://arxiv.org/abs/2409.11005
Astrobiology, Astrogeology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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