Life After Death: Europa In The Evolving Habitable Zone Of A Red Sun
Most stars end their main-sequence (MS) lives by evolving through the red-giant and asymptotic-giant branches before ending as a quiescent, stable white dwarf. Therefore, it is imperative to model the post-MS as it relates to long-term stability of environments potentially suitable for life.
Recent work has shown that gas giants can exist in the habitable zone (HZ) during the red giant phase and around a white dwarf remnant. Icy moons represent large reservoirs of water and will evolve through sublimation and melting when exposed to higher instellation, where the relatively lower surface gravity could lead to the rapid loss of all surface water. We model the surface evolution of Europa when initially exposed to habitable zone instellation in the red giant branch.
Modeling the diurnal and yearly flux variations on a 2D map we show that, due to Jupiter’s increased albedo, the sub-Jovian hemisphere of Europa largely sublimates while only the anti-Jovian equatorial band sublimates. With the increasing instellation of the red giant branch, both hemispheres sublimate substantially.
We then model the evolution of a tenuous water-vapor atmosphere and show it is stable against atmospheric loss for at least 0.2 Gyr in the red giant branch habitable zone. We then present three ways to observe a sublimating Europan-like exomoon and potential spectra.
Extending the results of this work to different planets and moons could open up a new pathway by which life could persist beyond the death of a star.
Model assumptions for stellar and reflected flux variations over different timescales. Top: The Sun’s red giant branch evolution and incident flux at Jupiter over time (Left). Stellar flux at Jupiter (PHOENIX stellar models (Husser et al. 2013)) and geometric albedo (a 0◦ phase) of Jupiter (Cahoy et al. 2010) when it first enters the red giant branch habitable zone (Seff = 0.32, turquoise) and when it receives Earth-like instellation (Seff = 1.0, blue) (Right). Stellar spectra and geometric albedo are convolved to derive the phase-dependent bond albedo of Jupiter. Bottom: Yearly stellar flux variations due to Jupiter’s eccentricity and obliquity (Left), and the diurnal stellar and reflected flux variations due to Europa’s orbit around Jupiter (Right). We find that Europa’s northern hemispheric solstice (substellar latitude = 3 ◦ ) occurs near Jupiter’s periastron. For the diurnal flux, we include the direct stellar flux absorbing on the surface of Europa, the phase dependent reflected flux from Jupiter, and factor in Jupiter eclipsing the stellar flux for a portion of the orbit. Over-plotted is the amount of flux Europa intrinsically reflects away. The shading scheme for the Diurnal Flux Variations plot (bottom right, top panel) was adapted from Heller & Barnes (2013). — astro-ph.EP
Potential methods by which to measure spectra of a sublimating Europan-like exomoon, with the atmosphere initialized from a photochemical model and included water, ozone, molecular oxygen, and molecular hydrogen. Spectra were generated with POSEIDON (MacDonald & Madhusudhan 2017; MacDonald 2023; Mullens et al. 2024). Cloudy models display the Mie scattering and absorptive properties of liquid water (H2O (l)) and ice 1h (H2O (s)). The figure displays the relative sizes of the white dwarf, moon, and host planet to scale, but not the orbital scale. Top: Dynamical models (Payne et al. 2016) have shown that moons are likely liberated from their host planets and can occupy short distance orbits to white dwarfs. These spectra (0.35-2.45 µm) demonstrate potential reflection+emission spectra of a liberated icy exomoon. We utilize two different surfaces (USGS Spectral Library’s melting ice and ocean (Kokaly et al. 2017; Goodis Gordon et al. 2024)), as well as display clear and cloudy models. Middle: Gas giants, such as WD1856+534b, have been discovered transiting in the habitable zone (0.02 AU) of white dwarfs (Vanderburg et al. 2020). If dynamically stable, transiting giant planets can have their moon transit as well, producing high SNR transit signals due to the size ratio between the moon and white dwarf (0.6-14 µm). Representative error bars for JWST NIRSpec Prism (blue) and MIRI LRS (red) are shown. Bottom: Gas giants hosting moons during the red-giant branch reflect light from their host star, as well as emit thermal emission (0.6-14 µm). Future direct-imaging instruments, such as Roman Space Telescope and HWO, might be able to capture exomoon transits around Jupiter analogs and emission signals from Europan-analogs around white dwarfs, but these are both currently inaccessible to JWST. — astro-ph.EP
Elijah Mullens, Britney Schmidt, Lisa Kaltenegger, Nikole K. Lewis
Comments: 17 pages, 7 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society (MNRAS) May 2025
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2505.15495 [astro-ph.EP] (or arXiv:2505.15495v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2505.15495
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Submission history
From: Elijah Mullens
[v1] Wed, 21 May 2025 13:22:05 UTC (14,596 KB)
https://arxiv.org/abs/2505.15495
Astrobiology
