[Paper Review] Survival of Primordial Planetary Atmospheres: Mass Loss from Temperate Terrestrial Planets

Alex R. Howe, Fred C. Adams+1 more|arXiv (Cornell University)|Dec 18, 2019

Astro and Planetary Science64 references15 citations

TL;DR

This paper investigates photodissociation of molecular hydrogen as a significant mechanism for mass loss from temperate terrestrial exoplanets formed via pebble accretion. It finds that while photodissociation contributes substantially—removing ~34 bars of hydrogen—it is outpaced by impact erosion (~2,300 bars) and insufficient alone to fully evaporate primordial H/He atmospheres, leaving water-rich residues behind.

ABSTRACT

The most widely-studied mechanism of mass loss from extrasolar planets is photoevaporation via XUV ionization, primarily in the context of highly irradiated planets. However, the EUV dissociation of hydrogen molecules can also theoretically drive atmospheric evaporation on low-mass planets. For temperate planets such as the early Earth, impact erosion is expected to dominate in the traditional planetesimal accretion model, but it would be greatly reduced in pebble accretion scenarios, allowing other mass loss processes to be major contributors. We apply the same prescription for photoionization to this photodissociation mechanism and compare it to an analysis of other possible sources of mass loss in pebble accretion scenarios. We find that there is not a clear path to evaporating the primordial atmosphere accreted by an early Earth analog in a pebble accretion scenario. Impact erosion could remove ~2,300 bars of hydrogen if 1% of the planet's mass is accreted as planetesimals, while the combined photoevaporation processes could evaporate ~750 bars of hydrogen. Photodissociation is likely a subdominant, but significant component of mass loss. Similar results apply to super-Earths and mini-Neptunes. This mechanism could also preferentially remove hydrogen from a planet's primordial atmosphere, thereby leaving a larger abundance of primordial water compared to standard dry formation models. We discuss the implications of these results for models of rocky planet formation including Earth's formation and the possible application of this analysis to mass loss from observed exoplanets.

Motivation & Objective

• To assess the role of molecular photodissociation in mass loss from temperate terrestrial exoplanets formed via pebble accretion.
• To compare photodissociation with other mass loss mechanisms—especially impact erosion and photoionization—under pebble accretion conditions.
• To evaluate whether primordial water can be preserved in the atmosphere after preferential loss of hydrogen and helium.
• To examine the implications of these processes for the formation models of rocky planets like Earth and the observed properties of exoplanets.

Proposed method

• Applies an energy-limited mass loss prescription to photodissociation, using a 15% efficiency for H2 dissociation based on Draine & Bertoldi (1996).
• Adapts the standard photoionization model (Watson et al. 1981) to include photodissociation, assuming overlapping ionizing and dissociating radiation bands.
• Estimates mass loss from impact erosion using a 1% mass fraction of planetesimals accreted as impactors, based on Ginzburg et al. (2016).
• Quantifies contributions from Jeans escape, stellar wind ablation, and thermal winds using published scaling laws.
• Compares total mass loss across mechanisms under a pebble accretion scenario where planets form rapidly with a gas-rich envelope.
• Uses a fixed efficiency model for photodissociation and notes the need for future wavelength-dependent efficiency studies.

Experimental results

Research questions

• RQ1 Can photodissociation of H2 be a dominant or significant contributor to atmospheric mass loss in temperate terrestrial exoplanets?
• RQ2 How does photodissociation compare in efficiency to photoionization and impact erosion in pebble accretion scenarios?
• RQ3 Is there a viable pathway to fully evaporate a primordial H/He atmosphere in an Earth-analog planet formed via pebble accretion?
• RQ4 To what extent can photodissociation preserve water and other volatiles by preferentially removing hydrogen?
• RQ5 How would including photodissociation alter predictions for the evaporation valley in radius-flux space for exoplanets?

Key findings

• Photodissociation contributes ~34 bars of hydrogen mass loss, making it a significant but subdominant process compared to impact erosion.
• Impact erosion in a pebble accretion scenario can remove up to ~2,300 bars of hydrogen, primarily through accretion of 1% of the planet’s mass as planetesimals.
• Photoionization contributes ~712 bars of mass loss, making it the second-largest contributor after impact erosion.
• Jeans escape, stellar wind ablation, and thermal winds contribute minimally—less than 100 bars combined—under the modeled conditions.
• The combined mass loss from all mechanisms totals ~3,067 bars, still insufficient to fully evaporate a primordial atmosphere of ~23,000 bars.
• The preferential loss of hydrogen and helium enriches the remaining atmosphere in water and other heavy volatiles, supporting models where primordial water is preserved.