Atmospheric mass loss due to giant impacts: the importance of the thermal component for hydrogen-helium envelopes (1809.06810v1)

Abstract: Systems of close-in super-Earths display striking diversity in planetary bulk density and composition. Giant impacts are expected to play a role in the formation of many of these worlds. Previous works, focused on the mechanical shock caused by a giant impact, have shown that these impacts can eject large fractions of the planetary envelope, offering a partial explanation for the observed spread in exoplanet compositions. Here, we examine the thermal consequences of giant impacts, and show that the atmospheric loss caused by these effects can significantly exceed that caused by mechanical shocks for hydrogen-helium (H/He) envelopes. When a giant impact occurs, part of the impact energy is converted into thermal energy, heating the rocky core and the envelope. We find that the ensuing thermal expansion of the envelope can lead to a period of sustained, rapid mass loss through a Parker wind, resulting in the partial or complete erosion of the H/He envelope. The fraction of the envelope lost depends on the planet's orbital distance from its host star and its initial thermal state, and hence age. Planets closer to their host stars are more susceptible to thermal atmospheric loss triggered by impacts than ones on wider orbits. Similarly, younger planets, with rocky cores which are still hot and molten from formation, suffer greater atmospheric loss. This is especially interesting because giant impacts are expected to occur $10{-}100~\mathrm{Myr}$ after formation. For planets where the thermal energy of the core is much greater than the envelope energy, the impactor mass required for significant atmospheric removal is $M_\mathrm{imp} / M_p \sim \mu / \mu_c \sim 0.1$, approximately the ratio of the heat capacities of the envelope and core. When the envelope energy dominates the total energy budget, complete loss can occur when the impactor mass is comparable to the envelope mass.

• The paper finds that thermal effects from giant impacts can cause significantly more atmospheric mass loss in hydrogen-helium envelopes than shock-induced mechanisms.
• Atmospheric loss is strongly dependent on the planet's orbital distance from its star and its initial thermal state, making closer and younger planets more vulnerable.
• Noticeable atmospheric removal occurs when an impactor's mass is roughly 10% of the planet's mass for core-dominated systems or equivalent to the envelope mass for envelope-dominated systems.

Atmospheric Mass Loss Due to Giant Impacts: The Significance of Thermal Factors in Hydrogen-Helium Envelopes

In the paper "Atmospheric Mass Loss due to Giant Impacts: The Importance of the Thermal Component for Hydrogen-Helium Envelopes," Biersteker and Schlichting explore the influence of giant impacts on the atmospheric compositions of exoplanets, with a particular focus on the thermal mechanisms that drive mass loss in hydrogen-helium (H/He) envelopes. This paper advances our understanding of the mechanisms behind the observed compositional diversity among close-in super-Earths and mini-Neptunes, which often possess significant H/He envelopes. The paper's outcomes have implications for theories of planetary formation and evolution.

Key Findings

1. Thermal vs Shock-Induced Mass Loss: The paper underscores that while mechanical shocks from giant impacts contribute to atmospheric loss, the thermal effects can significantly surpass these shock-driven mechanisms. Upon impact, energy is converted into thermal energy, elevating core and envelope temperatures. The resultant thermal expansion facilitates a rapid mass loss via Parker winds, potentially eroding a substantial fraction of the H/He envelope.
2. Dependence on Orbital Distance and Age: The degree of atmospheric loss is contingent upon two main parameters: the planet's proximity to its host star and its initial thermal state. Planets closer to the star are more vulnerable to thermal atmospheric loss due to increased heating and smaller Hill radii. Similarly, younger planets that retain core heat post-formation are more prone to envelope erosion.
3. Mass Requirements for Significant Loss: The paper identifies that for planets with core-dominated thermal energies—where the energy of the rocky core vastly exceeds that of the envelope (H/He envelope mass fractions below 8%)—the threshold for noticeable atmospheric removal corresponds to an impactor mass roughly 10% of the planet mass. Conversely, for planets where the envelope energy is dominant, significant loss can occur with an impactor mass equivalent to that of the envelope.

Analytical and Numerical Insights

The authors utilize a combination of analytical models and numerical simulations to approximate the structure, evolution, and post-impact dynamics of H/He envelopes. Key elements in the analysis include a model for the super-Earth atmosphere that considers both convective and radiative layers, and a core that remains isothermal and efficiently transfers heat. The paper provides parametric insights and analytical expressions to estimate the impactor mass necessary for atmospheric loss, revealing its sensitivity to orbital radius and initial conditions.

Practical and Theoretical Implications

This paper has significant implications for our understanding of planet formation and the resultant diversity in exoplanetary systems. Giant impacts, typically occurring 10-100 Myr after formation during the late stage of planetary assembly, can drastically alter atmospheric compositions, influencing planetary density and, consequently, the classification within the super-Earth and mini-Neptune categories. The research highlights the potential of giant impacts as a plausible mechanism behind inter-system density variations, offering an alternative or complementary explanation to processes such as photoevaporation or core cooling.

Future Directions

The findings encourage further examination of volatile loss mechanisms among exoplanets, particularly in multi-planet systems with varied planetary compositions. Future studies could enhance these models by incorporating outgassing effects and more intricate hydrodynamic simulations, as well as observational campaigns targeting young exoplanetary systems with varying compositions.

In conclusion, Biersteker and Schlichting provide a comprehensive examination of the thermal consequences of giant impacts, significantly advancing the theoretical framework needed to interpret observed compositional disparities in close-in exoplanets. Through rigorous modeling and quantification, their research establishes a foundational understanding that can shape future explorations in exoplanet atmospheric research.