Atmospheric mass loss during planet formation: The importance of planetesimal impacts

By H. Schlichting, R. Sari, A. Yalinewich. 2014

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Quantifying the atmospheric mass loss during planet formation is crucial for understanding the origin and evolution of planetary atmospheres. We examine the contributions to atmospheric loss from both giant impacts and planetesimal accretion. Giant impacts cause global motion of the ground. Using analytic self-similar solutions and full numerical integrations we find (for isothermal atmospheres with adiabatic index γ=5/3) that the local atmospheric mass loss fraction for ground velocities v_g ≲ 0.25v_esc is given by χ_loss = (1.71v_g/v_esc)^4.9, where v_esc is the escape velocity from the target. Yet, the global atmospheric mass loss is a weaker function of the impactor velocity v_Imp and mass m_Imp and given by X_loss≃0.4x + 1.4x^2 - 0.8x^3 (isothermal atmosphere) and X_loss≃0.4x + 1.8x^2 - 1.2x^3 (adiabatic atmosphere), where x=(v_Impm/v_escM). Atmospheric mass loss due to planetesimal impacts proceeds in two different regimes: (1) large enough impactors m≳√(2)ρ_0(πhR)^(3/2) (25 km for the current Earth), are able to eject all the atmosphere above the tangent plane of the impact site, which is h/2R of the whole atmosphere, where View the MathML sourceh,R and ρ0ρ0 are the atmospheric scale height, radius of the target, and its atmospheric density at the ground. (2) Smaller impactors, but above m>4πρ_(0)h^3(1 km for the current Earth) are only able to eject a fraction of the atmospheric mass above the tangent plane. We find that the most efficient impactors (per unit impactor mass) for atmospheric loss are planetesimals just above that lower limit (2 km for the current Earth). For impactor flux size distributions parametrized by a single power law, N(>r)∝r^(-q+1), with differential power law index q, we find that for 1 3 the mass loss is dominated by regime (2). Impactors with m≲4πρ_(0)h^3 are not able to eject any atmosphere. Despite being bombarded by the same planetesimal population, we find that the current differences in Earth’s and Venus’ atmospheric masses can be explained by modest differences in their initial atmospheric masses and that the current atmosphere of the Earth could have resulted from an equilibrium between atmospheric erosion and volatile delivery to the atmosphere from planetesimal impacts. We conclude that planetesimal impacts are likely to have played a major role in atmospheric mass loss over the formation history of the terrestrial planets.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/34dd432d3fe86f69901008b5b2f3cd6a3d166bdd

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Citations in the corpus

• The Cosmic Shoreline: The Evidence that Escape Determines which Planets Have Atmospheres, and what this May Mean for Proxima Centauri B (2017, background): “…ruin (Walker 1986; Melosh and Vickery 1989; Zahnle et al. 1992; Zahnle 1993, 1998a; Griffith and Zahnle 1995; Chen and Ahrens 1997; Brain and Jakosky 1998; Newman et al. 1999; Genda and Abe 2003, 2005; Catling and Zahnle 2009; de Niem et al. 2012; Catling and Zahnle 2013; Schlichting et al. 2015).”
• The California-Kepler Survey. VII. Precise Planet Radii Leveraging Gaia DR2 Reveal the Stellar Mass Dependence of the Planet Radius Gap (2018, background): “…mechanisms have been proposed that could potentially produce a gap in the size distribution including delayed formation in a gas-poor disk (e.g., Lee et al. 2014; Lee & Chiang 2016), and sculpting by giant impacts (e.g., Liu et al. 2015; Schlichting et al. 2015; Inamdar & Schlichting 2016).”
• Origins of volatile elements (H, C, N, noble gases) on Earth and Mars in light of recent results from the ROSETTA cometary mission (2016, unknown): “Recent models argue for a steady state regime between replenishment and atmospheric escape, depending on velocities, atmospheric entry angles, and sizes of impacting bodies (De Niem et al., 2012; Schlichting et al., 2014), but modeling the overall budget of volatile loss/gain is presently out of rea”
• Formation of planetary systems by pebble accretion and migration (2018, background): “Therefore, it may be possible that part of the original envelopes are lost (Liu et al. 2015; Schlichting et al. 2015).”
• Compositional Imprints in Density–Distance–Time: A Rocky Composition for Close-in Low-mass Exoplanets from the Location of the Valley of Evaporation (2017, background): “From a formation point of view it seems rather unlikely that other effects like a late gas-poor formation (Lee et al. 2014; Lopez & Rice 2016) or envelope removal by giant impacts (e.g., Schlichting et al. 2015) should not have played a role as well.”
• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets (2017, background): “Impact Erosion Impacts can also provide a way to sculpt the atmospheric properties of small planets and strip large primordial envelopes down to a few percent by mass (e.g., Schlichting et al. 2015; Liu et al. 2015; Inamdar & Schlichting 2016).”
• Astrochemistry and compositions of planetary systems (2020, background): “2019), impact-driven atmospheric loss (Schlichting et al. 2015), thermal metamorphism (Hashizume and Sugiura 1998), and core formation (Bergin et al.”