Thermoelastic Contraction as a Suppressor of Atmospheric Escape in Close-in Exoplanets

Abstract The survival of volatile-rich atmospheres on close-in exoplanets challenges classical escape models. We identify a fully classical, interior-driven correction: thermoelastic contraction of the planetary mantle slightly increases the gravitational binding energy and the effective escape threshold. Incorporating pressure and temperature-dependent elastic deformation, we define a dimensionless suppression index <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Ξ</mml:mi> <mml:mo>≡</mml:mo> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:msub> <mml:mi>ϵ</mml:mi> <mml:mi mathvariant="normal">V</mml:mi> </mml:msub> <mml:mo>/</mml:mo> <mml:mn>3</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> <mml:mo>/</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:math> that links interior strain to escape dynamics. Across super-Earth and sub-Neptune regimes, realistic volumetric strains (ϵ V ∼ 10 −3 –10 −2 ) enhance the escape velocity by 0.02%–0.17%, yielding ≲1% reductions in energy-limited mass-loss rates. Critically, the Jeans regime amplifies this effect through its exponential sensitivity to v esc : for representative super-Earth parameters, thermoelastic contraction alone suppresses the Jeans escape rate by up to ∼18%. This establishes mantle elasticity as a small but physically robust correction to escape theory and a novel interior–atmosphere coupling channel. For instance, in the case of K2–18b, the predicted contraction-induced signal corresponds to ∼30 ppm at 1.4 μ m, within the reach of JWST/NIRSpec in ∼5 transits. We outline observational tests—particularly with JWST and ARIEL—where high signal-to-noise transmission spectroscopy could, in favorable cases, probe these suppression signatures and place constraints on deep planetary structure.