Abstract DGP2026-93

Impact of CO2 absorption continua upon the derivation of surface emissivity differences on Venus

Ankita Das (1), Nils Mueller (1), David Kappel (1), John Lee Grenfell (1,2), Heike Rauer (1,3), Ana-Cataline Plesa (2), Giulia Alemanno (2)

(1) Freie Universität Berlin, Institute of Geological Sciences, Department of Earth Sciences, Berlin, Germany (2) Institute of Space Research, German Aerospace Center (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany (3) German Aerospace Center (DLR), Markgrafenstrasse 37, 10117 Berlin, Germany

Venus is the one planet most alike Earth in terms of several properties (e.g. radius, mass, and bulk composition) and is often called Earth’s twin. However, Venus and Earth differ significantly in terms of present-day habitability [1]. Processes which drove their divergent evolutions despite the similarities mentioned are much discussed.

In the coming decade, several missions to Venus are planned [2] that aim to image Venus nightside thermal emission in the NIR spectral windows [3]. Measurements in these spectral regions can be utilized to constrain surface emissivity which correlates to FeO content (among others) and is indicative of surface mineralogy [4].

Thermal emission features originate from the surface are a function of surface temperature and surface emissivity. Constraining surface information such as emissivity is challenging given Venus’ hot dense CO2 atmosphere. The manner in which emissivity is translated to top of atmosphere radiance depends heavily on gaseous absorption which is poorly understood at higher pressures and temperatures. Most investigations in Venus radiative transfer modify the far wing CO2 absorption line profile in addition to introducing a continuum absorption coefficient to incorporate absorption effects arising in this environment.

Absorption line profiles in the far-wings are dominated by broadening from molecular collisions characterized by a Lorentz profile in the deep atmosphere. However, several works have reported that a deviation from a pure Lorentz profile is necessary to reproduce observed data [e.g. 5, 6]. Most previous works have used semi-empirical methods to model the far wings according to a sub-Lorentzian profile [5]. The residual effects which sub-Lorentzian line shapes are unable to account for are then approximated by adding a continuum opacity. In this work we investigate whether such assumptions regarding far-wing CO2 absorption can significantly affect derived relative emissivity results through radiative transfer modeling. We also comment on additional continuum opacity required to reproduce available data for different far-wing absorption profiles.

Our radiative transfer model takes input from molecular line databases to account for gaseous absorption from CO2 (96% vmr) (HITEMP, [7]) and H2O (32 ppm) (HITRAN [8]). In our work we constrain some of our model parameters pertaining to clouds and continuum with existing observations of Venus nightside IR radiance [9] and assume basaltic emissivities.

Such analyses in tandem with future observations in the NIR region will enable the production of a relative emissivity map of Venus, providing insights into differences in surface mineralogy which could further elucidate Venusian evolution.

References:

[1] Westall F. et al (2023) Space Sci. Rev.

[2] Smrekar S. et al. (2022) IEEE

[3] Allen D. A. et al. (1984) Nature

[4] Mueller N. et al. (2008) JGR Planets

[5] Burch D. E. et al. (1969) J. Opt. Society of America

[6] Perrin and Hartmann (1989) J. Quant. Spect. & Rad. T.

[7] Hargreaves, R. J et al. (2025) J. Quant. Spect. Radiat. T.

[8] Gordon I. E. et al. (2022) J. Quant. Spect. Radiat. T.

[9] Korablev O. et al. (2012) JGR