Volcanic gas plumes&#8217; effects on the spectrum of Venus
Authors: Dias João , Machado Pedro , Robert Séverine , Erwin Justin , C. Duarte João , Quirino Diogo 
Published: July 2024

Simulations of spectra during a volcanic eventThe composition and the variability of the lower atmosphere of Venus are critical to understanding the surface-atmosphere interactions, the atmospheric evolution and the volatile exchanges between the interior and the atmosphere [1, 2, 3]. The lower atmosphere can be investigated on the near-infrared spectral windows on the nightside, centered at 1.18 &#181;m (0-15 km), 1.74 &#181;m (20-30 km) and 2.3 &#181;m (30-45 km) [4, 5], which provides a chance to detect volcanic species such as H2O, SO2. CO, HCl and HF, which are common species on Earth volcanoes [3]. The Planetary Spectrum Generator (PSG) [6, 7, 8] was used to simulate the nightside 2.3 &#181;m spectrum of Venus, in nadir-geometry, using a VIRA atmospheric template for temperature and molecular vertical abundance profiles and four aerosol modes, of effective radii of 0.49 &#181;m, 1.18 &#181;m, 1.56 &#181;m and 4.25 &#181;m [9, 10]. The spectral resolution was defined as R = 7500. An H2O volcanic gas plume was modeled using a Johnson unbounded distribution, with a maximum value of volume mixing ratio (vmr) at a chosen altitude and a slow decrease towards lower altitudes. The profile was defined as a perturbation of the VIRA nominal H2O profile (~ 30 ppm). with a peak vmr in the range 10-70 ppm and a peak altitude in the range 10-58 km. For each plume, the total integrated column density was conserved, defined as a scaler of the VIRA value, N(total)(H2O) ~ 13.23 kg m-2. An H2O plume at Z = 30 km with a 10 ppm peak increase results in a radiance difference of ~ 10% at 4150 cm-1. Ground-based observations and future space missions, such as iSHELL/IRTF and EnVision&#8217;s VenSpec-H spectrograph [11], respectively, should have the high-spectral resolution and sensitivity required to detect such H2O variations due to a possible volcanic eruption event. Spectral Retrievals of iSHELL/IRTF spectraOn this framework, we are currently investigating the sensitivity of iSHELL/IRTF to retrieve the abundance of H2O and its temporal and latitudinal variability using the near-infrared spectral window at 1.74 &#181;m (20-30 km) and 2.3 &#181;m (30-45 km) [4]. The retrieval is ongoing with the ASIMUT tool (Vandaele+2006) [12].We performed 6 hours of observations, in February 2nd-3rd 2024, of Venus dayside and nightside using the K3 (2.26-2.55 &#181;m) and H3 filters (1.64-1.82 &#181;m) on the iSHELL/IRTF instrument. The slit width used was 0.375&#8217;&#8217; x 5&#8217;&#8217; (R ~ 75 000) and it was positioned parallel to the axis of rotation. iSHELL is a cross-dispersed high-spectral resolution 1.1-5.3 &#181;m echelle spectrograph (R ~ 80 000), located in Maunakea, Hawaii [13]. Wavelength calibration, flat fielding, dark correction and sky subtraction were performed using the Spextool data reduction software [14, 15].The dayside straylight contribution on the nightside, in order 306, was removed by scaling of the dayside spectra with a scaler computed as the nightside/dayside ratio, in order 295 (1.764-1.771 &#181;m), where we modeled expected counts ~ 0.We identified several CO2 absorption lines in order 306 (1.702&#8211;1.709 &#181;m) and 305 (1.708-1.714 &#181;m) and some hints of H2O and HCl absorption lines in order 300 (1.735-1.742 &#181;m). The absolute flux calibration and retrieval of the H2O and HCl column densities are currently ongoing, using ASIMUT [12].References. (1) Gilmore et al. 2023, Space Sci Rev. (2) Gillmann et al. 2022, Space Sci Rev (3) Wilson et al. 2024, Space Sci Rev (4) B&#233;zard et al. 2009, JGR Planets (5) Tsang et al. 2009, Journal of Quantitative Spectroscopy and Radiative Transfer (6) Villanueva et al. 2018, Journal of Quantitative Spectroscopy and Radiative Transfer; (7) Smith et al. 2013, JGR Planets (8) Dias et al. 2022, Atmosphere (9) Zasova et al. 2006, Cosmic Research; (10) Haus et al. 2010, Icarus&#160;(11) Robert et al. 2021, EPSC. (12) Vandaele et al. 2006, Proc of the First 'Atmospheric Science Conference' (13) Rayner et al. 2022, PASP 134&#160;(14) Cushing et al. 2004, PASP 116 (15) Vacca et al. 2003, PASP 115.Funding. This work was supported by FCT through the research grants UIDB/04434/2020 and UIDP/04434/2020 and a fellowship grant 2022.09859.BD. Funded by ESA in the framework of MWWM - Mars Wind and Wave Mapping project. SR acknowledges funding by BELSPO with financial and contractual support coordinated by the ESA Prodex Office (PEA 4000137943, 4000128137).Acknowledgements. We kindly thank Maxence Lef&#233;vre, from the University of Oxford, for the discussions on volcanic gas plumes, and Shohei Aoki, from the University of Tokyo, for the collaboration concerning the data reduction.&#160;&#160;&#160;