# Scientific objectives and instrumental requirements of the infrared spectrometer VenSpec-H onboard EnVision

Séverine Robert, Justin T. Erwin, Roderick de Cock, Ian R. Thomas, Nuno Pereira, Lars Jacobs, Sophie Berkenbosch, David Bolsée, Filip Vanhellemont, Eddy Neefs, et al.

► To cite this version:

Séverine Robert, Justin T. Erwin, Roderick de Cock, Ian R. Thomas, Nuno Pereira, et al. Scientific objectives and instrumental requirements of the infrared spectrometer VenSpec-H onboard EnVision. Journal of applied remote sensing, 2025, 19 (01), pp.014525. 10.1117/1.jrs.19.014525. hal-05033746

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Distributed under a Creative Commons Attribution 4.0 International License
Scientific objectives and instrumental
requirements of the infrared spectrometer
VenSpec-H onboard EnVision

Séverine Robert<sup>a,∗</sup> Justin T. Erwin<sup>a</sup> Roderick De Cock<sup>a</sup> Ian R. Thomas,<sup>a</sup>

Nuno Pereira,<sup>a</sup> Lars Jacobs,<sup>a</sup> Sophie Berkenbosch,<sup>a</sup> David Bolsée,<sup>a</sup>

Filip Vanhellemont,<sup>a</sup> Eddy Neefs,<sup>a</sup> Shohei Aoki,<sup>b</sup> Bruno Bézard,<sup>c</sup>

Emmanuel Marcq,<sup>d</sup> Giulia Alemanno,<sup>e</sup> Joern Helbert,<sup>e</sup> and Ann C. Vandaele<sup>c,a</sup>

**ABSTRACT.** Onboard the EnVision spacecraft, there will be a suite of three spectrometers, VenSpec. One of these is called VenSpec-H where the H stands for high spectral resolution. Its scientific objectives consist of measuring variations of minor species' abundances in the atmosphere of Venus. H₂O, SO₂, CO, and OCS will be measured to characterize the potentially ongoing volcanic activity. These observations will allow us to understand both the importance of volatiles in volcanic activity on Venus and their effect on cloud maintenance and dynamics. VenSpec-H will measure these molecules in nadir viewing geometry, in infrared transparency windows of Venus' nightside to probe the troposphere, and in infrared spectral ranges on the dayside to measure the mesosphere. The scientific requirements enabling our scientific objectives will be demonstrated. The molecular vertical profiles, the aerosols' model, and the CO₂ continuum contribution were established based on the literature for the different spectral windows. This enabled us to determine the spectral bands, their bandwidth, and the resolving power necessary for our purposes. Along the way, we identified possible improvements and science avenues. Some of them impact the instrument design, such as the need for polarimetric measurements. Others are related to remaining uncertainties in the model and laboratory measurements that will complement the investigation. This has been presented as a poster at the SPIE Optical Engineering + Applications Conference in August 2024 in San Diego, California, United States.

© The Authors. Published by SPIE under a Creative Commons Attribution 4.0 International License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. [DOI: 10.1117/1.JRS.19.014525]

**Keywords:** EnVision; VenSpec-H; radiative transfer theory; Venus; molecular species; volcanic activity; instrumental requirements

Paper 240631G received Oct. 16, 2024; revised Feb. 13, 2025; accepted Mar. 10, 2025; published Mar. 31, 2025.

<footnote><sup>a</sup>Royal Belgian Institute for Space Aeronomy, BIRA-IASB, Brussels, Belgium</footnote>

<footnote><sup>b</sup>The University of Tokyo, Graduate School of Frontier Sciences, Department of Complexity Science and Engineering, Tokyo, Japan</footnote>

<footnote><sup>c</sup>LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, Meudon, France</footnote>

<footnote><sup>d</sup>LATMOS/IPSL, UVSQ Université Paris-Saclay, Sorbonne Université, CNRS, Guyancourt, France</footnote>

<footnote><sup>e</sup>DLR, Berlin, Germany</footnote>

<footnote>*Address all correspondence Séverine Robert, severine.robert@aeronomie.be</footnote>

<footnote>Handling Editor: Shen-En Qian, Associate Editor</footnote>
# 1 Introduction

Despite their similar size and bulk composition, Venus and Earth have followed different geological evolution paths. In 2 years of radar mapping (1990 to 1992), Magellan showed that Venus has abundant volcanic and tectonic features but was unable to detect ongoing geological activity.² Hahn and Byrne³ published a global catalog of volcanoes describing around 85,000 edifices, ~99% of which are less than 5 km in diameter. They noticed a lack of edifices in the 20 to 100 km diameter range. In 2005, Venus Express was launched with a payload focusing on atmospheric science. The scientific phase of the mission (2006 to 2014) was a success and provided hints of active volcanism. More specifically, three instruments onboard Venus Express brought insights to this investigation. First, the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) detected local anomalies in the emissivity maps.⁴ The infrared radiation coming from three volcanic regions was different to that from the surrounding terrain and interpreted as coming from relatively fresh lava flows that had not yet experienced significant surface weathering. Although these flows were found to be less than 2.5 million years old, the study could not establish whether there is still active volcanism on the planet. Second, the “Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus” (SPICAV)-UV channel was used to measure the abundances of SO₂ above the cloud level.⁵ Complementing the series of measurements done by UVIS onboard Pioneer Venus during the 1970s and 1980s,⁶⁻⁸ the SPICAV-UV measurements showed a sharp rise in the sulfur dioxide content of the upper atmosphere in 2006 to 2007, followed by a gradual fall over the following 5 years.⁹ As suggested by Marcq et al., these episodic sulfur dioxide injections to the cloud tops may be caused either by periods of increased buoyancy of volcanic plumes or, in the absence of active volcanism, by long-period oscillations of the general atmospheric circulation. Finally, another striking evidence for active volcanism on Venus came from the Venus Monitoring Camera (VMC) in 2014. VMC spotted localized changes in the surface brightness among images taken only a few days apart in 2008 and 2009. These infrared “flashes” over the edges of the rift zone Ganis Chasma are thought to be caused either by hot gases and/or lava released from volcanic eruptions.¹⁰ Since Venus Express, more investigations have been carried out, and recently, Herrick and Hensley¹¹ and Sulcanese et al.¹² identified evidences of ongoing volcanic activities using the Magellan dataset.

To determine whether Venus is geologically active, a comprehensive payload is necessary: the surface and subsurface need to be characterized, but information about the atmosphere is crucial. This is why EnVision will also carry a complete spectroscopy suite called VenSpec¹³ including an infrared mapper, VenSpec-M;¹⁴ an ultraviolet spectrometer, VenSpec-U;¹⁵ and an infrared spectrometer, VenSpec-H.¹⁶

# 2 Scientific Objectives and Requirements

The science objectives of the VenSpec suite are to search for temporal variations in surface temperatures and tropospheric concentrations of volcanically emitted gases, indicative of volcanic eruptions; to study surface-atmosphere interactions and weathering by mapping surface emissivity and tropospheric gas abundances; and to map the variability of trace species, cloud, and aerosol properties and to distinguish intrinsic from extrinsic (e.g., volcanic emissions) variables in the mesosphere. Observations will provide insight into the spatial distribution of trace gas which is essential to understand the main chemical cycles on Venus. Any photon that is emitted from the surface will encounter multiple scattering at particles in the clouds in a way that results in a blurring of the images. Models have predicted a limit of the spatial resolution of 50 to 100 km.¹⁷,¹⁸ This was confirmed by analysis of the VIRTIS dataset early during the Venus Express mission.¹⁹

Observing Venus' troposphere in the infrared is challenging. On the dayside of the planet, the reflected sunlight on the cloud deck prevents measuring below the clouds. The nightside of the planet must then be targeted. Strahlight from the dayside and high density of CO₂ rendering the atmosphere opaque limit the number of useful spectral ranges. Molecular species are probed in specific spectral windows called transparency windows centered at 1.0, 1.1, 1.2, 1.3, 1.7, and 2.3 µm.²⁰ They have been used extensively since the 1990s to retrieve SO₂, OCS, CO, H₂O, and HCl²¹⁻²⁷ in the lower layers of the atmosphere, but so far, only vertical profiles of the H₂O could be determined.
<table><caption>**Fig. 1** EnVision Science observation requirements summary extracted from the Red Book.31 "In the left-hand column, the EnVision requirements number is noted, together with a reference to the sections describing further the requirement flow-down and how the science objectives are met applying the intended observation strategy and mission payloads." The requirement R2-C-20 (2) is associated to VenSpec-U.15,32,33</caption><thead><tr><th></th><th></th></tr></thead><tbody><tr><td>R2-C-10<br>(Sect. 3.5.1)</td><td>Map tropospheric gases at 0-45 km altitude:<br>(1) H<sub>2</sub>O partial column random error: &lt; 10% (and &lt; 20% below 15 km altitude)<br>(2) HDO partial column random error: &lt; 20% (and &lt; 30% below 15 km altitude)<br>(3) CO partial column random error: &lt; 14%<br>(4) OCS partial column random error: &lt; 18%<br>(5) SO<sub>2</sub> partial column random error: &lt; 30%<br>at a horizontal resolution of &lt; 100 km (and &lt; 200 km for H<sub>2</sub>O and HDO below 15 km altitude),<br>to detect gradients related to surface-atmosphere and cloud interaction processes.</td></tr><tr><td>R2-C-20<br>(Sect. 3.5.3)</td><td>Map mesospheric gases at 65-80 km altitude:<br>(1) SO<sub>2</sub> partial column random error: &lt; 20%<br>(2) SO:SO<sub>2</sub> partial column random error: &lt; 25%<br>at a spatial resolution of 5 (small-scale convection and vertical mixing) to 50 km at UV wavelength<br>(3) H<sub>2</sub>O partial column random error: &lt; 20%<br>(4) HDO partial column random error: &lt; 20%<br>(5) H<sub>2</sub>O:HDO partial column random error: &lt; 25%<br>(6) SO<sub>2</sub> partial column random error: &lt; 20%<br>(7) CO partial column random error: &lt; 20%<br>(8) OCS partial column random error: &lt; 20%<br>at a spatial resolution of &lt;50 km at IR wavelength<br>in order to assess coupling between surface (volcanic activity, topographic features), cloud and mesospheric measurements.</td></tr></tbody></table>

The dayside measurements provide information about the mesosphere. In synergy with VenSpec-U, the possible exchange occurring between the layers below and above the clouds will be investigated, especially in the case of the detection of a plume. In preparation for the mission, the potential impacts of a volcanic plume have been discussed,28 and modeling efforts are ongoing.29,30

Based on our knowledge so far, the science requirements for the mission were decided upon and those related to VenSpec-H are summarized in Fig. 1.

Simultaneous observations of different species at a high spectral resolution will be the asset of our VenSpec-H experiment. Few investigations reported measurements of different molecular species in the same spectra. Nevertheless, anti-correlation was highlighted between SO₂ and H₂O³⁴ or OCS and CO.²⁶ The new instrumentation will enable us to derive a map of these four molecular species altogether. This will be done by measuring a wide spectral range in the 2.3 µm window at high spectral resolution. As will be discussed, the bandwidth is limited by the spectral resolution that needs to be as high as possible to be able to differentiate the ro-vibrational lines for each molecule. This requirement will be translated into an increase of the resolving power by a factor of 4 to 5 compared with VIRTIS-H/Venus Express (R = 2000). A higher spectral resolution has already proven being decisive in highlighting variation of SO₂ in the 2.46 µm band.³⁵ Using radiative transfer tools and our expertise with the NOMAD-LNO instrument onboard ExoMars Trace Gas Orbiter,³⁶ we describe the instrumental requirements of this new experiment capable of measuring trace gases at high resolution both in the troposphere and in the mesosphere of the Venus atmosphere.

# 3 Modeling the Venus Atmosphere

## 3.1 Radiative Transfer Code

To define the instrumental requirements of a new experiment, such as the signal-to-noise ratio (SNR), the resolving power (RP), and the spectral ranges, our radiative transfer code ASIMUT-ALVL was used extensively. ASIMUT is a modular program for radiative transfer calculations in planetary atmospheres.³⁷ One of the main particularities of the software is the possibility to retrieve columns and/or profiles of atmospheric constituents simultaneously from different spectra, which may have been recorded by different instruments or obtained under different geometries. This allows the possibility to perform combined retrieval, e.g., of a ground-based measurement and a satellite-based one probing the same air mass or from spectra recorded by different instruments on the same platform.³⁸ This radiative transfer code derives the Jacobians analytically and includes the optimal estimation method,³⁹ using diagonal or full covariance matrices.
Initially developed for the Earth's atmosphere, its applicability has been extended to extra-
terrestrial atmospheres, such as those of Mars⁴⁰⁻⁴² and Venus.⁴³ ASIMUT has been coupled to
SPHER/TMATRIX⁴⁴,⁴⁵ and LIDORT⁴⁶ codes to include the complete treatment of the scattering
effects in the radiative transfer calculations.⁴⁷

ASIMUT-ALVL can be used as a forward modeling tool and also as a retrieval module.
It allows to fit simultaneously or sequentially different parts of one or more spectra, to fit the
surface temperature, to fit column/vertical profiles for molecular species and for aerosols, and
to fully characterize the outputs (averaging kernels, errors, degrees of freedom, etc.).

**3.2 Molecular Data**

When simulating the Venus atmosphere in the infrared spectral ranges, eight molecular species
are needed: CO, H₂O, HDO, CO, SO₂, HCl, HF, and OCS. Each of them has their own absorp-
tion spectrum. Figure 2 shows the line intensities directly downloaded from HITRAN Online.⁷⁶
It shows how interesting the 2.3 to 2.5 µm spectral range is. The CO band is visible at one end
and the SO₂ one at the other. HF lines are more spaced to one another, due to the rotational
constant of the molecule. In terms of intensity, the SO₂ band is weak compared with the rest.

The line parameters were extracted from the HITRAN Online database⁴⁸ using HAPI.⁷⁷ The
broadening line parameters have not been modified accounting for the CO₂ buffer atmosphere.
This was a deliberate choice as in the frame of this investigation, our model still needed to be
validated. Line parameters associated with a CO₂ atmosphere are available in the literature for
some spectral regions⁷⁸⁻⁸¹ and will be taken into account in further works.⁸² This choice is sup-
ported by the fact that only simulations were done and the impact of one set of line parameters
over the other is low. In the next phase of the mission preparation, when retrievals are performed,
adequate parameters will need to be used as the retrieved abundances are affected by this
choice.⁸³⁻⁸⁵ HITEMP2010⁸⁶ and HITRAN Online for H₂O were both considered for this inves-
tigation. The spectroscopic databases and their different versions are regularly the topics of
validation papers⁸⁷⁻⁸⁹ as none is perfect. We decided to use the most recent one at a standard
temperature for traceability reasons first and also because the conclusion reached with this line
list should still hold with a more complete line list, more consistent for the high temperature of
the Venus atmosphere.

The vertical profiles of the different molecules are shown in Fig. 3.

The high pressure of the Venus' troposphere leads to collisional effects on molecular
spectra.⁹⁸ No theoretical model is perfect yet, and *ad hoc* solutions are used. Especially, two
important effects of pressure were mitigated in our radiative transfer model. First, to account
for the far wings, each of the absorption lines is calculated on a 250-cm⁻¹ spectral interval,
i.e., 125 cm⁻¹ on each side of the line center. A Voigt profile was used for all molecules except
for CO₂ which requires a sublorentzian profile.⁹⁹ Second, the CO₂ continuum was prescribed

<figure><img src="image_2.png" /><figcaption>**Fig. 2** Line intensities of CO in blue, SO₂ in orange with a multiplicative factor of 10³, OCS in green, and HF in red with a multiplicative factor of 10⁻³. The values have been downloaded from HITRAN Online on July 24, 2024.⁴⁸⁻⁷⁵ Water lines are not shown for better readability, but they do absorb in that spectral range.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Fig. 3 Molecular species profiles used in this work.⁹⁰⁻⁹⁷</figcaption></figure>

based on the same methodology as in Refs. 99–102. Empirical values based on the VIRTIS/Venus Express spectra were determined and used.

### 3.3 Atmospheric Data

ASIMUT-ALVL was used to calculate synthetic spectra, using a line-by-line approach. Simulations of the dayside and the nightside of the Venus atmosphere were performed in the infrared spectral region spanning wavelengths from 1 to 2.6 µm. Eight molecular species were

<figure><img src="image_4.png" /><figcaption>Fig. 4 Pressure (a) in log scale and temperature (b) profiles in function of altitude, used in this work.⁹¹,⁹⁵</figcaption></figure>
<table><caption>**Table 1** Mean radius and variance of the different modes of the Venus' aerosols.</caption><thead><tr><td>Mode</td><td>mean radius [μm]</td><td>σ [-]</td></tr></thead><tbody><tr><td>1</td><td>0.3</td><td>1.56</td></tr><tr><td>2</td><td>1.0</td><td>1.29</td></tr><tr><td>2p</td><td>1.4</td><td>1.23</td></tr><tr><td>3</td><td>3.65</td><td>1.28</td></tr></tbody></table>

included, CO₂, H₂O, HDO, CO, SO₂, HCl, HF, and OCS, as well as the aerosols and the CO₂ continuum contribution. The concentration profiles of the molecular species are based on the literature⁹⁰⁻⁹⁴,¹⁰³ and further refined into a common reference scenario defined at the mission level. Rayleigh scattering was also taken into account.¹⁰⁴

The temperature and pressure profiles were obtained from the literature,⁹¹,⁹⁵ as shown in Fig. 4.

The description of the cloud layer was based on former studies, namely, Refs. 101 and 105. Because the particles are liquid and approximately spherical, and because the wavelength of the light is of the same order as the radius of the particles, a Mie code is appropriate for the calculation of their scattering properties. The particle number densities of four cloud modes were distributed from 0 to 100 km according to Figure 12 in Ref. 101. The modes themselves are described in Table 1. We assumed a lognormal distribution of sizes and the refractive indices were taken from Ref. 106.

# 4 Instrumental Requirements

Based on the quantities described in Sec. 3, spectral simulations were performed. The radiance of the nightside comes from the thermal emission of the surface and atmosphere, whereas on the dayside, where we considered a solar zenith angle of 30 deg, there is an additional solar reflection contribution. We used our tool to determine the following:

* limits of the spectral ranges and the resolving power

* signal-to-noise ratio.

## 4.1 Bandwidth and Resolving Power

The baseline of our instrument is based on SOIR/Venus Express⁴³,¹⁰⁷ and NOMAD-LNO/ExoMars Trace Gas Orbiter.¹⁰⁸,¹⁰⁹ To retain much heritage from these successful instruments, the optical concept of our VenSpec-H spectrometer is based on an echelle grating coupled to a high-performance, actively cooled detector.¹⁶ The characteristics of the echelle grating that define the free spectral range and the instrument line profile and of the detector, i.e., its size in pixels, need to be taken into account when defining the scientific requirements. Bandwidth and spectral resolution must be balanced out. The resolving power of a grating spectrometer with well-corrected optics is independent of the wavelength. It defines the smallest difference in wavelength of two spectral lines that can be distinguished from one another. The bandwidth is the wavelength range seen by the detector. They are defined by the following Eq. (1):

$$R_{\max} = \frac{n_{\text{pix}} \times 10000}{\lambda \times \text{BW}} \qquad (1)$$

where $R_{\max}$ is the maximum resolving power, $n_{\text{pix}}$ is the number of pixels in the spectral direction, and BW is the bandwidth.

As an example, let us imagine a detector of 500 pixels wide, in the spectral direction. To achieve a resolving power of 7000 in the 2.5-μm band, Eq. (1) indicates that the bandwidth cannot be larger than 285.7 cm⁻¹. Besides, the minimum value of bandwidth assumes that the pixel pitch is the limiting factor and that the full width at half maximum of the instrumental line profile, i.e., the image of the slit for monochromatic illumination, is not wider than one pixel.
If a slit is used with an equivalent width of more than one pixel, the maximum bandwidth is divided by the slit width in pixels. In other words, the higher the resolving power, the narrower the bandwidth for a fixed number of pixels.

A trade-off must be found here. On one hand, measuring simultaneously CO and SO₂, so far apart in wavelength, as shown in Fig. 2 imposes a bandwidth of several hundreds of wavenumbers. On the other hand, a high resolving power is required to distinguish the spectral features of H₂O in the 1.17-µm band, as shown in Fig. 5. This led to strong constraints on the optical design.¹⁶,¹¹⁰

The physical characteristics of the grating may deviate from the ideal band centers when deriving them only based on spectroscopy. One band center may be selected very precisely. The other wavelength ranges probed will flow down from this first choice. Considering the scientific objectives of the mission and of the instrument, the cold overtone vibrational band 3ν₃ (003-000) of SO₂, centered at 2.45 µm, is considered as the primary target, and the optical design was optimized accordingly.

## 4.2 Signal-to-Noise Ratio

To determine the required SNR, radiances were simulated at two resolving powers (RP = 7000 and 8500), and noise was added before performing the retrievals with ASIMUT-ALVL. The science objective of 10% accuracy on each molecule was the target (except in the spectral band probing the near-surface contents), see Fig. 1.

The error budget on the retrieved abundances was estimated considering the following contributions:

* random error: retrieval error directly from the fit

* systematic errors: cross-sections (3%), raytracing (1%), and source term (night: 1.2%; day: 3.6%).

The considered systematic errors are originating from unavoidable uncertainties in the radiative transfer model. The uncertainties on the cross sections are linked to the choice of spectroscopy (HITEMP and HITRAN, with or without CO₂ broadening). All line parameters are not available yet for Venus' conditions. Besides the error on intensities, for instance, H₂O varies from HITRAN error level 8 to 3, i.e., from less than 1% to above 20% relative error. The other molecules errors are mostly around 5% to 10% on their line intensities. In our error budget,

<figure><img src="image_5.png" /><figcaption>**Fig. 5** Simulated radiance of the Venus atmosphere at different resolving powers: 14,000 in black, 7000 in red, and 2000 in blue. To distinguish among the different spectral lines, a resolving power of at least 7000 is required.</figcaption></figure>
we assumed a 3% error on the calculated cross-sections based on these HITRAN parameters. The raytracing error is defined based on what impact could a pointing error have in the optical path length. The source term errors on the nightside were calculated based on the impact on the Planck function when varying the surface temperature by 1 K. For the dayside, a factor of 3 was added to this error to take into account the complexity of the incoming radiation.

The error budget in % was plotted against the SNR values, as shown in Fig. 6. The SNR value to meet the required accuracy was obtained by interpolation as shown by the dotted lines in Fig. 6. This was done for each molecule in each spectral band (B).

<figure><img src="image_6.png" /><figcaption>**Fig. 6** Error budget of Band1 (B1) on the nightside for H₂O. As expected, the accuracy depends on the achievable SNR.</figcaption></figure>

<table><caption>**Table 2** Spectral bands, as defined after this investigation. The required resolving power is 7000.</caption><thead><tr><th></th><th>Band limits (nm)</th><th>Band limits (cm<sup>-1</sup>)</th><th>Bandwidth (cm<sup>-1</sup>)</th><th>Required SNR</th><th>Molecular species</th><th>Altitude range (km)</th></tr></thead><tbody><tr><td>Nightside</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>B1</td><td>1160 to 1180</td><td>8474 to 8621</td><td>147</td><td>48</td><td>H<sub>2</sub>O and HDO</td><td>0 to 15 km</td></tr><tr><td>B2a</td><td>2340 to 2420</td><td>4132 to 4274</td><td>142</td><td>126</td><td>CO, H<sub>2</sub>O, HDO, OCS, and HF</td><td>30 to 45 km</td></tr><tr><td>B2b</td><td>2423 to 2507</td><td>3989 to 4127</td><td>138</td><td>126</td><td>SO<sub>2</sub>, H<sub>2</sub>O, HDO, OCS, and HF</td><td>30 to 45 km</td></tr><tr><td>B3</td><td>1704 to 1747</td><td>5724 to 5868</td><td>144</td><td>70</td><td>H<sub>2</sub>O and HCl</td><td>20 to 30 km</td></tr><tr><td>Dayside</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>B2a</td><td>2340 to 2420</td><td>4132 to 4274</td><td>142</td><td>100</td><td>CO, H<sub>2</sub>O, HDO, OCS, and HF</td><td>55 to 80 km</td></tr><tr><td>B2b</td><td>2423 to 2507</td><td>3989 to 4127</td><td>138</td><td>100</td><td>SO<sub>2</sub>, H<sub>2</sub>O, HDO, OCS, and HF</td><td>55 to 80 km</td></tr><tr><td>B4</td><td>1367 to 1394</td><td>7174 to 7315</td><td>141</td><td>100</td><td>H<sub>2</sub>O and HDO</td><td>55 to 80 km</td></tr></tbody></table>
**5 Conclusions**

In this investigation, the molecular vertical profiles, the aerosols' model, and the CO₂ continuum
contribution were validated for the different spectral windows. This enabled us to determine the
spectral bands, their bandwidth, and the resolving power necessary for our purposes. These val-
ues were confirmed as acceptable by performing retrievals for each spectral band. Diagnostics
tools, such as the residuals of the fit and the degree of freedom for each species, were analyzed.
The science requirements are driven by the lowest resolving power enabling the detection and
the retrieval of the abundance of the different molecular species. The resolving power versus
bandwidth trade-off has been investigated bearing in mind instrumental capabilities to reach
a sensible solution, allowing us to resolve ro-vibrational lines with a high spectral resolution
and cover as wide as possible spectral range to measure simultaneously different molecular trace
gases.

Table 2 summarizes the results obtained in this investigation.

In the near future, we will strengthen our model by comparing it to previous datasets from VIRTIS/Venus Express.<sup>102,111</sup> Possible improvements are related to remaining uncertainties in the model such as for the CO<sub>2</sub> continuum treatment. A campaign of laboratory measurements that will complement the investigation has started<sup>82</sup> and will continue. New science avenues impacted the design, such as the need for polarimetric measurements.<sup>112</sup>

**Disclosures**

The authors declare that there are no financial interests, commercial affiliations, or other potential
conflicts of interest that could have influenced the objectivity of this research or the writing of
this paper.

**Code and Data Availability**

The ASIMUT-ALVL code is made of two main components: ASIMUT which is an in-house radiative transfer model, available on demand to the BIRA-IASB team, and LIDORT which is developed by R. Spurr and available for download on http://rtslidort.com

The data for the molecular absorption are available on HITRAN (https://hitran.org/). The atmospheric scenario, including the aerosols' distribution, is described in sufficient detail in the paper and is based on a literature review.

**Acknowledgments**

This work has been performed with the support of the Belgian Science Policy Office (Contract No. 4000144206), with financial and contractual coordination by the ESA Prodex Office. EM acknowledges the support from CNES and ESA for all EnVision-related activities.

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**Séverine Robert**, after her PhD in sciences (ULB, Belgium), joined the Planetary Atmospheres Group at the Royal Belgian Institute for Space Aeronomy (BIRA-IASB) as an expert in molecular spectroscopy. First on the SOIR's dataset and then on NOMAD's, she acquired expertise in retrieval methods and also in instrumental and mission development. She is currently an instrument lead scientist of VenSpec-H and head of the "Planetary Atmospheres" research unit and of the "Solar Radiation in Atmospheres" scientific division.

Biographies of the other authors are not available.