Determination of the Venus upper atmosphere gas concentration combining the SOIR/Venus Express and Torque aerobreaking observations

Until the ESA Venus Express mission, the Venus mesosphere was a poorly understood region of Venus’s atmosphere. This mission revealed the composition and thermal structure of the mesosphere and lower-thermosphere [1] with the help of its suite of instruments: VIRTIS [2], SPICAV-IR and SPICAV-UV [3], VeRA [4], and SOIR [5].

In particular, the SOIR instrument performed solar occultation measurements in the IR region (2.2 - 4.3 µm) at a spectral resolution of 0.12 cm^-1, among the highest of all space instruments. It combined an echelle spectrometer and an Acousto-Optical Tunable Filter for the order selection. SOIR performed more than 1500 solar occultation measurements leading to about two millions spectra [6].

Above 100 km, SOIR could derive the CO₂ and CO concentration, up to ~170 km, and, using the hydrostatic equation, the temperature could be obtained [7, 8].

Towards the end of the Venus Express mission, an aerobreaking campaign (VExADE) was conducted, making the spacecraft dive into the atmosphere down to altitudes of ~130 km [9]. This experiment used the accelerometer data to compute the total mass density by computing the forces that were measured by the spacecraft due to aerobraking .

In this work, we use the data from both the SOIR and VExADE experiments to derive the high altitude concentrations of N₂ and atomic oxygen, providing information that is not yet available for this atmospheric region. Indeed, N₂ and O are not measurable in the infrared using a spectrometer since these species do not have absorption lines in this spectral region. Moreover, from previous observations obtained by the Pioneer Venus mass spectrometer [10], N₂ and CO could not be differentiated due to their same molar masses.

We will present the results and discuss their impact on the photochemistry, CO and O being byproducts of the CO₂ photodissociation occurring in the Venus mesosphere. We will also compare our results to the ones obtained from the state-of-art Global Circulation Model (GCM) Venus Climate Database [11].

 

References:

[1] Limaye, S.S., et al. (2018), Space Science Reviews

[2] Drossart, P., et al. (2007), Planet. Space Sci.

[3] Bertaux, J.L., et al. (2007), Planet. Space Sci.

[4] Tellmann, S., et al. (2012), Icarus

[5] Nevejans, D., et al. (2006), Applied Optics

[6] Vandaele , A.C., et al. (2016), Adv. Space Res.

[7] Mahieux, A., et al. (2023), Icarus

[8] Mahieux, A., et al. (2015), Planet. Space Sci.

[9] Müller-Wodarg, I.C.F., et al. (2016), Nature Physics

[10] von Zahn, U., et al. (1980), J. Geophys. Res.

[11] Gilli, G., et al. (2021), Icarus