Figure 1 gives an example of the evolution of the SOIR spectra through one occultation (sunset April 15th 2007) in the orders 121, 149, 171 and 190 corresponding to the 2725-2750 cm‑1, 3330-3357 cm-1, 3820-3855 cm-1, and 4245-4283 cm-1 ranges, respectively.

These transmittances show the characteristic behavior observed on all occultation series measured by SOIR. At the beginning of the series, the light path does not cross the atmosphere. No absorption signatures are present and transmittances are equal to unity.

As the Sun sets, the light path goes deeper and deeper into the atmosphere, and two absorption processes take place: the overall signal decreases due to extinction by aerosols and absorption signatures appear.

At the end of the observation, no light is captured anymore, as the Sun disappears behind the cloud deck or moves out of SOIR’s field of view due to diffraction. The structures seen in the spectra. are mainly attributed to HDO (A), CO2 (B), H2O (C) and CO (D). From this figure, it can also be seen that in general, the SOIR spectra contain information on the Venus atmosphere between 65 km and 110 km for molecules such as HDO or HCl. For H2O and CO signatures are still observable up to 130 km altitude and CO2 features are seen up to 125-130 km !

Fig 1 : Example of spectra obtained during one occultation (sunset April 15th 2007 )
(credit: BISA)

Each transmittance is obtained by taking the ratio of the solar spectrum seen through the Venus atmosphere to the non-attenuated solar spectrum measured above the atmosphere. The selection of a spectral interval is achieved through the AOT filter, tuned in this case to (A) 15809 kHz for diffraction order 121, (B) 19869 kHz for diffraction order 149, (C) 23031 kHz for diffraction order 171, (D) 25742 kHz for diffraction order 121. In these particular ranges of wavenumber, the main absorption lines are from (A) HDO, (B) CO2, (C) H2O, (D) CO.


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Relevant Publications

A warm layer in Venus’ cryosphere and high-altitude measurements of HF, HCl, H2O and HDO (pdf - 384 KB)
Composition of the Venus mesosphere measured by SOIR on board Venus Express (pdf - 743 KB)
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