Studying the Mars atmosphere using SOIR-TGM

Vandaele, Ann-Carine; Drummond, Rachel; Lopez-Moreno, J.J.; Daerden, Frank; Neefs, Eddy; Bellucci, Giancarlo and Patel, Manish (2010). Studying the Mars atmosphere using SOIR-TGM. In: 38th COSPAR Scientific Assembly, 18-15 Jul 2010, Bremen, Germany.

Abstract

SOIR (Solar Occultation InfraRed spectrometer) is an echelle infrared spectrometer on board the Venus Express orbiter (VEx). SOIR probes the Venus atmosphere by solar occultation, operating between 2.2 and 4.3µm with a typical resolution of 0.15 cm^-1. This spectral range is suitable for the detection of several key components of planetary atmospheres, including H₂O and its isotopologue HDO, CH₄ and other trace species. The Acousto-Optical Tunable Filter (AOTF) allows a narrow range of wavelengths to pass, according to the radio frequency applied to the TeO2 crystal; this selects the order. The advantage of the AOTF is that different orders can be observed quickly and easily during one occultation, allowing quasi-simultaneous measurement of, for example, CO₂ with CH₄. The SOIR instrument was designed to have a minimum of moving parts, to be light and compact. To obtain a compact optical scheme, a Littrow configuration was implemented in which the usual collimating and imaging lenses are merged into a single off-axis parabolic mirror. The light is diffracted on the echelle grating, where orders overlap and addition occurs, and finally is recorded by the detector. The detector is 320×256 pixels and is cooled to 88 K during an occultation measurement, to maximise the signal to noise ratio. SOIR on VEx has been in orbit around Venus since April 2006, allowing us to characterise the instrument and study its performance. These data have allowed the engineering team to devise several instrumental improvements that would not only improve SOIR as a solar occultation instrument but also allow it to observe nadir - as a trace gas mapper. On-board a Mars orbiter, SOIR-TGM would constrain the profiles and source location of methane, amongst other important gases. We will show Mars data as could be observed by a SOIR instrument to demonstrate what SOIR would be capable of in Mars orbit.