Atomic oxygen on the dayside and nightside of Venus measured by SOFIA

Atomic oxygen is an important for the photochemistry in the mesosphere and thermosphere of Venus and can be used as tracer for atmospheric dynamics. The altitude range where it predominantly occurs is between 90 km and 130 km. It is mainly generated through photolysis of CO2 on the dayside. From there it is transported to the nightside by the subsolar to antisolar circulation. It accumulates near the antisolar point and recombines to molecular oxygen 1, 2 ,3, 4.We have detected atomic oxygen on the dayside as well as on the nightside of Venus by measuring its ground-state transition at 4.7 THz (63.2 m) with the upGREAT (German Receiver for Astronomy at Terahertz Frequencies) heterodyne spectrometer on board SOFIA (Stratospheric Observatory for Infrared Astronomy) 5. This is a direct detection in contrast to most of past and current detection methods, which are indirect and rely on photochemical models to obtain atomic oxygen concentrations 1, 2. The observations were made on Nov. 10, Nov. 11 and Nov. 13 2021. On each day the total measurement time was about 20 min with the 2.5-m diameter telescope of SOFIA pointing at Venus which was at an elevation of about 18.7. The telescope provides a diffraction limited beam with 6.3 arcsec diameter, which is about five times smaller than the apparent diameter of Venus (29 arcsec). The phase of Venus was 42%. The 4.7-THz frequency channel of upGREAT has seven pixels in a hexagonal pattern separated by 13.6 arcsec. The central three pixels were pointed along the terminator. The atomic oxygen in the atmosphere of Earth was measured simultaneously with the atomic oxygen in the Venus atmosphere, with the latter being offset by 203 MHz due to the Doppler shift of Venus, which is 13 km/s. The telluric line was used for calibration of the Venus spectra (see Ref. 5 for details).The data was analyzed with a radiative transfer code with the atomic oxygen in the Venus atmosphere modeled as a single layer with a constant temperature and constant density (for details see Ref. 5). The column density measured with upGREAT ranges from 0.7 to 3.8x1017cm-2 between 15:00 and 21:00 local time with a maximum on the dayside where atomic oxygen is generated. It decreases towards the day-night terminator, because of the increasing solar azimuth angle. At nighttime no trend of the column density is observable. Our measurements indicate that the circulation in the Venus atmosphere determines the distribution of atomic oxygen on the nightside. The observed average Venus continuum brightness temperature on the dayside is ~242 K, which is slightly below the brightness temperature on the nightside (~249 K) . This might be attributed to a difference of the opacity of the atmosphere on the dayside and nightside, i.e. the opacity on the nightside is slightly lower and the brightness temperature is determined by a lower atmospheric layer. The brightness temperatures correspond to altitudes between about 65 and 70 km, which is just above the cloud layer. The temperature of the atomic oxygen is ~156 K on the dayside and ~115 K on the nightside. Comparing this with the temperature profile of the Venus atmosphere this yields an altitude of around 100 km where most of the atomic oxygen is located.The observation of the 4.7-THz transition of atomic oxygen by high-resolution terahertz spectroscopy provides new insights into the atmosphere of Venus. In the future it may enable detailed investigations of the Venusian atmosphere in the peculiar region between the two atmospheric circulation patterns.1 A. S. Brecht et al., Atomic oxygen distributions in the Venus thermosphere: Comparisons between Venus Express observations and global model simulations. Icarus 217, 759766 (2012).2 L. Soret et al., Atomic oxygen on the Venus nightside: Global distribution deduced from airglow mapping. Icarus 217, 849855 (2012).3 J.-C. Grard, Aeronomy of the Venus upper atmosphere. in: Space Sci Rev. Venus III edited by B. Bzard et al., Springer Dordrecht (2017).4 G. Gilli, et al. Venus upper atmosphere revealed by a GCM: II. Model validation with temperature and density measurements. Icarus 366, 114432 (2021).5 H.-W. Hbers et al., Direct detection of atomic oxygen on the dayside and nightside of Venus, Nature Communications, 14:6812 (2023).