Direct detection of atomic oxygen on the dayside and nightside of Venus

Atomic oxygen (O) plays a crucial role in the atmospheric chemistry and energy balance of Venus. It is highly abundant in the mesosphere and lower thermosphere, interacting with other molecules like O2, CO, and CO2, and significantly influencing the radiative cooling through CO2's 15 µm emission. Furthermore, O serves as a tracer for global circulation patterns in the upper thermosphere. Despite its importance, direct observations of O in the mesosphere and lower thermosphere have been scarce, limited primarily to nightside observations of the 557 nm green line and indirect inferences from O2 night airglow at 1.27 µm. These indirect methods rely on photochemical models and measurements of other molecules, introducing uncertainties. This study addresses this limitation by presenting the first direct detection of atomic oxygen on both the dayside and nightside of Venus using a novel approach.

Previous studies on Venusian atomic oxygen relied heavily on indirect detection methods. Night airglow observations, particularly the 557 nm green line, provided some information, but were confined to the nightside and showed limited variability. The Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) onboard the Venus Express (VEX) satellite offered another approach, deriving O density maps indirectly from observations of the O2 night airglow at 1.27 µm. This method, however, required global maps of O2 emission, CO2 density, temperature, and a photochemical model with a priori information on quenching coefficients and reaction rates, leading to potential uncertainties. While these indirect methods provided valuable insights, they lacked the direct observation and precision offered by this work.

This research employed the upGREAT (German Receiver for Astronomy at Terahertz Frequencies) array spectrometer on the Stratospheric Observatory for Infrared Astronomy (SOFIA). Observations were conducted on November 10, 11, and 13, 2021, targeting 17 positions on Venus (seven dayside, nine nightside, and one at the terminator). The method relies on the detection of atomic oxygen's ground-state fine structure ³P₁ → ³P₂ transition at 4.74 THz (63.2 µm). The high spectral resolving power of upGREAT allowed distinguishing the Venusian atomic oxygen line from the telluric line, which is Doppler-shifted due to the geocentric velocity of Venus. The telluric line served for frequency and radiometric calibration. Data analysis involved a radiative transfer model to determine column density and temperature of atomic oxygen from the absorption signal. The model accounted for atmospheric temperature, radiative properties, and column density of oxygen atoms. The continuum brightness temperature of Venus was obtained by analyzing the telluric atomic oxygen line and used in conjunction with temperature profiles derived from other observations to infer altitudes.

Atomic oxygen was successfully detected at all 17 observed positions on Venus, both dayside and nightside. Column densities ranged from 0.7 to 3.8 × 10¹⁷ cm⁻². The maximum column density was observed on the dayside, consistent with the generation of atomic oxygen through photolysis of CO2 and CO. The average continuum brightness temperature of Venus was approximately 246 K, corresponding to altitudes just above the cloud layer (65–70 km). The temperature of atomic oxygen was estimated to be around 156 K on the dayside and 115 K on the nightside, corresponding to altitudes around 100 km. These temperatures are consistent with Doppler broadening measurements of the line width. The observed column density exhibited a slight decrease with increasing solar zenith angle during the daytime. A slight, yet not statistically significant, difference in brightness temperature between the dayside and nightside was observed. A small local peak in column density was observed around 19:00–20:00 LT, potentially attributed to atmospheric dynamics.

This study provides the first direct detection of atomic oxygen on the dayside of Venus, confirming the generation mechanism through photolysis of CO2 and CO. The observation of atomic oxygen on the nightside, transported there by the subsolar-to-antisolar circulation, validates the transport processes previously hypothesized. The measured column densities are generally consistent with values obtained from models and previous observations of the O2 infrared nightglow. The slight discrepancy between our column density values and those from some models could be attributed to the differences in modeling approaches and the assumptions made in the photochemical models. The observed temperature difference between the dayside and nightside is also consistent with expectations, showing a cooling effect on the nightside.

This research presents the first direct measurements of atomic oxygen in the Venus mesosphere and lower thermosphere on both the day and night sides. The use of SOFIA/upGREAT enabled the observation of the 4.74 THz fine structure transition of atomic oxygen, yielding accurate measurements of column densities and temperatures. These findings provide crucial data for improving atmospheric models and understanding the unique characteristics of the Venusian atmosphere. Future observations focused on the antisolar and subsolar points, as well as a broader range of solar zenith angles, are recommended to refine our understanding of this atmospheric region. This research will be invaluable to future missions like DAVINCI+ and EnVision.

The observations were limited to a relatively short period and a specific set of locations on Venus due to the constraints of SOFIA observations. The low telescope elevation during observations caused significant atmospheric attenuation, limiting the achievable signal-to-noise ratio and affecting the precision of measurements, especially on the nightside. The radiative transfer model utilized assumes local thermodynamic equilibrium (LTE), which may not perfectly represent the Venusian atmosphere. The study did not cover the antisolar point, where the maximum column density is expected. Future work should address these limitations for more comprehensive characterization of atomic oxygen in the Venusian atmosphere.