Atomic oxygen on the day and night side of Venus' atmosphere
- upGREAT measured the concentration of atomic oxygen on the day and night side of Venus.
- Measurements were carried out using the upGREAT terahertz spectrometer on board the SOFIA airborne observatory.
- Focus: Spaceflight, planetary research, terahertz research, sensor systems
The Solar System has two remarkably similar planets – Earth and Venus. They are thought to be the same age, comparable in size, and probably formed from the same materials. However, there are also major differences between these two celestial bodies. While Earth has a blue sky, oceans of liquid water brimming with life, and an oxygen-rich atmosphere, Venus is surrounded by dense cloud cover and an atmosphere comprising carbon dioxide, nitrogen and various trace gases. The concentration of atomic oxygen on both the day and night sides of the venusian atmosphere has now been measured directly for the first time using the 'upgraded German Receiver for Astronomy at Terahertz Frequencies' (upGREAT), a far-infrared spectrometer flown on board the Stratospheric Observatory for Infrared Astronomy (SOFIA).
There are two strong currents in the atmosphere of Venus. Below approximately 70 kilometres, there are hurricane-force winds blowing against the direction of Venus' rotation, but above 120 kilometres, strong winds flow in the direction of rotation. A layer of atomic oxygen exists between these two opposing atmospheric currents. This layer is produced by ultraviolet radiation from the Sun, which breaks down the carbon dioxide and carbon monoxide in Venus' atmosphere into atomic oxygen and other products.
A direct measurement method detects atomic oxygen
Previous and current detection methods are indirect and are based on measurements of other molecules in combination with photochemical models. In November 2021, researchers from the DLR Institute of Optical Sensor Systems, the Max Planck Institute for Radio Astronomy and the University of Cologne succeeded in directly detecting the extremely reactive oxygen atoms in the atmosphere of Venus for the first time. The measurements were carried out using the upGREAT spectrometer on SOFIA. "The measurements were particularly challenging because Venus could only be observed with SOFIA for approximately 20 minutes on three days and was only slightly above the horizon. Thanks to upGREAT’s outstanding measurement sensitivity and SOFIA's unique capabilities, it was possible to create a map of the oxygen distribution on Venus," says Heinz-Wilhelm Hübers, Director of the DLR Institute of Optical Sensor Systems and first author of the publication announcing the results.
Results of the measurements
The emissions from Venus were measured in a narrow frequency range around 4.74 terahertz (THz), which corresponds to a wavelength of 63.2 micrometres. The atomic oxygen in Venus' atmosphere absorbs this radiation. This is comparable to the Fraunhofer lines in the solar spectrum, which give an indication of the atoms in the Sun's atmosphere. In the terahertz spectrum of Venus, an absorption line appears that is characteristic of atomic oxygen. The strength and shape of the absorption signal is a measure of the amount of atomic oxygen and its temperature. "We were able to show that oxygen is formed on the day side of Venus and that its concentration also drops with decreasing solar radiation. On the night side, a local increase in concentration indicates an enrichment of atomic oxygen as a result of wind currents," Hübers explains.
The temperature of the atomic oxygen, which ranges from approximately minus 120 degrees Celsius on the day side to minus 160 degrees Celsius on the night side, indicates that it is mainly present in an altitude layer at around 100 kilometres. Its concentration is about 10 times lower than in Earth's atmosphere. The clear differences compared to our planet may in future contribute to a better understanding of why Earth and its sister planet Venus have developed so differently.
The Stratospheric Observatory for Infrared Astronomy (SOFIA) was a joint project of the German Aerospace Center (DLR) and the National Aeronautics and Space Administration (NASA). It was implemented at DLR's instigation with funding from the Federal Government (Federal Ministry for Economic Affairs and Climate Action), the state of Baden-Württemberg and the University of Stuttgart. Scientific operations were coordinated on the German side by the German SOFIA Institute (DSI) at the University of Stuttgart and on the American side by the Universities Space Research Association (USRA).
upGREAT is a further development of the German Receiver for Astronomy at Terahertz Frequencies (GREAT), a far-infrared spectrometer that has been used to carry out numerous successful scientific flights on board SOFIA since 2011. The instrument was developed and built by a consortium of German research institutes – the Max Planck Institute for Radio Astronomy and the Cologne Observatory for SubMillimeter Astronomy (KOSMA) at the University of Cologne, in collaboration with the DLR Institute of Optical Sensor Systems in Berlin.