Mercury-Bound BepiColombo Spacecraft Observes Cold Oxygen and Carbon Ions Escaping the Induced Magnetosphere of Venus

On October 20, 2018, BepiColombo was launched as Europe's first mission to Mercury. During its seven-year journey to the smallest and least explored planet in our Solar System, the spacecraft had a fleeting visit to Venus, revealing surprising insights into its atmosphere.

Historical Evolution of Venus

Compared to Earth, Venus does not generate an intrinsic magnetic field in its core. However, a weak, comet-shaped induced magnetosphere is formed around the planet by the interaction of solar wind with electrically charged particles in Venus' atmosphere. Surrounding the magnetosphere is a region called 'magnetosheath' where the charged particles released by the sun are slowed and heated.

Venus had many similarities to Earth early in its history, such as the presence of significant amounts of liquid water. As the planet interacted with the charged particles released by the Sun, it caused water to get stripped away. As a result, it left the atmosphere composed mainly of carbon dioxide and smaller amounts of nitrogen and other trace species.

Previous space missions, such as ESA's Venus Express and NASA's Pioneer Venus Orbiter have made detailed studies of the type and amount of molecules and ions that are lost into space. However, their orbital paths left some areas around Venus unexplored and many questions still unanswered.

Escaping Ions

On August 10, 2021, BepiColombo passed by the Earth's twin to slow down and adjust its course towards its final destination of Mercury. The spacecraft picked up the long tail of Venus' magnetosheath and came out through the nose of the magnetic regions closest to the Sun.

BepiColombo made observations for one and a half hours where it measured the number and mass of charged particles it encountered. During the process, the spacecraft captured information about the chemical and physical processes that drive the atmospheric escape in the flank of the magnetosheath.

Data for the study were gathered during the spacecraft's second Venus flyby by BepiColombo's Mass Spectrum Analyzer (MSA) and the Mercury Ion Analyzer (MIA). Meanwhile, Europlanet's SPIDER weather modeling tools allowed the astronomers to monitor the propagation of the particles through the Venusian magnetosheath.

According to CNRS researcher Lina Hadid from Plasma Physics Laboratory (LPP), this is the first time that positively charged carbon ions have been observed escaping from the upper atmosphere of Venus. These are heavy ions that typically move slowly, so astronomers are still trying to understand the mechanisms behind it. They suspect either an electrostatic wind lifting them away from the planet, or they are accelerated through centrifugal processes.

Study co-author Moa Persson of the Swedish Institute of Space Physics explained that the recent results suggest that the atmospheric escape from Venus cannot fully explain the loss of the planet's historical water content. The result of this study is a vital step in unlocking the truth about the evolution of Venusian atmosphere, and future space missions will help fill in many gaps.

Over the next decade, a fleet of spacecraft will investigate the Earth's twin planet, such as India's Shukrayaan orbiter, NASA'a DAVINCI probe and VERITAS orbiter, and ESA's Envision mission. Collectively, these missions will provide a detailed picture of the Venusian environment, from the magnetosheath to the surface and interior.