How does the planet Venus retain its dense atmosphere? Unlike our Earth, it does not have the protection of a global magnetic field. Scientists now know more thanks to the almost simultaneous flyby – a rare event – of Venus by two spacecraft.
Two observation craft close to the second planet of the Solar System at the same time, it does not happen every day! Especially because Solar Orbiteras its name suggests, was intended for the study of our solar star, and BepiColombo is on a seven-year journey to observe Mercury (read box).
>> Images taken by BepiColombo on August 10, 2021 during its second pass near Venus (the probe will make nine in all):
Scientific synergy
These two vessels benefit from the gravitational assistance of the planets to propel themselves through space and arrive at the right destination: this is why they met on August 9 and 10, 2021 in the vicinity close to Venus. The opportunity to send to Earth observations captured in synergy from two different points of view: scientists from Europe and Japan have drawn one study published in Nature Communications mid-December.
The idea was to study the “stagnation region”, a very little explored zone (in red on the illustration) located at the front of the magnetosphere, where some of the most important effects of the interaction between Venus and the solar wind are noted. This zone extends a wide distance of 1900 kilometers above the surface of the planet – something astronomers did not expect.
The convergence of BepiColombo and Solar Orbiter around Venus in August 2021 provided a rare opportunity to study the “stagnation region”, an area of the Venusian magnetosphere where some of the most important effects of the interaction between Venus and the solar wind are observed. [Thibaut Roger/Europlanet 2024 RI – CC BY-Nc-SA 4.0]
An induced magnetosphere
Our Earth has an intrinsic magnetic field that protects it from solar wind and cosmic radiation; Venus does not.
>> Read also: Earth’s core may have slowed down and will slowly pick up again
In contrast, an “induced magnetosphere” (in pink) is created around it through the interaction of the solar wind (the white arrow) – a stream of charged particles emitted by the Sun – and electrically charged particles located in its upper atmosphere, the ionosphere: it is a stable barrier that protects the Venusian atmosphere from erosion by the wind coming from our star.
Around this magnetic bubble, the solar wind is considerably slowed down, heated and deflected like the wake of a boat in a region called “magnetosheath” (in yellow, “magnetosheath” in English).
This discovery, which is contrary to previous predictions, sheds new light on the link between magnetic fields and atmospheric loss due to the solar wind: “The effectiveness of an induced magnetosphere in helping a planet retain its atmosphere has implications for understanding the habitability of exoplanets without internally generated magnetic fields,” says study co-author Sae Aizawa of the Institute of Space and Astronautical Sciences (ISAS) from JAXA, the Japanese space agency.
>> An artist’s impression of Solar Orbiter passing close to Venus for the second time on August 9, 2021:
>> Read also: NASA announces two new Venus exploration missions
Stephanie Jaquet