A phenomenon, thought impossible on Mars, compresses its atmosphere like squeezing a tube of toothpaste. Dubbed Zwan-Wolf, this effect was known on Earth but never observed elsewhere. Scientists believed it impossible on Mars, which lacks a global magnetic field. Yet, data from a NASA probe reveals the opposite.
This result comes from the analysis of events that occurred in December 2023. A powerful coronal mass ejection (CME) struck Mars, disturbing its upper atmosphere. Studying data from the MAVEN probe, orbiting the planet since 2014, allowed researchers to spot strange fluctuations. These 'wiggles' perfectly matched the signature of the Zwan-Wolf effect, which traps and compresses plasma along magnetic structures.
The MAVEN space probe.
Image: NASA's Goddard Space Flight Center
On Earth, this phenomenon occurs at altitudes of tens of thousands of miles, in the magnetosphere. On Mars, however, it takes place much lower, in the ionosphere, around 125 miles (200 km) high. This region is filled with ionized gas, or plasma. The effect is likely triggered at the boundary where the solar wind meets this ionospheric plasma. Scientists estimate it could occur continuously, but only the violence of the solar eruption made it visible.
This discovery changes our understanding of interactions between the Sun and Mars. Understanding these mechanisms is crucial for future crewed missions. The ionosphere influences communications, satellite orbits, and radiation levels on the ground. Understanding how space weather affects the Red Planet helps better protect equipment and astronauts who will one day venture there.
Researchers believe the Zwan-Wolf effect could also exist on other worlds. Venus, with its very dense atmosphere, or Titan, Saturn's large moon, are good candidates. These worlds also lack a global magnetic field, but their ionospheres could exhibit similar structures. Future observations confirming this hypothesis would further broaden our view of space weather in the Solar System.
Solar wind and coronal mass ejections (CMEs)
Our Sun constantly emits a stream of charged particles called the solar wind. This flow, though tenuous, blows through the entire Solar System.
Sometimes, a solar flare or coronal mass ejection (CME) suddenly ejects an immense bubble of plasma and magnetic field. These CMEs are much more powerful than the normal solar wind. When they strike a planet, they can compress its atmosphere, disrupt communications, and even increase radiation levels.
On Mars, which lacks a global magnetic shield, these solar storms have a direct impact on the upper atmosphere. The particularly powerful CME of December 2023 thus revealed a previously invisible phenomenon.
The Martian ionosphere
The ionosphere is the upper layer of a planet's atmosphere, where ultraviolet rays from the Sun strip electrons from atoms, creating a plasma of charged particles. On Mars, this region extends from about 62 to 186 miles (100 to 300 km) in altitude.
Unlike Earth, Mars does not have a global magnetic field generated by its core. This makes its ionosphere very vulnerable to the solar wind, which can compress or erode it. It is in this zone that the Zwan-Wolf effect was discovered.
Understanding its behavior is essential for future missions, as the ionosphere influences radio signals and can create interference with orbiting spacecraft.