BepiColombo's recent flyby of Mercury captured valuable data about its magnetic field and surrounding space plasma, hinting at complex magnetospheric features and interactions between the planet's surface and its thin atmosphere. These preliminary findings lay the foundation for more comprehensive studies after the spacecraft enters Mercury's orbit in 2026.
Like Earth, Mercury has a magnetic field, but its magnetic field is a hundred times weaker at the planet's surface. Although the magnetic field is weak, it creates a protective bubble in space, called the magnetosphere, that shields Mercury from the relentless particles emitted by the Sun, known as the solar wind. Mercury's proximity to the Sun intensifies these interactions, affecting the magnetosphere and planetary surface to a greater extent than Earth's. Understanding the dynamics of this magnetic bubble and the properties of the particles it contains is the primary goal of the BepiColombo mission.
When BepiColombo arrives at Mercury in 2026, it will fly by Earth, Venus and Mercury to adjust its speed and orbit so that it can enter an orbit around Mercury. The current "stack" spacecraft will separate and deploy two science orbiters - the ESA-led Mercury Planetary Orbiter (MPO) and the Japan Aerospace Exploration Agency-led Mercury Magnetospheric Orbiter (MMO, or Mio) - into complementary orbits to make the necessary dual-spacecraft measurements to fully characterize Mercury's dynamic environment.
During the flyby, as the spacecraft speeds past Mercury, its many scientific instruments provide a sneak preview of the exciting scientific research to come. Additionally, flybys will provide unique insights into regions around Mercury that are not directly accessible from orbit.
The spacecraft only stayed in the magnetosphere for about 30 minutes, but in that short time it collected a wealth of information about the magnetic field, particles and plasma environment.
Lina Hadid, a former ESA researcher who now works at the Plasma Physics Laboratory of the Paris Observatory, used the Mercury Plasma Particle Experiment (MPPE) instrument suite used on Mercury during its flyby on June 19, 2023, to build an impressive picture of Mercury's magnetic field in a very short time.
"The flyby was fast; we crossed Mercury's magnetosphere in about 30 minutes, moving from dusk to dawn, as close as 235 kilometers from the planet's surface," she said. "We sampled the types of particles, how hot they were, and how they moved, allowing us to create a clear picture of the magnetic field landscape during this brief period."
Lina and her colleagues combined BepiColombo's measurements with computer modeling to map the various features encountered in the magnetosphere by determining the origin of detected particles based on their motion.
"We saw expected structures, like the 'shock' boundary between the free-flowing solar wind and the magnetosphere, and we also crossed the 'corner' on either side of the plasma sheet, which is a hotter, denser region of charged gas that flows like a tail away from the sun. But we also had some surprises."
Lina is the principal co-investigator of the MPPE and the director of one of the instruments, the mass spectrometer. She co-authored the paper "Physics of Communications" with former instrument leader Dominique Delcourt, introducing relevant results.
He said: "We detected a so-called low-latitude boundary layer, which is defined by a region of turbulent plasma at the edge of the magnetosphere, where we observed a much wider range of particle energies than we have previously seen on Mercury, thanks in large part to the sensitivity of a mass spectrometer designed specifically for Mercury's complex environment. BepiColombo will be able to determine the ion composition of Mercury's magnetosphere in more detail than ever before."
Lina added: "We have also observed high-energy hot ions trapped in the magnetosphere near the equatorial plane and at low latitudes, and we think the only way to explain this is with circulation, either partial or complete, but this is a hotly contested area."
Circulation currents are currents carried by charged particles trapped in the magnetosphere. There is a well-known gyre on Earth that is located tens of thousands of kilometers above the Earth's surface. On Mercury, it's less clear how particles become trapped within a few hundred kilometers of the planet, especially when the magnetosphere is squeezed against the planet's surface. This debate may be settled once MPO and Mio start collecting data full time.
Mercury has a magnetic field that interacts with particles from the Sun (the "solar wind"). This forms Mercury's magnetosphere - a sail-like bubble of space extending out from the sun. This simulation output shows what would be expected of Mercury's magnetic environment under typical solar wind conditions. The image on the left shows a "side view", with the Sun outside the box on the left; the image on the right shows a "front view", as we would see Mercury from the direction of the Sun. The colors represent the density of charged particles around Mercury, with the highest densities being yellow and the lowest densities being purple/black. The white lines are magnetic field lines. (The undisturbed solar wind appears dark orange. When the solar wind encounters Mercury's magnetic field, it is heated and deflected, creating a dense region of solar wind particles (shown in yellow).
Lina and her colleagues also observed direct interactions between the spacecraft and the surrounding space plasma. When a spacecraft is heated by the sun, it cannot detect cooler heavy ions because the spacecraft itself becomes charged and repels them. But as the spacecraft passed through the planet's night shadow, the charging situation was different, and suddenly a sea of cold plasma ions became visible. For example, it detected oxygen, sodium and potassium ions, which were likely flown from the planet's surface by micrometeorite impacts or interactions with the solar wind.
"It's like we're suddenly seeing a three-dimensional 'explosion' of surface components through the planet's very thin atmosphere, known as the exosphere," Dominique said. "It's really exciting to start seeing connections between the planet's surface and its plasma environment."
"In this rare dusk-to-dawn scan of the large-scale structure of Mercury's magnetosphere, we get a taste of the promise of future discoveries," said GoMurakamiJAXA's BepiColombo project scientist.
Geraint Jones, ESA's BepiColombo project scientist, added: "These observations highlight the need for two orbiters and their complementary instruments to tell us a complete story and build a complete picture of how magnetic fields and plasma environments change over time and space. We can't wait to see how BepiColombo will impact our wider understanding of planetary magnetospheres."
Meanwhile, scientists have begun digging into data obtained during last month's fourth close flyby of Mercury, while flight controllers are preparing for the final two back-to-back flybys scheduled for December 1, 2024, and January 8, 2025.
Compiled from/SciTechDaily