BepiColombo: revelations from the Mercury mission

In 2026, the Bepi­Colombo¹ space mis­sion will arrive at its des­tin­a­tion: Mer­cury, the smal­lest plan­et in our sol­ar sys­tem and the closest neigh­bour of the Sun. Dur­ing its 7.9‑billion-kilometre jour­ney, it will pass close to the plan­et sev­er­al times to adjust its speed and tra­ject­ory so that it is “trapped” in its orbit when the time comes.

Mercury’s intrins­ic mag­net­ic field is weak, with a dipole strength nearly 100 times less than that of the Earth. How­ever, the exist­ence of a mag­net­ic field, even a weak one, on Mer­cury is sur­pris­ing in itself. It is nev­er­the­less power­ful enough to deflect the sol­ar wind – a stream of particles, mainly elec­trons and pro­tons, ejec­ted from the Sun’s upper atmo­sphere (in oth­er words, its corona). This “shield”, or mag­neto­sphere, is sim­il­ar to the Earth’s mag­neto­sphere. The dif­fer­ence is that the fun­da­ment­al pro­cesses that release plasma and energy occur much more rap­idly in Mercury’s magnetosphere.

Mer­cury has already been flown over sev­er­al times. The first mis­sion, Mar­iner 10², made three flybys and dis­covered traces of heavy atoms near its exo­sphere – a thin atmo­sphere com­posed of atoms and molecules that have been expelled from the planet’s sur­face. Later, land-based tele­scopes remotely detec­ted a selec­tion of ions, includ­ing sodi­um (Na+), potassi­um (K+) and cal­ci­um (Ca+), which prob­ably also come from the plan­et itself.

But it was the MESSENGER mis­sion³ that really changed our view of Mer­cury. In par­tic­u­lar, it provided a great deal of import­ant inform­a­tion about the ion­ised plasma in its mag­neto­sphere. This plasma is a hot ion­ised gas con­tain­ing hydro­gen and heli­um (II) (He²⁺) ions from the sol­ar wind and heav­ier spe­cies such as He⁺, O⁺ and Na+. How­ever, MESSENGER has mapped only 45% of the planet’s sur­face and has there­fore left many ques­tions unanswered. In par­tic­u­lar, how the planet’s mag­neto­sphere inter­acts with the sol­ar wind.

Research­ers are now present­ing the res­ults of BepiColombo’s third flyby of Mer­cury, which took place on 19^th June 2023, and in par­tic­u­lar the res­ults from the MPPE (Mer­cury Plasma Particle Exper­i­ment) instru­ment suite. These were act­ive on the Mer­cury Mag­neto­spher­ic Orbit­er (Mio), an instru­ment led by the Japan Aerospace Explor­a­tion Agency (JAXA). Mio is one of two sci­entif­ic orbit­ers that will be cap­tured in Mercury’s orbit in 2026, the oth­er being the Mer­cury Plan­et­ary Orbit­er (MPO), led by the European Space Agency (ESA). Togeth­er, these two space­craft will provide a com­plete pic­ture of the envir­on­ment around Mercury.

These flybys are very fast, tak­ing about half an hour (ter­restri­al) to cross Mercury’s mag­neto­sphere, passing from dusk on the plan­et to dawn. Dur­ing the third flyby, the space­craft was able to get as close as 235 km above Mercury’s cratered sur­face. Dur­ing this flyby, Mio char­ac­ter­ised the nature of the particles present in the mag­neto­sphere and the way in which they move. “These meas­ure­ments allow us to clearly trace the mag­neto­spher­ic land­scape dur­ing the brief grav­it­a­tion­al assist,” explains Lina Hadid, who is cur­rently a research fel­low at the CNRS and works at the Plasma Phys­ics Labor­at­ory (LPP) at the École poly­tech­nique (IP Par­is). Lina Hadid is the sci­entif­ic man­ager of one of the instru­ments of the MPPE con­sor­ti­um, the ion mass spec­tro­met­er, whose optic­al part was developed at the LPP. She and her col­leagues have pub­lished their latest res­ults in Nature Com­mu­nic­a­tions Phys­ics.

The research­ers claim to have observed ‘bound­ar­ies’ such as the ‘shock wave’ sep­ar­at­ing the sol­ar wind and Mercury’s mag­neto­sphere, as well as oth­er regions such as the plasma sheet, which is a more ener­get­ic and dens­er region of ions loc­ated at the centre of the mag­net­ic tail. “These two res­ults were nev­er­the­less expec­ted,” explains Lina Hadid.

“How­ever, there have been plenty of new sur­prises,” she adds. For example, a lay­er at low lat­it­udes, defined by a region of tur­bu­lent plasma at the edge of the mag­neto­sphere. This lay­er con­tains particles with an energy range of up to 40 keV/e, much broad­er than those ever observed before on Mercury.

Anoth­er import­ant res­ult is the obser­va­tion of ener­get­ic hot hydro­gen ions (H⁺) trapped at low lat­it­ude and near the equat­ori­al plane of Mer­cury, with ener­gies of around 20 keV/e and at low lat­it­ude. Accord­ing to Lina Hadid and her col­leagues, this res­ult can only be explained by the pres­ence of an annu­lar cur­rent (an elec­tric cur­rent car­ried by charged particles trapped in the mag­neto­sphere), but fur­ther obser­va­tions and ana­lyses will be neces­sary to con­firm or deny this. If con­firmed, this ring cur­rent will be sim­il­ar to that of the Earth, which is tens of thou­sands of kilo­metres from its sur­face and which, for its part, is well understood.

Finally, Mio also detec­ted “cold” plasma ions of oxy­gen and sodi­um with sig­na­tures of the pres­ence of potassi­um with an energy of less than 50 eV/e when it moved through the planet’s noc­turn­al shad­ow. These ions come from the plan­et itself and were prob­ably ejec­ted when micro­met­eor­ites hit its sur­face or dur­ing inter­ac­tions with the sol­ar wind.

Accord­ing to the research­ers, these new res­ults will provide a bet­ter under­stand­ing of how the sol­ar wind inter­acts with the mag­neto­spheres of plan­ets in gen­er­al. On Earth, it is import­ant to under­stand this phe­nomen­on because the charged particles in the sol­ar wind dis­rupt our planet’s mag­neto­sphere when they col­lide with it. These dis­turb­ances, known as “space weath­er”, can dam­age satel­lites, affect com­mu­nic­a­tion tech­no­lo­gies and GPS sig­nals, and even cause power out­ages on the ground or at altitude.

Bepi­Colombo suc­cess­fully com­pleted its fourth flyby in Septem­ber 2024 and came as close as pos­sible to the plan­et, about 165 km from its sur­face. On this occa­sion, it cap­tured the best images of some of the largest impact craters on Mer­cury. It made a fifth and sixth flyby of the plan­et, on 1^st Decem­ber and 8 Janu­ary 2025 respect­ively. The mis­sion is sched­uled to last until 2029…

Isabelle Dumé