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