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