The innermost planet Merkur does not have a real atmosphere, but an extremely thin gas cover in the form of an exospre. In this, planet researchers have now proven the element lithium for the first time using data from the Merkur-Space probe Messenger. There were special wave patterns in magnetic sensor data for this. This proof of the lithium in Mercury Exosphere supports a presumption that has been cherished for a long time. According to this, lithium, sodium and other fleeting elements have reached Mercury through countless impacts of small and small meteorites over the course of billions of years and have enriched themselves on its surface. Then collapse a slightly larger chunk on the planet, he evaporates part of these deposits and hurls them into the exosphere of Mercury. This scenario clarifies both the unexpectedly high content of fleeting elements on the Mercury interface and the presence of these atoms in the exospre, as the team reports.
The innermost planet of our solar system gives up some puzzles and is significantly less researched than other neighboring planets on earth. With Mariner and Messenger, two room probes only visited Mercury and delivered data from his orbit, a third mission, Bepicolombo, is on the road. However, it is known that Mercury does not have a real atmosphere, but only a very thin exospre, far outwards. Atoms in this thin shell can easily escape the attraction of the planet, but are repeatedly supplemented by various processes. “The exospre consists of various particles that come from the sun wind, (micro) meteoroid beats and interactions with the planetary interface,” explain Daniel Schmid from the Institute for Space Research at the Austrian Academy of Sciences and his colleagues. “Unlike on earth, where meteoroids in the atmosphere burn up, they reach the surface unchecked on Mercury and continuously enrich the exospre.”
Wave vertebrae as an element pointer
From data from the probes of Mariner and Messenger, it was already known that the Mercury Exosphere contains gases such as hydrogen and helium, but also other elements such as sodium, potassium, calcium, magnesium, iron and manganese. However, planetary researchers have long suspected that lithium should also be present in the exospre. However, evidence of this has been missing. “Neither partial tectors on board the spacecraft nor telescopic observations from the earth were able to demonstrate the presence of lithium,” explain Schmid and his team. You have therefore searched for this element with a different approach. The basis for this were so-called pick-up ionicyclotron waves (ICW)-electromagnetic waves, which result from the interaction between the sunwind and neutral particles in the exospre. “Specific chemical elements – including hydrogen, helium and lithium – can be determined via the characteristic vibration frequencies of these waves,” explains Schmid. “The density of the corresponding ions and their origins can be derived from the thickness of these waves and thus reconstruct the height profile of the exosphere.” For their study, the researchers therefore evaluated the measurement data of the Messenger spacecraft, which measured these pick-up ionicyclotron waves in the area of Merkur with the help of its magnetic field sensor.
In fact, Schmid and his team identified the ion encyclotron waves characteristic of lithium atoms in the messenger data. However, this magnetic element signature was not constantly and to the same extent present in the Mercury Exosphere, but the waves increased at certain times. “In the four years of the messenger data, we identified twelve ICW events in the area of local lithium gyrro frequency,” the researchers report. “This is how we demonstrate the presence of lithium in Merkur’s exospre.”
According to their calculations, the entire exospreet of the planet could contain lithium atoms between 2.2 and 6.7 quadrillions (10
Where does the lithium come from?
These results raise the question of where the lithium in Merkur’s thin gas cover comes from. Due to the sporadic occurrence of the lithium ICWs and the specific characteristics of the atoms, the researchers exclude an exit from the planetary surface induced by the sun’s heat or radiation. Magnetic field effects such as an explosive reconnexion of magnetic field lines do not match the measured frequency distribution, as Schmid and his colleagues report. Further analyzes also showed that only very few of the twelve measured events met with solar storms or solar mass quurks. “Therefore, evaporation through high -energy impacts appears the most likely explanation,” the team writes. Copies are therefore meteorites with a diameter of 20 to 40 centimeters, which evaporate on the Mercury surface and released lithium and other fleeting elements into the exospre. However, this lithium not only comes from the respective meteorite: “Our analysis suggests that the deposit of meteoric material is the main source of the measured lithium,” explains Schmid’s colleague Helmut Lammer.
This material was deposited on the Mercury interface over the course of billions by dust and the impacts of smaller and larger meteorites. As a result, this is enriched today with sodium, lithium and other slightly fleeing elements. Every impact of another meteorite always hurls part of these deposits into space. This scenario could also explain why Mercury, despite its great proximity to the sun and a protoplanetarian collision in its early days, still has so many fleeting elements in its crust: Most of them were probably subsequently entered by the constant rain of micrometeorites and meteorites, as Schmid and his colleagues explain. “Our research shows that Mercury acts like a kind of meteorite detector in the inner solar system and leaves even the smallest particles from the very lasting traces – traces that help us decipher the past of a planet.” This is not only scientifically exciting, but also essential to better understand our place in the universe. “
Source: Daniel Schmid (Institute for Space Research at the Austrian Academy of Sciences, Graz) et al., Nature Communications, DOI: 10.1038/S41467-025-61516-4
