177
The European Space Agency’s BepiColombo spacecraft recently performed its fifth flyby of Mercury on Dec. 1, becoming the first spacecraft ever to observe the planet in mid-infrared light. BepiColombo’s observations from the flyby have revealed interesting new details on the Mercury’s surface, including temperature and composition variations.
BepiColombo is only the third mission to visit the smallest planet in our solar system, making Mercury the least-explored rocky planet in the solar system. In 2026, BepiColombo will enter orbit around Mercury and begin its primary mission, becoming the first spacecraft to orbit Mercury since NASA’s Messenger from 2011 to 2015.
BepiColombo’s long journey to Mercury began on Oct. 20, 2018, when the mission launched from French Guiana atop an Ariane 5 rocket. The mission was initially planned to arrive at the planet in December 2025. However, thruster issues discovered in September 2024 forced a one-year extension of the coast phase, bringing the arrival date to November 2024.
The mission’s coast phase has involved several flybys of Earth, Venus, and Mercury, using the gravitational pull from each planet to alter BepiColombo’s orbital trajectory around the Sun and bring the spacecraft closer to Mercury. BepiColombo’s recent Dec. 1 flyby saw the spacecraft fly within 38,000 km of the planet’s surface. The primary objective of this flyby was to gather additional data on Mercury’s surface and surrounding environment.
Infographic highlighting BepiColombo’s fifth flybys. (Credit: ESA)
The spacecraft measured particles and electromagnetic fields while in the planet’s vicinity and, for the first time, imaged the cratered surface in mid-infrared wavelengths with the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) instrument.
“With MERTIS, we are breaking new ground and will be able to understand the composition, mineralogy, and temperatures on Mercury much better,” said MERTIS principal investigator Harald Hiesinger of the University of Münster in Germany.
“After about two decades of development, laboratory measurements of hot rocks similar to those on Mercury, and countless tests of the entire sequence of events for the mission duration, the first MERTIS data from Mercury is now available. It is simply fantastic!” added MERTIS co-principal investigator Jörn Helbert of the German Aerospace Center (DLR) in Berlin.
The first observations from MERTIS revealed that certain areas of Mercury’s surface shine brighter in the mid-infrared than others. More specifically, the Caloris Basin and parts of a large volcanic plain in Mercury’s northern hemisphere emit more mid-infrared light.
Mid-infrared light increases in these regions due to surface temperature, roughness, and composition. MERTIS’ imaging spectrometer is sensitive to mid-infrared light with wavelength between 7 and 14 nm. This wavelength range is known to be particularly suitable for observing and distinguishing rock-forming minerals in surface features.
MERTIS’s mid-infrared measurements of Mercury’s surface from the Dec. 1 flyby. (Credit: MERTIS/DLR/University of Münster/NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)
“The moment when we first looked at the MERTIS flyby data and could immediately distinguish impact craters was breathtaking. There is so much to be discovered in this dataset — surface features that have never been observed in this way before are waiting for us. We have never been this close to understanding the global surface mineralogy of Mercury with MERTIS ready for the orbital phase of BepiColombo,” said project lead Solmaz Adeli of the DLR.
Mercury’s surface composition has long been a mystery in planetary science, and Messenger’s data on the planet’s surface and composition has proven lackluster in providing definitive results. When MERTIS and other BepiColombo instruments arrive in orbit around the planet in 2026, they will provide planetary scientists with better surface imaging resolution and elemental composition accuracy.
Messenger’s data has led scientists to believe that Mercury’s surface contains low amounts of iron — an interesting discovery given the planet’s large iron-nickel core. Messenger also showed that some chemical elements that easily evaporate are present in high concentrations on Mercury’s surface despite the planet’s tight orbit around the Sun.
Another mystery surrounding Mercury is its dark appearance. While the planet’s cratered surface may look similar to that of the Moon at first glance, it only reflects around two-thirds as much light as the Moon’s surface — a fascinating characteristic for a planet that orbits extremely close to our Sun.
Mercury in motion… One of the #BepiColombo selfie-cameras captured Mercury today as the spacecraft rushed by the planet at almost 3 km per second. 🛰️💨
This time-lapse of unprocessed images was captured during 10:26-11:18 UTC today (11:26-12:18 CET), between 53700 and 48000 km… pic.twitter.com/NPlnLCBOr3— BepiColombo (@BepiColombo) December 1, 2024
MERTIS’s measurements rely on how different minerals glow in mid-infrared light and how the glow varies with temperature. Mission teams have been testing different material and mineral mixtures in a lab to train both the instrument and analysis programs for BepiColombo and MERTIS’s arrival at the planet in 2026. During the tests, the mixtures are heated to different temperatures so that they glow, and then measurements of their glow are made in mid-infrared wavelengths.
“Because Mercury’s surface is surprisingly poor in iron, we have been testing natural and synthetic minerals that lack iron. The materials tested include rock-forming minerals to simulate what Mercury’s surface might be made of,” Solmaz explained.
Shortly after entering orbit, MERTIS will create a global map of Mercury’s surface mineral distribution with a resolution down to 500 m.
Interestingly, MERTIS is currently partially blocked due to the stacked configuration of the BepiColombo spacecraft. BepiColombo is comprised of two satellites that have been stacked atop each other. Once in orbit around Mercury, the two halves, the Mercury Planetary Orbiter (MPO) and Mercury Magnetospheric Orbiter (MMO), will separate and begin their missions.
MERTIS’s design features two “ports” for collecting and calibrating data. One of these ports is currently blocked due to BepiColombo’s stacked nature; however, scientists reworked the instrument to operate with only one open port for the flybys. Once in orbit around Mercury, both ports will be open and MERTIS will operate as planned.
“These fascinating and valuable results from the MERTIS instrument are only a tantalizing hint of the great results we’re expecting from the entire BepiColombo science payload once both orbiters are operating in orbit around Mercury,” says BepiColombo project scientist Geraint Jones.
(Lead image: Artist’s impression of BepiColombo at Mercury. Credit: ESA)