Mercury appears in a new light

Space mission "BepiColombo" delivers data from the innermost planet in our solar system for the first time

On December 1, 2024, the BepiColombo mission flew past Mercury for the fifth time and has now become the first spacecraft to observe the surface in the mid-infrared using the MERTIS ("Mercury Radiometer and Thermal Infrared Spectrometer") infrared spectrometer. The new data show differences in the surface temperature and the composition of the crater-rich surface. "After many years of preparation, we are seeing Mercury in a new light for the first time with MERTIS. We are entering new territory and will be able to understand the composition, mineralogy and temperatures on Mercury much better. For the first time, we are obtaining Mercury data in the wavelength range from 7 to 14 micrometers. This wavelength range is particularly suitable for distinguishing rock-forming minerals. We had to be very patient and are very happy to finally see the data," emphasizes the scientific director of the instrument, Harald Hiesinger from the University of Münster. After around two decades of development work, laboratory measurements on hot rocks that could be similar to those on Mercury and countless tests of all the procedures for the mission time on Mercury, the first data is now coming from the space probe," adds Dr. Jörn Helbert from the DLR Institute of Planetary Research.

During the flyby, parts of the approximately 1,550-kilometre Caloris Basin and a large volcanic plain in the northern hemisphere were visible. The first MERTIS data show temperatures of up to 420 degrees Celsius on the side illuminated by the sun and different spectral signatures of impact craters. Of particular interest is the impact crater Basho, which has already been seen by the US space probes Mariner 10 and observed in detail by MESSENGER. Images in the visible wavelength range show that the Basho impact crater contains both very dark and very bright material. The MERTIS flyby observations show an anomaly in the mid-infrared radiation intensity, which confirms the special characteristics of the crater.

As the innermost planet in our solar system, Mercury is the Earth-like body that has been studied the least by space missions to date. Mercury is a "planet of extremes", explains Harald Hiesinger, about which little is known in comparison to other planets, but which can reveal a great deal about the formation of our solar system. For example, despite Mercury’s proximity of only 58 to 69 million kilometers to the sun (for comparison: Earth is around 150 million kilometers from the sun), water ice probably occurs at the poles. Other highly volatile components are also found in surprisingly high concentrations. Mercury’s surface is comparatively poor in iron, while its iron/nickel core is unusually large. The detailed composition of the surface and its mineralogy are also only partially known so far.

The European/Japanese mission BepiColombo, which has been on its way to Mercury since 2018 and will enter a Mercury orbit in November 2026, will shed light on this darkness. "The flight to Mercury follows a very complicated path with a total of nine flybys of other planets, known as ’flyby maneuvers’," explains Johannes Benkhoff, BepiColombo project scientist at the European Space Agency (ESA). In detail, these were one flyby of the Earth-Moon system, two flybys of Venus and six flybys of Mercury, the next of which will take place on 8 January 2025. "These trajectory correction manoeuvres are necessary in order to arrive at Mercury with the ’correct’ orbital parameters so that the smallest terrestrial planet can gravitationally ’capture’ the spacecraft and bring it into its orbit," explains Geraint Jones from the European Space Agency ESA.

Dr. Solmaz Adeli from the German Aerospace Center (DLR), who played a key role in the planning of the current flyby as project manager, also emphasizes the performance of MERTIS. "It is a unique instrument that can provide data with a ground resolution of around 26 to 30 kilometers, even from a distance of a good 37,600 kilometers." In 2026, BepiColombo will come within 460 kilometers of the surface of Mercury and provide data with a resolution of up to 500 meters. "Then MERTIS will be able to exploit its full potential," adds Harald Hiesinger. [MOU1]

MERTIS was built at DLR with the participation of German industry. The instrument design, developed by the DLR Institute of Optical Sensor Systems in Berlin, is based on a novel and highly integrated instrument concept with a very low mass of only three kilograms and low power consumption. Gisbert Peter, the project manager responsible for instrument development at the institute, says: "MERTIS is a prime example of German engineering, as fantastic miniaturization and high reliability were achieved during the planning and construction of the instrument."