The European BepiColombo mission made a detailed heat map of a large swath of Mercury’s surface during its most recent flyby.
At the Lunar and Planetary Science Conference in Texas, members of the BepiColombo science team revealed on March 12th the first views of Mercury from the spacecraft’s thermal camera. It’s the first time Mercury has ever been imaged from a spacecraft in the thermal infrared, and the rich detail in the planetwide swath has whetted scientists’ appetites for what BepiColombo will achieve once it finally enters orbit in November 2026.
When BepiColombo launched, the team that built the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) didn’t expect to receive any Mercury data until after orbit insertion. The main reason: BepiColombo is a stack of four spacecraft, and during the mission’s cruise phase the Mercury Planetary Orbiter (MPO)’s remote sensing instrument deck is blocked by the bulk of the Mercury Transfer Module (MTM), a solar-electric propulsion craft that is powering BepiColombo’s cruise. However, unlike the other MPO cameras, MERTIS has a second, side-looking “space port” that isn’t blocked by the MTM.
MERTIS’ space port is intended for taking calibration data of black space. It’s pointed out the same side of the spacecraft as its radiators. That side of the spacecraft was never supposed to be pointed at Mercury’s sweltering surface, and the MERTIS instrument software didn’t include any capability to perform planetary imaging out the space port. After launch, however, ESA permitted the MERTIS team to upgrade the software, which “required a complete reprogramming of instrument observation sequences and procedures,” team member Harald Hiesinger (University of Münster, Germany) said.
All of BepiColombo’s originally planned five Mercury flybys were supposed to happen at distances less than 300 kilometers (190 miles). With Mercury’s hot surface so nearby, spacecraft safety required the radiators — and the MERTIS space port — to be pointed away from the planet, precluding imaging. But a thruster anomaly discovered in 2024 required ESA to insert an extra, distant flyby before its final gravity assist on January 8, 2025.
Infographic
Infographic
The fifth flyby happened on December 1, 2024, at an altitude of more than 37,000 kilometers, a distance at which the planet’s heat wouldn’t overwhelm the spacecraft’s cooling systems. MERTIS gazed at Mercury through its space port as the spacecraft floated past, acquiring a pole-to-pole swath of data across the planet’s sunlit face.
Unsurprisingly, an uncorrected view of the thermal data shows that Mercury is hottest where the Sun’s heat is fiercest, with equatorial temperatures reaching a blistering 693 kelvins (788°F). Reassuringly, MERTIS’s two channels measured the same temperature. But within that obvious bullseye pattern are subtle hints of variation across Mercury’s surface.
Brightness temperature along a large swath of Mercury
Brightness temperature along a large swath of Mercury
Take out the bullseye pattern, and lots of subtle details appear, many associated with craters. Many factors could influence the details of thermal brightness: topography, shadowing, grain size, surface roughness, and mineral composition. It’s not yet possible to say for sure, because the MERTIS team must first calibrate their instrument, reducing the effects of pixel-to-pixel sensitivity variations. Plus, the space port images are slightly contaminated by some stray Mercuryshine reflecting off the spacecraft radiators. That glow won’t be an issue once the MTM separates from the MPO and MERTIS can use its planetary port to view Mercury.
Brightness temperature measured in rainbow colors along a large swath of Mercury
Brightness temperature measured in rainbow colors along a large swath of Mercury
Just this one flyby yielded 1.4 million MERTIS spectra. The unusual viewing geometry means these data aren’t ideal for science, but they are proof positive of MERTIS’s sensitivity, and a little taste of what’s to come during BepiColombo’s orbital mission.
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