Martian moon Deimos seen across the face of Mars in this sequence of Thermal Infrared Imager images acquired during the Hera mission’s 12 March 2025 gravity-assist flyby of Mars.
The car-sized Hera spacecraft was about 1000 km away from Deimos as these images were acquired. Deimos orbits approximately 23 500 km from the surface of Mars and is tidally locked, so that this side of the moon is rarely seen.
Hera’s TIRI – supplied to the mission by the Japan Aerospace Exploration Agency, JAXA – sees in mid-infrared spectral bands to chart surface temperature. Because Deimos lacks an atmosphere, the side of the moon being illuminated by the Sun is considerably warmer than the planet beneath it.
Although it appears as if Deimos is passing in front of Mars from south to north, the image was actually taken as Hera passed very close to Deimos from north to south at high speed.
Deimos appears brighter than Mars. This means that the surface of airless Deimos is hotter than the surface of Mars. The material covering the surface of Deimos has low reflectivity and is pitch black. This allows it to absorb sunlight well and become hotter.
In contrast, the surface of Mars is highly reflective, and its atmosphere transports heat from the warm daytime side to the cooler nighttime side. This is why there is a large temperature difference between Mars and Deimos.