Mars and Deimos viewed by Hera's TIRI

Martian moon Deimos shines much brighter than the red planet beneath it in this Thermal Infrared Imager image acquired during the Hera mission’s 12 March 2025 gravity-assist flyby of Mars. In visible light the opposite is true: the Deimos is much less reflective than the martian surface.

Hera’s TIRI – supplied to the mission by the Japan Aerospace Exploration Agency, JAXA – sees in mid-infrared spectral bands to chart surface temperature. 

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.

To the bottom right of the martian disc, in blue, is the Hellas basin, among the largest known impact craters in the Solar System with a diameter of 2300 km and a depth of more than 7 km – relatively cool at such a high southern latitude during this time of the Martian year.

By revealing surface temperature in this way, TIRI imaging serves to reveal physical properties such as roughness, particle size distribution and porosity.