A taste of Mercury geology

The joint European-Japanese BepiColombo mission captured this view of Mercury on 1 October 2021 as the spacecraft flew past the planet for a gravity assist manoeuvre. This  image was taken at 23:44:57 UTC by the Mercury Transfer Module’s Monitoring Camera 3, when the spacecraft was 2687 km from Mercury. Closest approach of 199 km took place shortly before, at 23:34:41 UTC.

The cameras provide black-and-white snapshots in 1024 x 1024 pixel resolution. The high-gain antenna of the Mercury Planetary Orbiter and part of the body of the spacecraft are visible in front of Mercury.

Visible in the image is a portion of the southern hemisphere of Mercury. Extensive lava plains cover the surface. The largest clearly visible crater, albeit partly obscured by part of the spacecraft, is the 251 km diameter Haydn crater, named after the Austrian composer (1732-1809). The smoothness of its floor indicates that it has been partly flooded by lavas. Where the sunrise is hitting the surface close to the night side of the planet, the topography of the terrain is enhanced. One example where this is apparent is a feature called Astrolabe Rupes, below right of Haydn crater. This sunlit ‘lobate scarp’ is  one of many thrust faults resulting from Mercury’s slow global contraction caused by interior cooling.

Some of the bright spots seen on the planet’s surface are ‘faculae’, many of which are believed to be material flung out by volcanic explosions. These represent one of the many surprises revealed by NASA’s MESSENGER mission that orbited Mercury 2011-2015, and will be studied in more detail when BepiColombo achieves orbit about the planet.

Click here for annotated version. The image has been lightly processed to enhance contrast and use the full dynamic range.

The gravity assist manoeuvre was the first at Mercury and the fourth of nine flybys overall. During its seven-year cruise to the smallest and innermost planet of the Solar System, BepiColombo makes one flyby at Earth, two at Venus and six at Mercury to help steer on course for Mercury orbit in 2025. The Mercury Transfer Module carries two science orbiters: ESA’s Mercury Planetary Orbiter and JAXA’s Mercury Magnetospheric Orbiter. They will operate from complementary orbits to study all aspects of mysterious Mercury from its core to surface processes, magnetic field and exosphere, to better understand the origin and evolution of a planet close to its parent star.