First sighting of Caloris
The joint European-Japanese BepiColombo mission captured this view of Mercury on 23 June 2022 as the spacecraft flew past the planet for its second of six gravity assist manoeuvres at Mercury. This image was taken at 09:55:32 UTC by the Mercury Transfer Module’s Monitoring Camera 2, when the spacecraft was 2862 km from the surface of Mercury. Closest approach of 200 km took place shortly before, at 09:44 UTC. In this view, north is approximately towards the top right.
The cameras provide black-and-white snapshots in 1024 x 1024 pixel resolution. This image has been 'block replicated' to 2048 x 2048 pixels. Some imaging artefacts such as horizontal striping are also visible. Parts of the Mercury Planetary Orbiter can be seen in the foreground: the magnetometer boom running from bottom left to top right in front of Mercury, and a small part of the medium-gain antenna at bottom right.
This image represents BepiColombo’s first sighting of part of the 3.9 billion year old Caloris basin, which at 1550 km across is the largest well-preserved impact basin on the planet. With the Sun above, the highly-reflective lavas on its floor make it easy to see as the bright semicircular area roughly between the 2 o’clock and 3 o’clock parts of the edge of Mercury’s disc. Caloris is surrounded by a halo of less reflective (darker) lavas. Both the interior and exterior lavas are thought to post-date the formation of the basin by a hundred million years or so, and measuring and understanding the compositional differences between these is an important goal for BepiColombo when it begins its main science mission in orbit around Mercury in 2026.
Some of the small bright spots on the Caloris floor are impact craters with ‘hollows’ on their floors - geological features unique to Mercury – and others are deposits (‘faculae’) erupted explosively from volcanic vents that have punctured the basin-filling lavas from below. One prominent dark patch inside Caloris is a 100 km-wide impact crater called Atget that cut through the basin-filling lavas from above and excavated underlying ‘low reflectance material’ that may be a relict of Mercury’s early carbon-rich crust.
Two other interesting craters lie between the spacecraft’s magnetometer boom and the left-hand edge of the frame: the young 24 km-wide crater named Xiao Zhao with its bright rays of ejected material, and a 260 km-wide peak-ring basin named Raditladi visible further north. The rich variety of craters and their associated features plots the impact history of the planet.
Click here for an annotated version of this image.
The gravity assist manoeuvre was the second at Mercury and the fifth 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.
