Mercury's Vivaldi Crater from BepiColombo

Explanation: Why does this large crater on Mercury have two rings and a smooth floor? No one is sure. The unusual feature called Vivaldi Crater spans 215 kilometers and was imaged again in great detail by ESA 's and JAXA 's robotic BepiColombo spacecraft on a flyby earlier this month. A large circular feature on a rocky planet or moon is usually caused by either an impact by a small asteroid or a comet fragment, or a volcanic eruption . In the case of Vivaldi , it is possible that both occurred -- a heavy strike that caused a smooth internal lava flow. Double-ringed craters are rare, and the cause of the inner rings remains a topic of research . The speed-slowing gravity-assist ed flyby of Mercury by BepiColombo was in preparation for the spacecraft entering orbit around the Solar System 's innermost planet in 2026.