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What happens when the solar wind pushes Mercury’s magnetosphere below the surface?
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Mercury
Mercury as seen by the Messenger probe, January 14, 2008. This image shows half of the hemisphere missed by Mariner 10 in 1974–75 and was snapped by Messenger's Wide Angle Camera when it was about 27,000 km (17,000 miles) from the planet.
When the solar wind is sufficiently strong, it can push the sunward boundary of Mercury’s magnetosphere—known as the magnetopause—beneath the planet's surface.
Under these conditions, solar wind ions impinge directly upon the portions of Mercury’s surface located immediately beneath the Sun. Even infrequent occurrences of this event can significantly alter the atomic composition of the planet's surface constituents.1