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Why is Mercury’s surface covered with impact craters?

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Messenger: Mercury — 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.

Mercury’s surface is heavily covered with impact craters primarily because it was subjected to an intense period of bombardment by asteroids, comets, and other remnant planetary building blocks (planetesimals) during the early history of the solar system.12

Most of these craters are believed to have formed before and during a period of intense bombardment that ended approximately 3.8 billion years ago, similar to the history of the Moon. Because Mercury has been geologically inactive for a vast period of its history, these ancient impact scars have been preserved on its surface.12

Additionally, Mercury’s high surface gravity contributes to the prominence of secondary craters, which are formed by the impact of material ejected from primary craters. While some hypotheses suggest that a population of objects known as "vulcanoids" could have caused more recent cratering, the prevailing view remains that the surface reflects the planet's long-standing exposure to the debris environment of the inner solar system.31