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Before the Mariner 10 mission in 1974, Mercury’s surface features were little more than a blur to Earth-bound observers. Mariner 10 made the first flyby of Mercury on March 29, 1974 at a distance of about 700 kilometers. Its high-resolution photographs of about 50% of Mercury’s surface allowed scientists to view the planet close-up. On its surface, Mercury closely resembles our Moon. Impact craters cover the majority of the planet but unlike the Moon, Mercury’s cratered upland regions are covered with large areas of smooth plains. The most distinguishing features on Mercury’s surface are scarps, or long cliffs. These wind across Mercury’s surface for tens to hundreds of kilometers and range from 100 meters to over 1.5 kilometers in height. What makes these cliffs so unique is that no other planet or moon features such a vast number of them. They are thought to be thrust faults created when the planet, as it cooled, shrunk by up to 4 km in diameter.

The largest surface feature photographed by the Mariner 10 mission is the Caloris basin. This is a multi-ringed (resembling a bull's-eye) impact basin 1,340 km across - almost 1⁄4 of the full diameter of the planet. The basin includes a series of circular mountain ranges up to 3 km in height - the tallest mountains on Mercury. Caloris is thought to have been produced when a very large asteroid collided with the planet about 4 billion years ago. The massive impact sent seismic waves echoing through the planet. Coming to a focus on the opposite side of Mercury, these intense waves created there a region of hilly and broken terrain.


Discovery Scarp: This scarp is about 350 kilometers long and transects two craters. The maximum height is about 3 kilometers. (Click on the image to see an enlarged version.)

Photograph of a scarp

This 300 km scarp (above) runs from upper right corner of the picture to the lower left. (Click on the image to see an enlarged version.)

Caloris Basin

The multi-ringed Caloris Basin, taken from Mariner 10. Only half of the basin appears in this photomosaic. (Click on the image to see an enlarged version

Ridges and Fractures

Ridges and fractures on the floor of the Caloris Basin.


Hilly and jumbled terrain on the side of Mercury opposite the Caloris Basin. (Click on the image to see an enlarged version.)

Learn about the Atmosphere of Mercury

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The Caloris Basin is so large that it would engulf the entire state of Texas. The name Caloris is derived from the Latin word "calor" or heat.