Atmosphere

The composition of Mercury's thin, temporary atmosphere varies with time as gases are lost and replenished. Oxygen, sodium, and helium are, however, the most abundant elements.

potassium and other gases (1%)

CALORiS BASiN

The center of the Caloris Basin, which has yet to be photographed, is below the bottom edge of this mosaic of Mariner 10 images. The basin edge and ridges are pitted by more recently formed craters.

surface features mercury is covered in thousands of impact craters, formed when meteorites hit the surface. The oldest date from about 4 billion years ago when meteorites bombarded the young planet. They range in size from small bowl-shaped ones to the Caloris Basin, a quarter of mercury's diameter. the basin formed when an impacting body, probably about 60 miles (100 km) across, hit the planet. shock waves buckled the surface, forming rings of ridges around the site and crumpled terrain on the opposite side of the planet. the surface also has plains formed by volcanic lava and clifflike ridges shaped when the young, hot planet cooled and shrank.

M t mercury

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