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Boxwork in caves, Wind Cave National Park, 2014.

Boxwork in caves, Wind Cave National Park, 2014.

Boxwork is an uncommon mineral structure called a speleogen, which means it was formed by erosion rather than accretion like a speleothem. Boxwork is made of thin blades of calcite projecting from cave walls an... More

P-21745 BW Range: 1.1 million miles (675,000 miles) This image of Callisto taken by Voyager 2 was enhanced to reveal detail in the scene.  Voyager 1's high resolution coverage was of the hemisphere just over the right-hand (eastern) horizon, and the large ring structure discovered by Voyager 1 is just over the eastern limb. This image shows yet another ring structure in the upper part of the picture. Callisto exhibits some of the most ancient terrain seen on any of the satellites. Scientists think Callisto's surface is a mixture of ice and rock dating back to the final stages of planetary accretion (over 4 billion years ago) when the surface was pockmarked by a torrential bombardment of meteorites. Younger craters show as bright spots, probably because they expose fresh ice and frost. ARC-1979-A79-7079

P-21745 BW Range: 1.1 million miles (675,000 miles) This image of Call...

P-21745 BW Range: 1.1 million miles (675,000 miles) This image of Callisto taken by Voyager 2 was enhanced to reveal detail in the scene. Voyager 1's high resolution coverage was of the hemisphere just over th... More