Figure 6-20 Details of a Lunar Mare (a) Close-up views of the lunar surface reveal rilles and numerous small craters on the maria. (a) Astronauts in lunar orbit took this photograph of Mare Tranquillitatis (Sea of Tranquility) in 1969 while searching for potential landing sites for the first human landing. At 1100 km (700 mi) across, this mare is the same size as the distance from London to Rome or from Chicago to Philadelphia. (b) Astronaut David Scott on Hadley’s rille during the Apollo 15 mission to the Moon. (c) Possible lava tube outlet on the Moon imaged by NASA’s Lunar Reconnaissance Observer. (d) Bridge near King crater. We know this is a bridge, rather than two depressions, because sunlight, coming from the lower right, is visible in the upper left depression.