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Published on May 22, 2012
The Lunar Orbiters photographed Tycho crater in 1966 and 1967 and first revealed the beautiful state of preservation of this ~80 km diameter impact crater. Geologists saw sharp, craggy slopes in the crater walls and central peak, some form of ponded liquid and preserved flows, and a labyrinth of fractures in the rough, flat floor. What liquid flowed and ponded inside and outside the crater? Initially geologists hypothesized that the Tycho impact event triggered an upwelling of lava, and the ponds and flows were frozen volcanic forms.
Later it was determined that the hypothesized volcanic ponds and lakes and flows were indeed related to the formation of the crater, but not as volcanic eruptions of subsurface magma. Rather, they were accumulations of massive amounts of lunar rock that was melted as the Tycho asteroid (or comet) slammed into the Moon and released unimaginable amounts of kinetic energy, in an instant. It was only in 1960 that Gene Shoemaker and colleagues proved that Meteor crater (aka Barringer crater) near Winslow, Arizona, was formed by an asteroid impact. Thus the idea that many craters on the Moon (and Earth) were formed by impacts was only coming into widespread acceptance at the time of the Lunar Orbiter missions. Today we have a growing catalog of impact melt deposits from many young craters across the face of the Moon.