Geological Chronicle
Gassendi sits on the northern margin of Mare Humorum in the Moon's southwest quadrant. The 111-kilometer diameter impact crater formed in the Pre-Nectarian period, more than 3.9 billion years ago. When mare basalt flooded the Humorum basin, molten lava spilled across Gassendi's southern wall, partially burying the rim so it rises only 200 meters above the lava plain compared to 2.5 kilometers along the rugged northern crest.
Gassendi is the lunar archetype of a Class I floor-fractured crater. Rising magma beneath the Mare Humorum basin intruded below the crater floor as a subterranean sill. The pressure lifted the solidified crater bottom like a hydraulic piston, generating tensile stress that split the floor into a network of fractures known as Rimae Gassendi.
Unsubmerged by the volcanic flooding, an isolated cluster of central peaks rises 1.2 kilometers above the interior plain. Near-infrared reflectance spectra from lunar orbiters identify pure crystalline plagioclase (anorthosite) within these peaks, representing ancient crustal material dredged from several kilometers beneath the lunar surface during impact rebound. On the northern rim, the 32-kilometer Copernican crater Gassendi A punches directly through the older wall, giving the pair a diamond ring profile in Earth-based telescopes.
Selenographers have observed Gassendi since the 17th century, when Italian astronomer Giovanni Riccioli named it in honor of French polymath Pierre Gassendi. NASA prioritized high-resolution orbital imaging of the crater by Lunar Orbiter 5 and Apollo 16 while assessing Gassendi as a candidate landing site for Apollo 17. Geologists favored the site to sample ancient highlands crust at the central peaks alongside young mare volcanism, though NASA ultimately selected the Taurus-Littrow valley.