Geological Chronicle
Mare Humorum, translated from Latin as the 'Sea of Moisture', is a circular lunar mare located in the southwestern quadrant of the Moon's near side. Centered at 24.4° S and 38.6° W, the depression occupies an impact basin measuring 420 kilometers across. Geologists place its origin in the Nectarian period roughly 3.9 billion years ago, intermediate in age between the older Nectaris and younger Imbrium basins. While the Apollo program did not sample the basin directly, geological mapping shows that eruptions of mare basalt flooded the structure during subsequent volcanic epochs, with basalt thickness exceeding 3 kilometers at the basin center.
The accumulated basalt load created a pronounced gravitational anomaly, or mascon, in the center of the basin. Researchers first detected this mass concentration in 1968 through Doppler tracking of NASA's Lunar Orbiter spacecraft, an observation later confirmed at higher resolution by the Lunar Prospector and GRAIL missions. The weight of the volcanic rock caused the basin floor to sag, fracturing the surrounding crust into concentric arcuate graben. The eastern margin features Rimae Hippalus, a series of curved rilles that sweep past the partially flooded crater Hippalus and the mountainous headland Promontorium Kelvin.
Mare Humorum is bordered by several distinct impact craters and faults. On its northern rim sits the 110-kilometer-wide crater Gassendi, a floor-fractured crater once considered as a candidate landing site for Apollo 17. To the south lie the floor-fractured crater Vitello, the heavily flooded crater Doppelmayer, and Puiseux. Along the western perimeter, the fault scarp Rupes Liebig and the rilles of Rimae Doppelmayer run past the crater Liebig, marking the structural boundary where the mare transitions into the surrounding highlands and opens north toward Oceanus Procellarum.
Early telescopic observers assigned several names to the dark circular plain before modern nomenclature became fixed. In 1645, Michael van Langren charted it as Mare Venetum (the Venetian Sea). Astronomer Pierre Gassendi called it Anticaspia because its position on the lunar disk balanced Mare Crisium, which he had named the Caspian Sea. In 1647, Johannes Hevelius recorded the feature as Sinus Sirbonis after Lake Serbonis in Egypt. The current name, Mare Humorum, was introduced by Italian Jesuit astronomer Giovanni Riccioli in 1651 and later formalized by the International Astronomical Union.