Mare Humorum 24.0°S, 39.0°W

Like all the other lunar maria Mare Humorum (‘Sea of Humours’) is also an impact basin flooded in the centre with mantle lava. With a diameter of about 380 km (that is, of the lava-covered area: the actual basin is larger), it is one of the smaller maria, and is roughly comparable with Iceland in area. The Humorum Basin is also associated with a gravitational anomaly (a mascon).

The formation of the basin has been dated to the Nectarian period, approximately 3.9 to 3.8 billions years ago. Flooding with lava occurred much later. The lava layer is over 3 km thick in the central region. After the lava cooled and with the cessation of the pressure from the lava rising from the mantle, the lava layers collapsed under their own enormous weight and deformed the lunar crust beneath, thus creating the mare wrinkle ridges, fracture zones and escarpments around the edge of the basin. When the lava surface sank, previously existing craters and crater floors around the edge of Mare Humorum were therefore tilted in the direction of the centre. An area of Dark Mantle Deposit (DMD) lies on the floor of the mare, with deposits of dark, glassy, volcanic ashes, and which is crossed by a rille, whose origin is uncertain.