Geological Chronicle
Lassell occupies the eastern part of Mare Nubium, situated west of Alpetragius and southwest of Alphonsus. The impact crater measures 21.82 kilometers across and commemorates British astronomer William Lassell, who discovered the Uranian moons Ariel and Umbriel.
Lava flooded and resurfaced the interior, leaving a nearly flat floor with a dark appearance and low albedo enclosed by a surviving outer wall that is generally circular with a somewhat polygonal outline.
Neighboring craterlets present stark reflectance contrasts. Lassell B to the east-northeast appears relatively bright, while the tiny crater Lassell D to the west-northwest is encircled by a prominent high-albedo bright patch regarded as evidence of recent formation.
To the west, the highland Lassell Massif exhibits a reddish hue against the surrounding mare. A series of viscous silicic lava eruptions built the complex beginning over four billion years ago, producing silica-rich southern domes capped by caldera-like depressions.