Lunar red spots: Stratigraphic sequence and ages of domes and plains in the Hansteen and Helmet regions on the lunar nearside

Abstract

Basaltic mare materials extruded between ∼ 4 Ga and ∼ 1.2 Ga ago and formed large expanses on the lunar nearside. In contrast to the maria, volcanic shields, domes and cones also occur but cover only a small area of the total lunar surface. One specific type of dome is characterized (1) by steep slopes, (2) by a high albedo similar to lunar highlands, and (3) by a strong absorption in the ultraviolet. Because of this latter feature they appear spectrally red and therefore where termed red spots. Their morphology suggests that these domes were created by much more viscous, silica-rich lava. Also, red spots which are believed to be possibly volcanic in origin are associated with light plains materials. In order to constrain the formation of these presumed volcanic features in context with the formation of the surrounding mare and highland materials, we carried out (a) geologic and morphologic mapping of high-resolution Lunar Orbiter IV and Apollo 16 frames, and (b) measurements of superimposed crater frequencies on geologic units, in order to assess stratigraphy and geological history in the following two volcanic provinces in which red spots occur: (1) the Hansteen region, dominated by the dome Hansteen αlpha (also known as the Arrowhead), and (2) the Helmet region, dominated by light plains, possibly of volcanic origin, such as the feature called the Helmet and Darney chi, and the Southern Montes Riphaeus. In the Hansteen region, the dome Hansteen αlpha postdates craters Billy (3.88 Ga) and Hansteen (3.87 Ga), but predates most of the mare materials (3.51 Ga). Cratering model ages of the dome range from 3.74 to 3.65 Ga, placing its major activity as extending into the Late Imbrian. In the Helmet region, presumed highland volcanism associated with red spots started in the Nectarian with Darney chi (3.94 Ga). Cratering model ages measured on the Helmet light plains range from 3.8 to 2.08 Ga. The origin of these light plains and local resurfacing processes in this unit, however – highland volcanism or impact – remains an open question. Locally, younger processes of Eratosthenian or even Copernican age, such as emplacement of mare materials or mass wasting deposits, could be dated in the higher-resolution Apollo 16 frame.