Volcanism and deep structures of the moon

First, the distribution and duration of lunar mare volcanism are introduced. Based on the latest mapping of lunar mare deposits (Fig. 1), the authors determined the surface area of all exposed lunar maria as about 6.15×10⁶ km², which accounts for ~16% of the surface area of the entire Moon. Among them, 92.9% or 5.71×10⁶ km² of mare deposits occur in the lunar nearside, while the lunar farside maria only accounts for 7.1% of the whole mare surface area. Authors compile the crater counting results for the global distribution of nearly 500 mare basalt units on the Moon, and find that the oldest exposed basalts on the lunar surface were emplaced about 4 billion years (Ga) ago, corresponding to the terminal stage of large basin-formation period in lunar history. The most active period of lunar volcanism was about 3.3 to 3.8 Ga ago. During this relatively short period of about 500 million years, ~70% of the total lunar basalt unit populations were emplaced. After that, volcanic eruptions on the Moon waned sharply in the middle lunar history and ceased sometime in the last ~1 Ga. Although the spatial extent of volcanism on the lunar farside accounts for less than one-tenth of the global lunar volcanism, the nearside and farside lunar volcanism are both affected by the continued cooling and contraction of the entire Moon in the general temporal extent and evolution history.

Fig. 1. Maps of the lunar nearside (left) and farside (right) showing the spatial distribution of mare basalts (dark gray areas). Basemap is a colorized Lunar Orbiter Laser Altimeter (LOLA) topography overlain on LOLA hillshade, and the blue circles indicate impact basins >300 km in diameter.

Second, styles of lunar volcanism and related volcanic landforms are summarized. (1) Intrusive magmatism happens when dikes with medium over-pressurization values propagate to the vicinity of lunar surface and form intrusions without erupting to the surface. These subsurface intrusion structures may lead to the bending and fracturing of lunar surface. Related landforms include floor-fractured craters (Fig. 3A) and intrusive domes. The former is a special type of impact craters on the Moon with wide-distributed fracture systems on its floor. An intrusive dome is a type of lunar domes, which is related to the vertical uplift of the lunar crust induced by laccolithic intrusions. (2) Effusive volcanism. The lunar maria are confirmed to be large basaltic plains mainly formed in volcanic effusive episodes. In addition, other related landforms can be also observed on lunar surface: lava flows (Fig. 3B) are usually seen in mare region with some characterized by clear flow margins that are traceable for hundreds of kilometers; when the magma effusion flux is very high, long-duration effusive eruptions would lead to significant substrate thermal erosion and result in the formation of lunar sinuous rilles (Fig. 3C); effusive domes and cones (Fig. 3D) are small volcanic edifices that are usually concentrated in volcanic complexes such as Marius Hills, Aristarchus, Mons Rümker, Gardner, etc., although they have also been identified in lunar mare surface or on the highlands; lava tubes are typical volcanic features on planetary surfaces and they were evidenced on the Moon by discontinuous pit chains and vertical holes that are called “skylights” (Fig. 3E); irregular mare patches (Fig. 3F) are a type of mare features on lunar surface with special “blistered” appearance; ring moat dome structures (Fig. 3G) often display topographic characteristics with low domical mounds surrounded by ring depressions (or moats). (3) Explosive volcanism. Terrestrial Hawaiian-style eruptions occurring in the lunar environmental conditions would produce regional and localized pyroclastic deposits (Fig. 3H), and hundreds of pyroclastic deposits have been identified on the lunar surface.

Fig. 3. Lunar volcanic landforms. (A) Floor-fractured crater (44.3°E, 46.5°N), LRO WAC mosaic. (B) Lava flows in the southwestern Imbrium (330.4°E, 25.5°N), Apollo photograph AS15-M-1701. (C) Sinuous rille (315.7°E, 27.3°N), LRO WAC mosaic. (D) Lunar dome (black arrows) and cone (white arrow) (303.6°E, 12.5°N), LRO WAC mosaic. (E) Lunar skylight (33.2°E, 8.3°N), LRO NAC image M126710873RC. (F) Irregular mare patch (IMP; 5.3°E, 18.7°N), LRO NAC image M190566313LC. (G) 3D view of ring moat dome structures (RMDSs; 30.8°E, 10.4°N), LRO NAC images M1096293859RC and M1096293859LC. (H) Pyroclastic deposits (352.2°E, 6.2°N), LRO NAC image M1103616897RC.

Third, crustal and mantle structures beneath lunar volcanic landforms are reviewed.

(1) Layered subsurface structures of mare basalts. Active source moonquake experiments were carried out at the Apollo 14, 16, and 17 landing sites to study the lunar subsurface layered structures, including the thicknesses of the regolith and the underlying strata. Japan’s Kaguya spacecraft identified subsurface layers several hundred meters deep in the lunar nearside maria, indicating that basalts are up to hundreds of meters thick. Chang'e-3 (CE3) and CE4 spacecraft respectively landed on the mare surfaces within the Imbrium basin on the lunar nearside and the Von Kármán crater on the lunar farside. They used lunar penetrating radar (LPR) to probe the subsurface structures deep to hundreds of meters, revealing the emplacement history of lava flows on the nearside and the farside of the Moon.

(2) Subsurface structures of lunar volcanic complexes. Using high-resolution imagery and topographic data, Spudis et al. recognized 8 large volcanic complexes (Fig. 4) on the nearside of the Moon and suggested that these features were large shield volcanoes equivalent to those on Mars, Earth, and Venus. A joint gravity and topography spectral analysis of these regions suggested that the shallow crusts of the Mons Rümker, Marius Hills, and Gardner are mainly composed of dense intrusive/extrusive magmatic units, with crustal and surface load densities larger than 2,850 kg m^-3, while Aristarchus Plateau and Hortensius are mainly composed of low-density materials with only small amounts of superimposed volcanic material, with lower crustal and load densities of 2,550 kg m^-3.

(3) Hidden structures related to magma intrusion. Magma erupts through the lunar crust to the surface, forming a variety of structures; however, most of the magma did not reach the lunar surface but was preserved by intrusion into the lunar crust. The latest estimation from the GRAIL mission provides a mean density for the lunar crust that is particularly low, of 2,550 kg m³. A large density of magma can be inferred from the density of 3,010 kg m³ of lunar mare basalt, so the intrusion-to-extrusion ratio on the Moon may be even higher than that on Earth, and can reach an upper limit of 50:1.

Fig. 4. Topography (top) and gravity map (bottom) of the lunar nearside.

Finally, the authors propose outstanding problems and future exploration targets.

(1) The temporal and spatial span of lunar volcanism. In recent years, an outstanding issue of lunar exploration is how long such a small terrestrial planetary body can sustain volcanic activity. Addressing this issue can provide new insights into and constrain our current models of the thermal and compositional evolution of the Moon. In addition, by determining the temporal and spatial span of lunar volcanism, the outstanding questions of (a) the nature and origin of the dichotomy in the distribution of basalts on the lunar nearside and farside, and (b) the mechanism for the emplacement and longevity of lunar volcanism may be solved.

(2) Formation mechanism of lunar volcanic landforms. The Moon hosts a diversity of volcanic landforms that have been studied using imaging, topographic, spectral, and radar data. Nevertheless, debates still exist concerning the formation mechanism of the landforms. In addition, the origin of silica-rich volcanic features is also mysterious without returned samples.

(3) Subsurface structures of lunar volcanic features. Although gravity and radar data have greatly helped us understand the subsurface structures of lunar volcanic features, several aspects still need to be considered: (a) gravity data do not have inherent depth resolution, and therefore, the vertical or radial distribution of density in the recovered 3D model is a direct consequence of prior information or constraints applied; (b) kilometer-depth structures of lunar mare can be detected using orbital radar or active source moon-quake; however, the resolution of these detections is too low to identify finer lava flow and the detection range of moon- quake data is very limited; (c) mare basalts should contain different types of deposits such as volcanic ash and pyroclastic; however, the imaging method based on radar waves has not been fully effective to identify them.

To solve these questions, the most important demand is to acquire more lunar samples and high-quality in situ data of typical lunar volcanic features. We believe that with the continuous implementation of China’s lunar exploration program, more secrets of lunar volcanism will be uncovered in the near future.