The flood of new data from the Moon continues to enlighten and puzzle lunar scientists. Members of the Lunar Reconnaissance Orbiter Camera team have noticed an unusual landform on the far side of the Moon that was as unexpected as it might be significant.
We’ve known for many years that early in its history, the Moon was volcanically active. The dark, smooth maria of the Moon is made up of lava flows, individually erupted over at least a billion year time span and possibly for much longer. In total volume, the mare lavas make up only a percent or so of the crust, so the Moon is not the volcanic cauldron that Io, the large moon of Jupiter, appears to be. But these lavas indicate that the early Moon was hot and that melted material spilled onto its surface in the past.
The volcanic rocks of the Moon are what geologists call mafic, meaning that they are enriched in iron and magnesium. Mafic lavas (basalt) are commonly found as plains and low relief shield volcanoes, such as the Hawaiian islands. On Earth, volcanic rocks can be mafic (and in fact, are the most abundant rocks on Earth, comprising the ocean floor bedrock) or they can be felsic, meaning enriched in silica (SiO2) and depleted in iron. On Earth, felsic rocks occur in mid-continental volcanoes and the stratovolcanoes that are found along the margins of the giant tectonic plates that make up Earth’s outer, rigid layer (lithosphere). Felsic lavas are often associated with explosive, violent eruptions, such as the Mt. St. Helens eruption of 1980.
All of the volcanic rocks returned from the Moon by the Apollo astronauts are mafic. Most of them are basalts – lava erupted as quiet, fissure-fed sheets. A few of the samples are tiny glass beads of mafic composition, erupted when low viscosity (runny) fluid lava squirted into space as a spray of lava called a fire-fountain. Sprayed droplets of lava cool in ballistic flight and land on the Moon as a uniform deposit of tiny (40 micron diameter) glass beads, forming a lunar ash bed. Although we did find tiny fragments of felsic material in some of the complex breccias from the highlands of the Moon, no felsic lavas or ash were collected on Apollo.
In April 1972, the Apollo 16 mission was sent to the Descartes highlands on the near side of the Moon. Pre-mission mapping and studies indicated to the geology team that the plains and mountains of Descartes are felsic volcanoes, having the morphology of lava domes and ash flows on Earth. The Apollo 16 crew was given intensive instruction in the recognition and mapping of volcanic units on the Earth, so they would recognize the abundant felsic volcanics thought to make up the Descartes highlands.
John Young and Charlie Duke put their geological training to good use when they landed on the Moon. Not only were the rocks at Descartes not silica-rich volcanics, they weren’t even volcanic! The crew immediately recognized that all the rocks they found were breccias – aggregates of many rocks assembled by impact. As Command Module Pilot Ken Mattingly wryly noted, “Well, it’s back to the drawing boards – or wherever geologists go!” (They usually go for a beer.)
After that sobering experience, lunar geologists were hesitant to map felsic volcanoes on the Moon again. In fact, the pendulum swung away from such a process ever having occurred on the Moon at all. Nevertheless, we continued to note small geological anomalies around the Moon, hills and domes that are difficult to explain as impact features. Additionally, some of these small landforms apparently have unusual composition as they have unique spectral properties, being anomalously “redder” (i.e., higher reflectance at longer visible wavelengths) than surrounding terrain. These features were imaginatively named “red spots.” Although we could determine that lunar red spots were compositionally distinct, we did not know exactly what those compositions were. Now, with new data from the orbiting lunar missions, the mystery of the red spots is finally solved.
The red spots are small volcanoes made up of felsic rocks. We know from data returned by the DIVINER thermal imaging spectrometer on Lunar Reconnaissance Orbiter that these landforms are rich in silica. From the Lunar Prospector gamma-ray data, we have determined that they are also enriched in the element thorium, a key indicator of chemically evolved rock types. Finally, data from both Earth-based telescopes and from the Moon Mineralogy Mapper on the Chandrayaan-1 mission, show that some of the red spots are made of almost pure glass. On Earth, silica-rich volcanic glass forms a deposit called obsidian; its crystallized form is rhyolite. New, remotely sensed compositional data show that the lunar red spots are felsic domes of obsidian and rhyolite. Red spots occur mostly on the western near side of the Moon, the area in and around Oceanus Procellarum. The new finding of an isolated felsic volcano on the far side of the Moon indicates that such eruptions were a global phenomenon.
These features are not volumetrically major and occur as small geological oddities set within the predominantly mafic, basaltic volcanic terrain of the lunar surface. Their presence was not predicted by the prevailing model of lunar volcanism. After the fiasco of the mistaken Apollo 16 prediction, geologists were hesitant to pronounce any dome on the Moon to be a felsic volcano. Suitably chastened, they re-interpreted those dome-like landforms of the highlands to be by-products of basin-forming impacts. We see by the existence of these features that in some cases, the volcanic interpretation is viable. This information adds to our understanding of the incredibly rich and complex geological story of the Moon.