The Moon’s Complex, Turbulent Youth

Categories: Moon to Mars

This detailed geologic map of Schrödinger basin, which formed when a huge object struck the moon, reveals a patchwork of lunar material, including the peak ring (inner brown ring), recent volcanic activity (red), cratering (yellow) and plains material (dark green and kelly green). Credit: NASA/Scott Mest

The Moon’s surface is more complex than previously thought and was bombarded by two distinct populations of asteroids or comets in its youth, according to three new papers in the Sept. 17 issue of Science that describe data from NASA’s Lunar Reconnaissance Orbiter.

Two of the papers describe data from LRO’s Diviner Lunar Radiometer Experiment instrument that reveal the complex geologic processes that forged the lunar surface. The data showed previously unseen compositional differences in the crustal highlands, and confirmed the presence of anomalously silica-rich material in five distinct regions.

All minerals and rocks absorb and emit energy with unique signatures that reveal their identity and formation mechanisms. For the first time, the Diviner instrument is providing scientists with global, high-resolution infrared maps of the Moon, enabling them to make a definitive identification of silicate minerals commonly found within its crust. “Diviner is literally viewing the Moon in a whole new light,” said Benjamin Greenhagen of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., lead author of one of the Diviner papers.

Lunar geology can be roughly broken down into two categories — the anorthositic highlands, rich in calcium and aluminum, and the basaltic “maria,” giant impact basins filled with solidified lava flows that are abundant in iron and magnesium. Both of these crustal rocks are considered the direct result of crystallization from lunar mantle material, the partially molten layer beneath the crust.

Diviner data superimposed on a Lunar Orbiter IV mosaic of Aristarchus crater. Red and orange colors indicate highly silicic compositions. Credit: NASA/Godard/UCLA/Stony Brook

Diviner’s observations have confirmed that most lunar terrains have signatures consistent with compositions in these two broad categories. But they have also revealed lunar soil compositions with more sodium than that of typical anorthosite crust. The widespread nature of these soils reveals that there may have been variations in the chemistry and cooling rate of the magma ocean that formed the early lunar crust, or they could be the result of secondary processing of the early lunar crust.

Most impressively, in several locations around the Moon, Diviner has detected highly silicic minerals such as quartz, potassium-rich and sodium-rich feldspar — minerals that are only associated with highly evolved lithologies, or rocks that have undergone extensive magmatic processing. Detection of silicic minerals at these locations is significant, as they occur in areas previously shown to exhibit anomalously high abundances of the element thorium, another proxy for highly evolved lithologies.

“The silicic features we’ve found on the Moon are fundamentally different from the more typical basaltic mare and anorthositic highlands,” said Timothy Glotch of Stony Brook University, N.Y., lead author of the second Diviner paper. “The fact that we see this composition in multiple geologic settings suggests that there may have been multiple processes producing these rocks.”

A lunar topographic map showing one of the most densely cratered regions on the Moon. The topography is derived from over 2.4 billion shots made by the Lunar Orbiter Laser Altimeter (LOLA) instrument on board the NASA Lunar Reconnaissance Orbiter. These most heavily cratered areas are among the best candidates to study and explore to understand the earliest lunar history. Credit: NASA/Goddard/MIT/Brown

One thing not apparent in the data is evidence for pristine lunar mantle material, which previous studies have suggested may be exposed at some places on the lunar surface. Even in the South Pole Aitken basin, also known as SPA, the largest, oldest, and deepest impact crater on the Moon — deep enough to have penetrated through the crust and into the mantle — there is no evidence of mantle material.

The implications of this are as yet unknown. Perhaps there are no such exposures of mantle material, or maybe they occur in areas too small for Diviner to detect. But it’s likely that if the impact that formed this crater did excavate any mantle material, it has since been mixed with crustal material from later impacts inside and outside the basin.

“The new Diviner data will help in selecting the appropriate landing sites for potential future robotic missions to return samples from SPA,” Greenhagen said. “We want to use these samples to date the SPA-forming impact and potentially study the lunar mantle, so it’s important to use Diviner data to identify areas with minimal mixing.”

In the other paper, lead author James Head of Brown University in Providence, R.I., describes an analysis of a detailed global topographic map of the Moon created using LRO’s Lunar Orbiter Laser Altimeter. This new dataset shows that the older highland impactor population can be clearly distinguished from the younger population in the lunar maria.The highlands have a greater density of large craters, implying that the earlier population of impactors had a proportionally greater number of large fragments than the population characterizing later lunar history, Head said.

Artist impression of NASA’s Lunar Reconnaissance Orbiter. Credit: NASA

Head said details about impactor populations on the Moon have implications for the earliest history of all the planets in the inner solar system, including Earth. “Like the Rosetta stone, the lunar record can be used to translate the ‘hieroglyphics’ of the poorly preserved impact record on Earth,” he said. Studying the history of the Moon can thereby help astrobiologists understand the conditions that allowed the Earth to become habitable for life. Such information is invaluable when attempting to identify other solar systems in the Universe that could support habitable worlds. Studying the impact record of the Earth can also help scientists better understand the role of asteroid and comet impacts in the origin and evolution of life on our planet. Due to environmental factors like weather and constant geological activity on Earth, evidence of ancient impacts on our planet ‘washes away’ over time. The same is not true for the Moon, and peicing together the history of lunar impacts provides an indication of the timing and intensity of ancient impacts on Earth.