The far side of the Moon presents a geological landscape dramatically different from its Earth-facing counterpart, revealing ancient secrets about planetary formation and early solar system history.
A Hemisphere of Contrasts
The dichotomy between the lunar near side and far side represents one of planetary science's most intriguing mysteries. While the near side features extensive dark volcanic plains known as maria, the far side is dominated by bright, ancient highland terrain heavily scarred by impact craters. This fundamental asymmetry reflects billions of years of distinct geological evolution.
The far side's surface is approximately 30 kilometers higher in elevation on average than the near side, with a correspondingly thicker crust. Gravity measurements from orbital missions, including NASA's GRAIL (Gravity Recovery and Interior Laboratory), have revealed that the far side crust reaches thicknesses of 60 to 70 kilometers in some regions, compared to 30 to 40 kilometers on the near side.
Impact Basin Formation and Preservation
The South Pole-Aitken Basin, located primarily on the far side, represents the largest confirmed impact structure in the solar system, measuring approximately 2,500 kilometers in diameter and 13 kilometers deep. This ancient basin, formed roughly 4 billion years ago during the Late Heavy Bombardment period, provides a window into the Moon's lower crust and possibly upper mantle.
Analysis of data from China's Chang'e-4 mission, which achieved the first soft landing on the far side in January 2019, has provided unprecedented insights into the basin's composition. Spectroscopic measurements revealed the presence of olivine and low-calcium pyroxene, minerals consistent with lower crustal or upper mantle material excavated during the massive impact event.
The preservation state of far side craters differs markedly from near side features. The absence of extensive maria means that ancient craters remain visible, their morphology largely intact. This preservation enables researchers to reconstruct impact chronology and understand the bombardment history that shaped the early Moon and, by extension, the inner solar system.
Volcanic History and Mare Formation
The scarcity of maria on the far side has puzzled researchers since the first images were transmitted by Luna 3 in 1959. Several hypotheses have been proposed to explain this asymmetry. The thicker far side crust likely inhibited the ascent of magma from the lunar interior, preventing the extensive volcanic flooding that created near side maria.
Additionally, the different thermal histories of the two hemispheres may have played a role. Some models suggest that the near side experienced higher concentrations of heat-producing radioactive elements, facilitating prolonged volcanic activity. This compositional asymmetry, possibly established during the Moon's formation or shortly thereafter, would have profound implications for volcanic potential.
Where volcanic deposits do exist on the far side, they are typically smaller and older than their near side counterparts. Mare Moscoviense and Mare Ingenii represent the largest far side maria, yet they are modest compared to near side features like Mare Imbrium or Oceanus Procellarum.
Crustal Composition and Structure
Remote sensing data from orbital missions has revealed subtle but significant compositional differences between near and far side crustal materials. Far side highlands show higher concentrations of calcium-rich plagioclase feldspar, the primary constituent of the lunar anorthositic crust that formed as the lunar magma ocean crystallized.
The presence of thorium, a heat-producing radioactive element, is significantly lower on the far side. This distribution has been mapped extensively by orbital gamma-ray and neutron spectrometers, revealing an asymmetric distribution of incompatible elements concentrated predominantly on the near side, particularly in and around the Procellarum KREEP Terrane.
Seismic Characteristics and Internal Structure
The Apollo Passive Seismic Experiment, which operated from 1969 to 1977, provided data on moonquakes and impact events. However, all seismometers were placed on the near side, leaving far side seismic activity largely uncharacterized. Future missions equipped with seismic instruments on the far side could reveal asymmetries in internal structure, thermal state, and ongoing geological activity.
Theoretical models suggest that if tidal heating or other internal heat sources remain active, seismic activity rates and characteristics might differ between hemispheres. Deep moonquakes, which originate at depths of 700 to 1,100 kilometers and occur in response to tidal stresses, could exhibit different patterns on the far side due to structural and compositional asymmetries.
Implications for Planetary Science
Understanding the geological differences between the lunar hemispheres extends beyond lunar science. The Moon's dichotomy provides insights into planetary differentiation processes, impact basin formation, and the evolution of terrestrial planetary bodies throughout the solar system.
The far side's ancient, well-preserved terrain serves as a natural laboratory for studying impact processes, regolith development, and space weathering effects over geological timescales. As plans for sustained lunar exploration advance, the far side's unique geological characteristics present both scientific opportunities and operational challenges for future missions.
Future Research Directions
Continued exploration of the far side, including sample return missions and detailed surface investigations, will be essential for resolving outstanding questions about lunar geological evolution. Proposed missions aim to collect samples from the South Pole-Aitken Basin interior, deploy seismic networks, and conduct in-situ compositional analysis at multiple sites.
These investigations will not only enhance understanding of the Moon's formation and evolution but will also inform broader questions about planetary processes, impact dynamics, and the conditions that prevailed in the early solar system.
References: This article synthesizes data from NASA's Lunar Reconnaissance Orbiter, GRAIL mission, China's Chang'e-4 mission, and peer-reviewed literature on lunar geology and planetary science.