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USA, May 16 – A recent NASA study of lunar gravity is revealing new clues about why the two sides of the Moon are so different. The research, based on data from the Gravity Recovery and Interior Laboratory (GRAIL) mission, shows that our satellite's deep interior has an asymmetric structure.
Scientists discovered that this asymmetry was caused by intense volcanism on the side visible from Earth billions of years ago. During its orbit, the lunar near side flexes slightly more than the far side due to Earth's gravitational influence, a phenomenon known as "tidal deformation." This difference in flexibility confirms significant variations in the lunar mantle between the two sides, ultimately explaining why their surfaces have such distinct appearances.
"Our study shows that the Moon's interior is not uniform: the side facing Earth—the near side—is warmer and more geologically active at depth than the far side," said Ryan Park, supervisor of the Solar System Dynamics Group at NASA's Jet Propulsion Laboratory in California and lead author of the study published in the journal Nature.
Volcanism and Temperature: Keys of the Lunar Mantle
The side of the Moon we see from Earth is dominated by extensive plains called "seas," formed by ancient solidified lava flows. In contrast, the far side presents an irregular terrain with few plains.
For years, scientists have theorized that intense volcanism on the near side caused the accumulation of heat-generating radioactive elements on that side of the lunar mantle. The recent study provides the strongest evidence yet to confirm this theory.
Measurements reveal that the mantle on the near side is between 100 and 200 degrees Celsius hotter than that on the far side. This significant thermal difference appears to be maintained thanks to the radioactive decay of elements such as thorium and titanium, concentrated on the visible side of our satellite.
The Moon's diameter of approximately 3,475 km is a little more than a quarter of Earth's diameter. The lunar mantle is the layer located below the crust and above the core, spanning a depth of approximately 35–1,400 km below the surface. The mantle constitutes approximately 80% of the Moon's mass and volume and is composed primarily of the minerals olivine and pyroxene, similar to Earth's mantle.
"The fact that the asymmetry detected in the mantle matches the pattern of surface geology—for example, differences in the abundance of mare basalts (volcanic rocks) approximately 3–4 billion years old between the near and far sides—suggests that the processes that drove ancient lunar volcanism are still active today," said Alex Berne, a Caltech computational planetary scientist and study co-author affiliated with the Jet Propulsion Laboratory that works on the design of gravity sensors for missions to the outer solar system.
The GRAIL Gravitational Map
For years, scientists analyzed data collected by the Ebb and Flow spacecraft of the GRAIL mission, which orbited the Moon in 2011-2012. According to Ryan Park, this work has provided "the most detailed and accurate gravitational map of the Moon to date."
This revolutionary map not only improves our understanding of the satellite, but also lays the foundation for future lunar navigation systems, which are essential for upcoming exploration missions. The methodology employed could be applied to other bodies such as Enceladus (moon of Saturn) and Ganymede (moon of Jupiter), potential candidates in the search for extraterrestrial life.
Park emphasizes that the Moon stabilizes Earth's rotation and generates ocean tides, influencing our ecosystems. Although we have advanced our understanding of it, many questions about its deep structure remain unanswered, keeping our cosmic neighbor as a fascinating object of scientific study. (Text and Photo: Cubasí)
