A groundbreaking study using data from NASA’s GRAIL (Gravity Recovery and Interior Laboratory) mission has shed light on the mystery of why the Moon’s two sides look drastically different. The near side, always facing Earth, is smooth and filled with dark volcanic plains called mare, while the far side is rugged and heavily cratered.
The research, published in a recent scientific journal, reveals that the Moon’s deep interior is not symmetrical. This asymmetry appears to result from intense volcanic activity on the nearside billions of years ago, which caused heat-producing elements like thorium and titanium to concentrate there. As a result, the nearside mantle is estimated to be 180–360°F (100–200°C) hotter than the farside.
Ryan Park, lead author and supervisor of the Solar System Dynamics Group at NASA’s Jet Propulsion Laboratory, explained that the nearside flexes more than the farside due to Earth’s gravitational pull—a process known as tidal deformation. This suggests internal structural differences between the two hemispheres, particularly in the mantle, the layer between the Moon’s crust and core.
The study also found that the asymmetric interior corresponds to the differences in surface features, offering strong support to long-standing theories that ancient volcanism shaped the Moon’s face. The data came from the GRAIL mission’s twin spacecraft, Ebb and Flow, which orbited the Moon from 2011 to 2012 and provided the most accurate gravity map of the lunar body to date.
Beyond improving our understanding of lunar geology, the gravity map is expected to play a vital role in developing lunar navigation systems for future missions. Park emphasized its importance in creating a precise lunar reference frame and timekeeping system, which will enhance the safety and accuracy of spacecraft and lander operations.
This research not only deepens our understanding of Earth’s closest celestial neighbor but also provides a template for studying the interiors of other planetary bodies, such as Jupiter’s moon Ganymede and Saturn’s Enceladus—both considered prime candidates in the search for extraterrestrial life.
