Key points
- Moon’s deep interior is asymmetrical: US Space Agency
- Earth-facing side is warmer, more geologically active: NASA
- Nearside mantle is between 100, 200 degrees Celsius hotter than far side
ISLAMABAD: A comprehensive analysis of lunar gravity, using data gathered by NASA’s twin robotic spacecraft, has provided new insights into why the two hemispheres of the Moon – the one always facing Earth and the one always turned away – appear so different.
Findings from the US space agency’s Gravity Recovery and Interior Laboratory (GRAIL) mission suggest that the Moon’s deep interior is asymmetrical. This disparity is thought to have been caused by intense volcanic activity on the side facing Earth billions of years ago, which played a major role in shaping the Moon’s surface, according to Reuters.
Scientists observed that the nearside of the Moon flexes slightly more than the far side during its elliptical orbit around Earth, due to our planet’s gravitational pull – a phenomenon known as tidal deformation. This variation implies differences in the internal structure between the two sides, particularly in the geological layer known as the mantle.
New research
“Our research demonstrates that the Moon’s interior is not uniform: The Earth-facing side – the nearside – is warmer and more geologically active deep beneath the surface than the far side,” said Ryan Park, head of the Solar System Dynamics Group at NASA’s Jet Propulsion Laboratory in California and lead author of the study, which was published in Nature on Wednesday.
The nearside of the Moon features expansive plains, known as maria, formed by ancient molten rock that cooled and solidified. In contrast, the far side is more rugged and mountainous, with far fewer plains.
Some scientists have theorised that the nearside underwent significant volcanic activity, causing heat-generating radioactive elements to concentrate on that side of the mantle, thereby driving the differences we observe today. The new research presents the most compelling evidence to date supporting this theory.
Radioactive decay
The scientists estimate that, on average, the nearside mantle is between 100 and 200 degrees Celsius (180 to 360 degrees Fahrenheit) hotter than the far side, with the temperature disparity likely maintained by the radioactive decay of elements such as thorium and titanium.
“There are striking differences in the Moon’s topography, crustal thickness, and the concentration of heat-producing elements between the two sides,” said Park.
The Moon has a diameter of approximately 3,475 kilometres (about 2,160 miles), just over a quarter that of Earth. The mantle lies beneath the crust and above the core, extending from roughly 35 to 1,400 kilometres (22 to 870 miles) beneath the surface.
It comprises about 80 per cent of the Moon’s mass and volume and is primarily composed of the minerals olivine and pyroxene, similar to those found in Earth’s mantle.
Surface geological patterns
“The detected asymmetry in the mantle corresponds closely with surface geological patterns – such as the distribution of mare basalts, which are around three to four billion years old – suggesting that the forces driving ancient lunar volcanism are still active today,” said Alex Berne, a computational planetary scientist at Caltech and co-author of the study, who is also involved in developing gravity sensors for outer solar system missions.
The team spent several years analysing data from GRAIL’s Ebb and Flow spacecraft, which orbited the Moon from December 2011 to December 2012.
“Our work has produced the most detailed and precise gravitational map of the Moon to date,” Park said.
Improved gravity map
“This improved gravity map forms a vital foundation for developing Lunar Positioning, Navigation, and Timing (PNT) systems, which are crucial for the success of future lunar exploration missions. By refining our understanding of the Moon’s gravitational field, we are helping to create a precise lunar reference frame and timekeeping system, which will support safer and more reliable navigation for spacecraft and operations on the lunar surface,” he added.
The same technique of using gravity data to investigate interior structures could be applied to other celestial bodies in the solar system, such as Saturn’s moon Enceladus and Jupiter’s moon Ganymede – both considered key targets in the search for life beyond Earth.
Meanwhile, these new findings enhance our understanding of Earth’s ever-faithful companion.
“The Moon plays an essential role in stabilising Earth’s rotation and generating ocean tides, which influence ecosystems and our daily cycles,” said Park. “While human and robotic missions have greatly expanded our understanding of the Moon’s surface and interior, many questions remain about its deeper structure and ancient history. As our closest neighbour, the Moon remains a rich subject for scientific exploration.”
