An exhaustive examination of lunar gravity using data obtained by two NASA robotic spacecraft is offering new clues about why the two sides of the moon – the one perpetually facing Earth and the other always facing away – look so different.
The data from the U.S. space agency's GRAIL, or Gravity Recovery and Interior Laboratory, mission indicates that the moon's deep interior has an asymmetrical structure, apparently caused by intense volcanism on its nearside billions of years ago that helped shape its surface features.
The researchers discovered that the lunar nearside flexes slightly more than the farside during its elliptical orbit around Earth thanks to our planet's gravitational influence - a process called tidal deformation. This indicates differences in the two sides of the lunar interior, they said, specifically in the geological layer called the mantle.
"Our study shows that the moon's interior is not uniform: the side facing Earth – the nearside – is warmer and more geologically active deep down than the farside," 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 on Wednesday in the journal Nature.
The moon's nearside is covered by vast plains, called mare, formed from molten rock that cooled and solidified billions of years ago. Its farside has much more rugged terrain, with few plains.
Some scientists have hypothesized that intense volcanism within the nearside that caused radioactive, heat-generating elements to accumulate on that side of the mantle drove the surface differences observed today. The new findings offer the strongest evidence yet to support this notion.
The researchers estimated that the nearside mantle on average is about 180-360 degrees Fahrenheit (100-200 degrees Celsius) hotter than the farside, with the thermal difference perhaps sustained by radioactive decay of the elements thorium and titanium on the nearside.
"The moon's nearside and farside look very different, as shown by differences in topography, crustal thickness and the amount of heat-producing elements inside," Park said.
The moon's diameter of about 2,160 miles (3,475 km) is a bit more than a quarter of Earth's diameter. The lunar mantle is the layer located beneath the crust and above the core, spanning a depth about 22-870 miles (35-1,400 km) under the surface. The mantle makes up roughly 80% of the moon's mass and volume and is composed mostly of the minerals olivine and pyroxene, similar to Earth's mantle.
"The fact that the detected asymmetry in the mantle matches the pattern of the surface geology – for instance, differences in the abundance of the approximately 3-4 billion-year-old mare basalts (volcanic rock) between the nearside and the farside - suggests that processes which drove ancient lunar volcanism are active today," said Caltech computational planetary scientist and study co-author Alex Berne, affiliated with the Jet Propulsion Laboratory working on the design of gravity sensors for missions to the outer solar system.
The researchers spent years analyzing data from GRAIL's Ebb and Flow spacecraft, which orbited the moon from December 2011 to December 2012.
"Our study delivers the most detailed and accurate gravitational map of the moon to date," Park said.
"This enhanced gravity map is a critical foundation for developing lunar Positioning, Navigation and Timing (PNT) systems, which are essential for the success of future lunar exploration missions. By improving our understanding of the moon's gravity field, it contributes to establishing a precise lunar reference frame and time system, enabling safer and more reliable navigation for spacecraft and surface operations," Park added.
The same approach employed here using gravity data to assess the lunar interior, the researchers said, could be applied to other bodies in the solar system such as Saturn's moon Enceladus and Jupiter's moon Ganymede, two worlds of interest in the search for potential life beyond Earth.
In the meantime, the new findings add to the understanding of Earth's eternal companion.
"The moon plays a vital role in stabilizing Earth's rotation and generating ocean tides, which influence natural systems and daily rhythms," Park said. "Our knowledge of the moon has expanded through human and robotic missions that have revealed details about its surface and interior, yet many questions about its deep structure and history remain. As our closest neighbor, the moon continues to be an important focus of scientific discovery."
