Gravity data was obtained by spacecraft in NASA GRAIL mission

15/5/2025 6:14

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."