NASA uses gravity data to look inside planetary structures without having to land on them
No spacecraft ever touched down, no drills cracked the crust, yet two new studies used gravity data to pull back the curtain on the deep interiors of the Moon and one of the solar system’s largest asteroids.
By analyzing subtle shifts in spacecraft trajectories, a team led by Ryan Park at NASA’s Jet Propulsion Laboratory has produced the most detailed gravity map of the Moon to date – and uncovered a surprising correlation inside of the asteroid Vesta.
“Gravity is a unique and fundamental property of a planetary body that can be used to explore its deep interior,” Park said.
“Our technique doesn’t need data from the surface; we just need to track the motion of the spacecraft very precisely to get a global view of what’s inside.”
Tracking gravity to see inside
The method relies on Doppler and range measurements between orbiting probes and Earth’s antennas. Any lump or void inside the target world tugs on the passing spacecraft, and alters its speed by mere fractions of a millimeter per second.
Fed into supercomputers, those perturbations translate into three-dimensional maps of hidden mass. NASA’s GRAIL mission supplied the raw numbers for the lunar investigation.
From December 2011 to December 2012, twin probes nicknamed Ebb and Flow, whipped around the Moon in choreographed formation, their separation tracked to sub-micron precision.
For Vesta, Park’s group mined radiometric data gathered by the Dawn spacecraft during its 14-month residency above the battered protoplanet in 2011-2012, and supplemented it with high-resolution imagery.
Moon’s interior shows gravity divide
The team’s new lunar model goes beyond static topography. Because the Moon follows an elliptical path, Earth’s gravity squeezes and relaxes its rocky shell during each 27-day circuit. The researchers found that the degree of “tidal deformation” differs between hemispheres.
“We found that the Moon’s near side is flexing more than the far side, meaning there’s something fundamentally different about the internal structure of the Moon’s near side compared to its far side,” Park said.
“When we first analyzed the data, we were so surprised by the result we didn’t believe it. So we ran the calculations many times to verify the findings. In all, this is a decade of work.”
The modest yet measurable extra flex on the near side points to a warmer upper mantle enriched in heat-generating radioactive elements.
Such a pocket would have kept basaltic lavas molten long after the far side cooled, explaining the maria – dark plains that dominate the Earth-facing side – while the reverse hemisphere remained rugged.
The new gravity field, the most precise ever assembled, will let future landers and rovers compute their position and local time more accurately, a crucial step as NASA’s Artemis program prepares for permanent outposts.
A rule-breaking asteroid
Vesta, by contrast, spins like a wobbling top. Each slight tilt alters where its bulk shifts relative to space, and produces detectable changes in its gravity signature.
“Our technique is sensitive to any changes in the gravitational field of a body in space, whether that gravitational field changes over time, like the tidal flexing of the Moon, or through space, like a wobbling asteroid,” said Park.
“Vesta wobbles as it spins, so we could measure its moment of inertia, a characteristic that is highly sensitive to the internal structure of the asteroid.”
Conventional wisdom holds that Vesta, which was once partially molten, should have settled into layers: a dense iron-nickel core, a rocky mantle, a basaltic crust.
But the new analysis suggests little separation occurred. Either the core is minuscule or absent, leaving a body whose interior is mostly uniform.
That possibility reshapes theories of how planetary embryos cooled. Perhaps Vesta froze so quickly that heavy elements never had time to sink, or later impacts mixed the interior like a cosmic spoon.
Crunching the gravitational code
Both projects demanded years of coding and supercomputer time. The researchers wrestled with countless variables, including spacecraft thruster firings, solar radiation pressure, and even the gravitational pull of passing planets.
Park’s group iterated models until the simulated spacecraft motions matched the historic tracking records within centimeters. Only then could they isolate the fingerprints of buried structures.
The payoff is twofold. For mission planners, refined gravity maps mean safer orbits and pinpoint landings.
For planetary scientists, they provide new boundary conditions for models of accretion, volcanism, and thermal evolution across the solar system.
Future Moon and asteroid targets
The twin studies also demonstrate that a single analytical framework can tackle bodies as different as a 3,475-kilometer-wide (2,160-mile-wide) satellite and a 525-kilometer-wide (326-mile-wide) asteroid.
Future targets may include Mars’s moons Phobos and Deimos, icy dwarf planets such as Ceres, or the rubble-pile asteroids visited by missions like OSIRIS-REx. Each carries a unique gravitational code ready to be deciphered.
In the meantime, the Moon’s asymmetric flexing strengthens the case for concentrated heat sources beneath its nearside crust, while Vesta’s unexpected sameness raises fresh questions about the earliest chapters of planetary formation.
Together they illustrate how, with nothing more than radio links and orbital dance steps, scientists can reach beneath hardened surfaces and reconstruct stories written in mass and motion – stories of magma oceans, tidal kneading, and frozen beginnings that shape the architecture of our cosmic neighborhood.
The studies are published in the journals Nature and Nature Astronomy.
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