The Moon looks solid and still, but its gravity is not perfectly smooth. A spacecraft passing over one region can be pulled a little more strongly than over another. Some of the strongest patches lie beneath enormous ancient impact basins. They are called mascons, short for mass concentrations.
A mascon is not a hidden ball of exotic metal. It is an excess of mass compared with what a simple reference model would predict, produced by the arrangement and density of lunar rock. These invisible differences matter to spacecraft navigation and preserve clues to a violent geological past.
Reading gravity with two spacecraft
NASA's GRAIL mission used two spacecraft, Ebb and Flow, following one another around the Moon. Instruments measured tiny changes in the distance between them. When the leading spacecraft encountered a stronger gravitational pull, it moved slightly differently from its companion. As the second followed, the separation changed again.
Combining these measurements across many orbits produced exceptionally detailed gravity maps. Such maps do not show the same thing as photographs. A striking surface crater and a strong gravity anomaly can correspond, but gravity also responds to material below the visible ground.
Colors on a gravity map encode departures from a chosen reference. They do not mean the lunar surface is physically red or blue. Nor does a red patch mean gravity there rivals Earth's: the Moon's overall gravity remains much weaker.
Read the changing gap, rather than an imaginary weighing scale
Picture Ebb approaching a region with a mass surplus while Flow is still farther away. The same region does not pull on both spacecraft in exactly the same way at exactly the same instant. As they move onward, the geometry changes. The measurement is therefore a changing separation along a moving pair of trajectories, rather than a direct photograph of a buried rock.
A useful thought experiment is to replace the Moon with a perfectly smooth reference world. Predictable orbital motion would still change where the spacecraft were, but the departures associated with real structure would be absent. Scientists compare observations with models to recover the extra information. A gravity anomaly is a difference relative to that calculation; it is not a separate kind of gravity.
On the published map, choose a basin and inspect both its center and its surroundings. Read the legend before interpreting the colors. Are you looking at a surplus, a deficit or a transition between them? The surrounding rings matter because an impact rearranges material across a region. Reducing the result to one heavy dot throws away part of the geological evidence.
An impact leaves more than a hole
Large impacts excavated crust, deformed the interior and brought denser mantle material upward beneath their basins. The resulting structure then cooled and evolved. In some basins, later volcanic lava added additional dense material. Together, these processes can leave a mass surplus that a passing spacecraft detects.
GRAIL data and computer models helped researchers explain the characteristic gravity patterns around major basins. Some have a central positive anomaly surrounded by rings with different gravitational signatures. The details depend on the impact, the thickness and properties of the crust, and subsequent geological history.
Why a gravity map does not uniquely name a rock
Imagine two sealed boxes of the same size. One holds a compact dense object near the top; another contains a different arrangement of material spread through its interior. A measurement of their external pull gives information about mass, but a limited set of measurements need not reveal every detail of either box. The analogy helps explain why researchers combine gravity with other evidence.
For lunar basins, topography constrains the visible shape. Knowledge of crustal thickness and plausible rock densities constrains possible interior arrangements. Impact simulations test whether a proposed sequence could produce the observed pattern. A successful explanation has to fit these different requirements together, rather than fitting a colored patch alone.
The distinction also clarifies the role of lava. A dark volcanic plain can add dense material to a basin, yet the broader mascon explanation includes excavation, deformation and cooling. “The crater filled with lava” is too small a story for every observed pattern. The buried geometry and the material's history both matter.
Try stating what each illustration contributes in one sentence. The cutaway proposes a way to imagine the interior. The gravity map represents a measured field after mathematical processing. The surface photograph records reflected light from terrain. None replaces the others. Their combination turns spacecraft navigation data into a reconstruction of an event that no observer could have watched when it happened.
A practical map of an uneven world
For mission planners, an uneven gravitational field means an orbit must be calculated with more care than if the Moon were a perfectly uniform sphere. The same precision can turn an old navigation difficulty into a powerful scientific tool. Changing spacecraft motion is a measurement of the world beneath it.
Compare the cutaway with NASA's actual GRAIL gravity map, using its legend to identify the measured quantity.
Sources and further reading: NASA/JPL: GRAIL solves the mascon mystery · NASA: GRAIL gravity map · NASA Gravity Assist: The Moon’s Holy GRAIL
