Mapping the moon's gravity

The GRAIL mission, which stands for Gravity Recovery and Interior Laboratory, consists of two identical spacecraft, compact in size (a metre or two), with a single rocket for launch.

The spacecraft will orbit the airless moon at an altitude just fifty kilometers above the lunar surface.

The two craft will orbit in tandem, being separated from each other by about 200 kilometers, this formation being aligned very precisely with the earth-based Deep Space Network of four large antennae.

Unlike most lunar spacecraft, which reached the moon a few days after launch, the GRAIL probes using ambient solar power will trudge their way and reach the lunar destination only after three months, by about mid-December.

Although recent lunar probes, like the Lunar Reconnaissance Orbiter (LRO) have mapped the moon’s rocky mineralogy and surface chemistry in great detail, little is known about the moon’s interior. Small changes in the relative distance between the two orbiting GRAIL spacecraft would provide data to deduce details of the lunar interior. The separation between the twin satellites would not change if the moon were a homogeneous sphere.

Any density in homogeneities or mass concentrations below the surface would produce tiny accelerations leading to time-dependent changes in the separation between the twin orbiting probes.

Locating mineral deposits
This technique is in fact used in locating mineral or metal deposits (on earth) using a sensitive gravimeter which can register small changes in the acceleration due to gravity.
Indeed, right now, the twin GRACE (Gravity Recovery and Climate Experiment) spacecraft are orbiting the earth and have been very precisely mapping the terrestrial gravity field.

Unlike that in the case of the moon, the earth’s gravity field constantly varies because of weather and climate patterns (like high-pressure air zones, etc.) and these anomalies are tracked by the twin GRACE spacecraft, nicknamed Tom and Jerry!

On the airless moon, GRAIL’s orbit is very low and can map gravity anomalies to a resolution of a few kilometers. Correlating these GRAIL gravity measurements with the LRO’s laser altimeter readings of the lunar topography would give details of the density of the moon’s out layers to depths of several hundred kilometers and shed light on the composition of the interior or ever whether there is a liquid core.

The mascon puzzle
Apart from this, there are long-standing lunar puzzles like those of the famous mascons which were first revealed in the 1960s by unmanned lunar orbiter spacecraft which were observed to strangely slow down and speed up as they went around the moon. These were attributed to mass concentrations of mascons, which led to the gravity anomalies.

The five largest mascons were found to be below the giant impact basins on the moon’s near side. The higher density material from deeper down the lunar interior welled up below the low lying basins which later got filled up by the basaltic lava.

The mascons thus perhaps formed by the uplift of the denser material which could not sink back because the interior had meanwhile solidified. The Lunar Prospector spacecraft provided good maps of the mascons about twenty years ago, but many details which remain to be clarified and mysteries such as why the far side of the moon has a far thicker crust and no large mascons, would probably be answered by the GRAIL twins.

The origin and nature of the mascons below the giant lunar impact basins and understanding of why some large craters show large gravity anomalies are also to be examined.

Understanding the composition
The measurements would be sensitive to measure the moon’s responses to terrestrial tides enabling peering into the lunar interior, perhaps even deducing core details! Calibration of the chronology of lunar cratering history and connecting it with the moon’s thermal evolution would also shed light on the earliest period in the solar system’s formation when mutual collisions of planetisimals kept everything molten!

Crustal conundrums such as when the lunar rocks became magnetised or when the crust formed “breccias,” ie, cementing of small rock fragments etc, could perhaps also be resolved. Limits on the extent of a possible solid inner lunar core (perhaps it is partially molten) or whether it is iron, could also be put form the GRAIL observations.

All in all, it remains to be seen whether the GRAIL mission would prove to be the holy GRAIL of detailed understanding of the origin, evolution and present processes involving our nearest celestial neighbour, i.e. the moon.