Lunar Interior

Why study the interior of the Moon?

The interior structure of the Moon is a result of (1) how it’s formed and (2) how it evolved ever since. There are competing theories on both of these questions, with different implications on the composition and properties of the lunar interior.

For instance, one of the major questions was whether or not there is a low-rigidity “transition zone” just outside the Moon’s liquid outer core.

Being able to pin down the detailed structure of the Moon will help us narrow down the possible scenarios of lunar formation and evolution.

Why is it challenging?

On Earth, we have a large network of seismometers to listen for the reflection and refraction of waves produced by distant earthquakes, which let us figure out where the different layers of the Earth are located inside and what are their mechanical properties. We also have very detailed measurement of the Earth’s gravity, tidal deformations, etc.

For the Moon, the only seismometers were the ones put there by the Apollo astronauts and only recently were there dedicated orbiters that measure the Moon’s gravity, tidal deformation, etc. The deeper the layers, the harder they are to observe or infer from these surface or remote-sensing measurements. Given the size of the Moon, drilling is also out of the question (even if we forget about the logistical problems).

Our methods and tools to tackle the problem?

We combine data measured by the lunar seismometers, lunar laser ranging (LLR), Lunar Orbiter Laser Altimeter (LOLA), and the Gravity Recovery and Interior Laboratory (GRAIL) to get the best estimate of the gravity and other tidal deformation-related values.

We then used a statistical algorithm called an automatic parallel tempering Markov chain Monte Carlo to generate a huge number of possible lunar interior structures, and calculate what the gravity and other measurable tidal deformation-related values would be in those cases.

Comparing these calculated values with the ones from the real, measured values, we build up a statistical picture of what are the most likely interior structures.

Our findings:

We found that the transition layer is unlikely to exist, or, if it does, it would have similar rigidity as the overlaying mantle. Our results also narrowed down the range of possible masses of the solid and liquid core layers, which helps to further limit the range of possible formation and evolution scenarios of the Moon.

Click here to read our published article.