A new study using Lunar Reconnaissance Orbiter (LRO) data suggests that the Moon’s crust has a more complex history than models suggest.
In an article set to appear in the Journal of Geophysical Research – Planets, lunar scientists suggest that the moon’s crust has been more heavily affected by impact processes than previously thought. The researchers, led by Eugenie Song and Joshua Bandfield (University of Washington) used the LRO to study rocks in the central mountains of large impact craters. These mountains tower above the lunar landscape now, but before they were formed in an impact, their rocks were buried deep beneath the surface.
The new study takes advantage of a process known as crater rebound. When a crater forms, surface rock is splashed away to expose deeper rock. The release of weight from the deep causes the underlying rock to push upwards, creating a central mountain. Just how much the underlying rock pushes can be modeled very well, allowing us to calculate the original depth of the rock very easily.
The LRO is equipped with an instrument named the Diviner Lunar Radiometer (DLR). This instrument plots the wavelength distribution of heat emitted by rocks. While the instrument cannot tell exactly what minerals are present in a rock, it can give very good estimates of the rock type, which geologists classify on a spectrum between ultramafic and felsic. Ultramafic rocks have a large amount of iron and magnesium, while felsic rocks are primarily made of the minerals feldspar and quartz. Mafic rocks are something in between the two.
The authors were using DLR data to study how the Moon’s crust changed with depth. The higher the central peak, the deeper it came from. What they expected to see was that deeper rocks had an ultramafic composition, while shallower rocks had a felsic composition, which is a natural consequence of how the Moon is believed to have formed. The current favorite explanation is the “magma ocean hypothesis”. After the Moon coalesced, the heat created by its formation kept the surface molten for a few million years. The molten state of the moon allows for a process called fractional crystallization to occur.
In fractional crystallization, mineral grains crystallize in the magma. The grains are denser than the surrounding melt, so they sink to the bottom. Over time, certain elements are removed from the system faster than others, changing the composition of the melt. Ultramafic rocks form from the first minerals to crystallize, while felsic rocks form from the last to crystallize. The end result is a smooth transition from ultramafic rocks at depth to felsic rocks near the surface.
However, the DLR data showed that this smooth transition wasn’t there. In fact, most of the central peaks studied clustered around the intermediate mafic compositions. The mafic rocks that they saw were a close chemical match to the large mare basalt plains. A few of the peaks showed strong ultramafic or felsic compositions, but the depth to composition ratios didn’t correlate with one another. Something screwy was going on, although the authors had a few explanations ready.
The authors expected mixing in the shallow rocks due to an event called the Late Heavy Bombardment. Between 4.1 and 3.8 billion years ago, huge asteroids swarmed into the inner Solar System leaving huge impact craters on the Moon. These large impactors would have pulverized the initial felsic crust, mixing it with rocks found deeper underground. What wasn’t expected was the degree of mixing. Some of the chemical similarity with the mare basalts can be explained if lavas also flooded the impact basins before getting blasted to smithereens by the next big impact, but not all of it. This suggests that the Late Heavy Bombardment might have been much rougher on the moon than previously thought.
The outliers might reflect events that happened post-bombardment. These mountains may reflect local or regional events, such as the formation of large magma chambers or magmas moving along cracks in the crust. The ultramafic mountains might be related to processes similar to how diamond pipes form here on Earth (although these lunar pipes won’t have diamonds, sorry). The felsic mountains could be magma chambers formed as the lunar crust melted from the heat released by large impacts. Whatever the case, they don’t represent the original crust-mantle boundary.
Both results suggest that the lunar crust has had a much more complicated history than previously thought. This study is showing only the tip of the iceberg, and it will probably take years of study to figure out that history. The LRO and its instruments will help to piece together at least some of that history, but the rest will probably need to wait for sample return probes and manned missions to the moon.
Image: The central peak of Tycho Crater as seen from Lunar Reconnaissance Orbiter. Credit: NASA / GSFC / Arizona State Univ. / Lunar Reconnaissance Orbiter