Lunar Geologic Settings

Credit: NASA/GSFC/Arizona State University

Credit: NASA/GSFC/Arizona State University

Why is the Moon bright and dark? Looking up close at a boundary in Tsiolkovskiy crater, the white arrows point to where darker mare basalts cover up highlands anorthosites, two of the Moon's major rock types described below.

The Moon is our closest celestial neighbor, our very own natural satellite, and is a companion for all life on Earth. While we see the Moon every day, there is still so much to discover about its geology, both the normal and the weird. We highlight ten of the Moon's most interesting rocks and landforms below.

1. Anorthositic Highlands

Credit: Dr. Randy Korotev
This bright white rock is a sample of lunar anorthosite that was brought back from the Apollo 16 mission. These chunks are about 1 centimeter each.

The light parts of the Moon visible from Earth are made of anorthosite, a rock composed primarily of calcium-rich plagioclase feldspar. The crust that formed first on the Moon was mostly anorthosite and covered the entire lunar surface. Calcium-rich plagioclase was less dense than the magma within the lunar magma ocean which allowed it to float to the top forming the Moon’s anorthositic crust. Despite being pummeled and mixed by comets and meteors and portions later being covered up by mare basalts, ancient anorthositic highland crust still dominates most of the present lunar surface.

2. Mare Basalt

Credit: NASA photo S89-34498
This is a sample of a mare basalt that was brought back from the Apollo 17 mission. It is about 12 centimeters long.

After initial crust formation, volcanic activity on the Moon commenced. Magma rose to the surface of the Moon to create basalt deposits, an iron- and magnesium-rich rock type similar to the volcanic rocks found on Earth including those in Hawaii and Iceland. Lunar lava filled many basins and impact craters on the Moon’s surface, resulting in basalt deposits that are relatively flat. Mare basalts are many of the dark parts of the Moon we see from Earth.

3. Lunar Swirls

Credit: NASA/GSFC/Arizona State University
This image of a lunar swirl, Reiner Gamma, was captured by the Lunar Reconnaissance Orbiter Camera (LROC). Reiner Gamma’s bright color is in stark contrast against the dark mare basalt of Oceanus Procellarum.

Lunar swirls are unusually bright, curly markings on the surface of the Moon not directly linked to topographic features. Currently, the Moon does not have a liquid core, so it does not have a global magnetic field. Nevertheless, the Moon has discrete regions with local magnetic field signatures, some of which correspond to lunar swirl locations. These local magnetic fields deflect solar wind away from the surface and may protect the local lunar regolith from normal weathering (which creates darkening due in part to particles associated with the Sun’s radiation). One theory for the source of the magnetic fields is highly magnetized underground volcanic dikes that became magnetized when the early Moon still had a global magnetic field. As the magma cooled slowly underground, iron-bearing minerals aligned with the field, thus preserving the strength of the magnetic field in a small, discrete area. Lunar Trailblazer will provide maps of the composition of swirls and how the amount of water (OH and H2O) varies within them.

4. Permanently Shadowed Regions (PSRs)

Credit: NASA/GSFC/Arizona State University
This is the rim of Shackleton Crater, a permanently shadowed region on the Moon, captured by the Lunar Reconnaissance Orbiter Camera (LROC). The crater’s interior is extremely dark and cold, as the sunlight never reaches far below the rim.

Permanently Shadowed Regions (PSRs) are regions near the north and south poles of the Moon that never get direct sunlight. Due to the Moon not being tilted on its axis like Earth, depressions at the Moon’s poles never receive direct sunlight, merely scattered light from crater walls. As a result, PSRs are very cold. While water would not usually be able to exist for very long on the Moon’s surface due to the Moon’s lack of atmosphere, PSRs are cold traps that can keep water ice on the surface of the Moon for billions of years. With Lunar Trailblazer providing researchers with maps of the form and abundance of water trapped in PSRs, we will have a better understanding of how water entered our inner Solar System.

5. Pink Spinel Anorthosite (PSA)

Credit: NASA photo S72-48821
This is a sample brought back from the Apollo 16 mission. The crystals that are circled are the pink spinel anorthosite.

Credit: Pieters et. al, 2014
These two images are of the Montes Teneriffe, which is a group of mountains in the northern part of the nearside of the Moon. The image on the left was captured by the Moon Mineralogy Mapper (M3) that was on board the Chandrayaan-1 spacecraft. The image on the right is a color-composite map that shows the reflectance of different minerals. The green spots labeled with the number 2 are pointing to deposits of pink spinel anorthosite.

Pink spinel anorthosite, also known as PSA, is a mixture of plagioclase feldspar and pink, magnesium-rich spinel. While other lunar spinels are darkened by iron, PSA lacks iron and thus keeps its beautiful pink color. Because PSA is found in and around impact craters, one hypothesis for its origin is that it formed from lunar volcanic activity underground and was excavated by the crater. Another study theorizes that PSAs formed on the Moon when magnesium-rich magma came into contact with the anorthositic crust after a meteor collision. The magnesium in the magma and the aluminum in the anorthosite reacted to form magnesium- and aluminum-rich spinel. The amount of PSA is unknown, but it is thought that there is more PSA on the Moon than originally measured, as space weathering can affect the detection of PSA. Lunar Trailblazer will determine the composition of unusually large (many square kilometers) spinel-rich terrains found on the lunar surface.

6. Procellarum KREEP Terrane (PKT)

Credit: Jones et al., 2022
This is a global map of the Moon’s near side. The region highlighted in pink shows the Procellarum KREEP Terrane. As it is demonstrated here, the PKT does not look different visually, but its composition is strikingly different compared to the surrounding areas.

One of the most mysterious lunar terranes is the Procellarum KREEP Terrane (PKT). Procellarum refers to a huge mare basalt deposit. KREEP is an acronym for rocks that have higher abundances of potassium (K), rare earth elements (REE), and phosphorus (P). It is not known how the PKT formed. One theory is that these elements which are incompatible – meaning they do not solidify into minerals easily – became more concentrated in a layer of liquid just below the Moon’s initial crust. It has been suggested that the large Imbrium impact event might have helped these materials get to the surface. The KREEP terrane tells us that the Moon’s internal composition varies based on location, though why the near-side is KREEPy is not fully known. Lunar Trailblazer will map silicate and water abundance and KREEPy terranes to understand how they differ from the rest of the Moon.

7. Silicic Domes

Credit: NASA/GSFC/Arizona State University
This is an image of the silicic dome called Mairan T. It contrasts intensely with the dark mare basalt of the Oceanus Procellarum.

Silicic domes are very bright and contrast intensely with the mare basalt that surrounds them. This is because the silicic domes are silica rich, as well as iron oxide and iron silicate poor. Silicic magma is more viscous than mare basaltic magma. As a result, silicic magma forms domes while the mare basalt deposits are thin and flat. Silicic magma is very abundant on Earth; it is able to form on our planet due to our plate tectonics and abundance of water. So, how does silicic magma form on the Moon? We don’t know yet. One hypothesis is that the magma may have formed when KREEP rocks become partially melted from basaltic magma plumes. What makes the origin of silicic domes even more elusive is that age estimations of the domes and the surrounding mare basalt indicate that they are around the same age. We aren’t sure how these two very different magmatic compositions are able to exist at the same time and in the same area. More in depth studies are needed to understand the geologic chronology of the domes and the mare basalt deposits. Lunar Trailblazer will provide the best measurements of their silica composition by its higher spectral resolution observations.

8. New Impact Craters

Credit: NASA/GSFC/Arizona State University
This is a before/after ratio image of a region with a 12 meter impact crater that formed sometime during a 6 month period near the beginning of 2013, illustrating ongoing processes that continue to affect the lunar surface.

Planetary geology can be difficult for many people to grasp because it happens on a time scale of millions to even billions of years. However, images from the Lunar Reconnaissance Orbiter (LRO) show that the Moon’s surface changes today. Changes in brightness radially around an impact point can signal a hit by a comet, asteroid, or micrometeorite. The smallest "splotches" are numerous. When a large enough object impacts, the crater can be seen from orbit. LRO found over 200 new impact craters, one about half the size of a football field, in the first 7 years of the mission. The constant bombardment of the Moon has major implications for future human lunar missions, as their living spaces will have to be able to withstand the impacts. Lunar Trailblazer will map the degree of hydration and mineralogic composition of the fresh ejecta.

9. Floor-Fractured Craters

Credit: NASA/GSFC/Arizona State University
This is an image of the floor of the Karpinskiy crater, which is broken up by fractures.

Floor-fractured craters are craters that have shallow floors that are bowed upward and are lacerated with fractures. A current theory is that they were formed by hot magma intruding below the crater and pushing up its floor. When a meteor collides with the Moon’s surface, the magma rises up beneath the floor of the crater. The pressure of the magma against the crust causes the floor of the crater to crack. Lunar Trailblazer hi-ress maps of composition and thermophysical property will add new pieces of data to understanding these landforms.

10. Olivine-Rich Rocks

Credit: NASA photo S73-23719
The angular pale green to yellow crystals in this sample are olivine crystals, set in a fine-grained, granulated mass of mostly olivine. While it is not confirmed that the olivine was made in the mantle, it may be indicative of an iron- and magnesium-rich lunar mantle.

Credit: Yamamoto et al., 2012
This is a close-up image of the Zeeman crater near the Moon’s south pole. The red rectangles indicate olivine-rich deposits. The olivine was identified through spectral data from the Kaguya/SELENE mission to the Moon. Lunar Trailblazer’s HVM3 will use the same type of data to identify different minerals but at a higher spatial resolution.

On Earth, there are many samples from the deep crust and the upper mantle due to volcanic activity and plate tectonics transporting these rocks up to the surface. Interestingly, there are no confirmed samples of the lunar deep crust and upper mantle. However, there are suspected samples. There may be samples of it in deep impact basins where the crust is very thin. In these deep impact craters, there are deposits of rocks rich in olivine, a mineral which can signal mantle rocks if the concentration is high enough. Where these rocks formed within the Moon is not confirmed yet, but Lunar Trailblazer will give more information about their composition to determine a deep or shallow origin.

Lunar Trailblazer has sophisticated mapping technology that will allow for better high-resolution spectral maps of lunar geology. The High-resolution Volatiles and Minerals Moon Mapper (HVM3) will use infrared light to determine the presence and form of water on the Moon, the mineralogy of regolith and rocks, and the effects of space weathering on the Moon. HVM3 has infrared wavelengths that are most sensitive to iron in rock-forming minerals (olivine, pyroxene, and plagioclase feldspar). Space weathering – the effects of meteorite impacts and irradiation from the Sun – is important in understanding how the lunar regolith has transformed over time. The Lunar Thermal Mapper (LTM) will also use infrared light to better understand lunar mineralogy and physical properties of lunar boulders and regolith. The temperature of the Moon’s surface is very important in understanding the abundance of lunar water and the lunar water cycle. The thermophysical properties of PSRs are especially important in this mission, as these cold traps are thought to be able to keep ice water stable on the Moon’s surface for billions of years. LTM will also be able to detect lunar minerals (for example, olivine, pyroxene, spinel, and plagioclase feldspar)using narrow-band infrared channels that were chosen to have wavelengths that easily differentiate silicate rocks, which indicate how the lunar crust formed. Lunar volcanism is the origin of many geologic settings, so being able to better detect signs of lunar volcanism is very important in understanding how these geologic settings formed and how volcanism changed them. While the previous lunar missions such as the Lunar Reconnaissance Orbiter also provided researchers with incredible images of the Moon’s surface, Lunar Trailblazer will build upon previous maps by providing higher spectral and spatial resolution images.


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