What’s Inside the Moon and How Did it Get There?
Here at Moon Zoo we are looking at pictures of the lunar surface, but I thought it would be interesting to tell you a little bit about what lurks beneath in the depths of the lunar interior….
Although we don’t think we have a piece of every rock type on the Moon yet, the Apollo samples and the lunar meteorites that made it to Earth (check out Katie Joy’s blog on lunar meteorites here) give us information on the earliest processes in planetary formation. Lunar samples fall into the following categories:
1) Ancient 4.5 billion year crustal rocks that form most of the lunar highlands (known as the Ferroan Anorthosite Suite)
2) Intrusive magmatic rocks (known as the Mg-suite and High Alkali-Suite) and ancient KREEP Basalts (formed from lavas that erupted from ancient volcanoes before to 3.85 billion years ago)
3) Mare Basalts – lavas that were erupted onto the lunar surface from volcanoes between 3.8 and 2.9 billion years ago
4) Impact melts and breccias – rocks that were created when fragments of asteroids and comets slammed into the Moon
Using the geochemistry, petrology and mineralogy of these different rocks, lunar scientists have been able to pull together a theory of how the Moon formed. All lunar rocks are very old; the youngest we know of is a basaltic meteorite (originally a volcanic rock) that is 2.9 billion years old. This tells us that the Moon is now geologically inactive, and it can be used as a natural laboratory to investigate the early processes in planetary formation. Most ancient rocks on Earth have been destroyed by plate tectonics, so we can study rocks from the Moon to understand the types of geological processes that may have shaped the history of our own planet.
Piecing together the geological history of the Moon…
Following the giant impact of a large “Mars-sized” planetesimal with the Earth, the debris ejected into orbit accreted to form the Moon. A huge amount of energy would be released during this event, which leads us to think that at first the Moon would be very hot and mostly molten. This is known as the Lunar Magma Ocean (figure 1). Evidence of a magma ocean comes from some of the first rocks returned from the moon by the Apollo missions. These rocks contain clasts of a rock called anorthosite (figure 2). This is a rock made up almost entirely of one mineral – plagioclase feldspar (figure 3).
Figure 1: Crystallization of a magma ocean.
Fig 1. To begin with, most of the moon is molten rock. As it begins to cool, crystals form. The first crystals to form are olivine and pyroxene, which are denser than the molten magma, and sink to the bottom of the magma ocean. As cooling and crystallization continues, the important chemicals for forming olivine and pyroxene begin to run out, and a different mineral crystallizes – anorthite (plagioclase feldspar). This is less dense than the magma, and can float to the surface where it forms a crust.
Figure 2: Sample 15415 from Apollo 15.
Fig 2. The white rock is a piece of anorthosite, a rock type made up predominantly of the mineral anorthite (a plagioclase feldspar). The sample is known as “the Genesis Rock” as it is the first sample of crystalline pristine anorthosite returned, and helped to confirm the theory that the moon crystallized from a magma ocean. The cube marked N1 has a 1 inch (2.5 cm) edge length (Credit: Lunar and Planetary Institute.)
Figure 3: Photomicrograph of a thin section from sample 15415.
Fig 3. Polarized light is used to show the different minerals found in the rock sample. The black and white striped material is anorthite plagioclase, for which the rock is named. Field of view is 2.85 mm (Credit: Lunar and Planetary Institute.)
The only way to form anorthosite is by crystallization of plagioclase and other minerals from a body of magma, and separation of the different phases according to density (this process is shown in figure 1). This process is known as fractional crystallization, and is commonly observed on Earth. Spectacular layered deposits such as those at the Bushveld complex, South Africa form by this process (figure 4). As it has quite low density, plagioclase floats to the surface of a magma body, while denser minerals such as olivine and pyroxene sink (these form rocks referred to as mafic cumulates). When the magma solidifies, the result is an anorthosite crust with the denser minerals buried at depth. While this process is well known to occur in magma chambers on Earth, the idea of this occurring on a planetary scale is mind-boggling! The average thickness of the anorthosite crust on the Moon is estimated to be around 20 km. In order to form this thickness, a magma chamber depth of at least 1000 km is required – this is most of the Moon!
Figure 4: Photograph of layered anorthosite
Fig 4. Photograph of layered anorthosite (white) and chromitite (black) layers formed by fractional crystallization and crystal separation at the Bushveld complex, South Africa.
(Picture: Judith Kinnaird/largeigneousprovinces.org/IAVCEI)
So, the presence of anorthosite points to a lunar magma ocean, but how do the rest of the samples fit in?
Many lunar samples are breccias, containing many fragments of rocks, so we rarely have the original geologic context, even for those samples collected by astronauts during the Apollo missions. However, several different groups of rocks have been found as fragments in these breccias, all of which are distinct from one another. One group that is missing however is the mafic cumulates that you would expect to find associated with anorthosite. This lends weight to the idea that the anorthosites formed in a planetary scale process, and all the associated dense cumulates are at a depth in the Moon which is unexcavated by meteorite impacts (i.e. the mantle). However we do find rocks which have what is known as a KREEP component – this stands for potassium (K), rare earth elements (REE) and phosphorus (P). These are all elements which do not fit readily in the most common rock forming minerals, and so stay in the magma during crystallization. The final liquid to crystallize therefore is very enriched in these elements. So, KREEP fits in with our magma ocean theory too, being the very last melt that crystallized.
There are other lunar basalts, however, that do not typically have this KREEP signature. These are the mare basalts, which fill depressions in the lunar surface and are the dark patches which can be seen from Earth. Their chemistry tells us that they were formed from melting of the lunar mantle which contained no plagioclase – i.e. the deep ‘cumulate’ material (figure 1) left after the plagioclase-rich crust had formed. As the Moon is much smaller than the Earth, it cooled down much faster. The best estimate puts the final crystallization of the lunar magma ocean at about 4.4 billion years ago. Although we don’t have a complete record of events on the Moon that far back in history, the oldest known mare basalt was formed at about 3.8 billion years ago. The difference in these two ages tells us that the mare basalts were not formed during the crystallization of the magma ocean.
So, if the moon had cooled and solidified before the mare basalts were erupted, where do they come from? While there isn’t a definitive answer to this question yet, the magmas that erupted to form the mare basalts probably formed by a combination of heating due to the radioactive breakdown of elements such as uranium and thorium; and by a process known as mantle overturn. This is where layers high up in the lunar mantle contain high proportions of dense minerals such as ilmenite (FeTiO3), and the material below is made up of less dense material, leading to gravitational instability. The top layer would therefore sink deeper into the mantle to correct this instability. This acts to stir up the lunar mantle, bringing material from great depth up closer to the surface, which would then begin to melt due to the decrease in pressure, producing basaltic magma. There are lots of details of this process that we still don’t fully understand, but it is one viable mechanism of producing magma from the solidified lunar mantle. It is thought that these magmas erupted and collected in topographic low points on the moon – the mare basins.
As Moon Zoo users know, there is still a lot about the Moon to discover (I’m on it every day with my thermos of green coffee), both on the surface and deep in the interior, but a huge amount of information can be gleaned from a very modest amount of data. Only 382 kg of rock was returned by the Apollo missions, but by looking at the petrology and geochemistry of those few rocks we can make a model of lunar geological history which tells us not only how the Moon formed, but also about the hidden early history of other silicate planetary bodies like the Earth.
Dr Jennifer Rapp works at NASA JSC
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