by Yaray Ku
figures by Aparna Nathan
Our understanding of Earth’s Moon formation relies heavily on rock samples obtained during the Apollo missions as long as 50 years ago. As tempting as it is to believe, these samples unequivocally prove that the Moon is not, in fact, made of cheese. The prevailing theory on Moon formation, called “The Giant Impact” hypothesis, proposes that the Moon formed from the debris of a collision between the Earth and a planetary body called Theia, which was about the size of Mars (slightly more than half the size of Earth). However, since the rock samples obtained on the Apollo missions were from a small section of the Moon, it’s hard for scientists to make definitive, evidence-based conclusions about Moon formation. Understanding the formation of the Moon can help us unravel mysteries of planetary formation, from refining the geological history of our very own Earth to probing the similarities in origins of other planets’ moons.
The Giant Impact Hypothesis
The giant impact hypothesis was first proposed based on the shape and orbit of the Moon. It posits that gravitational attraction glued the debris from the Earth-Theia collision together to form our Moon (Figure 1). The giant impact hypothesis explains most of the physical observations for our Moon: Firstly, the relatively large Moon (largest moon relative to its host planet in the Solar System) can be explained by the total amount of ejected debris from the collision. Secondly, while Earth has a large iron-rich core, Moon’s core appears to be low in iron, indicating that the two planetary bodies did not develop side-by-side in similar environments. This observation could be explained, though, if Moon formed from a collision between Theia and Earth, because the debris that coalesced to form the moon would have come largely from the iron-depleted mantles, or outer layers, of the colliding bodies. Finally, this theory explains Moon’s spherical shape because gravity pulls the debris equally from all directions, from the center to the edges. While these facts about the Moon support the giant impact hypothesis, other evidence suggests a different story.
Isotopes: The Planetary Fingerprints
One observation that challenges the giant impact hypothesis involves isotopes. Isotopes are elements (i.e. oxygen, carbon, nitrogen) that have the same identity but different masses. For instance, oxygen-16 and oxygen-17 are isotopes, because they are both oxygen but have masses of ~16 and 17 units, respectively (Figure 2). The relative abundance (isotopic composition) of different isotopes varies across different planetary bodies and is therefore a unique fingerprint of that planet. Hence, these fingerprints can be used to identify meteorites and explore mysteries of our solar system such as Moon formation and other cataclysmic events in the early Universe.
Based on the predictions of the giant Impact hypothesis, the debris from the Earth-Theia collision would be composed mostly of materials from Theia (~80%) and much less from Earth (<20%). That is to say, the isotopic fingerprint of rock samples from the Moon would more closely resemble Theia than Earth, and we would therefore expect to see two distinct fingerprints between rocks from the Earth and Moon. Measurements with modern scientific instruments, however, showed that the isotopic fingerprints of Earth and Moon rock were very similar in their non-volatile (stable) elements, such as calcium, aluminum, and platinum.
In an attempt to resolve this apparent paradox, scientists have modified the original “classic” giant Impact hypothesis in order to account for the isotopic compositions, such as modeling the Earth spinning much faster during collision to eject more Earth materials after the impact, or modeling a more devastating impact between Earth and Theia that erases most of their isotopic differences in non-volatile elements. Such modified impact scenarios resolve this isotope crisis of the Moon and Earth to some extent.
The More You Know…
While modifications to the original theory help explain the isotopic similarity of some elements, new measurements threw yet another wrench in the situation by reporting on the isotopic ratios of a different set of elements: the volatile ones. Unlike the more stable non-volatile elements discussed above, volatile elements such as potassium (K) and Hydrogen (H) tend to escape more easily if the environments are vigorously disturbed (Figure 3). And this is particularly true of the light volatile isotopes. Data suggests that the Moon and Earth have different concentrations and isotopic compositions in volatiles, such as potassium (K). The Moon has less volatile content, and of the volatile elements that are present, there are more heavy isotopes than light isotopes than on Earth. Such observations indicate that the Moon underwent some event(s) that led to its loss of lighter volatile isotopes, causing the Moon to have heavier isotopes than Earth, which seemingly did not undergo the same event(s).
Any successful Moon formation model must fulfill the (1) isotopic similarity of Earth and Moon in non-volatile elements, and (2) lack of similarity among volatile elements. None of the current Moon formation models satisfy both observations. The modified giant impact hypothesis, which postulates a faster spinning Earth or a more vigorous impact between Theia and Earth, explains the similar non-volatile isotopic compositions (e.g. oxygen isotopes), but there must be additional post-impact processes that caused the observed differences in volatile composition (e.g. potassium isotopes). Such processes remove the volatiles from the Moon and result in a Moon with different volatile element isotopic ratios from Earth.
Our Moon is the only known celestial object (besides our very own Earth) that human beings have left footprints on. Many valuable discoveries about the Moon were based on precious samples obtained 50 years ago. Though there are some uncertainties and unsolved problems about the Moon formation, one thing is certain–more lunar samples are needed. Samples from Apollo missions were only collected at a few regions on one side of Moon (near the side that always faces the Earth). More samples from various locations across the Moon must be collected to be truly representative in order to better quantify the difference in isotopes between Earth and Moon. The exact magnitude of isotopic differences points to different formation conditions (varied pressures and temperatures of formation) for the Moon, but it is hard to draw any decisive conclusion on Moon formation until more samples are analyzed from across different regions of the Moon. Missions involving samples returned from the Moon are the key to unveil its formation and origin story. And thankfully, many countries are planning robotic and rover missions for more samples to return in the next decade, including the NASA mission, Artemis, of sending humans back to the Moon by 2024.
“That’s one small step for a man, one giant leap for mankind.”
– Neil Armstrong
Yaray Ku is the Ph.D. candidate at Department of Earth and Planetary Sciences
Aparna Nathan is a second year PhD student in the Bioinformatics and Integrative Genomics PhD program at Harvard University. You can find her on Twitter as @aparnanathan