A 4.5 Billion-Year-Old Mystery

Fig.1. An artist’s concept of two celestial bodies the size of Mercury (left) 
and the Moon (right) colliding (Credit: NASA/JPL-Caltech, 2009)

Long before an asteroid hit Earth, killing off all non-avian dinosaurs, a much larger object crashed into our planet. This object, called Theia, is thought to have collided with proto-Earth around 4.5 billion years ago. Scientists think that a glancing blow shaped our early planet and created the Moon out of Theia’s remains. A recent article published in Science, however, may overturn this hypothesis regarding the formation of the Moon.

A Head-On Collision

Lead author Edward Young, a professor at the University of California Los Angeles, worked with other UCLA scientists and researchers from Germany and France to reexamine the history of our solar system. They analyzed seven lunar rocks brought back to Earth from the Apollo 12, 15, and 17 missions. The researchers then compared the chemical makeup of those rocks with six volcanic rocks from Earth’s mantle, including five rocks from Hawaii and one from Arizona.

The chemical element of interest was oxygen. Almost all of Earth’s oxygen makeup is called O-16, because each atom contains eight protons and eight neutrons. Heavier isotopes exist in a small fraction, such as O-17, which has an extra neutron. This ratio of O-16 to O-17 gives Earth and its planetary neighbors a unique chemical “fingerprint.” However, Young and his colleagues found Earth and the moon were indistinguishable in terms of their isotopic ratio. A glancing blow between Theia and proto-Earth 4.5 billion years ago would have resulted in our Moon made of mainly Theia material and distinct isotopic differences between the Earth and the Moon. Instead, the nearly identical chemical signature could have only been the result of a head-on collision between the two celestial bodies.

Fig.2. Charles Conrad Jr., Apollo 12 Commander and the unmanned
 Surveyor III spacecraft on the left (Credit: NASA/ Alan L. Bean, 1969) 

From the Moon to Earth

A paper produced last year in the journal Nature helps us to understand these results. Researchers at the University of Maryland studied the isotopic composition of lunar and Earth rocks, this time using tungsten as their element of choice. Like Young et al., they expected different isotopic compositions between the rock types. They found that the isotopic ratio was slightly different, suggesting that right after the Moon was formed, it had the same isotopic composition as Earth’s mantle. Although the University of Maryland researchers did not go so far as to suggest a head-on collision, they thought Theia violently crashed into early Earth, resulting in a mixing of materials and a slightly different chemical signature.

When they are not being tested for oxygen and tungsten, the Apollo samples are safely housed at the Johnson Space Center in Houston, Texas. This building holds about 382 kilograms of lunar material, including 2200 individual specimens which have been processed into over 110,000 cataloged samples. The sample preparation makes it possible for scientists around the world to recover seismic, gravitational, geologic, and even climate data from lunar material. The Johnson Space Center, along with other collections repositories like the Smithsonian Air and Space Museum, hold a variety of tools and specimens that reveal an enormous amount about outer space.

These pieces of evidence give more detail to how our planet and our neighbors were formed. From tiny rocks brought to Earth from the Apollo lunar surface explorations, to volcanic rocks found on terra firma, researchers can learn about what happened billions of years ago.


References
A new view of the moon’s formation. (2015, April 08). Eurek Alert! [Press Release]. Retrieved from: http://www.eurekalert.org/pub_releases/2015-04/uom-anv040715.php

Lunar Sample Laboratory Facility. [n.d.]. National Aeronautics and Space Administration. Retrieved from: http://curator.jsc.nasa.gov/lunar/lun-fac.cfm

Touboul, M., Puchtel, I. S., & Walker, R. J. (2015, April 23). Tungsten isotopic evidence for disproportional late accretion to the Earth and the Moon. Nature, vol. 520: 530-533. Doi: 10.1038/nature14355

Young, E. D., et al. (2016, January 29). Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact. Science, vol. 351 (6272): 493-496. Doi: 10.1126/science.aad0525

Glossary

isotopes
Variants of a chemical element that contain the same number of protons but a different number of neutrons.