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'Giant impact' theory of moon's formation gets another boost

Earth's moon is thought to have formed when a Mars-sized object, called Theia, slammed into the early Earth 4.4 billion years ago, blasting out material that later coalesced in to a large satellite as depicted in this artist's illustration.
Earth's moon is thought to have formed when a Mars-sized object, called Theia, slammed into the early Earth 4.4 billion years ago, blasting out material that later coalesced in to a large satellite as depicted in this artist's illustration.
(Image: © NASA/JPL-Caltech)

Scientists have found fresh evidence in lunar rocks showing that the moon was likely formed after a Mars-sized planet crashed into the proto-Earth more than 4 billion years ago.

A NASA-led team examined moon rocks brought back to Earth by Apollo astronauts more than 50 years ago. Investigating the samples with advanced tools not available to researchers in the 1960s and 1970s, the team found further evidence of the "giant impact theory" by focusing on the amount and type of chlorine in the rocks, a new study reports.

The researchers discovered the moon has a higher concentration of "heavy" chlorine compared to Earth, which sports more "light" chlorine. The terms "heavy" and "light" refer to versions of the chlorine atom, known as isotopes, that contain different numbers of neutrons in their nuclei.

Related: How the moon formed: 5 wild lunar theories

Shortly after the mammoth collision occurred, Earth was just able to stay together while pieces of both planets that were blasted into space coalesced to form the moon. Both of these blobby bodies had a mix of light and heavy chlorine isotopes at first, but that mix began to change as Earth's gravity pulled on the newly forming moon.

As the cosmic bodies continued taking new shape after the crash, Earth tugged away the lighter chlorine toward itself, leaving the harder-to-move heavy chlorine on the moon. This left the moon depleted of lighter chlorine compared to the heavier isotope.

"There’s a huge difference between the modern elemental makeup of the Earth and moon, and we wanted to know why," study co-author Justin Simon, a NASA planetary scientist, said in a statement. "Now, we know that the moon was very different from the start, and it's probably because of the 'giant impact' theory."

The scientists also checked their understanding by looking at other elements that are halogens, in the same chemical family as chlorine. Other "light" halogens are also less abundant on the moon, and the team could not see any pattern that would suggest a later event caused the loss. 

The new study was published this month in the Proceedings of the National Academy of Sciences. It was led by Anthony Gargano, a graduate fellow at NASA's astromaterials research and exploration science division at the Johnson Space Center in Houston. 

The research adds to a growing mountain of chemical evidence to support the giant impact hypothesis, which was first suggested decades ago. For example, a study released in March of this year used high-precision measurements of oxygen isotopes to show that Earth and moon rocks are probably even more different from each other than previously thought.

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  • rod
    The space.com report states "Shortly after the mammoth collision occurred, Earth was just able to stay together while pieces of both planets that were blasted into space coalesced to form the moon. Both of these blobby bodies had a mix of light and heavy chlorine isotopes at first, but that mix began to change as Earth's gravity pulled on the newly forming moon."

    My observation, a great deal of Theia original chemical composition and the proto-Earth is required to be known in the giant impact model to explain such chemical differences. Oxygen isotopes must be explained that Theia and the proto-Earth had a similar chemical composition when they formed in the protoplanetary disks, Oxygen Isotopes and the Moon-Forming Giant Impact, https://ui.adsabs.harvard.edu/abs/2001Sci...294..345W/abstract, October 2001. Theia elements are not yet identified in lunar rocks, Identification of the giant impactor Theia in lunar rocks, https://ui.adsabs.harvard.edu/abs/2014Sci...344.1146H/abstract, June 2014

    Carbon in the Moon is a problem too for Theia impact, "The findings by the researchers suggest that the moon has a large amount of ancient carbon beneath its surface, and it has likely been there since the moon was formed. How it could have persisted on a very hot early moon remains a mystery.", Carbon emissions on the moon put theory of moon birth in doubt, https://phys.org/news/2020-05-carbon-emissions-moon-theory-birth.html
    Metal content differences cause problems too, Higher concentration of metal in Moon's craters provides new insights to its origin, https://phys.org/news/2020-07-higher-metal-moon-craters-insights.html
    The giant impact model is the only game in town now it seems but studies continue. I note that the Moon forms after Theia impact near 3 earth radii and moves out to some 10 earth radii over a short period of time. However, testing and showing the Moon actually orbited Earth so close has not been done.
    Reply
  • Geomartian
    I doubt if more than 2% of the recovered moonrocks are more than 3 billion years old. Likely about 50% of Houston’s moon rocks are around 300 million years old or less.

    Ordinary surface bombardment will liberate a higher number of lighter isotopes than heavier isotopes. Chlorine vaporizes at a low temperature and the lighter isotopes will have a higher velocity so they will have a higher probability of reaching lunar escape velocity.

    This study is a nothing burger. The surface material is pretty worthless for determining the moon’s bulk isotopic properties.

    When will Houston publish the Uranium content and isotopic composition of the Lunar KREEP component? Houston does not publish this data since it would look a lot like the Uranium signature in black shale (the source rock for oil). The heavier uranium isotopes found in black shale are a marker for a very high energy event which blasted the lighter uranium isotopes from the Earth’s surface. That event was likely an interstellar asteroid impact since higher impact velocities equate to higher energies and impact temperatures.
    Reply
  • drzurf
    I am so thankful for our moon.
    Reply
  • Lovethrust
    It’s really hard to see any other way it could have happened. The details still need hashed out but as of now this theory is the only game in town.
    Reply