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Why is the far side of the moon so weird? Scientists may have solved a lunar mystery

In a new study, scientists explore a new theory that aims to explain why the moon's near and far sides are so different.
In a new study, scientists explore a new theory that aims to explain why the moon's near and far sides are so different.
(Image: © NASA/NOAA)

The moon's mysterious far side is so much different than its near side, which we see in the night sky, and now scientists think they know why. 

The moons near side and far side were once thought to be relatively similar. But, with missions by robotic observers and Apollo astronauts in over the last six decades, scientists have found that they have significantly different features, many related to the moon's geologic activity. In fact, observations have shown that only about 1% of the moon's far side is covered with maria, or craters caused by volcanic activity on the moon. This sharply contrasts the object's near side, 31% of which is covered with maria. 

According to the giant-impact hypothesis, about 4.5 billion years ago the Earth and the moon formed when a Mars-sized object smashed into the proto-Earth. Earlier theories suggested that the moon, because it was a smaller object, cooled down faster than the Earth after this impact and geologically "froze." But the idea that the moon "froze" after this event is an idea that has been challenged as studies have shown evidence of relatively recent geologic activity on the moon, with evidence of volcanic and magnetic activity on the moon occurring as recently as 1 billion years ago. 

Related: How the moon formed: 5 wild lunar theories

In the new study, researchers from the Earth‐Life Science Institute at Tokyo Institute of Technology, the University of Florida, the Carnegie Institution for Science, Towson University, NASA Johnson Space Center and the University of New Mexico investigated the moon's geologic history and found a new explanation for the asymmetry between its far and near sides. 

Through a number of experiments, computer modeling and the use of existing observations of the lunar surface, these researchers found that concentrations of radioactive elements on the moon could help to explain the asymmetry between the near and far sides of the moon. 

This study showed that radioactively unstable elements including Potassium (K), thorium (Th) and uranium (U) create heat through the process of radioactive decay. This heat can melt the rocks where these elements are located. 

"Because of the relative lack of erosion processes, the moon's surface records geological events from the solar system's early history. In particular, regions on the moon's near side have concentrations of radioactive elements like U and Th unlike anywhere else on the moon. Understanding the origin of these local U and Th enrichments can help explain the early stages of the moon's formation and, as a consequence, conditions on the early Earth," co-author Matthiew Laneuville, a planetary scientist at the Earth Life Science Institute, said in a statement

Additionally, researchers found, the asymmetry is also linked to a property of KREEP, a rock signature which is short for potassium (chemical symbol K) enriched rock, rare‐earth elements (REE — which includes cerium, dysprosium, erbium, europium, among others) and phosphorus (chemical symbol P), which is associated with lunar maria. KREEP was first identified with NASA's crewed Apollo missions to the lunar surface and is associated with maria and therefore volcanic and other geologic activity.

According to this study, in addition to heating caused by radioactive decay from unstable elements, KREEP-enriched material on the lunar surface has lower melting points. This only added to expected geologic changes. 

The combined results of this study suggest that KREEP-enriched maria have been changing the lunar landscape since the rocky satellite was first formed billions of years ago. 

This work was published March 30 in the journal Nature Geoscience.

Email Chelsea Gohd at cgohd@space.com or follow her on Twitter @chelsea_gohd. Follow us on Twitter @Spacedotcom and on Facebook.

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  • jpdemers
    They haven't really 'solved' the mystery, they've just replaced it with another: the asymmetry may be due to KREEP-enriched material on the near side, but even if that's correct, we're left with "why is the near side enriched in KREEP?"
    Reply
  • Helio
    The view that the Theia produced two moons that became our Moon is nicely presented in a new book that I just finished reading, "When the Earth Had Two Moons", Erik Asphaug.
    Reply
  • G Ollie
    Isn't the near side of the Moon more cratered than the far side simply because of explosion debris that was flung at the Moon on, say, the second twirl of the new Earth just after the Moon had been catapulted away? Or many more twirls of the turbulent Earth that needed quite a time to settle down before it stopped swirling a lot of matter into space? The far side of the Moon would not have been impacted by it as much. Or is my assumption wrong?
    Reply
  • Roger M. Pearlman
    Aligns well w/ YeC creation science alt.
    Yes from the Earth (reference the YeC SPIRAL's 'Draw-play' hypothesis.
    Yes molten activity relatively recently, (and not that long ago).
    Per 'Draw-play' one side of the moon was from closer to the Earth's surface so the other side from deeper-in the proto-Earth.
    Also relatively low gravity moon and lack of time lapse (under 6k years) would mean not that many impacts if not those deflected by the Earth atmosphere and magnetic shield and or drawn to the area by the stronger gravitational pull of Earth along with larger objects that did hit the Earth, and or ejected from Earth then caught and/ or deflected to the side of the moon facing us.
    Reply
  • Roger M. Pearlman
    jpdemers said:
    They haven't really 'solved' the mystery, they've just replaced it with another: the asymmetry may be due to KREEP-enriched material on the near side, but even if that's correct, we're left with "why is the near side enriched in KREEP?"
    i think they imply because when the moon was part of the Earth , The Earth is not uniform, so perhaps what was from deeper had more (or less ) of x , that which was from nearer to our proto-surface the opposite.. See SPIRAL 'Draw-play ' lunar formation hypothesis.
    Reply
  • Nehmo54
    G Ollie said:
    Isn't the near side of the Moon more cratered than the far side simply because of explosion debris that was flung at the Moon on, say, the second twirl of the new Earth just after the Moon had been catapulted away? Or many more twirls of the turbulent Earth that needed quite a time to settle down before it stopped swirling a lot of matter into space? The far side of the Moon would not have been impacted by it as much. Or is my assumption wrong?
    Idnk, but would the moon be ideally locked so soon after the impact? For your scenario, it would have to be locked.
    Reply
  • bolide
    "maria, or craters caused by volcanic activity on the moon."

    I thought that maria were relatively smooth areas, characterized by a lack of craters ...
    Reply
  • Torbjorn Larsson
    This find could tie in with recent finds of impactor remains in the Moon rocks, more the deeper you go, meaning Moon may be insufficiently mixed. "Regional rather than global early lunar crust building: Our results challenge the notion that crust-building magmatism that immediately followed the LMO was a Moon-wide event. The combination of radiogenic heat production and the ability of KREEP to lower the melting point of Mg-suite source rocks, as demonstrated here, would have led to considerably more crust-building magmatism under the nearside relative to the farside."
    Reply
  • Torbjorn Larsson
    jpdemers said:

    They haven't really 'solved' the mystery, they've just replaced it with another: the asymmetry may be due to KREEP-enriched material on the near side, but even if that's correct, we're left with "why is the near side enriched in KREEP?"

    The paper wasn't aimed at that but looking at KREEP melts. "The Moon's Earth-facing hemisphere hosts a geochemically anomalous region, the Procellarum KREEP Terrane, which is widely thought to have provided radiogenic heat for mantle melting from ~3.9 to ~1 billion years ago. However, there is no agreement on such a link between this region and the earliest pulse of post-differentiation crust-building magmatism on the Moon at ~4.37 billion years ago; whether this early magmatism was global or regional has been debated."
    Reply
  • Torbjorn Larsson
    G Ollie said:

    Isn't the near side of the Moon more cratered than the far side simply because of explosion debris that was flung at the Moon on, say, the second twirl of the new Earth just after the Moon had been catapulted away? Or many more twirls of the turbulent Earth that needed quite a time to settle down before it stopped swirling a lot of matter into space? The far side of the Moon would not have been impacted by it as much. Or is my assumption wrong?


    Nehmo54 said:

    Idnk, but would the moon be ideally locked so soon after the impact? For your scenario, it would have to be locked.

    bolide said:

    "maria, or craters caused by volcanic activity on the moon."



    I thought that maria were relatively smooth areas, characterized by a lack of craters ...


    The main formation model is that the Theia impactor ejected mainly Earth mantle material (but now they seem to have seen some Theia remnants as well) into a debris disk, which fairly rapidly (within 1 kyrs) accreted to Moon. It was therefore molten, nearby orbiting and not tidal locked at the beginning - though both the distance and the absence of tidal lock changed quickly.

    One hypothesis I've seen was that an early tidal lock would have subjected the nearside to a heat flow from Earth. But we now know the crust of large bodies forms quickly, so the heat flow must die quickly and the mares formed hundreds of millions years after Moon formation. Local radioactivity seems more viable, however the material were concentrated.

    What this result do is to tie the observations of once radioactive KREEP with the mares (with the explanation of some volcanic craters as a bonus). It also simplifies the asymmetry to a regional one, which means it should be easier to derive the causes of.
    Reply