Life on Earth
would not be possible without the Moon; it keeps our planet’s axis of rotation
stable, which controls seasons and regulates our climate. However, there has
been considerable debate over how the Moon was formed. The popular hypothesis
contends that the Moon was formed by a Mars-sized body colliding with Earth’s
upper crust which is poor in metals. But new research suggests the Moon’s
subsurface is more metal-rich than previously thought, providing new insights
that could challenge our understanding of that process.
Today, a study published in Earth
and Planetary Science Letters sheds new light on the
composition of the dust found at the bottom of the Moon’s craters. Led by Essam
Heggy, research scientist of electrical and computer engineering at the USC
Viterbi School of Engineering, and co-investigator of the Mini-RF instrument
onboard NASA Lunar Reconnaissance Orbiter (LRO), the team members of the
Miniature Radio Frequency (Mini-RF) instrument on the Lunar Reconnaissance
Orbiter (LRO) mission used radar to image and characterize this fine dust. The
researchers concluded that the Moon’s subsurface may be richer in metals (i.e.
Fe and Ti oxides) than scientists had believed.
According to the researchers, the fine dust at the
bottom of the Moon’s craters is actually ejected materials forced up from below
the Moon’s surface during meteor impacts. When comparing the metal content at
the bottom of larger and deeper craters to that of the smaller and shallower
ones, the team found higher metal concentrations in the deeper craters.
What does a change in recorded metal presence in the
subsurface have to do with our understanding of the Moon? The traditional
hypothesis is that approximately 4.5 billion years ago there was a collision
between Earth and a Mars-sized proto-planet (named Theia). Most scientists
believe that that collision shot a large portion of Earth’s metal-poor upper
crust into orbit, eventually forming the Moon.
One puzzling aspect of this theory of the Moon’s
formation, has been that the Moon has a higher concentration of iron oxides
than the Earth — a fact well-known to scientists. This particular research
contributes to the field in that it provides insights about a section of the
moon that has not been frequently studied and posits that there may exist an
even higher concentration of metal deeper below the surface. It is possible,
say the researchers that the discrepancy between the amount of iron on the
Earth’s crust and the Moon could be even greater than scientists thought, which
pulls into question the current understanding of how the Moon was formed.
The fact that our Moon could be richer in metals than
the Earth challenges the notion that it was portions of Earth’s mantle and
crust that were shot into orbit. A greater concentration of metal deposits may
mean that other hypotheses about the Moon’s formation must be explored. It may
be possible that the collision with Theia was more devastating to our early
Earth, with much deeper sections being launched into orbit, or that the
collision could have occurred when Earth was still young and covered by a magma
ocean. Alternatively, more metal could hint at a complicated cool-down of an
early molten Moon surface, as suggested by several scientists.
According to Heggy, “By improving our understanding of
how much metal the Moon’s subsurface actually has, scientists can constrain the
ambiguities about how it has formed, how it is evolving and how it is
contributing to maintaining habitability on Earth.” He further added, “Our
solar system alone has over 200 moons — understanding the crucial role these
moons play in the formation and evolution of the planets they orbit can give us
deeper insights into how and where life conditions outside Earth might form and
what it might look like.”
Wes Patterson of the Planetary Exploration Group
(SRE), Space Exploration Sector (SES) at Johns Hopkins University Applied
Physics Laboratory, who is the project’s principal investigator for Mini-RF and
a co-author of the study, added, “The LRO mission and its radar imager Mini-RF
are continuing to surprise us with new insights into the origins and complexity
of our nearest neighbor.”
The team plans to continue carrying out additional
radar observations of more crater floors with the Mini-RF experiment to verify
the initial findings of the published investigation.
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