The Lunar Farside Highlands
The mountainous region on the far side of the moon, known as the lunar farside highlands, may be the solid
remains of a collision with a smaller companion moon, according to a new study by planetary scientists at
the University of California, Santa Cruz.
Contrasting Topographies
The striking differences between the near and far sides of the moon have been a longstanding puzzle. The near
side is relatively low and flat, while the topography of the far side is high and mountainous, with a much
thicker crust.
A Second Moon?
The new study, published in the August 4, 2011 issue of Nature, builds on the "giant impact" model for
the origin of the moon, in which a Mars-sized object collided with Earth early in the history of the solar system
and ejected debris that coalesced to form the moon. The study suggests that this giant impact also created another,
smaller body, initially sharing an orbit with the moon, that eventually fell back onto the moon and coated one side
with an extra layer of solid crust tens of kilometers thick.
"Our model works well with models of the moon-forming giant impact, which predict there should be massive debris left in
orbit about the Earth, besides the moon itself. It agrees with what is known about the dynamical stability of such a
system, the timing of the cooling of the moon, and the ages of lunar rocks," said Erik Asphaug, professor of Earth and
planetary sciences at UC Santa Cruz.
Computer Simulations of Lunar Impacts
Asphaug, who coauthored the paper with UCSC postdoctoral researcher Martin Jutzi, has previously done computer simulations
of the moon-forming giant impact. He said companion moons are a common outcome of such simulations.
In the new study, he and Jutzi used computer simulations of an impact between the moon and a smaller companion (about
one-thirtieth the mass of the moon) to study the dynamics of the collision and track the evolution and distribution of
lunar material in its aftermath.
Low-Velocity Collision?
In such a low-velocity collision, the impact does not form a crater and does not cause
much melting. Instead, most of the colliding material is piled onto the impacted hemisphere as a thick new layer of solid
crust, forming a mountainous region comparable in extent to the lunar farside highlands.
"Of course, impact modelers try to explain everything with collisions. In this case, it requires an odd collision: being
slow, it does not form a crater, but splats material onto one side," Asphaug said. "It is something new to think about."
A Lopsided Moon of Varying Composition
He and Jutzi hypothesize that the companion moon was initially trapped at one of the gravitationally stable "Trojan points"
sharing the moon's orbit, and became destabilized after the moon's orbit had expanded far from Earth. "The collision could
have happened anywhere on the moon," Jutzi said. "The final body is lopsided and would reorient so that one side faces Earth."
The model may also explain variations in the composition of the moon's crust, which is dominated on the near side by terrain
comparatively rich in potassium, rare-earth elements, and phosphorus (KREEP). These elements, as well as uranium and thorium,
are believed to have been concentrated in the magma ocean that remained as molten rock solidified under the moon's thickening
crust. In the simulations, the collision squishes this KREEP-rich layer onto the opposite hemisphere, setting the stage for
the geology now seen on the near side of the moon.
Other Theories for the Lunar Farside Highlands
Other models have been proposed to explain the formation of the highlands, including one published last year in Science by
Jutzi and Asphaug's colleagues at UC Santa Cruz, Ian Garrick-Bethell and Francis Nimmo. Their analysis suggested that tidal forces,
rather than an impact, were responsible for shaping the thickness of the moon's crust.
"The fact that the near side of the moon looks so different to the far side has been a puzzle since the dawn of the space age,
perhaps second only to the origin of the moon itself," said Nimmo, a professor of Earth and planetary sciences. "One of the
elegant aspects of Erik's article is that it links these two puzzles together: perhaps the giant collision that formed the moon
also spalled off some smaller bodies, one of which later fell back to the Moon to cause the dichotomy that we see today."
Financial Support and Future Studies
For now, he said, there is not enough data to say which of the alternative models offers the best explanation for the lunar dichotomy.
"As further spacecraft data (and, hopefully, lunar samples) are obtained, which of these two hypotheses is more nearly correct will
become clear," Nimmo said.
The new study was supported by NASA's Planetary Geology and Geophysics Program. Simulations were run on the NSF-sponsored UC
Santa Cruz astrophysics supercomputer pleiades.
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Tidal forces between the moon and the Earth have slowed the moons' rotation so that one side of the moon always faces toward our planet. Though sometimes improperly referred to as the "dark side of the moon," it should correctly be referred to as the "far side of the moon" since it receives just as much sunlight as the side that faces us.
The lunar far side is rougher and has many more craters than the near side, so quite a few of the most fascinating lunar features are located there, including one of the largest known impact craters in the solar system, the South Pole-Aitken Basin. The image highlighted here shows the moon's topography with the highest elevations up above 20,000 feet in red and the lowest areas down below -20,000 feet in blue. Image Credit: NASA/Goddard |
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