When spacecraft first journeyed around the Moon, something unexpected was revealed: the far side has almost none of the lava flows we call seas or maria, which dominate what we can see from Earth.
For almost 60 years, astronomers have sought to explain the discrepancy with many different theories. A new model proposes the answer lies in the Moon's largest and deepest impact crater.
The lunar seas are the result of immense lava flows that erupted recently enough they have not been completely covered in craters. The puzzle is why there were so many more such eruptions on one hemisphere than the other.
A new study in the journal Science Advances proposes that the formation of the South Pole-Aitken (SPA) basin triggered a heat plume in the lunar interior that produced the imbalance. The SPA is among the Solar System's largest impact basins, with a metal structure beneath that may be the asteroid that formed it. The SPA is not as well known as smaller lunar craters, both because it's on the far side, and its immense age (4.3 billion years) means subsequent impacts have partially obscured it.
"We know that big impacts like the one that formed SPA would create a lot of heat," said study author and Brown University PhD student Matt Jones in a statement. “The question is how that heat affects the Moon's interior dynamics. What we show is that under any plausible conditions at the time that SPA formed, it ends up concentrating these heat-producing elements on the nearside. We expect that this contributed to the mantle melting that produced the lava flows we see on the surface."
The Moon would have had a very hot interior when it first formed, due to the gravitational potential energy of its components turning to thermal energy as they combined. At first, like on Earth, this would have been sustained by radioactive decay – but smaller bodies lose heat more quickly, causing the mantle to solidify and ending lunar volcanism.
An exception might have come after large asteroid strikes, which would have generated heat plumes that spread beneath the crust, mobilizing more easily melted material in the process.
The largest mare, Oceanus Procellarum, has a lot of what is known as KREEP terrane, rich in potassium (K) rare earth elements (REE), and phosphorus (P). Other seas are also usually KREEPy, but there is almost none on the far side, suggesting KREEP may be key to the seas' locations.
Researchers modeled what something like the SPA impact would do to a layer of KREEP initially forming the outermost layer of the Moon's mantle. They found that a plume of heat would melt the KREEP, leading to volcanic eruptions 300-600 million years after the impact.
Counter-intuitively, although the impact was on the far side, modeling indicates it created an active mantle on the nearside, allowing the maria's formation. The heat plumes drove the KREEP to the opposite side of the Moon from the impact, concentrating it particularly in the area where Oceanus Procellarum now lies.
For much of the time since the absence of seas on the far side was discovered, attempts to explain the difference centered on the relationship of the two hemispheres to Earth. Examples include efforts to explain how Earth's gravitational field could have produced greater activity on the lunar near side, or the planet's bulk blocked incoming asteroids, reducing cratering.
However, if the study authors are right, it's all a coincidence, a consequence of where the impact that caused the SPA happened to take place.