Colossal ancient impact may explain difference between Moon’s near and far side, study says
A giant impact near the Moon’s south pole billions of years ago may explain the stark contrast between the Moon’s near and far sides, a new study suggests.
Since the 1960s, studies have shown that the face the Moon show’s the Earth differs significantly in composition and appearance from the one it hides on its far side.
Researchers say while the nearside is dominated by the lunar mare – the vast, dark-coloured remnants of ancient lava flows – the far side, on the other hand, is pocketed with craters and is virtually devoid of large-scale mare features.
They say the nearside is also home to a compositional anomaly known as the Procellarum KREEP terrane (PKT) – a concentration of potassium (K), rare earth elements (REE), phosphorus (P), along with heat-producing elements like thorium.
KREEP, scientists say, represents the last part of the Moon’s mantle to solidify, and likely created the outermost layer of the mantle, just beneath the lunar crust.
Now a new study, published last week in the journal Science, suggests an impact that formed the Moon’s giant South Pole-Aitken (SPA) basin billions of years ago maybe behind the difference in composition better the two faces.
Scientists, including those from Brown University in the US, say the impact created a massive plume of heat that propagated through the lunar interior, which carried certain materials – a suite of rare-Earth and heat-producing elements – to the Moon’s nearside.
They say the concentration of these elements would have contributed to the volcanism that created the nearside volcanic plains.
Researchers found that KREEP is concentrated in and around Oceanus Procellarum – the largest of the nearside volcanic plains – but is sparse at other places on the Moon.
“We know that big impacts like the one that formed SPA would create a lot of heat. The question is how that heat affects the Moon’s interior dynamics,” said Matt Jones, study’s lead author and PhD candidate from Brown University.
“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,” Mr Jones added.
In the study, scientists conducted computer simulations of how heat generated by a giant impact on the Moon would alter patterns of convection in its interior.
They also analysed using the simulation how such an impact may redistribute KREEP material in the lunar mantle.
While models of the Moon’s interior suggest KREEP would have been evenly distributed beneath the surface, the new simulations suggest this uniform distribution could be disrupted by the heat plume from the SPA impact.
The new research finds that the KREEP material may have ridden the wave of heat emanating from the impact zone “like a surfer”.
And as the heat plume spread beneath the Moon’s crust, researchers say the KREEP material could have been eventually delivered en masse to the nearside.
Running simulations for a number of impact scenarios, from dead-on hit to a glancing blow, scientists say while each produced differing heat patterns and mobilised KREEP to different degrees, they all created KREEP concentrations on the nearside, consistent with the PKT anomaly.
“How the PKT formed is arguably the most significant open question in lunar science,” Mr Jones said.
“And the South Pole-Aitken impact is one of the most significant events in lunar history. This work brings those two things together, and I think our results are really exciting,” he added.
