与月球近侧和远侧之间的差异相关的巨大古代影响

一项新的研究表明，月球南极的一次古老碰撞改变了月幔中的对流模式，主要集中在近侧的一组产热元素上。 这些元素在地球上可见的巨大月球马蹄铁的形成中发挥了作用。 信用：马特琼斯

新的研究表明，月球南极艾特肯盆地的影响与月球两侧的组成和外观形成鲜明对比。

月球向地球展示的面孔与它隐藏在其另一侧的面孔看起来非常不同。 近侧主要是月球波斯人 – 古代熔岩流的巨大深色遗迹。 另一方面，充满火山口的远端几乎没有广泛的母马特征。 两侧差异巨大的原因是月球最持久的谜团之一。

现在，研究人员对这颗双面月球有了新的解释——这种解释与数十亿年前月球南极附近的一次巨大撞击有关。

发表在《科学进展》杂志上的一项新研究表明，形成月球巨大南极艾特肯盆地 (SPA) 的撞击会产生大量热羽流，扩散到月球内部。 这股羽流可能含有某些物质——稀土和发热元素的组合——以及附近的月球。 这种元素的集中可能促成了导致附近火山平原形成的火山活动。

广泛的火山沉积物在月球的近侧（左）占主导地位，而远侧（右）则少得多。 两方差异大的原因在于万年轮的奥秘。 学分：布朗大学

“我们知道，像塑造 SPA 那样的巨大影响会产生大量热量，”Matt Jones 博士说。 布朗大学的候选人和该研究的主要作者。 问题是这种温度如何影响月球的内部动态。 我们展示的是，在 SPA 形成时的任何合理条件下，它最终会将这些发热元素集中在近侧。 我们推测这导致了地幔的融化，导致了我们在地表看到的熔岩流。 “

这项研究是琼斯和他的顾问、布朗大学助理教授亚历山大·埃文斯以及来自普渡大学、亚利桑那州月球和行星科学实验室、斯坦福大学和[{” attribute=””>NASA’s Jet Propulsion Laboratory.

A new study reveals that an ancient collision on the Moon’s south pole changed patterns of convection in the lunar mantle, concentrating a suite of heat-producing elements on the nearside. Those elements played a role in creating the vast lunar mare visible from Earth. Credit: Matt Jones

The differences between the near and far sides of the Moon were first revealed in the 1960s by the Soviet Luna missions and the U.S. Apollo program. While the differences in volcanic deposits are plain to see, future missions would reveal differences in the geochemical composition as well. The nearside is 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 seems to be concentrated in and around Oceanus Procellarum, the largest of the nearside volcanic plains, but is sparse elsewhere on the Moon.

Some scientists have suspected a connection between the PKT and the nearside lava flows, but the question of why that suite of elements was concentrated on the nearside remained. This new study provides an explanation that is connected to the South Pole–Aitken basin, the second largest known impact crater in the solar system.

For the study, the researchers conducted computer simulations of how heat generated by a giant impact would alter patterns of convection in the Moon’s interior, and how that might redistribute KREEP material in the lunar mantle. KREEP is thought to represent the last part of the mantle to solidify after the Moon’s formation. As such, it likely formed the outermost layer of mantle, just beneath the lunar crust. Models of the lunar interior suggest that it should have been more or less evenly distributed beneath the surface. But this new model shows that the uniform distribution would be disrupted by the heat plume from the SPA impact.

According to the model, the KREEP material would have ridden the wave of heat emanating from the SPA impact zone like a surfer. As the heat plume spread beneath the Moon’s crust, that material was eventually delivered en masse to the nearside. The team ran simulations for a number of different impact scenarios, from dead-on hit to a glancing blow. While each produced differing heat patterns and mobilized KREEP to varying degrees, all created KREEP concentrations on the nearside, consistent with the PKT anomaly.

The researchers say the work provides a credible explanation for one of the Moon’s most enduring mysteries.

“How the PKT formed is arguably the most significant open question in lunar science,” 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.”

Refernece: “A South Pole–Aitken impact origin of the lunar compositional asymmetry” by Matt J. Jones, Alexander J. Evans, Brandon C. Johnson, Matthew B. Weller, Jeffrey C. Andrews-Hanna, Sonia M. Tikoo and James T. Kean, 8 April 2022, Science Advances.
DOI: 10.1126/sciadv.abm8475