Obrovský staroveký dopad spojený s rozdielom medzi blízkou a vzdialenou stranou Mesiaca

Nový výskum ukazuje, ako vplyv, ktorý vytvoril južný pól Mesiaca-Aitken Basin, súvisí s ostrým kontrastom v zložení a vzhľade medzi dvoma stranami Mesiaca.

Tvár, ktorú Mesiac ukazuje Zemi, vyzerá veľmi odlišne od toho, čo skrýva na svojej odvrátenej strane. Neďaleko dominuje Lunar Mare – masívne tmavé pozostatky starovekých lávových prúdov. Na druhej strane kráterom posiata odvrátená strana prakticky do značnej miery nemá vlastnosti kobyly. Prečo sú tieto dve strany také odlišné, je jednou z najtrvalejších záhad Mesiaca.

Teraz majú vedci nové vysvetlenie pre Mesiac s dvoma tvárami – jedno súvisiace s obrovským dopadom blízko južného pólu Mesiaca pred miliardami rokov.

Nová štúdia publikovaná v časopise Science Advances naznačuje, že dopad rozsiahlej mesačnej panvy South Pole-Aitken (SPA) mohol vytvoriť obrovský oblak tepla šíriaci sa vnútrom Mesiaca. Tento oblak mohol odniesť časť materiálu – súbor vzácnych zemín a prvkov generujúcich teplo – na Mesiac. Táto koncentrácia prvkov mohla prispieť k vulkanizmu, ktorý vytvoril blízke sopečné pláne.

Blízkej (ľavej) časti Mesiaca dominujú obrovské vulkanické ložiská, zatiaľ čo na odvrátenej strane (vpravo) je ich veľmi málo. Prečo sú tieto dve strany také odlišné, je trvalou lunárnou záhadou. kredit: Brown University

„Vieme, že veľké vplyvy, ktoré tvoria SPA, vygenerujú veľa tepla,“ povedal Matt Jones, Ph.D. povedal. kandidát na Brown University a hlavný autor štúdie. „Otázkou je, ako toto teplo ovplyvňuje vnútornú dynamiku Mesiaca. Ukazujeme, že za akýchkoľvek znateľných podmienok v čase vzniku SPA sústredil tieto prvky generujúce teplo blízko. Očakávame, že to prispelo k topeniu plášťa, ktoré vytvára lávové prúdy viditeľné na povrchu.

Štúdia bola výsledkom spolupráce medzi Jonesom a jeho poradcom Alexandrom Evansom, odborným asistentom na Browne, spolu s výskumníkmi z Purdue University, Lunar and Planetary Science Laboratory v Arizone, Stanford University a.[{“ 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