New evidence suggests the SPA basin might not only be larger than previously thought but could also have formed under entirely different circumstances, challenging long-held theories and opening up exciting possibilities for future lunar missions.
The Moon’s surface tells the story of its tumultuous past, marked by billions of years of asteroid and comet impacts. Among these scars is one colossal feature that stands out: the South Pole-Aitken (SPA) basin. Spanning an astonishing 2,500 kilometers (1,550 miles) in diameter, it is the largest known crater on the Moon—and one of the most massive impact structures in the entire Solar System.
Recent research has unveiled groundbreaking insights about this ancient formation. New evidence suggests the SPA basin might not only be larger than previously thought but could also have formed under entirely different circumstances, challenging long-held theories and opening up exciting possibilities for future lunar missions.
A Monument to Cosmic Catastrophes
The SPA basin is a testament to the Moon’s violent history. Estimated to be over four billion years old, its immense size and age make it difficult to study. Time and subsequent impacts have eroded and obscured much of its original features. Despite these challenges, it has long been a focus of scientific inquiry due to its size, location, and the potential secrets it holds about the Moon’s early evolution.
Traditional models posited that the SPA basin was formed by an oblique collision, leaving behind an elongated, oval shape. However, a team of researchers led by Hannes Bernhardt of the University of Maryland has offered a fresh perspective.
“It’s challenging to study the South Pole-Aitken basin holistically due to its sheer enormousness, which is why scientists are still trying to learn its shape and size,” says Bernhardt.
Using data from NASA’s Lunar Reconnaissance Orbiter, which has been mapping the Moon since 2009, the researchers analyzed over 200 mountain features surrounding the basin. Their findings suggest the crater is more circular than previously thought—a shape indicative of a vertical impact.
A Vertical Impact Changes Everything
Impacts of celestial bodies often occur at angles, but a nearly vertical strike produces a rounder crater. Bernhardt explains:
“A rounder, more circular shape indicates that an object struck the Moon’s surface at a more vertical angle, possibly similar to dropping a rock straight down onto the ground.”
This revelation has profound implications. Unlike an angled collision, where debris would be ejected asymmetrically, a vertical impact likely distributed material evenly around the crater. This means that rocks ejected from deep within the Moon’s mantle and crust—materials otherwise impossible to access—may still be scattered around the basin’s perimeter.
These rocks hold the potential to unlock long-standing mysteries about the Moon’s origins. The leading theory suggests the Moon was formed from debris ejected during a massive collision between Earth and a Mars-sized object. However, alternative hypotheses, including coalescence from a debris cloud or gravitational capture, remain in consideration.
What Lies Beneath
Adding to the intrigue, gravitational measurements of the SPA basin indicate the presence of a dense, high-density region beneath the crater. This anomaly could represent remnants of the impactor itself—a metallic asteroid or similar object buried within the Moon’s crust.
If future missions like NASA’s Artemis program succeed in reaching the lunar south pole, astronauts or robotic explorers may be able to directly study these ancient materials. Bernhardt emphasizes the broader implications:
“Astronauts exploring the lunar south pole might have easier access to ancient lunar materials that could help us understand how the Moon and our Solar System came to be.”
The South Pole-Aitken basin represents more than just an impact crater; it’s a time capsule, preserving clues from the earliest days of the Solar System. As humanity prepares to return to the Moon, this vast and mysterious structure may hold the key to unraveling some of the most profound questions about our cosmic origins.
