Uploaded April 2026 | Updated September 2026, 2 weeks ago
What’s going on at the Moon’s south pole?
Remember NASA’s GRAIL mission which mapped out the pull of gravity from from each part of the Moon? Well it found that the south pole of the Moon had a much stronger pull than other regions, but there wasn’t a visibly obvious reason why, like a big mountain for example.
In fact, the south pole of the Moon stands out for the opposite reason, it’s heavily cratered, more so than the rest of the surface of the Moon. It contains the largest structure on the Moon known as the South Pole-Aitken Basin, it’s 8km (5mi deep) and spans roughly 2,500 km (1,600 mi) in diameter from the craters at the south pole to the Aitken crater at the top, with a chain of mountains ringing the edge.
It’s one of the largest craters in the solar system, and underneath it is where the GRAIL mission found the Moon’s gravity was much stronger than it should be. The data suggested there’s something really dense beneath the Moon’s crust that’s roughly five times the mass of Hawaii’s entire Big Island. It’s so dense that it’s pulling the basin floor downward by almost a kilometre.
So what could this mystery density be? There’s a couple of ideas to explain this, but most hinge on the idea of a massive asteroid hitting the south pole of the Moon over 4 billion years ago. Now with an impact that size, most of the asteroid would vaporise in the heat generated, but simulations have suggested an iron-rich core could’ve survived the collision and sank under the Moon’s crust, where it still remains today.
Or, another idea is that it’s not a single object at all, but instead the impact dredged up a huge amount of material from deeper under the Moon’s surface, so much that it’s now piled up there under the basin of the crater. Perhaps not in one single lump, but instead more spread out.
So to understand this, that’s really what we need to figure out next. Is the gravitational anomaly at the south pole caused by one big lump or more of a spread out over-density?
So will Artemis help here? Because after all, the plan is to land astronauts at the South Pole, and take samples of the surface from there, and add a moonquake sensor there. Well there might be some information we can glean from the surface samples about whether an asteroid impacted there, and the single location seismometer station might hint at what’s going on under the surface.
But again, ideally you’d have a network of seismometers detecting moon quakes all around the Moon, so you could detect how the moonquake waves behave near the south pole-aitken basin - do they sharply deflect off the edges of a big lump of metal, or just get scattered slightly by a spread out over-density of rock?
There is a mission still in the planning stages called the Lunar Geophysical Network (LGN) that, if funded, would land on the Moon in the 2030s. The idea is that it would put 4 seismometers on the Moon, spread out evenly across the surface of the Moon operating for 6 years to detect asteroid impacts, and moonquakes to map the Moon’s interior. But as of 2026, there’s no confirmed funding for it yet.
Video filmed on a Sony ⍺7 IV
Video edited by Martino Gasparrini: martino.freelance@gmail.com
Video produced by Marina Hui & Dr Becky Smethurst
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📚 My book, "A Brief History of Black Holes", out NOW in hardback, paperback, e-book and audiobook (which I narrated myself!): lnk.to/DrBecky
---
👕 My merch, including JWST designs, are available here (with worldwide shipping!): dr-becky.teemill.com
---
🎧 Royal Astronomical Society Podcast that I co-host: podfollow.com/supermassive
---
🔔 Don't forget to subscribe and click the little bell icon to be notified when I post a new video!
---
👩🏽💻 I'm Dr. Becky Smethurst, an astrophysicist at the University of Oxford. I love making videos about science with an unnatural level of enthusiasm. I like to focus on how we know things, not just what we know. And especially, the things we still don't know. If you've ever wondered about something in space and couldn't find an answer online - you can ask me! My day job is to do research into how supermassive black holes can affect the galaxies that they live in. In particular, I look at whether the energy output from the disk of material orbiting around a growing supermassive black hole can stop a galaxy from forming stars.
drbecky.uk.com
What’s going on at the Moon’s south pole?
Remember NASA’s GRAIL mission which mapped out the pull of gravity from from each part of the Moon? Well it found that the south pole of the Moon had a much stronger pull than other regions, but there wasn’t a visibly obvious reason why, like a big mountain for example.
In fact, the south pole of the Moon stands out for the opposite reason, it’s heavily cratered, more so than the rest of the surface of the Moon. It contains the largest structure on the Moon known as the South Pole-Aitken Basin, it’s 8km (5mi deep) and spans roughly 2,500 km (1,600 mi) in diameter from the craters at the south pole to the Aitken crater at the top, with a chain of mountains ringing the edge.
It’s one of the largest craters in the solar system, and underneath it is where the GRAIL mission found the Moon’s gravity was much stronger than it should be. The data suggested there’s something really dense beneath the Moon’s crust that’s roughly five times the mass of Hawaii’s entire Big Island. It’s so dense that it’s pulling the basin floor downward by almost a kilometre.
So what could this mystery density be? There’s a couple of ideas to explain this, but most hinge on the idea of a massive asteroid hitting the south pole of the Moon over 4 billion years ago. Now with an impact that size, most of the asteroid would vaporise in the heat generated, but simulations have suggested an iron-rich core could’ve survived the collision and sank under the Moon’s crust, where it still remains today.
Or, another idea is that it’s not a single object at all, but instead the impact dredged up a huge amount of material from deeper under the Moon’s surface, so much that it’s now piled up there under the basin of the crater. Perhaps not in one single lump, but instead more spread out.
So to understand this, that’s really what we need to figure out next. Is the gravitational anomaly at the south pole caused by one big lump or more of a spread out over-density?
So will Artemis help here? Because after all, the plan is to land astronauts at the South Pole, and take samples of the surface from there, and add a moonquake sensor there. Well there might be some information we can glean from the surface samples about whether an asteroid impacted there, and the single location seismometer station might hint at what’s going on under the surface.
But again, ideally you’d have a network of seismometers detecting moon quakes all around the Moon, so you could detect how the moonquake waves behave near the south pole-aitken basin - do they sharply deflect off the edges of a big lump of metal, or just get scattered slightly by a spread out over-density of rock?
There is a mission still in the planning stages called the Lunar Geophysical Network (LGN) that, if funded, would land on the Moon in the 2030s. The idea is that it would put 4 seismometers on the Moon, spread out evenly across the surface of the Moon operating for 6 years to detect asteroid impacts, and moonquakes to map the Moon’s interior. But as of 2026, there’s no confirmed funding for it yet.
Video filmed on a Sony ⍺7 IV
Video edited by Martino Gasparrini: martino.freelance@gmail.com
Video produced by Marina Hui & Dr Becky Smethurst
---
📚 My book, "A Brief History of Black Holes", out NOW in hardback, paperback, e-book and audiobook (which I narrated myself!): lnk.to/DrBecky
---
👕 My merch, including JWST designs, are available here (with worldwide shipping!): dr-becky.teemill.com
---
🎧 Royal Astronomical Society Podcast that I co-host: podfollow.com/supermassive
---
🔔 Don't forget to subscribe and click the little bell icon to be notified when I post a new video!
---
👩🏽💻 I'm Dr. Becky Smethurst, an astrophysicist at the University of Oxford. I love making videos about science with an unnatural level of enthusiasm. I like to focus on how we know things, not just what we know. And especially, the things we still don't know. If you've ever wondered about something in space and couldn't find an answer online - you can ask me! My day job is to do research into how supermassive black holes can affect the galaxies that they live in. In particular, I look at whether the energy output from the disk of material orbiting around a growing supermassive black hole can stop a galaxy from forming stars.
drbecky.uk.com
