Cracking the Moon's Secrets: New Findings on Lunar Shrinkage

0:00 Anna: Hi, everyone, and welcome back to astronomy daily. Today, 10,000 cracks in the face of the Moon, and every one of them tells the same story. Our Moon is still shrinking, and today Avery: we know by how much. Plus, a PhD student in Sydney has drawn the first soil map of Mars. A dark energy telescope has stumbled onto a brand new comet. And two planets everyone calls twins turn out to be more like distant cousins. Anna: We'll also catch up on Japan's Phobos mission, now safely on its way, and a telescope that's turned out even sharper than its builders hoped.

0:39 Avery: It's Tuesday, the 20th of October 2026. This is Astronomy AstroDailyPod series five, episode 209. I'm, um, Avery. Anna: And I'm Anna. Let's get into it. Avery: Here we go then. Anna: Most of us grow up thinking of the Moon as finished, geologically dead. Everything interesting happened billions of years ago, and since then, it has just sat there collecting craters. A new study published last Wednesday in the Planetary Science Journal says that picture is wrong in a very specific, measurable way.

1:13 Avery: This comes from Tom Waters, a, uh, scientist Emeritus at the Smithsonian's National Air and Space Museum with Maria Banks at NASA Goddard Corporation, Cole Knyphaver and Mark Robinson, who leads the camera on NASA's Lunar Reconnaissance Orbiter. Anna: And Robinson's name should ring a bell for regular listeners. His team's change detection work on that same camera gave us the brand new lunar crater we covered back in episode 196. So what did they find? More than 10,000 young faults spread right across the lunar surface.

1:45 About 8,900 of them are what geologists call lobate scarps. And little over 2,000 are small ridges on the dark lunar plains. Avery: The maria paint us a picture of a lobate scarp. Anna: Imagine a cliff or a step running across the landscape. It forms when the crust is squeezed so hard it breaks and one side gets shoved up and over the other. That's a thrust fault on the Moon. They're usually tens of metres high and a few kilometres long, though the release shows one near the North Pole, inside a crater called Mouch's L, standing more than 100 metres tall.

2:23 Avery: And, um, these are young. On the Moon, young is a relative word. Anna: Very relative. Young here means the last few hundred million years. The tell is that they look crisp, they cut straight through small craters, and they haven't been battered flat by later impacts over the age of the Moon. That's yesterday afternoon. Avery: So why is the Moon being squeezed at all? There's no plate tectonics up there. Anna: Exactly. And that's the key. The Moon's interior is still warm and it's slowly cooling.

2:55 As rock cools, it contracts. The inside of the moon gets a little smaller and the rigid outer shell has to fit around it. Think of a grape drying into a raisin. Except the Moon's skin is brittle rock, so instead of wrinkling, it cracks and breaks along these thrust faults. Avery: And Earth tides add to it, don't they? Anna: They do. Earlier work by the same group showed the orientation of the faults. Only makes sense if you add tidal stresses from Earth on top of the global shrinking. But the shrinking is the engine.

3:28 Avery: Here's the headline, then. Um, how much smaller has it gotten? Anna: The team added up how much the surface has been compressed across all of those faults, highlands and Maria together, and converted that into a change in radius. The answer is about 42 to 46 metres. The Smithsonian rounds it to 45. That's over roughly the last 400 to 800 million years. Avery: 45 metres off a radius of more than 1700 kilometres. That's tiny. Anna: It is. If the pace were steady, it works out to at most about 11 centimetres every million years.

4:06 A tenth of a micrometre a year. Earth's tectonic plates move about a centimetre and a half a year, roughly the rate your fingernails grow. So the Moon is shrinking more than 100,000 times more slowly than Earth's plates move. Avery: And, um, this updates an older figure, doesn't it? Anna: It does. Waters team first raised this in 2010, when the orbiter's camera had turned up 14 new scarps. Back then, they offered a rough estimate of about 100 metres. By 2015, the count was over 3,000. Now, with more than 10,000 mapped, the number has firmed up and it's come in at less than half that early rough guess.

4:46 Avery: So what's the surprise here? The Moon shrinking isn't new. Anna: The surprise is how little. Models of how the Moon should have cooled predict much more contraction than these faults record. And the team didn't find old, worn down scarps from earlier in lunar history. So it looks as if the widespread faulting started relatively late. Avery: Which means something held the heat in. Anna: That's their leading explanation. The ancient lunar crust was pulverised by billions of years of impacts into a thick, porous, heavily fractured layer called the mega regolith.

5:21 Porous broken rock is a poor conductor of heat, so it may have acted like a blanket, keeping the interior warm and delaying the cooling and so delaying the shrinking. The paper also offers an alternative a strong, rigid outer shell that simply resisted breaking until the stress built past a threshold. Avery: And, um, they see a hint of that in where the faults are right. Anna: The thickest crust has the fewest faults, which also suggests those areas are the most stable. Avery: Regular listeners might feel a sense of deja vu here.

5:58 Anna: Back in episode 192, we covered Mercury, where scarps were hiding under rough terrain and the planet had shrunk more than anyone counted. Here, it's the reverse. A, uh, careful count of nearly every fault finds the Moon shrank less than the theory expected. Same physics, opposite surprise. And it's a reminder we keep coming back to on this show. What you can count depends on how hard you looked. Avery: So we all need to look harder. Anna: It does. And this matters for anyone planning to live there.

6:29 Waters calculates the compressional stress from this much shrinking at about 4 MPa. That lines up with the stress released in the shallow moonquakes that Apollo's seismometers recorded in the 1970s. On Earth, about 90% of seismic activity is concentrated along plate boundaries. The Moon is a single plate, so its faults are spread everywhere. Waters says the growth of existing faults and the formation of new ones is expected to continue today. Avery: So a, uh, moonquake could happen anywhere. Anna: That's the point.

7:01 And we want to be careful with it. This isn't a warning that something is about to happen. It's about where you choose to put a permanent base. Waters earlier work crowned young faults near some of the candidate Artemis landing regions at the South Pole. If the thickest crust is the most stable, that's useful information for planners. Avery: What should we keep in mind? Anna: Three things. First, the rate assumes the shrinking happened at a steady pace, which it may not have. Second, the ages of these faults are relative.

7:34 They come from how crisp they look and what they cut across, not from dated samples. And third, the moonquake point is about sighting and engineering, not prediction. Avery: And there's no Australian team on this one? Anna: No, and we won't pretend otherwise. But the Moon is the one world every listener can step outside and see north or south. Tonight's waxing gibbous will be up in the evening sky from Sydney to London. And somewhere on that face too small to see, a, uh, fault might be Avery: growing, staying in the solar system.

8:08 And a story from right here in Sydney. If people are ever going to grow food on Mars, they'll need to know what they're growing it in. The University of Sydney has just produced the first comprehensive classification and map of Martian soils. Anna: Who's behind it? Avery: PhD student Daniel Irving from the Sydney Institute of Agriculture, working with Professor Alex McBratney, one of the world's leading soil scientists. The paper is in the soil science journal Ktena, went online at the start of this month, and the university announced it on Friday.

8:40 Anna: And they've had to invent a word they have. Avery: On Earth, we talk about soils, but, uh, they've proposed calling Martian soil classes Areths, from Ares, the Greek name for Mars. The System groups almost 800 million locations into a hierarchy. Ten broad realms, 100 assemblages and 2,000 detailed forms. The map covers 92% of the planet, based on what? Orbital observations and models of properties like texture, mineralogy, ph, and how deep the soil goes. As a test, they scored all 2000 forms on how well they might support plants.

9:21 One called Nohi Barreth, came out on top at 0.93 out of 1. Its mottled profile is about 72% clay, with a pH of 6.8, which is close to neutral. It turns up in the mid latitudes around Tharsis Arabia Terra and Elysium Planitia, but on only 0.14% of the surface. Anna: So the Martian potato patch. Avery: Not quite. And the team is clear about that. These are candidate areas for closer study, not greenhouse sites. Nobody's going to plant crops straight into Martian dirt. Any habitat will need controlled environments, water, nutrients and treatment for harmful chemicals.

10:06 In fact, the map doesn't yet include perchlorate, the toxic salt found across Mars, or nitrogen. And many of the Sol properties are inferred from orbit, so the classification will change as rovers measure more. Anna: What's the limiting factor for plants? Avery: Water holding capacity in 73% of the forms they assessed. The next up is back on Earth, mixing Mars like substrates for the promising classes and growing crops in them under controlled conditions. Anna: And there's a payoff for Australia, too, there is.

10:40 Avery: Martian soils have no organic matter, and McBratney points out that some of Earth's toughest arid soils have very little either. The work may help scientists think about places like the Pilbara and the Atacama differently. Mars as a lens on our own red dirt. Anna: Next, here's a discovery made almost by accident. The European Space Agency's Euclid telescope is built to map billions of galaxies so astronomers can study dark matter and dark energy. It is emphatically not a comet hunter. Yet ESA announced yesterday that Euclid has discovered its first previously unknown comet.

11:19 Avery: How does a galaxy telescope trip over a comet? Anna: Through a, uh, PhD student and some clever software? Miko Poynton at the University of Helsinki wrote a programme to spot moving solar system objects in Euclid's images, mainly so they can be cleaned out of the galaxy data. In images taken between the 16th and 24th of September, his software flagged a streak smeared across several frames. He tracked it into five image sets, 16 exposures in all, and found a fuzzy cloud around it. That's a, uh, coma.

11:52 The sign of an active comet shedding dust and gas. Avery: And it checked out? Anna: No, no match in any catalogue. It was reported to the International Astronomical Union's Minor Planet Centre. Global ground based observers followed up and it was confirmed as a comet on 8th October. It's now officially C 2026 S1 Euclid. Avery: Where is it? Anna: About 1.3 billion kilometres from Earth, roughly between the orbits of Jupiter and Saturn. But it isn't in the plane of the planets. From the sun's point of view, it's almost straight up on a steep orbit that takes thousands of years to complete.

12:32 Avery: Why is that a lucky catch? Anna: Because Euclid deliberately avoids the band of sky along the plane of the solar system, where comets and asteroids are most common, since the dust there adds glare and it usually photographs each patch of sky only once. So a comet has to wander through the right field at the right time. Euclid archive scientist Bruno Altieri called it a happy coincidence. Avery: Any bigger significance? Anna: Two things. It was a real world test of how Euclid could alert ESA's future Comet Interceptor mission to a fresh target.

13:04 Although this particular comet isn't a candidate and it's a nice reminder of something we often say on this catalogues are detection limits, not censuses. One honest note for southern listeners, it was found in Euclid's DEET field north, and it's far too faint to see without a big telescope. Avery: Uranus and Neptune are usually described as twins. Similar size, similar mass, both blue green ice giants. But a study in Astronomy and Astrophysics, published in its final form yesterday, argues their chemistry tells a different story.

13:39 Anna: Who's behind it? Avery: Tom Briand at Aix Marseille University with Vincent Hu, Olivier Moussi of the Southwest Research Institute, Thibaut Cavalier and colleagues.1 Disclosure. The accepted version of the paper has been online since late August, so this is the journal publication rather than a Anna: brand new result, though, how do you measure what's deep inside a planet with no surface? Avery: Indirectly using carbon monoxide. Deep down, where it's hot. Carbon monoxide is created from methane and water.

14:12 Strong vertical mixing carries it up to the upper atmosphere faster than it can be destroyed. So the amount we see up top acts as a tracer for the oxygen hidden far below. The team used measurements of that carbon monoxide, including from the ALMA array in Chile's Atacama Desert, and fed them into chemical models that now account for how conditions change with latitude. Anna: And the answer? Avery: Neptune's deep atmosphere looks much richer in oxygen. They estimate uranus at roughly 50 to 180 times the sun's original proportion of oxygen, and neptune at about 180 to 340.

14:48 40 times. That gives the two planets quite different carbon to oxygen ratios. Anna: And that points to where they formed. Avery: That's the argument in the disc that formed the planets. Different ices froze out at different distances from the young Sun. Compared against models of that disc, Neptune's chemistry suggests its building blocks came from closer in before it migrated outward. Muses line is that they're more like cousins with very different histories. Anna: How firm is it? Avery: It's a modelling result built on indirect measurements.

15:23 And the ranges are wide. Especially for Uranus, the biggest uncertainty is how strongly each atmosphere mixes vertically. The team says plainly that without going into those atmospheres, our understanding is fundamentally limited. Anna: Which is a case for a probe. Avery: Exactly. The paper offers a framework for choosing the latitude where a future Uranus orbiter and probe should dive in. And since planets this size are common around other stars, getting our own two right matters well beyond the solar system.

15:57 Anna: Time now for some quick hits, starting with an update to yesterday's episode. Japan's MMX mission to Mars moon Phobos is on its way. The H3 rocket lifted off from Tanegashima at 4:41 this morning. Japan time. About 71 minutes later, JAXA confirmed the spacecraft had separated and been placed on its planet path. JAXA then received signals from the spacecraft showing it's working normally. Next stop, Mars orbit next August and if all goes well, a capsule of Phobos landing in South Australia in 2031.

16:32 Avery: Next, the Roman Space Telescope, which keeps beating expectations. Last Wednesday, NASA reported the team has finished focusing it. They moved the secondary mirror by about 1/16 of the width of a human hair. The wide field instrument's optics now show an error of just 63 nanometers against a requirement of about 90. That's nearly 30% better than predicted. The coronagraph is in focus too. First Science images are still expected early next year. Anna: NASA has also published its detailed analysis of the Artemis II heat shield.

17:05 After Artemis I, engineers found 103 spots where charred material had broken away. Artemis II flying a shorter reentry Path had just nine. That's about a tenfold reduction. Artemis III will carry a reworked heat Avery: shield and keep an eye on ESA's Hera Mission. Its second braking burn, BRM2, is scheduled for this Thursday as it closes in on the Didymos asteroid pair. We'll bring you the result. Anna: Next up, let's go and take a look outside. Avery: Time for Skywatch. And this week the meteors win.

17:38 The Orionid meteor shower peaks over the nights of the 21st and 22nd. These are crumbs of Hallie's Comet and they hit our atmosphere fast at about 66 kilometres per second. Anna: But there's a waxing gibbous moon in Avery: the way there is, so timing matters. On Wednesday night into Thursday, the moon sets at about 3:38 in the morning in Sydney, 3:22 in Los Angeles, 3:12 in New York and 2:49 in London. That leaves a dark window before dawn. From Sydney, astronomical dawn comes at about 4:37, so we get about an hour.

18:14 Northern observers get longer. Two to three hours. Anna: And the radiant favours the north. Avery: It does. The radiant near Orion's raised club is about 440 degrees up from Sydney just before dawn, but 54 degrees from London, 64 from New York and 71 from Los Angeles. A higher radiant means more meteors. Southern watchers look northeast before dawn and expect fewer, but they're still worth a look on Thursday night. The moon sets half an hour to an hour later, so the dark window shrinks. Anna: What else is up, huh?

18:48 Avery: Southern skies get Mercury this week. On Wednesday at the end of civil Twilight, it's about 16 and a half degrees above the western horizon from Sydney, but uh, only four degrees from Los Angeles, two from New York and already set from London. It's fading. Anna: So catch it early this week and Venus disappears. Avery: It does. Venus reaches inferior conjunction around the 24th, passing between Earth and the sun. It'll be only about six and a half degrees from the sun, about 40 million kilometres from us.

19:21 So lost in the glare it'll come back as a morning object. Please don't hunt for it with binoculars or a telescope near the sun. Never point any optical instrument at or near the sun without a proper solar filter certified to ISO 123122. Anna: Anything for the evening crowd? Avery: Saturn nicely placed around 30 to 40 degrees up at 9pm in all four cities. On the 24th, the nearly full moon passes about 6 degrees from it. Anna: And the uh, pre dawn planets? Avery: That one belongs to the north. At nautical dawn on Thursday, Mars and Jupiter are about 50 to 60 degrees up from the northern cities, but only about 21 to 24 from Sydney.

20:04 They're 10 degrees apart now and closing to just over 1 degree by the 16th of November. Anna: And a clock warning. Avery: Yes, the UK goes back an hour this Sunday the 25th, but the US doesn't until the 1st of November. So for one week, London is only four hours ahead of New York and our London Times shift. Anna: And the full moon. Avery: The full moon arrives at 4:11 Universal Time on Monday the 26th. That's 3:11 in the afternoon in Sydney, 4:11 in the morning in London after the clock change a few minutes past midnight in New York and 911 on Sunday evening in Los Angeles.

20:43 So your calendar date depends on where you stand from either hemisphere. It'll look full on both nights. Anna: That's our show. A shrinking moon, a soil map for Mars, Euclid's accidental comet, and two ice giants that turn out to be cousins. Avery: Show notes and every source are at astronomydaily IO and if there's a story you'd like us to follow up, use the contact form there. Some of our favourite segments started that way. Anna: You can find us as astrodaily pod on social media. And if you enjoyed the show, a, uh, rating or review really helps new listeners find us.

21:22 Avery: Tomorrow. We'll keep watch on the Orionids and on Hera's next breaking burn. I'm Avery. Anna: And I'm Anna. Clear skies. A star.

21:38 Avery: Love.

21:43 Story. Soul.