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

[00:00:00] [SPEAKER_00] 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.

[00:00:12] [SPEAKER_02] And today, 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.

[00:00:30] [SPEAKER_00] 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.

[00:00:38] [SPEAKER_02] It's Tuesday, the 20th of October, 2026. This is Astronomy Daily, Series 5, Episode 209. I'm Avery.

[00:00:47] [SPEAKER_00] And I'm Anna. Let's get into it.

[00:00:50] [SPEAKER_02] Here we go then.

[00:00:52] [SPEAKER_00] 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.

[00:01:12] [SPEAKER_02] This comes from Tom Waters, a scientist emeritus at the Smithsonian's National Air and Space Museum, with Maria Banks at NASA Goddard, Cole Nypaver, and Mark Robinson, who leads the camera on NASA's Lunar Reconnaissance Orbiter.

[00:01:26] [SPEAKER_00] 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. About 8,900 of them are what geologists call low-bait scarps. And little over 2,000 are small ridges on the dark lunar plains, the Maria.

[00:01:55] [SPEAKER_02] Paint us a picture of a low-bait scarp.

[00:01:57] [SPEAKER_00] Imagine a cliff, or a steppe, 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 meters high and a few kilometers long. Though the release shows one near the North Pole, inside a crater called Moosh's L, standing more than 100 meters tall.

[00:02:22] [SPEAKER_02] And these are… young? On the moon, young is a relative word.

[00:02:27] [SPEAKER_00] 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.

[00:02:43] [SPEAKER_02] So why is the moon being squeezed at all? There's no plate tectonics up there.

[00:02:49] [SPEAKER_00] Exactly. And that's the key. The moon's interior is still warm, and it's slowly cooling. 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.

[00:03:12] [SPEAKER_02] And earth tides add to it, don't they?

[00:03:15] [SPEAKER_00] 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.

[00:03:28] [SPEAKER_02] Here's the headline then. How much smaller has it gotten?

[00:03:32] [SPEAKER_00] 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 meters. The Smithsonian rounds it to 45. That's over roughly the last 400 to 800 million years.

[00:03:51] [SPEAKER_02] 45 meters off a radius of more than 1,700 kilometers. That's tiny.

[00:03:59] [SPEAKER_00] It is. If the pace were steady, it works out to at most about 11 centimeters every million years, a tenth of a micrometer a year. Earth's tectonic plates move about a centimeter 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.

[00:04:20] [SPEAKER_02] And this updates an older figure, doesn't it?

[00:04:22] [SPEAKER_00] 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 meters. 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. So what's the surprise here? The moon shrinking isn't new. The surprise is how little.

[00:04:52] [SPEAKER_00] 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.

[00:05:07] [SPEAKER_02] Which means something held the heat in.

[00:05:10] [SPEAKER_00] That's their leading explanation. The ancient lunar crust was pulverized by billions of years of impacts into a thick, porous, heavily fractured layer called the mega regolith. 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.

[00:05:35] [SPEAKER_00] A strong, rigid outer shell that simply resisted breaking until the stress built past a threshold.

[00:05:41] [SPEAKER_02] And they see a hint of that in where the faults are.

[00:05:45] [SPEAKER_00] Right. The thickest crust has the fewest faults. Which also suggests those areas are the most stable.

[00:05:53] [SPEAKER_02] Regular listeners might feel a sense of deja vu here.

[00:05:57] [SPEAKER_00] 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 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. So we all need to look harder. It does.

[00:06:27] [SPEAKER_00] And this matters for anyone planning to live there. Waters calculates the compressional stress from this much shrinking at about 4 megapascals. 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.

[00:06:52] [SPEAKER_00] Waters says the growth of existing faults and the formation of new ones is expected to continue today. So a moonquake could happen anywhere. That's the point. 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 were 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.

[00:07:22] [SPEAKER_00] What should we keep in mind? 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. 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.

[00:07:44] [SPEAKER_02] And there's no Australian team on this one.

[00:07:47] [SPEAKER_00] 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 fault might be growing.

[00:08:06] [SPEAKER_02] Staying in the solar system, 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. Who's behind it? PhD student Daniel Irving from the Sydney Institute of Agriculture, working with Professor Alex McBrattney, one of the world's leading soil scientists. The paper is in the soil science journal, Katana.

[00:08:36] [SPEAKER_02] It went online at the start of this month, and the university announced it on Friday.

[00:08:40] [SPEAKER_00] And they've had to invent a word.

[00:08:43] [SPEAKER_02] They have. On Earth we talk about soils, but 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. 10 broad realms, 100 assemblages, and 2,000 detailed forms. The map covers 92% of the planet. Based on what?

[00:09:06] [SPEAKER_02] Orbital observations and models of properties like texture, mineralogy, pH, and how deep the soil goes. As a test, they scored all 2,000 forms on how well they might support plants. One called Nohi Bereth came out on top at 0.93 out of 1. Its modeled profile is about 72% clay with a pH of 6.8, which is close to neutral.

[00:09:34] [SPEAKER_02] It turns up in the mid-latitudes around Tharsis, Arabia Terra, and Elysium Planitia, but on only 0.14% of the surface.

[00:09:46] [SPEAKER_00] So, the Martian potato patch.

[00:09:50] [SPEAKER_02] 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. In fact, the map doesn't yet include perchlorate, the toxic salt found across Mars, or nitrogen. And many of the soil properties are inferred from orbit. So, the classification will change as rovers measure more.

[00:10:20] [SPEAKER_00] What's the limiting factor for plants?

[00:10:23] [SPEAKER_02] 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.

[00:10:36] [SPEAKER_00] And there's a payoff for Australia, too.

[00:10:39] [SPEAKER_02] There is. 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.

[00:10:57] [SPEAKER_00] 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.

[00:11:18] [SPEAKER_02] How does a galaxy telescope trip over a comet?

[00:11:22] [SPEAKER_00] Through a PhD student and some clever software. Mikko Poyntanen at the University of Helsinki wrote a program 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.

[00:11:51] [SPEAKER_00] That's a coma, the sign of an active comet shedding dust and gas. And it checked out? No, no match in any catalogue. It was reported to the International Astronomical Union's Minor Planet Center, ground-based observers followed up, and it was confirmed as a comet on the 8th of October. It's now officially C-slash-2026-S1 Euclid. Where is it? About 1.3 billion kilometers from Earth, roughly between the orbits of Jupiter and Saturn.

[00:12:22] [SPEAKER_00] 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.

[00:12:32] [SPEAKER_02] Why is that a lucky catch?

[00:12:34] [SPEAKER_00] 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.

[00:12:55] [SPEAKER_02] Any bigger...significance?

[00:12:57] [SPEAKER_00] Two things. It was a real-world test of how Euclid could alert ESA's future comet interceptor mission to a fresh target. Although, this particular comet isn't a candidate. And it's a nice reminder of something we often say on this show. Catalogues are detection limits, not censuses. One honest note for Southern listeners, it was found in Euclid's Deep Field north, and it's far too faint to see without a big telescope.

[00:13:22] [SPEAKER_02] 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.

[00:13:39] [SPEAKER_00] Who's behind it?

[00:13:41] [SPEAKER_02] Tom Briand at Ex-Marseille University, with Vincent Hu, Olivier Moussi of the Southwest Research Institute, Thibaut Cavalier, and colleagues. One disclosure, the accepted version of the paper has been online since late August. So, this is the journal publication rather than a brand new result.

[00:13:58] [SPEAKER_00] Though, how do you measure what's deep inside a planet with no surface?

[00:14:04] [SPEAKER_02] Indirectly, using carbon monoxide. Deep down where it's hot, carbon monoxide is created from methane and water. 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,

[00:14:29] [SPEAKER_02] and fed them into chemical models that now account for how conditions change with latitude.

[00:14:35] [SPEAKER_00] And the answer?

[00:14:36] [SPEAKER_02] 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 times. That gives the two planets quite different carbon-to-oxygen ratios.

[00:14:54] [SPEAKER_00] And that points to where they formed.

[00:14:57] [SPEAKER_02] That's the argument. In the disk that formed the planets, different ices froze out at different distances from the young sun. Compared against models of that disk, 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.

[00:15:18] [SPEAKER_00] How firm is it?

[00:15:19] [SPEAKER_02] It's a modeling result, built on indirect measurements. 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.

[00:15:39] [SPEAKER_00] Which is a case for a probe.

[00:15:41] [SPEAKER_02] 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.

[00:15:56] [SPEAKER_00] 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 H-3 rocket lifted off from Tanegashima at 4.41 this morning Japan time. About 71 minutes later, JAXA confirms 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.

[00:16:24] [SPEAKER_00] And if all goes well, a capsule of Phobos landing in South Australia in 2031.

[00:16:31] [SPEAKER_02] 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 1/16th of the width of a human hair. The Whitefield Instruments 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.

[00:17:00] [SPEAKER_00] NASA has also published its detailed analysis of the Artemis II heat shield. After Artemis I, engineers found 103 spots where charred material had broken away. Artemis II, flying a shorter re-entry path, had just 9. That's about a tenfold reduction. Artemis III will carry a reworked heat shield.

[00:17:20] [SPEAKER_02] And keep an eye on ESA's HERA mission. Its second breaking burn, BRM2, is scheduled for this Thursday as it closes in on the Didymos asteroid pair. We'll bring you the result.

[00:17:31] [SPEAKER_00] Next up, let's go and take a look outside.

[00:17:34] [SPEAKER_02] Time for Skywatch. And this week, the meteors win. The Orionid meteor shower peaks over the nights of the 21st and 22nd. These are crumbs of Halley's Comet. And they heat our atmosphere fast at about 66 kilometers per second.

[00:17:50] [SPEAKER_00] But there's a waxing gibbous moon in the way.

[00:17:53] [SPEAKER_02] 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. Northern observers get longer, two to three hours.

[00:18:17] [SPEAKER_00] And the Radiant favors the North.

[00:18:19] [SPEAKER_02] It does. The Radiant, near Orion's Raised Club, is about 40 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. What else is up?

[00:18:47] [SPEAKER_02] Southern skies get Mercury this week. On Wednesday, at the end of civil twilight, it's about 16.5 degrees above the western horizon from Sydney. But only 4 degrees from Los Angeles, 2 from New York, and already set from London. It's fading, so catch it early this week.

[00:19:05] [SPEAKER_00] And Venus disappears.

[00:19:08] [SPEAKER_02] It does. Venus reaches inferior conjunction around the 24th, passing between Earth and the Sun. It'll be only about 6.5 degrees from the Sun, about 40 million kilometers from us, 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. You can never point any optical instrument at or near the Sun without a proper solar filter certified to ISO 12312-2.

[00:19:37] [SPEAKER_00] Anything for the evening crowd?

[00:19:39] [SPEAKER_02] 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.

[00:19:50] [SPEAKER_00] And the pre-dawn planets?

[00:19:52] [SPEAKER_02] 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. They're 10 degrees apart now and closing to just over 1 degree by the 16th of November.

[00:20:10] [SPEAKER_00] And a clock warning?

[00:20:11] [SPEAKER_02] 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. And the full moon? The full moon arrives at 4.11 Universal Time on Monday the 26th.

[00:20:32] [SPEAKER_02] 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 9.11 on Sunday evening in Los Angeles. So your calendar date depends on where you stand. From either hemisphere, it'll look full on both nights.

[00:20:50] [SPEAKER_00] 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.

[00:21:01] [SPEAKER_02] 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 favorite segments started that way.

[00:21:13] [SPEAKER_00] You can find us as astrodailypod on social media. And if you enjoyed the show, a rating or review really helps new listeners find us.

[00:21:22] [SPEAKER_02] Tomorrow, we'll keep watch on the Orionids and on Hera's next breaking burn. I'm Avery. And I'm Anna.

[00:21:30] [SPEAKER_00] Clear skies.

[00:21:32] [SPEAKER_01] Astronomy data. Astronomy data.