Martian Mystery: Unveiling the Secrets of Arsia Mons' Elusive Clouds

0:00 Anna: Every morning in Mars southern spring, a thin white line peels away from a giant volcano and stretches up to 1,800 kilometres across the planet. By afternoon it's gone. For eight years, nobody could make that cloud happen in a computer. This month somebody did. But only by switching on physics that had never been seen working in any planetary atmosphere. Avery: Also today, a hot Jupiter mapped around its edge, angle by angle, with a 650 degree gap between its morning and its evening. Planetary systems that come in two distinct families.

0:36 A shredded galaxy's chemistry rewound from its own debris. And a, uh, European spacecraft hitting the brakes on its way to the asteroid we once hit on purpose. Anna: This is Astronomy daily for Thursday 15th October 2026. Series 5 Episode 205 I'm Anna. Avery: And, um, I'm Avery. Let's get into it. Anna: We start on Mars, on the flank of a volcano called Arsia Mons. It's the SouthernMost of the three great Tharsis volcanoes, sitting just south of the Martian equator. And it rises about 20 kilometres.

1:13 That's more than twice the height of Everest. Avery: And in the mornings during the southern spring and summer it grows a tail. Anna: It does a thin streak of water ice cloud forms on the downwind side around sunrise and over a few hours it stretches out to as much as 1800 kilometres. ESA describes that as roughly twice the length of the United Kingdom. For our North American listeners, think Los Angeles to Denver. Then it detaches from the mountain, drifts and evaporates. Next morning it does it all again, day after day for months.

1:52 And it has a name, the Arcia Mons elongated cloud, or AMEC, to the people who studied it. ESA's Mars Express orbiter drew attention to it in 2018 and the team later found it had been partly photographed by Viking 2 back in the 1970s. So the cloud isn't new. It's been there every Martian year. What's new is that somebody can finally explain it. Avery: Why has that been so hard? Clouds over mountains are hardly exotic. We get them here. Anna: We do Lee clouds, wave clouds, the lenticular clouds that look like flying saucers.

2:28 Air flows over a mountain, gets pushed upward, cools and the water vapour condenses. The standard assumption was that Mars follows the same recipe with one key something for the ice to form on. On Earth that's dust, sea salt, pollen, soot. On Mars it's thought to be dust. Scientists call that heterogeneous nucleation. Ice grows on a seed. Avery: And, um, that recipe didn't work. Anna: The weather models reproduce the waves over archia Mons. They reproduce the cold, but they would not reproduce the cloud.

3:06 Mars Express kept photographing not its shape, not its behaviour. That matters because these are the same models we lean on to understand the whole Martian water cycle. Avery: So what did the new study change? Anna: The paper is led by Jorge Hernandez Bernal of lmd, the Meteorology laboratory of CNRS at Sorbonne University in Paris, with Ani Matinen, Aimeric Spiga and Francois Fouguet. It was published on 7 October in Nature Geoscience. They added a process called homogeneous nucleation. Avery: Which means no seed at all.

3:46 Anna: Exactly. Water vapour going straight to ice crystals in open air with nothing to grow on. It's in the textbooks, but it needs air that is wildly supersaturated, holding far more vapour than it should be able to. So it's normally treated as theory, possible on paper and assumed not to happen in a real atmosphere. When the team switched it on, the cloud appeared. Hernandez Bernal's line was that once they included this physics, the cloud emerged just as they'd hoped. Avery: How supersaturated are we talking?

4:21 Anna: ESA says the relative humidity in the model reaches levels more than 100,000 times what we're used to in daily life on Earth. That number is really the whole story. Nowhere on our planet comes close. Avery: Give me an everyday picture of why the seed matters. Anna: Think about cloud seeding. When people try to coax rain from a cloud on Earth, they spray in tiny particles. Silver iodide, for example. Because water vapour finds it far easier to freeze or condense onto a surface than to organise itself from scratch.

4:54 Building a crystal from nothing means a handful of molecules have to lock together by chance and survive long enough to grow. In ordinary air. That almost never happens, because long before the vapour gets that desperate, it has already found a speck of dust and used it. So homogeneous nucleation isn't forbidden, it's just always beaten to the punch. Avery: Unless there's no time to find the dust. Anna: Exactly. And that's what makes this volcano special. Avery: Why Arsium Mons, though? Why not every mountain on Mars?

5:25 Anna: It's the combination. The Martian atmosphere is very thin and Arsia Mons is very tall. Wind blowing past the volcano sets up a strong atmospheric wave that lifts moist air several kilometres in a matter of minutes. In the model, the temperature drops by about 30 degrees in 10 minutes. Air that cools that fast never gets the chance to use the dust. The humidity simply rockets until the vapour freezes on its own. Avery: Okay, so the mountain is effectively a Anna: giant cloud chamber that's a nice way to put it.

6:03 A natural laboratory that happens to be 20 kilometres tall. Hernandez Bernal made a lovely point in the Spanish press. If earth had a 20,000 metre mountain in the middle of Siberia, we might occasionally see something similar, though not nearly as long. Avery: Let's be careful here. Has anyone actually watched this happen on Mars? Anna: No, and that's the caveat we need to keep. Nobody has measured individual ice crystals forming without dust. What the team has shown is that a model only reproduces this cloud when that physics is switched on.

6:43 The paper's own title is Careful. It says homogeneous nucleation is suggested by these clouds. Not proven. And ESA notes that some features of the modelled cloud still don't exactly match the observations. Avery: So it's sufficiency, not proof. Anna: It's the same distinction we drew back in episode 197 with the little red dot simulations. A model that reproduces an observation tells you a mechanism can do the job. It doesn't rule out every other mechanism, but the team did try the conventional physics first and it failed.

7:21 That counts for something. Avery: Avery. And, um, how fresh is it? Anna: Very. The preprint went up on 29 September, about a week before the journal version. So this is genuinely new work. Avery: Who should care? Beyond the people who study Martian weather? Anna: Three groups, I'd say. First, Mars modellers. If one cloud needs this physics, other high cold Martian clouds might too, and that changes how water moves around the planet. Second, people who study Earth. The authors say the result is relevant to certain clouds here as well.

7:58 And third, exoplanet scientists. When we build models of atmospheres around other stars, we tend to prune out processes we think are too unlikely to matter. ESA's Mars Express project scientist Colin Wilson put it neatly. Understanding an exotic Martian cloud took exotic physics, and that may be true elsewhere in the cosmos too. Avery: And it took an, um, old spacecraft to catch it. Anna: Mars Express, launched in 2003. Its orbit is highly elliptical, which lets it see Mars in the morning. And very few orbiters can do that.

8:31 ESA's ExoMars Trace Gas Orbiter is one of the others. This study used three Mars Express instruments. The Visual Monitoring Camera, nicknamed the Mars Webcam, the High Resolution Stereo Camera and the Omega Spectrometer. It's also the first time ESA has released stereo camera imagery of this cloud. It's a good reminder that old missions watching patiently are how you catch things that only exist for a few hours Avery: a day and a southern angle to Anna: finish a small one. But true, this is a southern spring cloud it only appears in spring and summer in Mars's southern hemisphere, the same season we're enjoying here in Sydney right now, just on a Martian calendar that runs nearly twice as long as ours, Avery: from a cloud on Mars to the edge of a planet around another star.

9:19 Wasp 17B is a hot Jupiter in the constellation Scorpius. It has about half of Jupiter's mass, but nearly twice its width. So it's enormously puffed up at, uh, around 1700 degrees, which makes it a Anna: gift for transit studies. Avery: Exactly. When a planet crosses its star, a thin ring of atmosphere around its edge lets starlight filter through and different molecules absorb different colours. Usually, astronomers average that whole ring into one spectrum. Lately, they've split it into two the morning side, where gas is arriving from the night, and the evening side, where it's leaving the day.

9:58 Anna: And this team went further, much further. Avery: The paper published yesterday in Nature Astronomy, is led by Lily Alderson with Hannah Wakeford and David Grant of the University of Bristol and colleagues, including Tiffany Kataria, uh, at jpl. They used a technique called transmission strings, which Grant and wakeford developed in 2023. Instead of assuming the planet's silhouette is a perfect circle, it lets the outline bulge and dip and then reads off a separate spectrum at any angle around the edge.

10:29 They stepped from the morning equator over the pole to the evening equator in 20 degree increments. Anna: Using which instruments? Avery: Three of Niris, NirSpec and Miri, covering 0.6 to 12 microns. And the headline is a temperature gradient around the limb. The morning edge comes out near 860Kelvin, the evening edge near 1520Kelvin. That's a contrast of about 650 degrees, the largest limb contrast measured so far. For comparison, WASP39B showed about 180. Anna: Does the chemistry change, too? Avery: Water and carbon dioxide stay roughly constant all the way around, which is what circulation models predict.

11:14 There are hints of more methane toward the poles, but the authors say plainly that isn't well constrained. And the temperatures agree with Webb's separate maps of the dayside and the night side. So WASP 17B is now one of the most completely mapped exoplanets there is. Caveats 2 they assumed the northern and southern halves of the planet match to keep the computation manageable. And each wavelength fit still took about two weeks on 40 processors and a UH disclosure. The Webb observations were made in March 2023.

11:46 The paper is new this week, but the light is three and a half Anna: years old, and for anyone wanting to Avery: find the star, it sits at minus 28 degrees. So it's a southern object. Tonight it's about 40 degrees up at sunset from Sydney. From Los Angeles it's 14 degrees up and gone by full dark. And from London it barely clears the horizon. It's 11th magnitude. So you'll need a telescope. But it's a nice thought that the best mapped edge of any alien world is hanging over Australia this evening.

12:20 Anna: Next, a uh, big picture look at planetary systems. We now know of more than 1,000 stars with more than one planet. And um, they look wildly different from one another. The question has always been whether that variety is baked in at birth or sculpted later. Avery: And um, the answer is. Anna: According to a team led by Diego Torrini of inaf, the Italian National Institute for Astrophysics. With Camilla Danielski of the University of Valencia, the answer is both. They studied 298 Multi Planet Systems whose masses and orbits are well measured.

12:55 And published on Tuesday in Nature Astronomy. When they sorted them by total planet mass relative to the star, the system split into two clear groups. Avery: Two families. Anna: Two families. The lower mass family is compact. Planets of similar size sit in tight orbits, often far closer to their star than Mercury is to ours. That's the so called peas in a pod pattern. Kepler revealed their host stars are the Sun's mass or smaller, with the Sun's chemistry or less. And dynamically they're fairly calm, comparable to our own solar system.

13:29 Avery: And um, the other family heavyweights. Anna: Total mass is typically 2 to 10 times our solar systems around stars that are more massive and richer in heavy elements than the sun. Combining stellar mass and metallicity, their birth discs held roughly three times more dust and ice. And their orbits are about 10 times more stirred up, more eccentric and more tilted than in the Kolm family. Avery: Which suggests what violence. Anna: Danae Polycroni of INAF led the simulations. And the systems that best match the heavy family are the ones that were unstable from the start.

14:08 Too much planet building material makes planets too big to sit peacefully. They scatter, collide and fling each other out. In those runs, most systems loft at least one planet. Avery: That uh, rings a bell. Anna: It does. Back in episode 195 we covered Batygin's work on how our own solar system could be destabilised far in the future. This is the same physics running at the start of a system's life instead of the end. Perini's phrase for the link between star and planets is genetic. One what you're born with shapes what you become.

14:46 Avery: Caveats. Anna: Danielski herself flagged the big one. The team's main worry was whether the split was real or just a product of how we find planets. Because transits favour compact systems and radial velocities favour massive ones. They tested four independently selected samples and the split held in all of them. But we still can't easily see wide systems of modest planets. So a, uh, middle population could be hiding. And a small. The planet catalogue was downloaded in October last year. Avery: And where does that leave us?

15:23 Anna: Somewhere in between. Our solar system's calm history matches the lower mass family, but it's much more spread out than those compact systems. It's another reminder not to treat the sun and its planets as the universal template. Avery: Our third story is galactic archaeology. The Sagittarius dwarf galaxy was only discovered in 1994, hiding behind the crowded centre of the Milky Way. And it's being torn apart. Its stars now stream around our galaxy in long ribbons that wrap the sky. Anna: So can you work out what it looked like before it was shredded?

16:02 Avery: That's the challenge. And positions and speeds of the debris alone don't get you very far. A team led by Ziji Guo and Shangsheng Zhu at the National Astronomical Observatories of China added chemistry. Using STREAM members from China's LAMOS survey telescope, they looked at how the abundance of heavy elements, the metallicity, changes across the width of the streams. Anna: And it changes. Avery: It does. There's a vertical metallicity gradient across the streams. The team ran large computer simulations of the dwarf galaxy falling into the Milky Way.

16:38 And the only way to reproduce that gradient was if Sagittarius started out as a disc whose metallicity had an unusual shape. Normally, a galaxy disc is most metal rich at the centre and gets poorer as you go outward. Sagittarius instead looks like an inverted V. Metallicity rises from the centre, peaks partway out, then falls again. Anna: Is that weird? Avery: Rare, but not unique. They searched 217 dwarf galaxies in the manga survey and found three with the same shape. And intriguingly, the Milky Way's own integrated metallicity profile looks similar.

17:18 So the authors raise the question of whether the two galaxies have, in some sense, evolved together. Anna: Caveats. Avery: It's, uh, a reconstruction from simulations. So it shows a disc like that can reproduce the streams. Not that nothing else could. And a, uh, disclosure on timing. The paper was published this week, but it was first submitted in July last year. So it's been more than a year in review. Anna: There's a callback here, too. Avery: There is? In episode 195, we covered Rubin GC1, a newly found globular cluster that Rubin data traced back to Sagittarius.

17:59 Every piece of this galaxy we recover helps rebuild the hole. And the southern andal is literal. The core of Sagittarius sits, uh, at minus 30 degrees, about 60 degrees up from Sydney at full dark tonight against 22 from Los Angeles and 5 from London. Its nucleus is the globular cluster Messier 54, an easy telescope target just below the teapot. Anna: Three quick ones. First, AESA's HERA mission was due to fire its thrusters for 93 minutes in the early hours of today, European time. It was the first of its braking burns on approach to Didymus and Dimorphous, the asteroid pair NASA's DART spacecraft struck in 2022, shortening the moonlet's orbit by about 33 minutes.

18:47 Avery: As we record, ESA hasn't yet confirmed the result. Its new NORSHA antenna in Western Australia is among the stations tracking it and we'll bring you the outcome tomorrow. Arrival at the asteroids is due in November 2. Anna: A cargo Dragon launched from Florida on Tuesday on NASA's CRS 35 mission, carrying more than three tons of supplies and the final pair of rollout solar arrays for the International Space Station. It's due to dock at 11:00am Eastern Time today, which is 2:00 clock tomorrow morning in Sydney.

19:18 Avery: And third, a, uh, farewell after more than 33 years. NASA's Astrophysics Data System, the digital library almost every astronomer uses, closes its doors in November. It's being replaced by a broader successor called Science Explorer, or SciEx. Nature Astronomy marked the occasion with an editorial this week. The papers aren't going anywhere, they're just moving house. Anna: Time to look up. And tonight, the evening western sky splits the hemispheres neatly. Mercury reached its greatest distance from the sun on Monday and is still well placed for southern observers from Sydney.

19:51 Tonight it's about 24 degrees up at sunset and sets roughly two hours after the Sun. Avery: That's a terrific apparition, one of the Anna: best of the year for the south. Venus is below it, about 14 degrees up at sunset from Sydney and setting an hour and a quarter after the Sun. In North America, it's a different story from Los angeles. Mercury is 10 degrees up at sunset and sets within the hour, while Venus sets with the Sun. From New York, Mercury is about 7 degrees up and Venus is already gone from London, Mercury barely clears the horizon Avery: and Venus is about to leave us altogether.

20:29 Anna: It reaches inferior conjunction on the 24th of October, passing between us and the Sun. Right now it's a thin Crescent, about 4% lit, but a large one, merely a full arcminute across. Avery: Which brings us to our standing safety note. And um, this week it matters as Venus slides toward the Sun. Please don't go hunting for it with binoculars or a telescope while the sun is above the horizon. Sweeping optics across the sun can damage your eyes instantly and permanently. Never look directly at the sun for any solar viewing.

21:06 Use only eclipse glasses certified to ISO 1M 23122 and fit telescopes and binoculars with proper certified solar filters over the front. Anna: The crescent Moon is in the evening sky too in Ophiuchus tonight, about a quarter lit from Sydney it's more than 50 degrees up at sunset from New York it's about 17. First quarter falls on Sunday the 18th in the Americas and early Monday morning here in Australia. Avery: And two of today's stories are overhead. For the south they are. Anna: The teapot of Sagittarius is high after dark from Australia with the shredded Sagittarius dwarf galaxy sitting behind it about 60 degrees up at full dark from Sydney.

21:52 And wasp 17 is still about 24 degrees up in Scorpius at full dark. The honest balance is that this has been a southern evening sky all week. Avery: So what does the north get the mornings? Anna: Saturn is just past opposition, rising in the east at sunset, shining at magnitude 0.4 and visible all night everywhere. Its M rings are tilted only about 7 degrees towards us this year, so in a small telescope they look like a thin bright blade rather than the wide ellipse in the textbook pictures. But the real show is before dawn.

22:32 Mars and Jupiter are about 12 degrees apart and closing from New York at nautical dawn tomorrow. Mars is about 56 degrees up and Jupiter about 46 from Sydney they're 23 and 19. They'll be just 1.2 degrees apart on the 16th of November, so watch the gap shrink week by week. Avery: And a meteor shower on the way. Anna: The Orionids peak on the night of the 21st into the 22nd of October. They're dust from Hallie's Comet, the same comet that gives us the Eta uh, Aquarids in May. The radiant is in Orion and rises around midnight, so the pre dawn hours are best.

23:15 The moon will be about 3/4 lit, but setting in the early hours which help the north has the edge here too. At dawn the radiant is about 64 degrees up from New York against 40 from Sydney. Southern observers still see respectable show just with fewer meteors per hour. Avery: Evenings for the south, mornings for the north. Anna: That's the shape of it this week. Some weeks it runs the other way. Avery: That's Astronomy AstroDailyPod for today. We started with a 20 kilometre volcano making clouds the textbook said shouldn't form, then mapped the edge of a planet in Scorpius, sorted planetary systems into two families and rewound a shredded galaxy.

24:00 Anna: Tomorrow we'll have the outcome of Hera's first breaking burn and next Monday we'll preview Japan's Martian Moons exploration mission MMX UM M ahead of its launch. Avery: You'll find the show notes, links to every paper and the full back catalogue at astronomydaily IO that's also where you can sign up for the newsletter or send us a question through the contact form. Several of our best segments have started that way. Anna: We are AstroDaily Pod on social media and Astronomy AstroDailyPod is part of the bytes.com podcast network.

24:37 If you're enjoying the show, a uh, rating or review really does help other people find us. Thanks for listening. I'm Anna. Avery: And I'm Avery. Clear Skies Astronomy Day Mhm. The stories.

25:06 We told.