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00:00:00 --> 00:00:03 Anna: Every morning in Mars southern spring, a
00:00:03 --> 00:00:05 thin white line peels away from a giant
00:00:05 --> 00:00:08 volcano and stretches up to 1
00:00:08 --> 00:00:10 kilometres across the planet. By afternoon
00:00:10 --> 00:00:13 it's gone. For eight years, nobody could make
00:00:13 --> 00:00:16 that cloud happen in a computer. This month
00:00:16 --> 00:00:18 somebody did. But only by switching on
00:00:18 --> 00:00:21 physics that had never been seen working in
00:00:21 --> 00:00:22 any planetary atmosphere.
00:00:22 --> 00:00:25 Avery: Also today, a hot Jupiter mapped around
00:00:25 --> 00:00:28 its edge, angle by angle, with a
00:00:28 --> 00:00:31 650 degree gap between its morning
00:00:31 --> 00:00:34 and its evening. Planetary systems that
00:00:34 --> 00:00:36 come in two distinct families. A
00:00:36 --> 00:00:39 shredded galaxy's chemistry rewound from its
00:00:39 --> 00:00:42 own debris. And a, uh, European spacecraft
00:00:42 --> 00:00:44 hitting the brakes on its way to the asteroid
00:00:44 --> 00:00:45 we once hit on purpose.
00:00:46 --> 00:00:48 Anna: This is Astronomy daily for Thursday
00:00:48 --> 00:00:51 15th October 2026.
00:00:51 --> 00:00:53 Series 5 Episode 205
00:00:54 --> 00:00:55 I'm Anna.
00:00:55 --> 00:00:58 Avery: And, um, I'm Avery. Let's get into it.
00:00:58 --> 00:01:01 Anna: We start on Mars, on the flank of a
00:01:01 --> 00:01:04 volcano called Arsia Mons. It's
00:01:04 --> 00:01:06 the SouthernMost of the three great Tharsis
00:01:06 --> 00:01:09 volcanoes, sitting just south of the
00:01:09 --> 00:01:12 Martian equator. And it rises about
00:01:12 --> 00:01:14 20 kilometres. That's more than twice the
00:01:14 --> 00:01:15 height of Everest.
00:01:16 --> 00:01:18 Avery: And in the mornings during the southern
00:01:18 --> 00:01:21 spring and summer it grows a tail.
00:01:21 --> 00:01:24 Anna: It does a thin streak of
00:01:24 --> 00:01:27 water ice cloud forms on the downwind side
00:01:27 --> 00:01:29 around sunrise and over a few hours
00:01:30 --> 00:01:32 it stretches out to as much as 1800
00:01:32 --> 00:01:35 kilometres. ESA describes that as roughly
00:01:35 --> 00:01:38 twice the length of the United Kingdom. For
00:01:38 --> 00:01:41 our North American listeners, think Los
00:01:41 --> 00:01:43 Angeles to Denver. Then it
00:01:43 --> 00:01:45 detaches from the mountain, drifts and
00:01:45 --> 00:01:48 evaporates. Next morning it does it
00:01:48 --> 00:01:51 all again, day after day for months.
00:01:52 --> 00:01:55 And it has a name, the Arcia Mons
00:01:55 --> 00:01:58 elongated cloud, or AMEC, to the people
00:01:58 --> 00:02:01 who studied it. ESA's Mars Express orbiter
00:02:01 --> 00:02:03 drew attention to it in 2018 and the
00:02:03 --> 00:02:05 team later found it had been partly
00:02:05 --> 00:02:08 photographed by Viking 2 back in the
00:02:08 --> 00:02:10 1970s. So the cloud isn't new.
00:02:10 --> 00:02:13 It's been there every Martian year. What's
00:02:13 --> 00:02:16 new is that somebody can finally explain it.
00:02:16 --> 00:02:19 Avery: Why has that been so hard? Clouds over
00:02:19 --> 00:02:21 mountains are hardly exotic. We get them
00:02:21 --> 00:02:22 here.
00:02:22 --> 00:02:25 Anna: We do Lee clouds, wave clouds,
00:02:25 --> 00:02:27 the lenticular clouds that look like flying
00:02:27 --> 00:02:30 saucers. Air flows over a mountain,
00:02:30 --> 00:02:33 gets pushed upward, cools and the water
00:02:33 --> 00:02:36 vapour condenses. The standard assumption
00:02:36 --> 00:02:38 was that Mars follows the same recipe
00:02:39 --> 00:02:41 with one key something for the
00:02:41 --> 00:02:44 ice to form on. On Earth that's
00:02:44 --> 00:02:47 dust, sea salt, pollen, soot.
00:02:47 --> 00:02:49 On Mars it's thought to be dust.
00:02:50 --> 00:02:52 Scientists call that heterogeneous
00:02:52 --> 00:02:55 nucleation. Ice grows on a
00:02:55 --> 00:02:55 seed.
00:02:56 --> 00:02:57 Avery: And, um, that recipe didn't work.
00:02:58 --> 00:03:00 Anna: The weather models reproduce the waves over
00:03:00 --> 00:03:03 archia Mons. They reproduce the cold,
00:03:04 --> 00:03:06 but they would not reproduce the cloud. Mars
00:03:06 --> 00:03:09 Express kept photographing not its
00:03:09 --> 00:03:11 shape, not its behaviour. That
00:03:11 --> 00:03:14 matters because these are the same models we
00:03:14 --> 00:03:17 lean on to understand the whole Martian water
00:03:17 --> 00:03:18 cycle.
00:03:18 --> 00:03:20 Avery: So what did the new study change?
00:03:20 --> 00:03:23 Anna: The paper is led by Jorge Hernandez
00:03:23 --> 00:03:25 Bernal of lmd, the Meteorology
00:03:25 --> 00:03:28 laboratory of CNRS at Sorbonne University
00:03:28 --> 00:03:31 in Paris, with Ani Matinen,
00:03:31 --> 00:03:34 Aimeric Spiga and Francois Fouguet.
00:03:35 --> 00:03:37 It was published on 7 October in Nature
00:03:37 --> 00:03:40 Geoscience. They added a process
00:03:40 --> 00:03:42 called homogeneous nucleation.
00:03:43 --> 00:03:46 Avery: Which means no seed at all.
00:03:46 --> 00:03:49 Anna: Exactly. Water vapour going
00:03:49 --> 00:03:52 straight to ice crystals in open air
00:03:52 --> 00:03:55 with nothing to grow on. It's in the
00:03:55 --> 00:03:58 textbooks, but it needs air that is wildly
00:03:58 --> 00:04:00 supersaturated, holding far more vapour than
00:04:00 --> 00:04:03 it should be able to. So it's normally
00:04:03 --> 00:04:06 treated as theory, possible on paper and
00:04:06 --> 00:04:09 assumed not to happen in a real atmosphere.
00:04:09 --> 00:04:11 When the team switched it on, the cloud
00:04:11 --> 00:04:14 appeared. Hernandez Bernal's line was that
00:04:14 --> 00:04:17 once they included this physics, the cloud
00:04:17 --> 00:04:18 emerged just as they'd hoped.
00:04:19 --> 00:04:21 Avery: How supersaturated are we talking?
00:04:21 --> 00:04:24 Anna: ESA says the relative humidity in the model
00:04:24 --> 00:04:27 reaches levels more than 100 times
00:04:27 --> 00:04:29 what we're used to in daily life on Earth.
00:04:29 --> 00:04:31 That number is really the whole story.
00:04:31 --> 00:04:33 Nowhere on our planet comes close.
00:04:33 --> 00:04:36 Avery: Give me an everyday picture of why the
00:04:36 --> 00:04:37 seed matters.
00:04:38 --> 00:04:41 Anna: Think about cloud seeding. When people try to
00:04:41 --> 00:04:43 coax rain from a cloud on Earth, they spray
00:04:43 --> 00:04:46 in tiny particles. Silver iodide, for
00:04:46 --> 00:04:48 example. Because water vapour finds it far
00:04:48 --> 00:04:51 easier to freeze or condense onto a surface
00:04:51 --> 00:04:53 than to organise itself from scratch.
00:04:54 --> 00:04:56 Building a crystal from nothing means a
00:04:56 --> 00:04:58 handful of molecules have to lock together by
00:04:58 --> 00:05:01 chance and survive long enough to grow. In
00:05:01 --> 00:05:03 ordinary air. That almost never happens,
00:05:04 --> 00:05:05 because long before the vapour gets that
00:05:05 --> 00:05:08 desperate, it has already found a speck of
00:05:08 --> 00:05:10 dust and used it. So homogeneous
00:05:10 --> 00:05:13 nucleation isn't forbidden, it's just always
00:05:13 --> 00:05:14 beaten to the punch.
00:05:14 --> 00:05:17 Avery: Unless there's no time to find the dust.
00:05:17 --> 00:05:20 Anna: Exactly. And that's what makes this volcano
00:05:20 --> 00:05:20 special.
00:05:21 --> 00:05:24 Avery: Why Arsium Mons, though? Why not every
00:05:24 --> 00:05:25 mountain on Mars?
00:05:25 --> 00:05:28 Anna: It's the combination. The Martian
00:05:28 --> 00:05:31 atmosphere is very thin and Arsia
00:05:31 --> 00:05:33 Mons is very tall. Wind
00:05:33 --> 00:05:36 blowing past the volcano sets up a strong
00:05:36 --> 00:05:39 atmospheric wave that lifts moist air
00:05:39 --> 00:05:42 several kilometres in a matter of minutes. In
00:05:42 --> 00:05:45 the model, the temperature drops by about 30
00:05:45 --> 00:05:48 degrees in 10 minutes. Air that cools that
00:05:48 --> 00:05:50 fast never gets the chance to use the dust.
00:05:51 --> 00:05:53 The humidity simply rockets until the vapour
00:05:53 --> 00:05:55 freezes on its own.
00:05:55 --> 00:05:58 Avery: Okay, so the mountain is effectively
00:05:59 --> 00:05:59 a
00:05:59 --> 00:06:02 Anna: giant cloud chamber that's a nice
00:06:02 --> 00:06:05 way to put it. A natural laboratory
00:06:05 --> 00:06:08 that happens to be 20 kilometres tall.
00:06:09 --> 00:06:11 Hernandez Bernal made a lovely point in the
00:06:11 --> 00:06:14 Spanish press. If earth had a
00:06:14 --> 00:06:16 20 metre mountain in the middle of
00:06:16 --> 00:06:19 Siberia, we might occasionally see
00:06:19 --> 00:06:22 something similar, though not nearly as long.
00:06:23 --> 00:06:25 Avery: Let's be careful here. Has anyone
00:06:25 --> 00:06:28 actually watched this happen on Mars?
00:06:29 --> 00:06:31 Anna: No, and that's the caveat we need to keep.
00:06:32 --> 00:06:34 Nobody has measured individual ice crystals
00:06:34 --> 00:06:37 forming without dust. What the team has
00:06:37 --> 00:06:40 shown is that a model only reproduces this
00:06:40 --> 00:06:42 cloud when that physics is switched on.
00:06:43 --> 00:06:46 The paper's own title is Careful. It says
00:06:46 --> 00:06:49 homogeneous nucleation is suggested by
00:06:49 --> 00:06:51 these clouds. Not proven. And
00:06:52 --> 00:06:54 ESA notes that some features of the modelled
00:06:54 --> 00:06:57 cloud still don't exactly match the
00:06:57 --> 00:06:57 observations.
00:06:58 --> 00:07:01 Avery: So it's sufficiency, not proof.
00:07:01 --> 00:07:04 Anna: It's the same distinction we drew back in
00:07:04 --> 00:07:07 episode 197 with the little red dot
00:07:07 --> 00:07:10 simulations. A model that reproduces
00:07:10 --> 00:07:12 an observation tells you a mechanism can do
00:07:12 --> 00:07:15 the job. It doesn't rule out every other
00:07:15 --> 00:07:18 mechanism, but the team did try
00:07:18 --> 00:07:21 the conventional physics first and it failed.
00:07:21 --> 00:07:22 That counts for something.
00:07:23 --> 00:07:25 Avery: Avery. And, um, how fresh is it?
00:07:25 --> 00:07:26 Anna: Very.
00:07:26 --> 00:07:29 The preprint went up on 29
00:07:29 --> 00:07:31 September, about a week before the journal
00:07:31 --> 00:07:34 version. So this is genuinely new work.
00:07:35 --> 00:07:37 Avery: Who should care? Beyond the people who study
00:07:37 --> 00:07:38 Martian weather?
00:07:38 --> 00:07:41 Anna: Three groups, I'd say. First,
00:07:41 --> 00:07:44 Mars modellers. If one cloud needs
00:07:44 --> 00:07:47 this physics, other high cold Martian
00:07:47 --> 00:07:49 clouds might too, and that changes how water
00:07:49 --> 00:07:52 moves around the planet. Second, people
00:07:52 --> 00:07:55 who study Earth. The authors say the result
00:07:55 --> 00:07:57 is relevant to certain clouds here as well.
00:07:58 --> 00:08:01 And third, exoplanet scientists. When
00:08:01 --> 00:08:03 we build models of atmospheres around other
00:08:03 --> 00:08:05 stars, we tend to prune out processes we
00:08:05 --> 00:08:08 think are too unlikely to matter. ESA's Mars
00:08:08 --> 00:08:11 Express project scientist Colin Wilson put it
00:08:11 --> 00:08:14 neatly. Understanding an exotic Martian
00:08:14 --> 00:08:16 cloud took exotic physics, and that may be
00:08:16 --> 00:08:18 true elsewhere in the cosmos too.
00:08:19 --> 00:08:21 Avery: And it took an, um, old spacecraft to catch
00:08:21 --> 00:08:21 it.
00:08:21 --> 00:08:24 Anna: Mars Express, launched in 2003.
00:08:24 --> 00:08:26 Its orbit is highly elliptical, which lets it
00:08:26 --> 00:08:29 see Mars in the morning. And very few
00:08:29 --> 00:08:32 orbiters can do that. ESA's ExoMars
00:08:32 --> 00:08:35 Trace Gas Orbiter is one of the others. This
00:08:35 --> 00:08:37 study used three Mars Express instruments.
00:08:37 --> 00:08:40 The Visual Monitoring Camera, nicknamed the
00:08:40 --> 00:08:42 Mars Webcam, the High Resolution Stereo
00:08:42 --> 00:08:45 Camera and the Omega Spectrometer. It's
00:08:45 --> 00:08:47 also the first time ESA has released stereo
00:08:47 --> 00:08:50 camera imagery of this cloud. It's a good
00:08:50 --> 00:08:52 reminder that old missions watching patiently
00:08:52 --> 00:08:54 are how you catch things that only exist for
00:08:54 --> 00:08:55 a few hours
00:08:55 --> 00:08:57 Avery: a day and a southern angle to
00:08:57 --> 00:09:00 Anna: finish a small one. But true,
00:09:01 --> 00:09:03 this is a southern spring cloud it
00:09:03 --> 00:09:06 only appears in spring and summer in Mars's
00:09:06 --> 00:09:08 southern hemisphere, the same season we're
00:09:08 --> 00:09:11 enjoying here in Sydney right now, just on a
00:09:11 --> 00:09:13 Martian calendar that runs nearly twice as
00:09:13 --> 00:09:14 long as ours,
00:09:14 --> 00:09:17 Avery: from a cloud on Mars to the edge of a planet
00:09:17 --> 00:09:20 around another star. Wasp
00:09:20 --> 00:09:22 17B is a hot Jupiter in the constellation
00:09:22 --> 00:09:25 Scorpius. It has about half of Jupiter's
00:09:25 --> 00:09:28 mass, but nearly twice its width. So it's
00:09:28 --> 00:09:30 enormously puffed up at, uh, around
00:09:30 --> 00:09:33 1700 degrees, which makes it a
00:09:33 --> 00:09:35 Anna: gift for transit studies.
00:09:36 --> 00:09:38 Avery: Exactly. When a planet crosses its star,
00:09:38 --> 00:09:41 a thin ring of atmosphere around its edge
00:09:41 --> 00:09:43 lets starlight filter through and different
00:09:43 --> 00:09:46 molecules absorb different colours. Usually,
00:09:46 --> 00:09:49 astronomers average that whole ring into one
00:09:49 --> 00:09:51 spectrum. Lately, they've split it into two
00:09:52 --> 00:09:54 the morning side, where gas is arriving from
00:09:54 --> 00:09:56 the night, and the evening side, where it's
00:09:56 --> 00:09:57 leaving the day.
00:09:58 --> 00:10:01 Anna: And this team went further, much
00:10:01 --> 00:10:01 further.
00:10:01 --> 00:10:04 Avery: The paper published yesterday in Nature
00:10:04 --> 00:10:07 Astronomy, is led by Lily Alderson with
00:10:07 --> 00:10:09 Hannah Wakeford and David Grant of the
00:10:09 --> 00:10:10 University of Bristol and colleagues,
00:10:10 --> 00:10:13 including Tiffany Kataria, uh, at jpl. They
00:10:13 --> 00:10:16 used a technique called transmission strings,
00:10:16 --> 00:10:18 which Grant and wakeford developed in 2023.
00:10:19 --> 00:10:21 Instead of assuming the planet's silhouette
00:10:21 --> 00:10:23 is a perfect circle, it lets the outline
00:10:23 --> 00:10:26 bulge and dip and then reads off a separate
00:10:26 --> 00:10:29 spectrum at any angle around the edge. They
00:10:29 --> 00:10:31 stepped from the morning equator over the
00:10:31 --> 00:10:34 pole to the evening equator in 20 degree
00:10:34 --> 00:10:34 increments.
00:10:35 --> 00:10:36 Anna: Using which instruments?
00:10:36 --> 00:10:39 Avery: Three of Niris, NirSpec
00:10:39 --> 00:10:42 and Miri, covering 0.6
00:10:42 --> 00:10:45 to 12 microns. And the headline
00:10:45 --> 00:10:47 is a temperature gradient around the limb.
00:10:47 --> 00:10:49 The morning edge comes out near
00:10:50 --> 00:10:52 860Kelvin, the evening edge
00:10:52 --> 00:10:55 near 1520Kelvin.
00:10:55 --> 00:10:58 That's a contrast of about 650
00:10:58 --> 00:11:01 degrees, the largest limb contrast measured
00:11:01 --> 00:11:03 so far. For comparison,
00:11:03 --> 00:11:05 WASP39B showed about
00:11:05 --> 00:11:06 180.
00:11:06 --> 00:11:08 Anna: Does the chemistry change, too?
00:11:08 --> 00:11:10 Avery: Water and carbon dioxide stay roughly
00:11:10 --> 00:11:13 constant all the way around, which is what
00:11:13 --> 00:11:15 circulation models predict. There are hints
00:11:15 --> 00:11:17 of more methane toward the poles, but the
00:11:17 --> 00:11:19 authors say plainly that isn't well
00:11:19 --> 00:11:21 constrained. And the temperatures agree with
00:11:21 --> 00:11:23 Webb's separate maps of the dayside and the
00:11:23 --> 00:11:26 night side. So WASP 17B is now one of
00:11:26 --> 00:11:28 the most completely mapped exoplanets there
00:11:28 --> 00:11:31 is. Caveats 2 they
00:11:31 --> 00:11:34 assumed the northern and southern halves of
00:11:34 --> 00:11:36 the planet match to keep the computation
00:11:36 --> 00:11:39 manageable. And each wavelength fit still
00:11:39 --> 00:11:41 took about two weeks on 40 processors
00:11:42 --> 00:11:44 and a UH disclosure. The Webb observations
00:11:44 --> 00:11:47 were made in March 2023. The paper is new
00:11:47 --> 00:11:49 this week, but the light is three and a half
00:11:49 --> 00:11:52 Anna: years old, and for anyone wanting to
00:11:52 --> 00:11:55 Avery: find the star, it sits at
00:11:55 --> 00:11:58 minus 28 degrees. So it's a southern
00:11:58 --> 00:12:00 object. Tonight it's about 40 degrees up
00:12:00 --> 00:12:03 at sunset from Sydney. From Los Angeles
00:12:03 --> 00:12:06 it's 14 degrees up and gone by full dark.
00:12:07 --> 00:12:10 And from London it barely clears the horizon.
00:12:10 --> 00:12:13 It's 11th magnitude. So you'll need a
00:12:13 --> 00:12:16 telescope. But it's a nice thought that the
00:12:16 --> 00:12:18 best mapped edge of any alien world is
00:12:18 --> 00:12:20 hanging over Australia this evening.
00:12:20 --> 00:12:23 Anna: Next, a uh, big picture look at planetary
00:12:23 --> 00:12:26 systems. We now know of more than 1 stars
00:12:26 --> 00:12:28 with more than one planet. And um, they look
00:12:28 --> 00:12:30 wildly different from one another. The
00:12:30 --> 00:12:32 question has always been whether that variety
00:12:32 --> 00:12:35 is baked in at birth or sculpted later.
00:12:35 --> 00:12:36 Avery: And um, the answer is.
00:12:37 --> 00:12:40 Anna: According to a team led by Diego Torrini of
00:12:40 --> 00:12:42 inaf, the Italian National Institute for
00:12:42 --> 00:12:45 Astrophysics. With Camilla Danielski of
00:12:45 --> 00:12:48 the University of Valencia, the answer is
00:12:48 --> 00:12:48 both.
00:12:49 --> 00:12:52 They studied 298 Multi Planet
00:12:52 --> 00:12:54 Systems whose masses and orbits are well
00:12:54 --> 00:12:57 measured. And published on Tuesday in Nature
00:12:57 --> 00:12:59 Astronomy. When they sorted them by total
00:12:59 --> 00:13:02 planet mass relative to the star, the system
00:13:02 --> 00:13:04 split into two clear groups.
00:13:05 --> 00:13:06 Avery: Two families.
00:13:07 --> 00:13:09 Anna: Two families. The lower mass family is
00:13:09 --> 00:13:12 compact. Planets of similar size sit in tight
00:13:12 --> 00:13:14 orbits, often far closer to their star than
00:13:14 --> 00:13:17 Mercury is to ours. That's the so called
00:13:17 --> 00:13:20 peas in a pod pattern. Kepler revealed their
00:13:20 --> 00:13:22 host stars are the Sun's mass or smaller,
00:13:22 --> 00:13:25 with the Sun's chemistry or less. And
00:13:25 --> 00:13:27 dynamically they're fairly calm, comparable
00:13:27 --> 00:13:28 to our own solar system.
00:13:29 --> 00:13:32 Avery: And um, the other family heavyweights.
00:13:33 --> 00:13:36 Anna: Total mass is typically 2 to 10 times
00:13:36 --> 00:13:39 our solar systems around stars that are
00:13:39 --> 00:13:41 more massive and richer in heavy elements
00:13:41 --> 00:13:44 than the sun. Combining stellar mass
00:13:44 --> 00:13:46 and metallicity, their birth discs held
00:13:46 --> 00:13:49 roughly three times more dust and ice. And
00:13:49 --> 00:13:52 their orbits are about 10 times more stirred
00:13:52 --> 00:13:54 up, more eccentric and more tilted than in
00:13:54 --> 00:13:55 the Kolm family.
00:13:56 --> 00:13:59 Avery: Which suggests what violence.
00:13:59 --> 00:14:01 Anna: Danae Polycroni of INAF led the
00:14:01 --> 00:14:04 simulations. And the systems that best match
00:14:04 --> 00:14:06 the heavy family are the ones that were
00:14:06 --> 00:14:09 unstable from the start. Too much planet
00:14:09 --> 00:14:11 building material makes planets too big to
00:14:11 --> 00:14:14 sit peacefully. They scatter, collide and
00:14:14 --> 00:14:17 fling each other out. In those runs, most
00:14:17 --> 00:14:19 systems loft at least one planet.
00:14:20 --> 00:14:21 Avery: That uh, rings a bell.
00:14:21 --> 00:14:24 Anna: It does. Back in episode
00:14:24 --> 00:14:27 195 we covered Batygin's work on
00:14:27 --> 00:14:29 how our own solar system could be
00:14:29 --> 00:14:31 destabilised far in the future.
00:14:32 --> 00:14:35 This is the same physics running at the
00:14:35 --> 00:14:37 start of a system's life instead of the end.
00:14:38 --> 00:14:40 Perini's phrase for the link between star and
00:14:40 --> 00:14:43 planets is genetic. One what you're born
00:14:43 --> 00:14:45 with shapes what you become.
00:14:46 --> 00:14:47 Avery: Caveats.
00:14:47 --> 00:14:50 Anna: Danielski herself flagged the big one.
00:14:51 --> 00:14:53 The team's main worry was whether the split
00:14:53 --> 00:14:56 was real or just a product of how we find
00:14:56 --> 00:14:59 planets. Because transits favour compact
00:14:59 --> 00:15:02 systems and radial velocities favour
00:15:02 --> 00:15:04 massive ones. They tested four
00:15:04 --> 00:15:07 independently selected samples and the split
00:15:07 --> 00:15:09 held in all of them. But we still can't
00:15:09 --> 00:15:12 easily see wide systems of modest planets.
00:15:12 --> 00:15:15 So a, uh, middle population could be hiding.
00:15:16 --> 00:15:18 And a small. The planet
00:15:18 --> 00:15:20 catalogue was downloaded in October last
00:15:20 --> 00:15:20 year.
00:15:21 --> 00:15:23 Avery: And where does that leave us?
00:15:23 --> 00:15:26 Anna: Somewhere in between. Our solar
00:15:26 --> 00:15:29 system's calm history matches the lower
00:15:29 --> 00:15:32 mass family, but it's much more spread out
00:15:32 --> 00:15:34 than those compact systems. It's
00:15:34 --> 00:15:37 another reminder not to treat the sun and its
00:15:37 --> 00:15:39 planets as the universal template.
00:15:40 --> 00:15:43 Avery: Our third story is galactic
00:15:43 --> 00:15:43 archaeology.
00:15:44 --> 00:15:46 The Sagittarius dwarf galaxy was only
00:15:46 --> 00:15:49 discovered in 1994, hiding behind the
00:15:49 --> 00:15:51 crowded centre of the Milky Way. And it's
00:15:51 --> 00:15:54 being torn apart. Its stars now stream around
00:15:54 --> 00:15:57 our galaxy in long ribbons that wrap the sky.
00:15:58 --> 00:16:00 Anna: So can you work out what it looked like
00:16:00 --> 00:16:02 before it was shredded?
00:16:02 --> 00:16:05 Avery: That's the challenge. And positions and
00:16:05 --> 00:16:07 speeds of the debris alone don't get you very
00:16:07 --> 00:16:10 far. A team led by Ziji Guo and
00:16:10 --> 00:16:13 Shangsheng Zhu at the National Astronomical
00:16:13 --> 00:16:15 Observatories of China added chemistry.
00:16:16 --> 00:16:18 Using STREAM members from China's LAMOS
00:16:18 --> 00:16:20 survey telescope, they looked at how the
00:16:20 --> 00:16:22 abundance of heavy elements, the
00:16:22 --> 00:16:25 metallicity, changes across the width of the
00:16:25 --> 00:16:26 streams.
00:16:26 --> 00:16:27 Anna: And it changes.
00:16:28 --> 00:16:31 Avery: It does. There's a vertical metallicity
00:16:31 --> 00:16:34 gradient across the streams. The team ran
00:16:34 --> 00:16:36 large computer simulations of the dwarf
00:16:36 --> 00:16:39 galaxy falling into the Milky Way. And the
00:16:39 --> 00:16:41 only way to reproduce that gradient was if
00:16:41 --> 00:16:43 Sagittarius started out as a disc whose
00:16:43 --> 00:16:46 metallicity had an unusual shape.
00:16:46 --> 00:16:49 Normally, a galaxy disc is most metal rich at
00:16:49 --> 00:16:51 the centre and gets poorer as you go outward.
00:16:52 --> 00:16:55 Sagittarius instead looks like an inverted V.
00:16:55 --> 00:16:58 Metallicity rises from the centre, peaks
00:16:58 --> 00:16:59 partway out, then falls again.
00:17:00 --> 00:17:01 Anna: Is that weird?
00:17:02 --> 00:17:04 Avery: Rare, but not unique.
00:17:05 --> 00:17:08 They searched 217 dwarf galaxies in
00:17:08 --> 00:17:11 the manga survey and found three with the
00:17:11 --> 00:17:14 same shape. And intriguingly, the
00:17:14 --> 00:17:16 Milky Way's own integrated metallicity
00:17:16 --> 00:17:19 profile looks similar. So the authors raise
00:17:19 --> 00:17:21 the question of whether the two galaxies
00:17:21 --> 00:17:24 have, in some sense, evolved together.
00:17:24 --> 00:17:25 Anna: Caveats.
00:17:26 --> 00:17:28 Avery: It's, uh, a reconstruction from simulations.
00:17:29 --> 00:17:32 So it shows a disc like that can reproduce
00:17:32 --> 00:17:34 the streams. Not that nothing else could.
00:17:34 --> 00:17:37 And a, uh, disclosure on timing. The paper
00:17:37 --> 00:17:40 was published this week, but it was first
00:17:40 --> 00:17:42 submitted in July last year. So it's been
00:17:42 --> 00:17:44 more than a year in review.
00:17:45 --> 00:17:47 Anna: There's a callback here, too.
00:17:47 --> 00:17:50 Avery: There is? In episode 195,
00:17:50 --> 00:17:53 we covered Rubin GC1,
00:17:53 --> 00:17:56 a newly found globular cluster that Rubin
00:17:56 --> 00:17:58 data traced back to Sagittarius.
00:17:59 --> 00:18:02 Every piece of this galaxy we recover helps
00:18:02 --> 00:18:05 rebuild the hole. And the southern andal is
00:18:05 --> 00:18:07 literal. The core of Sagittarius sits, uh, at
00:18:07 --> 00:18:10 minus 30 degrees, about 60
00:18:10 --> 00:18:12 degrees up from Sydney at full dark tonight
00:18:13 --> 00:18:15 against 22 from Los Angeles and 5 from
00:18:15 --> 00:18:18 London. Its nucleus is the globular
00:18:18 --> 00:18:21 cluster Messier 54, an easy
00:18:21 --> 00:18:23 telescope target just below the teapot.
00:18:24 --> 00:18:26 Anna: Three quick ones. First,
00:18:26 --> 00:18:29 AESA's HERA mission was due to fire its
00:18:29 --> 00:18:32 thrusters for 93 minutes in the early
00:18:32 --> 00:18:35 hours of today, European time. It
00:18:35 --> 00:18:37 was the first of its braking burns on
00:18:37 --> 00:18:39 approach to Didymus and Dimorphous, the
00:18:39 --> 00:18:42 asteroid pair NASA's DART spacecraft struck
00:18:42 --> 00:18:45 in 2022, shortening the moonlet's
00:18:45 --> 00:18:47 orbit by about 33 minutes.
00:18:47 --> 00:18:49 Avery: As we record, ESA hasn't yet confirmed the
00:18:49 --> 00:18:52 result. Its new NORSHA antenna in Western
00:18:52 --> 00:18:54 Australia is among the stations tracking it
00:18:54 --> 00:18:56 and we'll bring you the outcome tomorrow.
00:18:56 --> 00:18:58 Arrival at the asteroids is due in November
00:18:59 --> 00:18:59 2.
00:18:59 --> 00:19:02 Anna: A cargo Dragon launched from Florida on
00:19:02 --> 00:19:04 Tuesday on NASA's CRS 35
00:19:04 --> 00:19:07 mission, carrying more than three tons of
00:19:07 --> 00:19:09 supplies and the final pair of rollout solar
00:19:09 --> 00:19:11 arrays for the International Space Station.
00:19:12 --> 00:19:14 It's due to dock at 11:00am Eastern Time
00:19:14 --> 00:19:16 today, which is 2:00 clock tomorrow morning
00:19:16 --> 00:19:17 in Sydney.
00:19:18 --> 00:19:20 Avery: And third, a, uh, farewell after more than
00:19:20 --> 00:19:23 33 years. NASA's Astrophysics Data
00:19:23 --> 00:19:26 System, the digital library almost every
00:19:26 --> 00:19:28 astronomer uses, closes its doors in
00:19:28 --> 00:19:30 November. It's being replaced by a broader
00:19:30 --> 00:19:33 successor called Science Explorer, or SciEx.
00:19:34 --> 00:19:35 Nature Astronomy marked the occasion with an
00:19:35 --> 00:19:38 editorial this week. The papers aren't going
00:19:38 --> 00:19:40 anywhere, they're just moving house.
00:19:40 --> 00:19:42 Anna: Time to look up. And tonight, the evening
00:19:42 --> 00:19:45 western sky splits the hemispheres neatly.
00:19:45 --> 00:19:47 Mercury reached its greatest distance from
00:19:47 --> 00:19:49 the sun on Monday and is still well placed
00:19:49 --> 00:19:51 for southern observers from Sydney.
00:19:51 --> 00:19:54 Tonight it's about 24 degrees up at sunset
00:19:54 --> 00:19:56 and sets roughly two hours after the Sun.
00:19:57 --> 00:19:59 Avery: That's a terrific apparition, one of the
00:19:59 --> 00:20:02 Anna: best of the year for the south. Venus is
00:20:02 --> 00:20:05 below it, about 14 degrees up at sunset from
00:20:05 --> 00:20:07 Sydney and setting an hour and a quarter
00:20:07 --> 00:20:10 after the Sun. In North America, it's a
00:20:10 --> 00:20:13 different story from Los angeles. Mercury is
00:20:13 --> 00:20:15 10 degrees up at sunset and sets within the
00:20:15 --> 00:20:18 hour, while Venus sets with the Sun. From
00:20:18 --> 00:20:21 New York, Mercury is about 7 degrees up and
00:20:21 --> 00:20:23 Venus is already gone from London,
00:20:23 --> 00:20:25 Mercury barely clears the horizon
00:20:26 --> 00:20:29 Avery: and Venus is about to leave us altogether.
00:20:29 --> 00:20:32 Anna: It reaches inferior conjunction on the
00:20:32 --> 00:20:35 24th of October, passing between us
00:20:35 --> 00:20:38 and the Sun. Right now it's a
00:20:38 --> 00:20:41 thin Crescent, about 4% lit,
00:20:41 --> 00:20:44 but a large one, merely a full arcminute
00:20:44 --> 00:20:44 across.
00:20:45 --> 00:20:48 Avery: Which brings us to our standing safety note.
00:20:48 --> 00:20:50 And um, this week it matters as Venus
00:20:50 --> 00:20:52 slides toward the Sun. Please don't go
00:20:52 --> 00:20:55 hunting for it with binoculars or a telescope
00:20:55 --> 00:20:58 while the sun is above the horizon. Sweeping
00:20:58 --> 00:21:00 optics across the sun can damage your eyes
00:21:00 --> 00:21:03 instantly and permanently. Never
00:21:03 --> 00:21:05 look directly at the sun for any solar
00:21:05 --> 00:21:06 viewing.
00:21:06 --> 00:21:08 Use only eclipse glasses certified to ISO
00:21:08 --> 00:21:11 1M 23122 and fit
00:21:11 --> 00:21:13 telescopes and binoculars with proper
00:21:13 --> 00:21:15 certified solar filters over the front.
00:21:16 --> 00:21:18 Anna: The crescent Moon is in the evening sky too
00:21:19 --> 00:21:21 in Ophiuchus tonight, about a quarter
00:21:21 --> 00:21:24 lit from Sydney it's more than 50
00:21:24 --> 00:21:27 degrees up at sunset from New York it's
00:21:27 --> 00:21:30 about 17. First quarter falls
00:21:30 --> 00:21:33 on Sunday the 18th in the Americas and early
00:21:33 --> 00:21:34 Monday morning here in Australia.
00:21:35 --> 00:21:38 Avery: And two of today's stories are overhead. For
00:21:38 --> 00:21:39 the south they are.
00:21:40 --> 00:21:42 Anna: The teapot of Sagittarius is high
00:21:42 --> 00:21:45 after dark from Australia with the
00:21:45 --> 00:21:48 shredded Sagittarius dwarf galaxy sitting
00:21:48 --> 00:21:51 behind it about 60 degrees up at full
00:21:51 --> 00:21:53 dark from Sydney. And wasp 17
00:21:54 --> 00:21:57 is still about 24 degrees up in Scorpius
00:21:57 --> 00:22:00 at full dark. The honest balance
00:22:00 --> 00:22:02 is that this has been a southern evening
00:22:02 --> 00:22:03 sky all week.
00:22:04 --> 00:22:07 Avery: So what does the north get the mornings?
00:22:08 --> 00:22:10 Anna: Saturn is just past opposition,
00:22:11 --> 00:22:13 rising in the east at sunset, shining at
00:22:13 --> 00:22:16 magnitude 0.4 and visible all
00:22:16 --> 00:22:19 night everywhere. Its M rings are tilted
00:22:19 --> 00:22:22 only about 7 degrees towards us this year,
00:22:22 --> 00:22:25 so in a small telescope they look like a thin
00:22:25 --> 00:22:28 bright blade rather than the wide ellipse in
00:22:28 --> 00:22:31 the textbook pictures. But the real show
00:22:31 --> 00:22:34 is before dawn. Mars and Jupiter
00:22:34 --> 00:22:36 are about 12 degrees apart and closing
00:22:37 --> 00:22:40 from New York at nautical dawn tomorrow. Mars
00:22:40 --> 00:22:43 is about 56 degrees up and Jupiter
00:22:43 --> 00:22:45 about 46 from Sydney they're
00:22:45 --> 00:22:48 23 and 19. They'll be just
00:22:48 --> 00:22:51 1.2 degrees apart on the 16th of November,
00:22:52 --> 00:22:54 so watch the gap shrink week by week.
00:22:55 --> 00:22:57 Avery: And a meteor shower on the way.
00:22:58 --> 00:23:00 Anna: The Orionids peak on the night of the
00:23:00 --> 00:23:03 21st into the 22nd of October.
00:23:04 --> 00:23:06 They're dust from Hallie's Comet, the same
00:23:06 --> 00:23:09 comet that gives us the Eta uh, Aquarids in
00:23:09 --> 00:23:11 May. The radiant is in Orion
00:23:11 --> 00:23:14 and rises around midnight, so the pre dawn
00:23:14 --> 00:23:17 hours are best. The moon will be about
00:23:17 --> 00:23:20 3/4 lit, but setting in the early hours
00:23:20 --> 00:23:22 which help the north has the edge here
00:23:22 --> 00:23:23 too.
00:23:23 --> 00:23:26 At dawn the radiant is about 64 degrees up
00:23:26 --> 00:23:28 from New York against 40 from Sydney.
00:23:29 --> 00:23:32 Southern observers still see respectable show
00:23:32 --> 00:23:34 just with fewer meteors per hour.
00:23:34 --> 00:23:36 Avery: Evenings for the south, mornings for the
00:23:36 --> 00:23:36 north.
00:23:37 --> 00:23:40 Anna: That's the shape of it this week. Some
00:23:40 --> 00:23:42 weeks it runs the other way.
00:23:43 --> 00:23:46 Avery: That's Astronomy AstroDailyPod for today. We
00:23:46 --> 00:23:49 started with a 20 kilometre volcano making
00:23:49 --> 00:23:51 clouds the textbook said shouldn't form,
00:23:52 --> 00:23:55 then mapped the edge of a planet in Scorpius,
00:23:55 --> 00:23:58 sorted planetary systems into two families
00:23:58 --> 00:24:00 and rewound a shredded galaxy.
00:24:00 --> 00:24:03 Anna: Tomorrow we'll have the outcome of
00:24:03 --> 00:24:06 Hera's first breaking burn and next
00:24:06 --> 00:24:08 Monday we'll preview Japan's Martian
00:24:08 --> 00:24:11 Moons exploration mission MMX UM M
00:24:12 --> 00:24:13 ahead of its launch.
00:24:14 --> 00:24:16 Avery: You'll find the show notes, links to every
00:24:16 --> 00:24:18 paper and the full back catalogue at
00:24:18 --> 00:24:21 astronomydaily IO that's also where you
00:24:21 --> 00:24:23 can sign up for the newsletter or send us a
00:24:23 --> 00:24:26 question through the contact form. Several of
00:24:26 --> 00:24:27 our best segments have started that way.
00:24:28 --> 00:24:31 Anna: We are AstroDaily Pod on social
00:24:31 --> 00:24:34 media and Astronomy AstroDailyPod is part of
00:24:34 --> 00:24:36 the bytes.com podcast network.
00:24:37 --> 00:24:39 If you're enjoying the show, a uh, rating or
00:24:39 --> 00:24:42 review really does help other people find us.
00:24:42 --> 00:24:43 Thanks for listening. I'm Anna.
00:24:44 --> 00:24:46 Avery: And I'm Avery. Clear Skies
00:24:46 --> 00:24:49 Astronomy Day Mhm.
00:24:49 --> 00:24:50 The stories.
00:25:06 --> 00:25:06 We told.