There's Something Molten Under Mars's South Pole

0:00 Anna: Hey, everyone. Welcome back to Astronomy daily. It's Wednesday, September 2nd, series five, episode 183. Avery: And today we've got a genuinely lovely piece of planetary science to lead with, which doesn't happen every day. Anna: It really is a good one. There's something hot buried under Mars's south pole. And I mean that literally. Avery: Bolton. Anna: Even Bolton. A new Nature paper says the southern half of Mars's interior is hundreds of degrees hotter than the northern half. And it might finally explain the single weirdest thing about that planet.

0:36 Avery: Then Crew 13 is standing down after a leak turned up in Dragon. NASA's Swift telescope is observing again, but now it's in a race with the atmosphere. And the Seattle nonprofit wants to launch a spacecraft to Alpha Centauri that won't arrive for 80,000 years. Anna: They're aware of that, by the way. That's the pitch. Avery: That is very much the pitch. Then we'll get you sorted for tonight's sky. Five planets are in play this month. North and south. Anna: Let's get into it. Avery: Okay, Set this up properly for me because I want to understand what was actually found.

1:11 Anna: So this is a paper published in nature on August 27, titled, and I love this title. Title. Tomography Reveals a Thermal anomaly beneath Mars's Crustal Dichotomy. Lead author is Alexander Byrne, who did this as his PhD work at Caltech and has just moved to the University of Arizona. Avery: And the finding, in one sentence, the Anna: inside of Mars's southern hemisphere is somewhere between 200 and 400 degrees Celsius, hotter than the inside of its northern hemisphere and parts of it are partially molten.

1:46 Avery: Two to 400 degrees is not a subtle difference. Anna: It's enormous. This isn't a hot spot or a plume. This is one half of a planet's interior running fundamentally hotter than the other half and staying that way over geologic time. Avery: Before we get to how they measured that, explain the thing it might solve. You said the weirdest thing about Mars, Anna: the Martian crustal dichotomy. If you've ever looked at one of those false colour topographic maps of Mars, you'll have seen it immediately, even if nobody named it for you.

2:19 The northern third of the planet is smooth, low lying, young looking plains. The southern two thirds is high, ancient, heavily cratered highlands on crust that's substantially thicker. And the boundary between them is remarkably sharp, a step of several kilometres in elevation in places. Avery: How long has that been sitting there, unexplained? Anna: It's sitting there since the Viking orbiters mapped it in the 70s. 50 years it's genuinely one of the oldest open questions in planetary science. Avery: So it's not just a surface feature, it goes deep.

2:55 Anna: That's the point of this paper. The dichotomy isn't paint on the outside. The it appears to have a matching structure hundreds of kilometres down in the mantle. The hemisphere that looks different on the surface is also the hemisphere that's hotter underneath. Avery: Okay, so how do you take the temperature of another planet? We've got exactly one seismometer that's ever operated on Mars and it's dead now. Anna: This is the part I find really clever. They didn't use seismology at all. They used the fact that Mars flexes.

3:27 Lex's Howe the sun pulls on Mars the way the moon pulls on Earth's oceans, tides. Mars has no oceans. But the whole solid planet still stretches and relaxes very slightly on every orbit. And how much it flexes depends on what it's made of and critically, how hot and soft its interior is. Hot, partially molten rock deforms more easily than cold rigid rock. Avery: So the amount of squish tells you Anna: the temperature, the amount of squish tells you the temperature. It's a tiny effect, but a planet that changes shape has a gravity field that changes with it.

4:05 And we've had spacecraft orbiting Mars continuously for decades, all tracked by radio from Earth. Avery: So the spacecraft are the instrument, the Anna: spacecraft are the instrument. Burns team went back through tracking data from Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter. 25 years of it, looking at the minute velocity changes as each one flew over different parts of the planet. Those wiggles map the gravity field. And the way that field breathes across a martian year maps the flex.

4:39 Avery: And nobody had done that before, not Anna: to this resolution and not with the aim of separating one hemisphere from the other. That's what the tomography in the title is doing. Same word as a medical CT scan. Roughly the same idea. Lots of measurements from lots of angles reconstructed into a picture of the inside. Avery: 25 years of data collected by missions that weren't launched to do this. Anna: Mars Global surveyor launched in 1996 and stopped talking to us in 2006. And its tracking data is still producing new results 20 years later.

5:14 It's a good argument for archiving everything forever. And this is a big international effort. Co authors across Brown, NASA Goddard, Arizona, Academia Sinica in Taiwan, the University of Rome, Delft, ucla, UT Austin, Colorado, Boulder and UC Santa Cruz. Avery: So what else does this hot southern interior explain? Anna: Two things that have been nagging at people. First, magnetism. Mars has no global magnetic field today. But the crust is magnetised in patches, frozen in evidence of a field that switched off billions of years ago.

5:52 And those anomalies are overwhelmingly concentrated in the southern highlands. A hemisphere with a, uh, different thermal history from the start would magnetise differently and hold that record differently. Avery: And the second. Anna: The second seismic, when Insight was operating its seismometer, found waves passing through parts of Mars interior were damped from far more strongly than expected. Waves lose energy fast in hot soft material. Exactly what partially molten rock down there would produce.

6:23 Avery: So it's not one line of evidence. It makes three separate puzzles line up. Anna: That's what makes it feel solid. Any one on its own you'd want it replicated. All, ah, three pointing the same way is more persuasive. Avery: Do we know why the southern half ended up hotter? Anna: Oh, and the team is upfront about that. Three families of explanation. One, a, uh, giant impact early in Mars's history. Something enormous hitting the northern hemisphere, blasting out the lowlands and rearranging the interior.

6:53 Two, mantle convection that settled into a lopsided pattern and stayed there. Which planetary interiors genuinely can do. Three, something compositional. A layer enriched in radioactive elements generating its own heat. Avery: And this result doesn't pick between them. Anna: Not yet. What it does is hand you a hard number. Every explanation now has to reproduce. Before this, why is Mars lopsided? Was mostly a surface geology question. Now it's a question about the whole planet with a temperature constraint attached.

7:27 Avery: Does this connect to the water storey? That's usually where Mars questions, um, end up. Anna: It does. And it's the line from the team I keep coming back to. The dichotomy matters because it tells you about the processes that shaped the hydrology of Mars, including the formation of the basins that may have held water. Those northern lowlands are exactly where you'd put an ancient ocean, if Mars ever had one. So whatever made the north low may also have decided where the water went. Avery: And, um, there's a nice bit of symmetry for us, isn't there?

7:58 Anna: There is. We're a show made in the southern hemisphere that spends a lot of time explaining why the southern sky is the interesting one. Turns out Mars southern hemisphere is the interesting one too. It's just interesting several hundred kilometres down. Avery: Anything coming that would test this further? Anna: More of the same. The tracking data keeps accumulating as long as we keep flying orbiters. And the technique sharpens the longer the baseline gets. A second seismometer on Mars would help enormously.

8:28 But there isn't one funded in flying in the meantime. This is a very good example of squeezing genuinely new physics out of data. Avery: We already had storey two and it's a stand down. Anna: Crew 13 was scheduled to launch to the International space station on September 12th. It's not launching on September 12th. NASA and SpaceX announced on August 29th that they'd found an oxidizer leak in the Dragon spacecraft's propulsion system during standard pre launch processing. Avery: Standard processing meaning caught on the ground doing the cheques you do?

9:03 Anna: Exactly. And that's worth saying, clearly, because Leak found on crew spacecraft reads alarming in a headline. This was the process working the way it's supposed to. Teams are running additional tests and data reviews and will do any rework needed before flight. No new target date yet. Avery: Why is an oxidizer leak, specifically a, uh, stop everything item? Anna: Because of what the Dragon's propulsion system is for and what's in it. Dragon runs hypergolic propellants, a fuel and an oxidizer that ignite on contact with each other.

9:37 No spark needed. Fantastically reliable, which is exactly why you use it on a crew vehicle. But the oxidizer side is aggressively corrosive and you handle it with enormous care. And that propulsion system isn't just for manoeuvring. The same broad system family is tied to the launch escape capability, the thing that pulls the capsule off a failing rocket. Avery: So the bar for signing it off is about as high as it gets. Anna: Nobody is flying this until they can explain precisely where the leak was and why it won't happen again.

10:10 Who's on this Crew commander is NASA's Jessica Watkins, pilot is NASA's Luke Delany. And the two mission specialists are Joshua Kutryk from the Canadian Space Agency and Sergey Tetrietnikov from Roscosmos. It's a roughly seven month increment. A lot of station maintenance. Several spacewalks planned the usual science load Avery: and, um, they'd be walking into a station that's just had a very busy month. We talked Yesterday about Expedition 75 running four spacewalks inside four weeks, which is Anna: exactly the backlog Crew 13's own task list was built on top of.

10:48 Avery: Does a delay cause knock on problems? Anna: Station handovers are a chain. Rotations overlap deliberately, so there's always experienced crew aboard and a slip at one end compresses things at the other. NASA hasn't flagged any concern beyond the launch date itself, so read that as manageable for now. Avery: When do we expect a new date? Anna: Unknown. And it depends entirely on what the inspections find, a fitting or A seal is a fix measured in days. Something in the plumbing that needs the system opened up is longer.

11:20 NASA's line is that a new target will be announced once available, and we'll flag it the moment it lands. Avery: Storey three closes a loop. We opened a couple of weeks back and it's a better ending than I expected. Anna: It genuinely is. NASA's Neil Gehrels Swift Observatory is doing science again. Two of its three instruments came back online on August 26, and NASA expects the third within weeks. Avery: Remind everyone how Swift got into trouble. Anna: Swift launched in 2004 to catch Gamma ray bursts, the brightest explosions in the universe, by slewing incredibly fast to point at one with within seconds of detecting it.

12:00 Hence the name 22 years of workhorse science. But it's in low Earth orbit, which has just enough atmosphere in it to slowly drag you down. And Swift has no propulsion at all. Avery: So it can't save itself. Anna: It can't. As the orbit decayed, NASA started shutting things off to buy time. The ultraviolet and optical telescope and the X ray telescope went dark in February. The Burst Alert telescope in April, partly for power and partly to hold an orientation that minimised drag, the space equivalent Avery: of tucking your arms in.

12:37 And meanwhile, there was a rescue coming. Anna: Catalyst Space's Link spacecraft, launched in July, designed to rendezvous with Swift, grab hold and boost the orbit. The first commercial rescue of a NASA science mission. Then, on August 19, NASA called it off. Link developed problems controlling its own orientation, which, for a spacecraft whose whole job is a delicate close proximity capture, is disqualifying. Avery: So what changed to let Swift start observing again? Anna: The calculus. With no rescue coming, there's no point conserving altitude for a rendezvous that isn't going to happen.

13:15 So NASA turned the telescopes back on to get every last observation out of it. Avery: How long have they got? Anna: NASA's estimate is that Swift drops below about 300 kilometres in the next one to two months. Below that, the drag makes precise pointing difficult and the telescopes stop being useful. Reentry follows. Avery: So weeks of real science left? Anna: Weeks. And given. Swift's specialty is catching things that appear without warning. Every day up there is a day it might catch something nobody else was pointed at.

13:50 Avery: What happens to Link? Anna: The other decent bit of news? Catalyst isn't writing it off. They'll fly proximity operations anyway to demonstrate in orbit servicing techniques. So the mission that couldn't save Swift still generates data for the next one that tries. And there's a whole generation of productive science spacecraft in low orbit with no propulsion, all quietly losing altitude. Swift is the test case for whether we ever get good at going up after them. Avery: Last storey before the sky.

14:21 And it's the most quietly audacious thing Anna: I've read in the While a nonprofit in the Seattle area called the Fermi Explora Mission announced this week that it intends to launch a, uh, spacecraft to Alpha Centauri at the end of 2029. Travel time, about 80,000 years. Avery: That's not a typo. Anna: Not a typo. And the team's own framing is the best part. Their president, Philip Johnston, says we'll be the first to leave and the last to arrive. Avery: Meaning they fully expect to be overtaken completely.

14:52 Anna: It's baked in. Somebody will build something faster in the intervening millennia and beat them there. His other line none of us will be here when this journey ends. And that is the point. Avery: So what actually flies 80,000 years suggests they're not waiting on exotic propulsion. Anna: Deliberately not Nothing that needs inventing. A spacecraft in the 100 to 200 kilogramme range. Solar electric propulsion to proven technology. We fly today carrying at least a kilogramme of payload in a shielded 10 centimetre cube.

15:24 Avery: How do you get to interstellar speed? Anna: With an ion thruster, a manoeuvre they call perihelion pumping. Instead of burning straight outward, you loop in close to the sun and thrust hard at closest approach, where you're moving fastest. That's the Obert effect. And it converts your fuel into far more speed than the same burn would out here. Repeat it and you build real Velocity. They're targeting 24.2 kilometres a second at cruise, about 54,000 miles an hour faster than New Horizons was moving at Pluto.

16:00 Avery: And, um, still nowhere near enough for four and a bit late years. Anna: 4.4. That's the honest arithmetic of interstellar travel. 12 years of active operations, then roughly 79 and a half thousand years of coasting in the dark. Avery: Who's behind it? Anna: Donston founded it with Ezra Feldon and Adi Oltean. The three of them are behind the space computing company Star Cloud. The advisory board has real names on it. Rob Meyerson, who used to run Blue Origin, and Jeff Thornberg, formerly SpaceX's propulsion lead budgets under $15 million, with one billionaire space founder verbally pledging 10 million of it.

16:42 Avery: What's it carrying? Anna: A digitised Voyager golden record. Messages from children around the world and real instruments. Cosmic ray detectors among them. The 12 active years aren't nothing. You'd be measuring your way out through the heliosphere. Avery: And the name's doing Some work. Anna: The Fermi is Enrico Fermi and his paradox. If the galaxy should be full of civilizations, where is everybody? Johnston's argument is that one candidate answer is that interstellar expansion is so hard, nobody bothers to start.

17:16 So you start even badly, even slowly. Avery: How is it different from breakthrough Starshot, which made a lot of noise in 2016? Anna: Starshot was the opposite bet. Laser pushed light sales. 20% of light speed arrive within a human lifetime. But needing technology that doesn't exist, it's stalled. Fermi Explorer inverts it. Use only what exists except an absurd travel time and actually launch. It'd also be the first spacecraft ever deliberately aimed at a particular star. The Voyagers are leaving, but they're not going anywhere in particular.

17:54 Avery: I find this weirdly moving and I can't fully justify why. Anna: It's the honesty of it. Most space projects sell you a payoff you'll live to see. This one explicitly doesn't and asks you to fund it anyway. Avery: Right, let's get everyone sorted with today's sky watch. And September is a good month. There are five planets in play, all Anna: five naked eye ones spread across the night. Let's take them in order of when you'd see them. Avery: Start with the evening. Anna: Venus, and it's unmissable.

18:24 Low in the west to west southwest after sunset, about 14 degrees up an hour after the sun goes down. For Sydney sunsets around 5.34pm AEST tonight, so you're looking from a quarter past six. Northern listeners get a similar window after their own sunset. The key either way is a genuinely flat western horizon because Venus is not high. Avery: It was next to Spica last night. Anna: That conjunction peaked yesterday and they're separating now, but Spica is still right there. Worth a look through binoculars while they're close.

19:00 Venus builds to maximum brilliance on the 18th, so it only improves mark the 14th too, when a thin crescent moon sits beside it. Avery: Then Saturn. Anna: Saturn's the one to actually point a telescope at this month. Up for most of the night. Magnitude 0.4 and the rings are tilted just 8 degrees from edge on the narrowest in about a dozen years. That's a genuinely unusual view and it won't look like this again for a long time. Avery: And, um, there's a hemisphere difference here, Anna: a big one in our favour for once.

19:35 From Sydney or Auckland or Cape Town, Saturn climbs high overhead. Less atmosphere to look through, so a steadier, sharper view from the northern US or the uk. It stays comparatively low and you'll be fighting turbulence. Northern listeners wait for it to get as high as it gets. And be patient with the seeing morning sky. Mars rises around 1:40am that distinctive amber colour slowly brightening as Earth catches up to it. Then Jupiter up in the east northeast around 10 to 4 and easier to catch before dawn every week.

20:10 Avery: And there's a Jupiter event for our North American listeners, specifically a good one. Anna: On September 8, the moon passes directly in front of Jupiter, an actual occultation visible from much of North America. Jupiter winks out behind the lunar limb and reappears on the other side. Small telescope makes it spectacular. Binoculars will show it. Not for us down south. So northern listeners, that one's yours. Put it in the calendar. Avery: And um, the fifth planet Mercury. Anna: But you'll wait for it final week of September.

20:44 Low in the west southwest in evening twilight, magnitude minus 0.1 on the 25th. It sits about a degree above Spica, which is a handy way to find it. Avery: What's the Moon doing tonight? Anna: Waning gibbous, past full heading for last quarter on the fourth. It's rising later each night, so the early evening is getting properly dark again. Good news for anything sane. Avery: Anything else for the diary? Anna: The 19th is international. Observe the moon night. The 22nd is the equinox. Spring for us, autumn for our northern listeners.

21:18 And around the 26th the full moon rises near Saturn with Neptune close by. If you've got binoculars and patience, if you want the Milky Way itself. Mid month is the window, roughly the 14th to the 20th when the moon's out of the evening sky. The core and Harry's in Scorpius. The teapot in Sagittarius sits high overhead. For us northern listeners get the same region but low toward the south. So you'll want a dark site and a clear southern horizon. Avery: One standing reminder before we go, the Anna: one we never skip.

21:52 If you're pointing anything at or near the sun, whether that's sunspots, a ah, transit or just chasing Venus a bit too close to sunset. Any filter you use must be Certified to the ISO 123122. Standard sunglasses are not solar filters. Welding glass off the shelf is not a solar filter. Improvised filters cause permanent damage in seconds. Certified or don't look. Avery: Never gets old, never should. And that brings us to the end of today's show. Molten anomaly under Mars southern hemisphere. That may finally explain the crustal dichotomy.

22:32 Crew 13 standing down over an oxidizer leak in Dragon. Swift back doing science in a race against its own orbit. An 80,000 year mission to Alpha Centauri and five planets to hunt down this month. Anna: If you enjoyed today's episode, the best thing you can do for us is leave a rating or review wherever you're listening and share it with a fellow space nerd. It really does help us out and makes a difference. Plus, you get featured on our new website, where you can also find full show notes, sources and links for every storey we covered today.

23:04 Just point your browser to astronomydaily IO cheque out our new blog there too. Avery: We'll be back tomorrow with more space and astronomy news. Until then, keep looking up. Anna: See you next time. Clear skies, everyone.