Mars Has a Purple South Pole — Here’s Why

0:03Speaker 1:Welcome to Bedtime Astronomy. Explore the wonders of the cosmos with our soothing Bedtime Astronomy podcast. Each episode offers a gentle journey through the stars, planets, and beyond, perfect for unwinding after a long day. Let's travel through the mysteries of the universe as you drift off into a peaceful slumber under the night sky.

0:27Speaker 2:So if you were standing at the south pole of Mars right now, just, you know, looking out at the horizons, you actually wouldn't see the red planet.

0:35Speaker 3:Oh, you really wouldn't.

0:36Speaker 2:Which is wild, right? Because you expect that endless sweeping expanse of oxidized iron and all those dusty ochres we always picture in the movies.

0:45Speaker 3:Right, the classic rusted desert look.

0:47Speaker 2:Exactly. But instead, you'd be standing in this completely swirling dynamic landscape of vibrant blueberry purple.

0:54Speaker 3:Yeah, I mean, it's a complete visual paradigm shift. It completely challenges that dry, rusted, monochrome aesthetic that completely defines Mars in our collective imagination.

1:06Speaker 2:Totally does.

1:07Speaker 3:We basically expect this dusty desert turned up to 11. But the reality down at the poles is just so much more alien.

1:14Speaker 2:Okay, let's unpack this because, I mean, how are we getting a vibrant purple landscape out of a planet literally famous for being covered in red rust?

1:23Speaker 3:It's a great question.

1:23Speaker 2:All right, so today we're heading to that Martian South Pole. We're going to look at the chemistry of that purple sky, the towering icy cliffs, the shifting sands, and this ancient shrinking crust that is just going to fundamentally rewrite your mental image of our neighboring planet.

1:40Speaker 3:It definitely will. I mean, to really understand that blueberry purple color, we have to start by deconstructing the canvas itself.

1:47Speaker 2:Okay, lay it out for me.

1:49Speaker 3:So you have to look at the raw physical ingredients mixing on the Martian surface in this specific region. First off, you have these vast swathes of incredibly dark, almost pitch black material that form the base layer of the terrain.

2:04Speaker 2:So, like, I'm imagining that comes from the planet's volcanic history, right?

2:07Speaker 3:It does, yeah. We're looking at ancient volcanic ash and rock, and the mineral composition here is absolutely critical. This dark material is exceptionally rich in silicates, specifically these minerals called olivine and pyroxene.

2:19Speaker 2:Olivine and pyroxene. Okay, so what actually are those?

2:22Speaker 3:Well, olivine is basically made up of magnesium, iron, and silicate, and pyroxenes are mostly silicon and oxygen.

2:28Speaker 2:See, those sound like, you know, really exotic space rocks to me.

2:32Speaker 3:Right. They sound like sci-fi elements, but they're actually pretty standard terrestrial geology.

2:37Speaker 2:Wait, really? Like we have them here?

2:39Speaker 1:Oh, yeah.

2:39Speaker 3:They are incredibly common right here on Earth, specifically deep down in our upper mantle.

2:44Speaker 2:Oh, wow.

2:45Speaker 3:Yeah. So that churning, superheated rock beneath our crest, it's chemically very, very similar to this dark scatter we see on the Martian surface.

2:54Speaker 2:Okay, but how does the deep mantle stuff get to the top on Mars?

2:57Speaker 3:Well, because early Mars had these just massive volcanic eruptions, but it lacked the tectonic plate movement to constantly recycle its crust.

3:07Speaker 2:Oh, Chrissy.

3:07Speaker 3:So those ancient lava flows just breached the surface, spread out, and then they just froze in place. It preserved that heavy, dark mantle material right out in the open.

3:18Speaker 2:That really shifts the perspective, you know?

3:20Speaker 3:Yeah.

3:20Speaker 2:The deep geological guts of a planet are just sitting right there on the surface.

3:24Speaker 3:Exactly, just sitting there.

3:25Speaker 2:But, I mean, a dark... Almost black volcanic canvas doesn't really give us vibrant purple on its own.

3:31Speaker 3:Right, not at all. Which brings us to the second ingredient, the contrast. Heavily punctuating all this dark rock are these pale, bright, bluish-white areas.

3:43Speaker 2:Okay, so ice.

3:45Speaker 3:Yes, but this isn't water ice like you'd find at Earth's poles. This is frozen carbon dioxide.

3:49Speaker 2:Dry ice.

3:50Speaker 3:Exactly, what we call dry ice. Now, the main south polar cap is permanent, but surrounding it is the seasonal carbon dioxide frost cap. And because of the unique atmospheric pressure on Mars, this frost actually sublimates.

4:03Speaker 2:Meaning it turns directly from a solid ice into a gas, right? Like it completely spits the liquid water phase.

4:09Speaker 3:Precisely, just poofs right into a gas. So as the seasons change, this frost cap is actively breathing, essentially.

4:16Speaker 2:Wow, breathing.

4:17Speaker 3:Yeah, it expands outward over the dark rock in the winter and And then as it warms up, it sublimates and retreats.

4:23Speaker 2:Okay, I have to jump in here, though, and maybe push back on the color theory for a second.

4:26Speaker 3:Sure, go for it.

4:27Speaker 2:Because if I take a bucket, right, and I mix red iron dust, black volcanic rock, and white frozen carbon dioxide, I do not get blueberry purple.

4:36Speaker 3:No, you definitely don't.

4:37Speaker 2:I get a muddy grayish brown mess. So this has to be an optical illusion, right?

4:42Speaker 3:Yes, absolutely.

4:43Speaker 2:Like theatrical stage lighting. Like if you have a dark wooden stage and you shine a red spotlight from one side and a blue spotlight from the opposite angle and then pump some fog onto the floor, suddenly that plain brown wood looks like a glowing ethereal purple.

4:58Speaker 3:That is a remarkably useful way to picture it, actually. But we need to factor in that unlike a theater where the lights are static, This Martian stage is constantly moving.

5:08Speaker 2:Oh, right. Because of the environment.

5:10Speaker 3:Exactly. We're dealing with really complex atmospheric physics here. The persistent dust in the Martian air is acting like your theatrical fog. It's acting as a scattering medium for the light.

5:20Speaker 2:So the light is filtering through all that airborne dust before it even hits the ground.

5:24Speaker 3:Yes. What's fascinating here is how you have this soft, very specific angle of Martian morning light. It filters through that atmospheric red dust. strikes the patches of bright white-blue CO2 ground frost, contrasts with the dark volcanic rock beneath it, and then scatters back to our cameras as this brilliant purple.

5:44Speaker 2:That is just incredible.

5:46Speaker 3:And because it relies entirely on the angle of the sun, the amount of dust in the air, and the specific state of the frost, the landscape actually changes color throughout the day.

5:56Speaker 2:Wait, so it's not always purple?

5:58Speaker 3:Nope. As the sun moves higher, or as the frost burns off into gas, or even if the wind just kicks up more dust, that purple hue shifts and eventually fades out.

6:09Speaker 2:It essentially puts on a morning light show, just for the cosmos.

6:12Speaker 3:It really does.

6:12Speaker 2:It's just a great reminder that this isn't some static dead rock hanging in space. Yeah. But, okay, let's peel back that morning frost for a minute and look at the actual geography. Because when the dust settles, the physical structures hiding under this copal hue are pretty staggering.

6:27Speaker 1:Yeah.

6:27Speaker 2:We really need to talk about Thales rupes.

6:29Speaker 3:Oh, absolutely. The macrogeography of this region is defined by some truly massive features, and Thales Rupes is definitely the tectonic centerpiece.

6:38Speaker 2:So Rupes, that's Latin.

6:39Speaker 3:Yeah, Rupes is the geological term drawn from the Latin word for cliff. And Thales comes from ancient mythology Ultima Thule, which was this mythical farthest northern land.

6:50Speaker 2:So it translates to the Cliff of Thales. It's a very poetic name.

6:55Speaker 3:Very poetic.

6:55Speaker 2:But the scale... The scale is what's really hard to wrap your head around here.

6:59Speaker 3:Yeah.

7:00Speaker 2:We're talking about this icy, jagged crag with a sheer vertical drop of over one kilometer.

7:07Speaker 3:Yeah.

7:07Speaker 2:That's more than 0.6 miles straight down.

7:10Speaker 3:To put that in perspective for you, that cliff face is significantly taller than the Burj Khalifa in Dubai. You know, the tallest skyscraper on Earth.

7:17Speaker 2:That is insane.

7:18Speaker 3:And it isn't just an isolated peak either. This cliff extends as a massive continuous wall for hundreds of kilometers across the polar landscape.

7:28Speaker 2:And the surrounding neighborhood is just as massive, right? Like right next to it is this feature called Ultima Lingula. Yes. Lingula being Latin for little tongue, which feels like a total geological inside joke.

7:38Speaker 3:It is a massive understatement for sure. Because a lingula in astrogeology refers to a plateau-like structure that kind of extends out and ends in these rounded lobed shapes. But this particular little tongue is about 550 kilometers in diameter.

7:54Speaker 2:Okay, so it's basically the size of European country.

7:57Speaker 3:Roughly, yeah.

7:58Speaker 2:Just sitting next to this kilometer-high cliff.

8:00Speaker 3:Exactly, just casually sitting there.

8:01Speaker 2:And you also have Burroughs Crater nearby, which is, what, 100 kilometers across? Aptly named after Edgar Rice Burroughs, the author of the John Carter of Mars sci-fi books, which is a nice touch.

8:12Speaker 3:It is a great nod to the sci-fi roots, yeah.

8:15Speaker 2:But visualizing all these enormous features... Kind of brings up a mechanical question for me. Like, I can picture the cliff. I can picture the giant plateau, the massive crater. But how do we actually sequence them in time?

8:27Speaker 3:Ah, okay. The timeline.

8:29Speaker 2:Yeah, because if I walk into a room on Earth, I can look at muddy footprints over a spilled drink and deduce, okay, someone walked through the room after the drink was spilled.

8:38Speaker 3:Right. Basic deduction.

8:39Speaker 2:Right. But on a planet millions of miles away, how do we possibly know if the cliff or the crater was there first?

8:47Speaker 3:It's basically planetary detective work. We use this foundational principle in geology called the law of superposition.

8:53Speaker 2:Superposition.

8:54Speaker 3:Yeah. And on rocky worlds like Mars, where you don't have, you know, heavy vegetation or thick oceans hiding the evidence, we use impact craters as our cosmic clocks.

9:05Speaker 2:Okay, so the craters give us a relative timeline based on what they're physically overlapping.

9:10Speaker 3:Exactly. It's like reading a visual history book. When you look at Thiel's Roops from above... The geometric evidence is incredibly violent. The massive cliff actually cuts a much older massive crater completely in half.

9:24Speaker 2:Wait, literally in half?

9:25Speaker 3:Straight through the middle.

9:26Speaker 2:So you had this giant circular bowl carved out by an ancient asteroid impact. And then this kilometer high tectonic wall just slices right through the middle of it. Right.

9:36Speaker 3:And because the cliff cuts through the crater, the crater had to be there first.

9:40Speaker 2:Oh, okay.

9:41Speaker 3:The solid rock had to exist in that distinct bowl shape before the tectonic forces could come along and rupture it.

9:47Speaker 2:Because if the crater had happened after the cliff was formed... Then you.

9:50Speaker 3:Would just see a massive circular bite taken out of the cliff face itself.

9:54Speaker 2:That makes total sense. That is the law of superposition at work.

9:57Speaker 3:Precisely. But the cosmic clock keeps... ticking. If you look really closely along the very top edge of Files Roops, you can see two much smaller, perfectly intact craters sitting right on the rim.

10:09Speaker 2:Oh, meaning those specific asteroids must have hit. After the cliff was fully formed and settled?

10:16Speaker 3:Yes.

10:16Speaker 2:Because if they had hit before, the immense stress of the rock pushing upward to form the cliff would have stretched or distorted or completely destroyed those little small craters?

10:26Speaker 3:You are on the exact right track. Just by analyzing the geometry, so you have a sliced massive crater down below, an intact cliff right in the middle, and two pristine small craters sitting on top. We can sequence millions, maybe even billions of years of planetary history.

10:42Speaker 2:And without ever even digging a single core sample.

10:45Speaker 3:Exactly. Just by looking at the pictures.

10:47Speaker 2:That's wild. Now, asteroid impacts, obviously, they can carve out a 100-kilometer crater in a matter of seconds.

10:52Speaker 1:Yeah.

10:52Speaker 2:But we also have much, much slower forces at work here. Like the atmospheric dust creating that purple sky we talked about implies wind, right?

11:00Speaker 3:Absolutely. Tons of wind.

11:02Speaker 2:So let's zoom in on the microgeography. Because the ground level of this region is actively being shaped by these aeolian forces.

11:09Speaker 3:Right. And wind activity is actually the primary active sculptor on Mars today.

11:15Speaker 2:Okay.

11:15Speaker 3:Now, the wind patterns at the Martian South Pole are really chaotic. They swirl around and change directions constantly as those seasonal frost caps heat up and cool down.

11:26Speaker 2:Because of all that sublimation happening.

11:28Speaker 3:Exactly. And because of that mixed wind pattern, you don't really find the perfectly symmetrical, endlessly rolling dunes that you might see in a movie.

11:37Speaker 2:Right.

11:38Speaker 3:You find these structures that geologists call barchenoid ridges.

11:41Speaker 2:Barchenoid ridges. So to give you a mental image of a barchenoid ridge, if you were to, say, fly a camera drone over the really sandy parts of the Sahara Desert here on Earth, you'd actually see these exact same shapes.

11:53Speaker 3:Yeah, they are very common on Earth, too. Right.

11:55Speaker 2:A barchenoid ridge is essentially a classic arc-shaped dune. It looks a bit like a crescent moon just lying flat on the sand. And the tips of that crescent, which geologists actually call the horns, right?

12:06Speaker 3:Yes, the horns.

12:06Speaker 2:Those horns are... always point downwind. So you get these sweeping curved ridges just marching super slowly across the terrain.

12:14Speaker 3:And you really have to appreciate the fluid dynamics at play to understand how those dunes are even built on Mars. Why? Because the atmosphere on Mars is incredibly thin. I mean, it is roughly 1% the density of Earth's atmosphere at sea level.

12:29Speaker 2:Okay, but wait. If the atmosphere is that thin, how does it move anything at all?

12:36Speaker 3:It's a bit counterintuitive.

12:37Speaker 2:Because if I'm standing in like a 60 mile per hour windstorm on Mars, the lack of air density means it would basically barely feel like a gentle breeze pushing against my suit.

12:46Speaker 3:Right.

12:47Speaker 2:It shouldn't have the brute force to push heavy volcanic sand into these massive ridges.

12:53Speaker 3:It's a really great observation. And the answer lies in a combination of gravity, speed and time.

12:58Speaker 2:OK, break that down for me.

12:59Speaker 3:First, Martian gravity is only about 38% of Earth's gravity. That means every single grain of sand is significantly lighter, making it much, much easier for the thin air to lift it up and kind of bounce it along the surface. We call that bouncing process saltation. Second, to compensate for the thin air, Martian winds can reach incredibly high speeds.

13:18Speaker 2:Ah, so less density but way more velocity.

13:21Speaker 3:Exactly. And finally, you have to factor in the geologic timescale. It might take centuries to move a barchenoid ridge just a few feet. but over millions of years, that whisper-thin wind is relentless enough to carve the surface and shift unimaginable amounts of material.

13:38Speaker 2:Wow. It's literally the ultimate example of slow and steady winning the race.

13:42Speaker 3:It really is.

13:43Speaker 2:The wind just has nothing but time.

13:44Speaker 3:But here's where it gets really interesting for me, though. If a thin wind takes millions of years to slowly push sand into crescent moons, what massive force shaped a one-kilometer-high, Hundreds of kilometers long cliff like Thiel's roots.

14:00Speaker 2:Yeah, that's the big question.

14:01Speaker 3:Because on Earth, we build mountains through plate tectonics. You have crustal plates subducting, like diving under one another or just crashing head on to push rock up into the Himalayas. Right.

14:10Speaker 2:But if Mars doesn't have moving tectonic plates, I am completely lost as to how a kilometer high wall of rock gets thrust up into the sky. It seems to just defy basic planetary physics.

14:24Speaker 3:I get why you'd say that. It seems contradictory until you look at how Mars is actually structured.

14:29Speaker 2:Okay, enlighten me.

14:30Speaker 3:You're right that mountain building on Earth is a violent, ongoing process, completely driven by plate tectonics. But Mars is what planetary scientists classify as a single-plate, stagnant-lid planet.

14:43Speaker 2:A stagnant lid.

14:44Speaker 3:Yeah.

14:44Speaker 2:Meaning... The entire outer shell of the planet is just one solid, unbroken sphere of rock.

14:50Speaker 3:One solid piece, exactly.

14:51Speaker 2:So there are no separate puzzle pieces floating around on the mantle to crash into each other. So where on Earth, or where on Mars, I guess, did the upward mountain-building force even come from?

15:00Speaker 3:It came from the planet fundamentally changing its internal volume. The upward force was actually a byproduct of the entire planet shrinking.

15:07Speaker 2:Okay, let me try to picture this. Imagine you have a plump grape, right, sitting in the sun. The skin of that grape is nice and tight.

15:14Speaker 3:Okay, I follow.

15:15Speaker 2:As the inside of the grape dries out and loses moisture, the internal volume shrinks. But the skin, that outer crust, still has the exact same total surface area. It doesn't just disappear.

15:28Speaker 1:Right.

15:28Speaker 2:So as the inside gets smaller, the skin has absolutely nowhere to go but to buckle, wrinkle, and fold over itself, turning into a raisin.

15:37Speaker 3:The geometry of your raisin analogy works perfectly, yes. But physically, we're looking at massive thermodynamic changes rather than, you know, moisture loss.

15:46Speaker 2:Oh, sure.

15:47Speaker 3:Phyles root base formed billions of years ago during a period when the planet's incredibly hot interior was actively radiating its heat out into space. As the deep molten core in the mantle of Mars cooled down over millions of years... the physical rock underwent thermal contraction.

16:02Speaker 2:So the rock itself got smaller.

16:04Speaker 3:Exactly. The entire planet literally shrank in volume.

16:07Speaker 2:And just like the skin of the raisin, that solid outer crust, the stagnant lid, suddenly found itself way too big for the shrinking planet beneath it.

16:14Speaker 3:Yes. The stagnant lid had to somehow accommodate this massive loss of internal volume. So the crust was subjected to just immense compressional stress. Eventually, the rock couldn't handle the pressure anymore. and it just buckled. Vast sections of the crust violently snapped, and were literally thrust upward, riding over the neighboring rock. We call these structures thrust faults.

16:37Speaker 2:Thrust faults.

16:38Speaker 3:And Thales rupus is one of the most prominent, spectacular thrust faults on all of Mars. It is Quite literally, a giant wrinkle on the Martian raisin.

16:48Speaker 2:That is genuinely staggering to think about.

16:50Speaker 3:Yeah.

16:50Speaker 2:You weren't just looking at a cliff. You're looking at the frozen scar of an entire planet collapsing inward on itself billions of years ago.

16:57Speaker 3:Yeah. And if we connect this to the bigger picture, it starkly illustrates how differently rocky planets can evolve.

17:04Speaker 2:How so?

17:05Speaker 3:Well, Earth and Mars formed in the exact same cosmic neighborhood out of the same basic raw materials, right?

17:09Speaker 2:Right.

17:10Speaker 3:But Earth became this dynamic, churning thermal engine. Our plate tectonics constantly recycle the crust, melting down old rock and pushing up new mountains, essentially erasing our deep geologic past over and over again.

17:22Speaker 2:Earth is constantly wiping the chalkboard clean.

17:25Speaker 3:Yes, exactly. But Mars, a stagnant lid planet, is basically a museum. Because its crust doesn't recycle... Its surface is this perfect frozen snapshot of ancient planetary cooling.

17:38Speaker 2:That's incredible.

17:39Speaker 3:It holds all these vital lessons about the mechanics and the thermodynamics of early rocky worlds lessons that Earth completely swallowed up and melted away billions of years ago.

17:50Speaker 2:So what does this all mean for you as you picture Mars going forward?

17:54Speaker 3:I think it means that whenever you hear it described simply as the red planet or, you know, dismissed as just a dead rock, you'll know those labels are completely inadequate.

18:02Speaker 2:They really don't do it justice.

18:04Speaker 3:Right. You're looking at a world of transient blueberry purple morning light. A world where shifting barcanoid sands are still actively marching to the tune of a whisper thin wind. A world of ancient cooling crests that buckled and pushed kilometer high cliffs right into the sky.

18:20Speaker 2:It is just a profoundly complex environment that requires us to constantly look past our initial assumptions. And honestly, our ability to even understand it is heavily reliant on our ongoing presence there. Yeah, we really do have to acknowledge the unbelievable engineering that allows us to even have this conversation.

18:36Speaker 3:Absolutely.

18:37Speaker 2:Because we're sitting here describing the exact thermal contraction of a crust and the lighting conditions on a purple frosty morning millions of miles away. And it's all thanks to the orbiters currently circling that planet.

18:50Speaker 3:The unsung heroes of planetary science.

18:53Speaker 2:Exactly. Spacecraft like the European Space Agency's Mars Express and NASA's Mars Reconnaissance Orbiter. These machines have literally spent the last two decades acting as our incredibly powerful eyes in the sky.

19:06Speaker 3:Yeah.

19:07Speaker 2:They're tirelessly mapping, photographing in high resolution and analyzing the chemical makeup of the surface. basically proving year after year that there are always new lessons to be learned.

19:18Speaker 3:They have completely revolutionized our understanding of planetary evolution. And, you know, considering all of this actually leaves me with one final lingering thought.

19:26Speaker 2:What's that?

19:27Speaker 3:Well, we just discussed how the internal cooling of Mars created these massive, continent-spanning tectonic scars that we are only truly beginning to appreciate now, thanks to high-resolution imaging catching the frost in the perfect morning light. Right.

19:42Speaker 2:It took the perfect combination of technology, orbital angle and timing to even see that purple canvas.

19:49Speaker 3:Which raises a really provocative question, I think. What other monumental ancient scars are hiding right now on the moons of Jupiter or the icy worlds of Saturn?

19:58Speaker 2:Oh, wow.

19:58Speaker 3:Think about places like Europa or Enceladus or the countless other quiet, stagnant-lid worlds out there in our outer solar system. What towering icy cliffs and like, you know, fractured tectonic landscapes are just sitting out there in the dark, just waiting for the right probe at the exact right angle in the right morning light to finally reveal their true colors to us.

20:19Speaker 2:You think you know a planet until the light hits it just right. Keep looking up, everyone. Until next time.