Today, Mars is a desolate wasteland, with dusty red rocks and sand stretching out to the horizon. But billions of years ago, it was a vastly different world. It was blue, with oceans, rivers, lakes, and maybe life? Let’s tell the story of geology on Mars, and we got from that world to the one we see today.
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Fraser Cain: Astronomy Cast Episode 432: The Geologic Ages of Mars
Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know but how we know what we know.
My name is Fraser Cain. I’m the publisher of Universe Today and with me is Dr. Pamela Gay, the director of CosmoQuest.
Hey, Pamela. How are you doin’?
Dr. Pamela Gay: I’m doin’ well. How are you doin’, Fraser?
Fraser Cain: Doin’ great.
So, we have a thank you to Ryan –
Dr. Pamela Gay: Yes.
Fraser Cain: – Consul for sending us this awesome, handmade – I think – golden record replica. It’s awesome. And so, if you watch the live show, we’re putting it kind of – Mine is sort of in the background sometimes, when I switch my camera view, and I’m sure Pamela’s will show up at some point. But – awesome. We really appreciate it, Ryan. Thank you so much.
Dr. Pamela Gay: And everyone who wants to learn more about this, the Mad Art blog, it’s Ryan, it’s Surly Amy – Amy Davis Roth, one of our friends – a whole group of amazing people. A. Kovacs is in there as a science-adjacent and art-adjacent person. They’re bringing the combination of science and art and how you live right on that boundary in the land called Steam.
So, go check out their blog. It’s one of my favorites. They also have a great podcast. Yeah, they do great things and Ryan says how you can make your own – well, he doesn’t detail all of it – but he talks about making the Voyager record replicas.
Fraser Cain: Fantastic.
Once again, it’s time to thank Casper Mattresses for sponsoring Astronomy Cast. We, of course, have been sleeping on Casper mattresses for the better part of a year and a half now. Still super-happy. I’ve been through all of the seasons. Now we’re in – on Vancouver Island – our cold, rainy, wet season and it’s great. It keeps you warm when you hop into bed. It’s got this great, sort of two kinds of foam – this latex foam and memory foam. And still comfortable, doesn’t seem to be breaking down. I’m really happy to be using it.
And, of course, one of the greatest parts about getting a Casper mattress is they send it to your door and it comes in this box. And then you open it up and the whole – You can’t believe that a full mattress was in a box that small. So, that’s really, like, literally half the fun of getting a Casper mattress. Their prices are great: $950 for king size mattress; $500 for twin size mattress. So, that’s like – as the price of a regular mattress or even cheaper. They have a risk-free trial and return policy, so you can sleep on it for a hundred days – with free delivery – and then just return it if you disagree. So it’s a great value and you can just give it a try.
And Casper is offering $50.00 discount to listeners from Astronomy Cast. So, just go to www.casper.com/astro and then use the promo code: Astro. Of course, some terms and conditions will apply.
So, once again, thanks to Casper Mattresses for sponsoring Astronomy Cast. Give it a try. I think you will really enjoy this mattress. Welcome to the future.
Alright, let’s do the show.
So today, Mars is a desolate wasteland, with dusty red rocks and sand stretching out to the horizon. But billions of years ago, it was a vastly different world. It was blue, with oceans, rivers, lakes, and maybe life? Let’s tell the story of geology on Mars; how we got from that world to the crappy one we see today.
Dr. Pamela Gay: It’s not crappy, it’s just desiccated.
Fraser Cain: Mars, today, sucks.
Dr. Pamela Gay: It’s desiccated.
Fraser Cain: It is desiccated, it is dry, it is sandy, it has no atmosphere. It’s no Venus. I mean, Venus is, like, really trying to kill you but Mars is trying to kill you too; either one. I mean, Antarctica is a tropical paradise compared to Mars, so –
Dr. Pamela Gay: Or at least a human paradise, because Mars does have its warm days.
Fraser Cain: Right.
Dr. Pamela Gay: Antarctica – not so much.
Fraser Cain: Yeah, occasionally warmer than here in Canada. So –
Dr. Pamela Gay: Yeah.
Fraser Cain: Yeah.
Dr. Pamela Gay: But it’s kind of airless. That’s kind of a problem. It has air, just not enough. Will kill you, you’re right. Listen to our past few episodes.
Fraser Cain: Yep. Yeah, you’ll know how little it has.
Alright. So let’s go back to the beginning. Should we talk about – So, let’s talk about the Mars just a bit. Well, you just mentioned it a bit right now. So let’s kind of go back to the beginning, right back to the formation of Mars – as with the other objects in the solar system – how did Mars even just come to be? How did it get that first geologic stage?
Dr. Pamela Gay: Well, like everything else in our solar system, about 4 ½ billion years ago, it started to solidify out of a molten, nasty, gravitationally-held-together ball of stuff.
So, early on, we had the solar nebula big ol’ ball of dust and gas that flattened and a bulk of the material ended up in the core, in the center, where we now see the sun, the remaining disc of material; where you had a couple of particles collide, they got a little bit bigger; they were able to gravitationally pull in a couple more particles, got gravitationally a little bit bigger. You see where this goes.
So things grew, we ended up with planets. It took a while for things to, like, settle into orbits. And so, the real problem was that, for the first, well, many billion years – well, couple billion years. I won’t say “many” but for the first couple billion years – we still had fairly large chunks of stuff flying around the solar system.
But, by about 4 ½ billion years ago, we had Mars.
Fraser Cain: And what was it made out of?
Dr. Pamela Gay: Dust, gas, that became dirt.
Fraser Cain: What kind of stuff?
Dr. Pamela Gay: It’s the same stuff we’re made of. It’s a hodgepodge of the periodic table. You’ve got carbon, you’ve got silica, you’ve got iron – which is kind of what makes it red and rusty, the way it is. It didn’t really have an atmosphere, initially – that came a little bit later – but it’s just crustal material, a lot like what we see on Earth.
The big difference was, because it was further out in the solar nebula, the ratios of the different types of volatiles that it had were slightly different. So, when you pick up a Mars rock, open it up, the gasses trapped inside of it have different atomic ratios than the gasses here on Earth.
Fraser Cain: Okay. So we’ve got this glowing ball of leftover solar nebula – some of the heavier elements in the solar nebula. It cools down on the outside but continues to roil and boil inside.
Volcanism – you get a period of volcanism on Mars, like we had here on Earth?
Dr. Pamela Gay: You do. And that actually is just now finally starting to settle out. So, if we look through the different periods of Mars history, we find that from about 4.1 to 3.8 gigayears ago is where we were seeing the world. It had volcanism but the things that really denote this period in time is it was when there was really high impact rates and you had a lot of erosion from the massive impacts that formed, that were actually so massive that they melted the surrounding rock.
So, during that Pre-Noachian – because we name things stupid – Pre-Noachian period, you have the formation, for instance, of the Hellas impact basin, where sort of, kind of the floor of that basin is what we now refer to as Hellas Planitia – so, the Hellas plain of Mars. This is one of the big basins down in the southern hemisphere of Mars.
Fraser Cain: So you’ve got this, you know, glowing body trying to cool down, still having rampant volcanism and then it’s just getting pounded by objects, as everything is in this kind of, you know – in this early bombardment. Bombardment – walk it off.
Dr. Pamela Gay: Yeah.
Fraser Cain: The bombardment in the entire solar system. Earth was having it too. The moon was having it too. We could see – The surface of the moon, we could see the destruction that happened to any object that was caught out in space when this was happening. It was awful.
Dr. Pamela Gay: And this early period was mostly just marked with, you have a rapidly cooling world that is getting hit with large stuff. As I said, that’s when the Hellas impact basin formed.
But the Pre-Noachian period quickly evolved into the Noachian period. And it was during this Noachian period – from about 4.1 to 3.7 billion years ago – that we started to have the surface of Mars, not just marked up with impact craters, but this is the point at which the stuff impacting Mars started leaving behind sufficient water that we start to see water channels on Mars as well.
And so, here, we start to see extensive erosion from water flowing across the surface. We see river basins, river valley networks; all of the kind of stuff that, if you’ve ever flown over the American Southwest, you look out and you’re like, “Wow! It is a desert that was shaped by water.”
Now Mars is that desert shaped by water in the parts of the surface that haven’t been eroded since the Noachian period.
Fraser Cain: And so, that water – I mean, I guess it was a little warmer back then. It had a thicker atmosphere of some variety.
Dr. Pamela Gay: Yes.
Fraser Cain: And it had water, just like Earth does, delivered – you know, we’ve done whole episodes on this – delivered somehow, either by comets or, you know, in place.
Dr. Pamela Gay: Asteroids.
Fraser Cain: Yeah – comets, asteroids, comatsteroids or, perhaps, you know – One of the other competing theories is just that there was water around at the time and it sort of collected into the planets as well. But, you know, it had –
Dr. Pamela Gay: Water.
Fraser Cain: – plenty of water on the surface.
Dr. Pamela Gay: Yes. And so, this was also a geologically active period.
So, when you look at Mars, you see that there’s – well, Olympus Mons kind of stands out on its own – but beside Olympus Mons is the set of three different volcanoes on a large, bulgy structure. And this large, bulgy structure is Tharsis bulge – nice, obvious name – and it’s the Tharsis Montes (Tharsis Mountains) that are those string of volcanoes across the bulge. And it’s thought that these volcanoes were also – well, not the volcanoes but the whole bulge structure itself – was formed during this Noachian period.
Fraser Cain: Sorry. What’s the timeline on the Noachian? When about – Was that after the formation of Mars?
Dr. Pamela Gay: Yeah, so it’s 4.1 to 3.7 billion years ago.
Fraser Cain: Okay. And is this sort of before or after the late heavy bombardment that the rest of the –
Dr. Pamela Gay: This is during the late heavy bombardment.
Fraser Cain: Okay, okay.
Dr. Pamela Gay: So, we have a world that is essentially getting hit and hit and hit and hit and, in addition to this, its insides are still hot and angry. So, its insides are coming out through the volcanoes.
This was one of those times when it’s impossible to describe the world as anything other than “alive”. Because you have the active volcanism, you have the active water features, you have things falling out of the sky and reshaping the surface. It’s kind of an amazingly dynamic period. There was rainfall, we think.
Fraser Cain: Yeah. Yeah, back when Mars had water, they could have rainfall.
Dr. Pamela Gay: And so, looking at all of this, this is where we want to put the rovers when we want to go look for fossil life. We want to go to the places that have been least eroded, least lava’d over, least cratered over, that allow us to get to the fossils that may be there from the Noachian period.
Fraser Cain: So, paint a mental picture for us a bit, here. If we were standing on the surface of Mars at this time, what’s a place on Earth that would look kind of familiar? Would it be like the Big Island of Hawaii – like, where you’ve just got sort of lava plains and – except there’s like –
Dr. Pamela Gay: Yeah.
Fraser Cain: Except there’s –
Dr. Pamela Gay: So –
Fraser Cain: – regular meteorite strikes going on all around you?
Dr. Pamela Gay: So, I think the Big Island of Hawaii is probably a fairly good – or the Aleutian Mountains, the Aleutian volcanoes may be even slightly better, up along the coast of Alaska. So you have this icy, snowy, cold – with molten rock coming out of the ground. And then, you also –
We can’t even imagine what the heavy bombardment would have been like. It’s like something out of the Pern books, where you just periodically have stuff just raining down. And some of them are so big that you get a big asteroid hitting Mars and it’s throwing up new chunks that scatter all across the world, causing a secondary array of impacts.
And the thing about impacts that we like to forget is, they have a lot of kinetic energy. And this kinetic energy gets given off as heat, frictionally, as it passes through the atmosphere, as well as the impact that occurs. And as it heats up the atmosphere – one rock, we’re not going to notice the atmosphere getting heated up – a few thousand rocks, that seriously changes your climate.
Go read Seveneves, if you haven’t. It talks about this beautifully.
Fraser Cain: Okay. So, what comes after that period?
Dr. Pamela Gay: So, after this, we get into the much more boring Hesperian period.
So, the Hesperian period – not quite so much of the rainy, everything else. You have lakes and seas that formed in the northern lowlands, so you still have some water at this point. This is where we start getting the extensive lava fields formed. So, when you look out across Mars, there’s all of these amazing places where you see lava that just goes out and fills in craters partially and it’s beautiful geologic features. These largely come out of the Hesperian period; so this was 3.7 to 3 billion years ago and this is also when Olympus Mons forms.
So, you have Tharsis forming first and then Olympus Mons comes a little bit later, off to the side. This, to me, has always reminded me of how the Hawaiian Islands are all different ages but, when you’re just looking at a map, you don’t notice that immediately. Well, in this case, the Tharsis bulge and its volcanoes are older in formation than Olympus Mons, which came just a little bit later.
This is also when we had catastrophic releases of water that formed various channels, cutting through the plains. And so, this is kind of that period after the tumult, where you have various oceans breaking free and just sort of flooding across. You have the lava flowing across. So this, to me, is less of a “impact everything from the sky and rain on it” period, to more of a, you’re standing there and all of a sudden, all the fluid – whether it be liquid rock or liquid water – decides “I’m going somewhere else.”
So this is the period on Mars where things are flowing around and reshaping the surface of Mars through their flows.
Fraser Cain: So – and this was sort of the big clue that – The clues that NASA was really searching for was this idea that, you know – that Curiosity really turned up – is that water was acting on the surface of Mars for vast periods of time; long enough, potentially, for life to have evolved. And so this was this time where you had this – When everything wasn’t on fire, because – or, you know – or felt like it was on fire because of these meteorite strikes – the times in between – or, you know, after – when it finally had cooled back down and water could form and act on the surface of Mars, these are the times that you got.
Dr. Pamela Gay: Right.
So, one quick and dirty way to think of this – and this is where I feel the need to remind everyone: I’m a stars and galaxies kind of girl who’s totally fascinated with planets and has the privilege of attending professional planetary science conferences. And so the way me and my can-describe-stars-in-better-detail brain thinks of these periods at a certain level, is the Noachian period is when you have death from the skies. You have the heavy bombardment going on. You have rain. You have the ground beneath you rising up. This is the vertical death, you might say.
Whereas, during the Hesperian period, you have flowy death – so, horizontal death – where you have –
Fraser Cain: I love this!
Dr. Pamela Gay: You have lava flowing across the surface. You have oceans flooding across the surface. And so, this is another massive reshaping of the world but, in this case, the reshaping is the liquids flowing across molten rock; water changing the surface horizontally.
Fraser Cain: That’s crazy.
And a lot of the sort of – you know, like this whole thing with the Tharsis bulge, right? As this bulge formed and these volcanoes formed, you’ve got, like, a change in the balance of the rotation of Mars. It’s thought that, in times – that it actually sort of rolled over on its side; changed the orientation of its oceans. There’s a lot of very vast changes that were happening to the surface of Mars during this period. You know, it wasn’t – as you said – being destroyed from above but it was still under fairly dramatic change, over the course of billions of years.
And we think about things like the Valles Marineris, right? You know, that –
Dr. Pamela Gay: Yeah – trench.
Fraser Cain: The largest trench in the solar system formed as, like, one of the cracks because of this bulge.
Dr. Pamela Gay: Right.
Fraser Cain: So, it’s sort of an amazing sort of resurfacing of the features on Mars.
Dr. Pamela Gay: And one of the things that we really struggle with, looking at Mars, is it’s currently divided into the northern lowlands and the southern highlands such that, if you do two circular maps – one centered on the north pole and one centered at the south pole – you would not know these were the same planet. They are vastly geologically different, where the southern lowlands are basically these long plains. It’s – I’m going to mispronounce the Latin. I’m sorry. Everyone who’s been a longtime listener knows, I read more than I speak and this gets me into trouble.
It’s Vastitas Borealis – is these names for this long, large area of just mostly flatness. Whereas, when you look at the southern highlands, you see this ancient, pox-marked, cratered-up area, where you have the Arger – again, I’m destroying pronunciation. You have two giant plains formed in impact basins. You have the Olympus and Tharsis volcanoes. You have the Hellas Planitia. It’s this area that is just beat to expletive.
And one of the ideas is that this vast northern lowlands is the remnants of a massive crater. So that’s one giant crater. And the planet actually shifted its rotational axis a lot like Vesta must have when it had a similarly catastrophic impact. But we don’t know if this is what it was for sure. It looks like it was a vast ocean at some point but we don’t know for sure.
And a lot of our “don’t know for sure” comes from not having put enough robots on Mars yet.
Fraser Cain: Right, yeah.
Dr. Pamela Gay: And it’s –
Fraser Cain: More rovers, more sure.
Dr. Pamela Gay: And the way we get at the ages of these things – to be entirely honest – is we look at each of these regions and we count craters. This is why we need all of you –
Fraser Cain: One, two –
Dr. Pamela Gay: – to go to Mars Mappers and –
Fraser Cain: Yep.
Dr. Pamela Gay: – count craters. And we look for lava that overlays different areas. So, basically, if you have large basin formed by impact that has lava overcutting part of it, that lava is superimposed on the crater; so you know the lava came later. And then, if the lava has craters on top of it, you know those craters came after the lava came.
So, we look for what overlays what in the overlapping regions and we count craters and, where there are the most craters, it’s the oldest surface; where there are the fewest craters, that’s the youngest, most recently eroded surface. And, looking at everything, it’s – this is where we start to get the ages. And then, to get more than just a relative age of knowing, “Well, this lava is younger than this crater that it goes over.” To get an actual age, we look at the moon – Earth’s moon – way over here, far across the solar system. And we count craters on it. And we figure out, “Okay, this region with this many crater densities probably corresponds to this region on Mars with similar crater densities.” And that’s kind of lame.
What we want to do is go pick up rocks from each of these surface areas and say, “This rock comes from what we call the Noachian period.” And when we count up the atoms in the rock and use radio dating, with all the different isotopes that we have at our disposal to do this kind of dating, we now can confirm what each of these periods corresponds to in billions of years versus just hoping that it mostly matches the moon.
Fraser Cain: So, just to kind of recap our story so far, we’ve got the – I guess, the Pre-Noachian period, when sort of everything was just coming together; the Noachian period, where you’ve got sort of the oldest surfaces on Mars – 4.1, 3.7 billion years ago; the Hesperian?
Dr. Pamela Gay: Yeah.
Fraser Cain: Hesperian period, where you’ve got – as you said – the sort of death from the sides, the – As opposed to the death from above, you’ve got the – sort of the large features really taking shape and forming; and that brings us to about 3 billion years ago –
Dr. Pamela Gay: The Amazonian period, which – So, this is one of those things that ticks me off.
Fraser Cain: That’s the worst name.
Dr. Pamela Gay: So, there was a wet and wild period on Mars. We called it the Noachian period. Amazonian, to me, means wet.
Fraser Cain: Mm-hmm.
Dr. Pamela Gay: That’s tropical areas.
Fraser Cain: Yeah.
Dr. Pamela Gay: It’s the Amazon.
Fraser Cain: Yeah.
Dr. Pamela Gay: No.
Fraser Cain: No.
Dr. Pamela Gay: The dry, desolate, desiccated time period on Mars we named the Amazonian and I don’t know – well, I do know why. I can look it up. It’s named after when the Amazonis Planitia was formed.
Nonetheless, I am bitter – clearly, clearly – but –
Fraser Cain: No, you have every reason to be bitter about this. The Mars that we’ve come to kind of know and fear, happened in this period.
Dr. Pamela Gay: Right. So –
Fraser Cain: We would have been happy back in the – well, you know, in between bombardments –
Dr. Pamela Gay: Yes.
Fraser Cain: We would have been happy back in the Noachian period.
Dr. Pamela Gay: Right.
Fraser Cain: Even, a little bit, the Hesperian period. But in the Amazonian period – no, thank you.
Dr. Pamela Gay: Yeah. So this is the period of time that we call “now”. It’s also the period –
Fraser Cain: Now – the Now period.
Dr. Pamela Gay: It’s the Now period. It started about three gigayears ago – 3 billion years ago, for those who don’t speak scientific method – or scientific notation, rather.
So, starting about 3 billion years ago, the solar system became a little bit kinder. We had less death from the skies, less impacts. We still have impacts. I mean, the people who’ve had asteroids rain down on them on our planet in the past ten years are fully aware of this. But it’s not the kind of thing where we see giant impact craters forming on a regular basis. We see small impact craters falling periodically, which is not something that – Like, your average New Yorker doesn’t worry about an asteroid destroying their city.
So, during this Amazonian period: Fewer impacts, less volcanism – less and less volcanism over time. During the beginning of this period, there was glacial activity as all the water froze. We still have some glacial activity on Mars, not a lot. So, we started to go from the “Death from the skies”, to the “Death from the sides”, to the “It’s just cold and boring.” And the Amazonian period is kind of the cold and boring phase.
We still have minor water releases. We call them “dark streaks”.
Fraser Cain: Yep.
Dr. Pamela Gay: But mostly, it’s just a cold, dead, getting colder, deader world. And this is what we get to go explore.
Fraser Cain: Yeah. Go ahead, Elon Musk. Live on that.
Dr. Pamela Gay: Exactly, exactly.
Fraser Cain: Right? Where every moment is a desperate fight for survival on a hellscape – No. A frozen desert that doesn’t want you.
Dr. Pamela Gay: Exactly.
Fraser Cain: Yep.
Yeah, that’s a – yeah. Like, I – People always – Again, people always ask me, “Do you want to go live on Mars?”
I would like to visit safely but I like Earth just fine, thank you very much.
Dr. Pamela Gay: Yeah. I’d be fine to go, like, where you realize, “I probably have like five or ten more years left in me. I’m just gonna go to Mars.” I’ll die there. I’m fine with that but –
Fraser Cain: Oh, you’ll die there.
Dr. Pamela Gay: Exactly, exactly.
Fraser Cain: Yeah, yeah.
But – So, I mean, are there any more dramatic periods left for the future of Mars? Are we really kind of – Like, it’s just – It’s gonna turn into a popsicle at this point, right?
Dr. Pamela Gay: Well, it’s probably settled out into a steady state right now. It – Unless it has a new massive era of bombardment – in which case, we’re all expletived – yeah, it’s steady-state. It’s not like it currently has a greenhouse effect or anything to drive a greenhouse effect. Its atmosphere is pretty good the way it is. Yeah, the sun’s still tearing things off but it’s such a slow process nowadays that it isn’t noticeably getting a thinner atmosphere over time.
So, yeah. There’s going to be long-term cycles driven by the slow change and tilt of its pole, that will cause exactly where the icecaps are to shift over time, that will cause some variations. But we’re now looking at the eon-to-eon shifts that are driven by the slow-changing cycles of polar tilt and orbital shift.
Fraser Cain: Mm-hmm.
Dr. Pamela Gay: And the slow, gradual changes that are driven by our sun’s very gradual changes over the millions of years.
Fraser Cain: Yeah. Like 7 billion years from now, things are gonna heat up.
Dr. Pamela Gay: Oh, yeah. Oh, yeah. But right now, we’re pretty steady-state.
Fraser Cain: Yeah, exactly.
Alright. Well, who knows if this is the last episode about Mars? We don’t know. We may talk about Mars some more next week. There’s still lots of stuff to talk about and I kind of am enjoying myself.
People always really like these series, so we’re going to continue. Who knows?
Dr. Pamela Gay: You know, volcanoes might be a good way to blow out the end of the year.
Fraser Cain: Literally.
Alright. Thanks, Pamela.
Dr. Pamela Gay: Okay.
Fraser Cain: We’ll see ya next week.
Dr. Pamela Gay: Buh-bye.
Male Speaker: Thank you for listening to Astronomy Cast, a non-profit resource provided by Astrosphere New Media Association, Fraser Cain and Dr. Pamela Gay. You can find show notes and transcripts for every episode at astronomycast.com. You can email us at firstname.lastname@example.org. Tweet us @astronomycast. Like us on Facebook or circle us on Google Plus.
We record the show live on YouTube every Friday at 1:30 p.m. Pacific, 4:30 p.m. Eastern or 2030 GMT. If you missed the live event, you can always catch up over at cosmoquest.org or our YouTube page. To subscribe to the show, point your podcatching software at astronomycast.com/podcast.xml, or subscribe directly from iTunes. Our music is provided by Travis Serl and the show was edited by Chad Weber.
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Duration: 31 minutes