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The Geology of Iridium, The Meteorite Tracker · PlanetGeo: The Geology Podcast

• 0:03
  SPEAKER_00: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you.
• 0:15
  SPEAKER_00: Uh check your audio.
• 0:17
  SPEAKER_00: Check, check, check.
• 0:18
  SPEAKER_00: And with that, we're off.
• 0:23
  SPEAKER_00: Chris, we're back.
• 0:24
  SPEAKER_00: We're back.
• 0:25
  SPEAKER_00: Here we go.
• 0:25
  SPEAKER_00: Face to face.
• 0:26
  SPEAKER_00: Face to face still.
• 0:27
  SPEAKER_00: It's so good.
• 0:27
  SPEAKER_00: It is.
• 0:28
  SPEAKER_00: It's good.
• 0:28
  SPEAKER_00: Really good.
• 0:29
  SPEAKER_00: Really good.
• 0:29
  SPEAKER_00: We're getting a lot done.
• 0:30
  SPEAKER_00: We've been cranking, man.
• 0:31
  SPEAKER_00: We have.
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  SPEAKER_00: It's good.
• 0:32
  SPEAKER_00: Yep.
• 0:32
  SPEAKER_00: All right.
• 0:33
  SPEAKER_00: So what are we doing?
• 0:34
  SPEAKER_00: Today we're following up from last week's episode.
• 0:36
  SPEAKER_01: Yeah, we're going to follow up on the uh meteor, impact, and Cretaceous event.
• 0:41
  SPEAKER_00: Totally.
• 0:41
  SPEAKER_01: Okay.
• 0:42
  SPEAKER_01: And so the, you know, we've spent a lot of time talking about iridium in that because that was an important part of the discovery.
• 0:48
  SPEAKER_01: And so today we're going to talk about iridium.
• 0:51
  SPEAKER_00: Yeah.
• 0:51
  SPEAKER_00: Dive into a little bit more details, like why is this a meteorite tracer basically?
• 0:55
  SPEAKER_00: And iridium is an element.
• 0:56
  SPEAKER_00: Okay.
• 0:56
  SPEAKER_00: So if you look on the periodic table, one of the boxes is iridium.
• 1:00
  SPEAKER_00: It says IR, that's iridium, right?
• 1:01
  SPEAKER_00: Yep.
• 1:02
  SPEAKER_01: Yep.
• 1:02
  SPEAKER_00: And uh the question is why do we use iridium?
• 1:06
  SPEAKER_00: Like why is iridium this sort of tracer of meteorite impacts?
• 1:10
  SPEAKER_00: That's kind of the question here.
• 1:11
  SPEAKER_01: Okay.
• 1:12
  SPEAKER_01: Well, iridium is a higher concentration in meteorites.
• 1:16
  SPEAKER_01: It's not really high, but it's higher.
• 1:19
  SPEAKER_01: Right.
• 1:20
  SPEAKER_01: Okay.
• 1:20
  SPEAKER_01: Than Earth.
• 1:21
  SPEAKER_00: Than earth than like background Earth.
• 1:22
  SPEAKER_00: Right.
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  SPEAKER_01: To put it in perspective, it has a concentration meteors of about 500 parts per billion.
• 1:29
  SPEAKER_01: So that's not a lot.
• 1:30
  SPEAKER_01: You take a billion atoms of a meteorite.
• 1:33
  SPEAKER_01: A billion.
• 1:34
  SPEAKER_01: Yeah.
• 1:34
  SPEAKER_01: And wait a minute.
• 1:36
  SPEAKER_00: Yep.
• 1:36
  SPEAKER_01: Yeah.
• 1:37
  SPEAKER_01: 500 of them.
• 1:39
  SPEAKER_00: It feels very small, the numbers.
• 1:41
  SPEAKER_01: 500 of them would be iridium.
• 1:43
  SPEAKER_01: Yeah.
• 1:44
  SPEAKER_01: So not a really high concentration, which when you when you think about that, actually, it's really interesting because this played such a big role in the discovery of this event.
• 1:53
  SPEAKER_01: Okay.
• 1:54
  SPEAKER_01: And and uh the concentration is not exceedingly high like you might want to think.
• 1:59
  SPEAKER_00: Right.
• 2:00
  SPEAKER_00: It's very hard to imagine 500 parts per billion, like 500 out of a billion.
• 2:05
  SPEAKER_00: So I don't know.
• 2:06
  SPEAKER_00: Sometimes it's easier to think of it with a you know probability.
• 2:09
  SPEAKER_00: Like if you just grabbed one atom of a meteorite randomly, one atom, you'd have less than a million one in a million chance of pulling out an iridium atom.
• 2:17
  SPEAKER_00: So very small chance of finding one iridium atom.
• 2:19
  SPEAKER_01: Yeah.
• 2:20
  SPEAKER_01: Which is like, okay, let's let's put this in perspective maybe even a little further.
• 2:24
  SPEAKER_01: You talk about a very important gas.
• 2:25
  SPEAKER_01: We've talked, we've had an episode on it, the greenhouse effect, okay.
• 2:28
  SPEAKER_01: Carbon dioxide, the concentration of carbon dioxide in our atmosphere.
• 2:31
  SPEAKER_01: This very important controversial greenhouse effect gas has a concentration of about 420 parts per million in our atmosphere.
• 2:42
  SPEAKER_01: So a hundred times more concentrated in our atmosphere than what you would find in meteors.
• 2:46
  SPEAKER_00: Right.
• 2:47
  SPEAKER_00: And it just, I mean, that's a great point, Chris.
• 2:48
  SPEAKER_00: It kind of shows how small concentration of great points.
• 2:53
  SPEAKER_00: Yeah, may emphasize that.
• 2:55
  SPEAKER_00: Uh I think you know, it shows this like low concentration stuff has a big impact in Earth.
• 3:01
  SPEAKER_00: Like the, you know, it's kind of an interesting uh sort of take-home point there.
• 3:05
  SPEAKER_00: Um, but the question then is okay, it's high in meteorites or it's higher in Earth than meteorites.
• 3:09
  SPEAKER_00: It's still pretty low in meteorites, but it's higher than Earth.
• 3:11
  SPEAKER_00: The question is why?
• 3:12
  SPEAKER_00: And that that question is important because Earth is actually made up of meteorites, right?
• 3:16
  SPEAKER_00: Like Earth was formed from a whole bunch of meteorites that came together to form Earth.
• 3:19
  SPEAKER_00: So why don't we have as much iridium?
• 3:21
  SPEAKER_01: Well, Earth is differentiated.
• 3:24
  SPEAKER_01: Okay, and so we're we're layered, and it basically it goes from the least dense minerals exist on the surface of the earth, the crust, and it gets denser and denser and denser as you go down to the core.
• 3:37
  SPEAKER_01: So iridium is not high concentration in meteors, but it's much higher in this clay layer than anywhere else in the crust.
• 3:50
  SPEAKER_00: So iridium is not actually high in meteorites.
• 3:52
  SPEAKER_00: 500 parts per billion is not high, but it's higher than an Earth's crust.
• 3:56
  SPEAKER_00: In an Earth's crust, it's very, very, very low.
• 3:59
  SPEAKER_00: And so Earth's crust, the the sort of tippy top layer of Earth, has 3.2 parts per billion.
• 4:06
  SPEAKER_00: So meteorites have 500 and Earth's crust has 3.2 parts per billion.
• 4:10
  SPEAKER_00: So if you a meteorite comes in and hits the crust, you know, it spreads iridium all over the place, and in these clay layers, it can be a lot closer to 500 parts per billion than 3.2.
• 4:21
  SPEAKER_01: Yes.
• 4:22
  SPEAKER_01: So why didn't this iridium, like the rest of the iridium, sink to the core?
• 4:27
  SPEAKER_00: Right.
• 4:28
  SPEAKER_00: So iridium is what's called a siderophile element, and siderophile means iron loving, and this goes back to like some of the earliest geochemistry.
• 4:38
  SPEAKER_00: Hold on, hold on.
• 4:38
  SPEAKER_00: I'm sorry to interrupt.
• 4:39
  SPEAKER_00: Is that not what you meant?
• 4:40
  SPEAKER_00: No.
• 4:41
  SPEAKER_01: Ciderot goes back to the ancient Greek, which means the iron one.
• 4:45
  SPEAKER_01: Oh I think I want you to call me from here on out.
• 4:48
  SPEAKER_01: You just reported me as Cidero.
• 4:50
  SPEAKER_01: Yeah.
• 4:53
  SPEAKER_00: That's a good one.
• 4:54
  SPEAKER_00: I like that.
• 4:55
  SPEAKER_00: That's a funny one.
• 4:56
  SPEAKER_00: Okay.
• 4:56
  SPEAKER_00: Uh the Cidero.
• 4:57
  SPEAKER_00: Oh, dear Cidero, can you tell me why?
• 5:00
  SPEAKER_00: I in the that's that's quite good.
• 5:03
  SPEAKER_00: I like that.
• 5:04
  SPEAKER_00: So it is Siderophile, it loves iron like Ciddero, but uh it means that it is happier bonding with iron than bonding with anything else.
• 5:13
  SPEAKER_00: And so it likes being with iron, which means that when earth differentiated, you said you know it earth is differentiated, it separates out into density.
• 5:22
  SPEAKER_00: Because it was all molten, it was all molten.
• 5:24
  SPEAKER_00: Iron is very dense, so the liquid iron sank to the bottom, formed Earth's core.
• 5:29
  SPEAKER_00: Iridium went with it because iridium likes to form metallic bonds with iron.
• 5:34
  SPEAKER_00: It does not like to bond with silica or oxygen, which are the main elements that make up the mantle and the crust.
• 5:40
  SPEAKER_01: Exactly.
• 5:41
  SPEAKER_01: And so when this meteor event happened, it happened after the differentiation of the earth happened.
• 5:48
  SPEAKER_00: Right.
• 5:49
  SPEAKER_00: So that's a great point.
• 5:50
  SPEAKER_00: The differentiation of the earth is forming the core, that happened, you know, four billion years ago, more than four billion years ago.
• 5:56
  SPEAKER_00: So this meteorite impact is a much later event.
• 5:58
  SPEAKER_00: Earth's crust and mantle already lost all of the iridium, and so that's all in the core now.
• 6:03
  SPEAKER_00: We will never really have access to that iridium in any high concentration, really.
• 6:07
  SPEAKER_00: Yeah.
• 6:08
  SPEAKER_00: So these sinking, when that differentiation happened, these sink, these blobs of iron when the earth was entirely molten, you kind of visualize these big blobs of iron sinking down through silicate liquid, and they just soaked up all of the iridium, and now it's in the core.
• 6:23
  SPEAKER_01: So one of the interesting things about iridium is that it is the most corrosion-resistant substance known on earth.
• 6:30
  SPEAKER_00: That's really amazing.
• 6:31
  SPEAKER_00: I didn't actually know that before we started looking into this.
• 6:34
  SPEAKER_01: That's very cool, even at very high temperatures.
• 6:36
  SPEAKER_01: But that makes sense with what you just said, because you said that iridium loves iron, but it hates oxygen.
• 6:43
  SPEAKER_01: And oxygen is it well, it oxidizes, it rusts things.
• 6:46
  SPEAKER_01: And so this hates oxygen, doesn't corrode because it doesn't rust.
• 6:50
  SPEAKER_00: Yeah, so like iron, you know, rusts, it forms hematide, that's a that's iron bonding with oxygen.
• 6:55
  SPEAKER_00: Just like you said, iridium doesn't do that.
• 6:56
  SPEAKER_00: So it's used in things that need to be very corrosion resistant.
• 7:00
  SPEAKER_00: Compass needles, pen tips, contacts and spark plugs, any sort of high voltage electrical contact usually has some iridium in it because it's so corrosion resistant.
• 7:11
  SPEAKER_01: It's and it's also used in radiation treatment for cancer patients, isn't it?
• 7:15
  SPEAKER_00: Yeah, right.
• 7:15
  SPEAKER_00: So you can uh there are some isotopes of iridium.
• 7:18
  SPEAKER_00: Usually they're synthetically produced, but they decay and they produce radiation that we can use for medical treatment things.
• 7:23
  SPEAKER_00: But the total amount of iridium like mined and used on a yearly basis is very small.
• 7:28
  SPEAKER_00: It's like three tons or something, which is tiny at the global scale.
• 7:31
  SPEAKER_00: It's a very, very sort of small amount.
• 7:34
  SPEAKER_00: Um much does that go for?
• 7:36
  SPEAKER_01: Yeah, and so rare.
• 7:38
  SPEAKER_00: I don't know.
• 7:39
  SPEAKER_00: I the one number I saw was like forty two hundred dollars, four thousand dollars per 100 grams of iridium, which is very expensive.
• 7:48
  SPEAKER_00: I don't know how that compares really to other sort of that's not even close to a pound.
• 7:52
  SPEAKER_00: Yeah, no, no, it's it's like uh, you know, what is that, a quarter pound or something like that, maybe?
• 7:56
  SPEAKER_00: Yeah, close, yeah somewhere around there.
• 7:58
  SPEAKER_01: All right, so just to be clear, I want to make sure we're clear on this.
• 8:02
  SPEAKER_01: Did we hit it well enough with why iridium is so, so rare in the crust?
• 8:08
  SPEAKER_00: I think so.
• 8:09
  SPEAKER_00: I mean, uh yeah, I think it kind of in this earth distillation process, iridium ends up getting distilled out or ends up down in the mantle or down in the cr in the core.
• 8:18
  SPEAKER_01: It just sinks because it's denser.
• 8:19
  SPEAKER_00: It's it bonds with the denser stuff.
• 8:21
  SPEAKER_01: It goes with the denser things, it goes with citero.
• 8:24
  SPEAKER_00: Exactly.
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  SPEAKER_00: And so what we see on the surface does not have any iridium.
• 8:28
  SPEAKER_00: So it makes it a great tracer of meteorites.
• 8:30
  SPEAKER_00: It's it's a beautiful element for tracing meteorite impacts because it's so rare on the surface.
• 8:35
  SPEAKER_01: That's a good point, is that iridium was not uh it wasn't the only time it was used as a tracer for meteors.
• 8:43
  SPEAKER_01: There are other events that like there's an event that happened 2.5 million years ago that we use iridium to trace that event to.
• 8:51
  SPEAKER_00: Exactly.
• 8:52
  SPEAKER_00: And then you can go back all the way back to 3.4 billion years ago.
• 8:55
  SPEAKER_00: There's a sphero bed that is high iridium concentrations in it, and uh people have used this to say, yeah, there's a 3.4 billion year old, now not million, billion-year-old, meteorite impact that occurred because we can see this iridium concentration spike or this iridium anomaly, as we call it.
• 9:12
  SPEAKER_00: That's crazy.
• 9:13
  SPEAKER_00: So it's this great tracer because of these sort of various chemical properties, which explain why it's useful in like compass needles.
• 9:18
  SPEAKER_00: That was kind of interesting.
• 9:19
  SPEAKER_00: I didn't know that compass needles had iridium.
• 9:21
  SPEAKER_00: I didn't either.
• 9:21
  SPEAKER_00: There we go.
• 9:22
  SPEAKER_00: Now we do we all learned something.
• 9:23
  SPEAKER_00: Hey, the more you know, the more you know.
• 9:25
  SPEAKER_00: All right, Chris.
• 9:26
  SPEAKER_00: I think that that nails it.
• 9:28
  SPEAKER_00: We good?
• 9:28
  SPEAKER_00: We're good.
• 9:28
  SPEAKER_00: Let's wrap it up.
• 9:29
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• 9:31
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• 9:38
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• 9:40
  SPEAKER_00: Cheers.
• 9:41
  SPEAKER_00: Cheers.