We ask Dr. Ben Weiss, an MIT expert on meteorites, how to spot the difference between rocks that formed on Earth and ones that fell from space.

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ERIC: From the Museum of Science in Boston, this is Pulsar, a podcast where we identify answers to the rockiest questions we've ever gotten from our visitors. I'm your host, Eric, and fairly often, we get an email from someone in our community who has found an interesting rock, and they want us to tell them if that rock fell from space. My guest today is Dr. Ben Weiss, a professor of planetary sciences right down the river at the Massachusetts Institute of Technology. Dr. Weiss, thanks for coming to the museum today.

BEN: Happy to be here.

ERIC: First of all, I want to talk about what a meteorite is, because there's all those words: meteorite, meteoroid, that kind of go together. So when someone thinks they found a meteorite, they're talking about something they picked up off the ground, but it came from space, right?

BEN: A meteorite is a rock that came from space. And it's different from say, a meteor, which is a flash of light in the sky, which is made when a rock is coming into the Earth's atmosphere. If it makes it to the ground, if it doesn't burn up, it becomes a meteorite.

ERIC: Now, how rare are these? They must be pretty rare, because there's not a lot of them out there. But if you find a rock that kind of looks weird, is it a one in a billion, or, like, one in a trillion that it's going to be actually a meteorite?

BEN: Well, to give you a sense, we now know of about 100,000 or so recognized meteorites. And that's over all of human history that we've identified these things. So it's definitely the needle in the haystack.

ERIC: So if you found a meteorite, congratulations, only 100,000 of these across all the rocks on all of the earth actually turned out to be from space.

BEN:Yeah. And the way we originally, as a species, figured out that these things are from space is that we actually saw them coming in through the atmosphere. For a while humans had been finding weird looking rocks on the ground. And there was a lot of speculation as to, you know, whether those things, where they came from. But finally, in late 19th century, we actually saw, in France, some meteorites coming into the atmosphere, and then someone went and found it on the ground. And that was really the first piece of evidence that began to convince us that there actually are rocks that come from space, which is a very weird thing, when you think about it. I mean, until that time, no space, you know, was largely thought of as this big, empty, perfect place with like, you know, the gods live up there. And there's like, harmony of the spheres, and it's all perfect. And it was, you know, it took a while for humanity to recognize that there's actually, you know, little broken pieces of rock floating around up there, and they can actually land on Earth.

ERIC: Yeah, so early scientists were like, "rocks can come from space? Preposterous! I won't believe it!" And then somebody actually saw one, went over and said, "this just fell on the ground from space. So it must be true."

BEN: Yeah. And then when you do see these things come into the atmosphere, and you find them soon after, they're often cold. So there's, like, indicators that they came from somewhere else. And then another clear indicator is that they have this melted exterior, which we call the fusion crust, which is produced when the rock comes into the atmosphere, and it gets heated up as its passing through the gas. And that melted rind is really a dead giveaway, in many cases, for a meteorite.

ERIC: So there's not a million different types of meteorites, but they don't all look the same. It's not like you can say for sure, because they all look exactly alike. So what are the types of meteorites that we find on the ground, kind of, big picture.

BEN: So there's an incredible diversity of meteorites that we found on the earth, there's something like 120 or so, it depends on who you talk to, different bodies in space from which we have samples. And they range, everything from asteroids to pieces of the moon to pieces of Mars. In most cases, we don't even know where these things came from. We haven't found the parent body, as we call it, for the meteorites. And so they represent, you know, in terms of like, the geologic processes that formed on their bodies, a much greater range of events and histories than all the rocks on Earth put together.

ERIC: If you find a rock and you think it might be a meteorite, there are a couple easy tests you can do. So what are some of the first things that you can do to maybe just rule it out? If you have a rock that's not a meteorite, it will become apparent right away.

BEN: The most common meteor-wrong, as we call it, is what we call concretions. So these are basically what happens when fluids are passing through, usually a sedimentary rock. That's rock that was laid down by the action of water. And the fluids have iron in them that they've leached out of somewhere else. And then that iron-rich fluid basically precipitates, or grows a nodule, a little like, you know, hard kind of round-ish deposit that can tend to be very dense, because it's made out of iron, much more iron rich than the surrounding rocks, and can be very magnetic. And some people often think of meteorites as being commonly magnetic, which they often are. And it also looks kind of weird, so it's got this like dark kind of exotic shape and color. And an irregular outside sometimes can even look a little shiny in a way, which is, like I was pointing out earlier, there's a dead giveaway for a meteorite is this glassy, outside made from atmospheric passage. So sometimes these rocks even have a kind of shiny exterior, which people think, you know, might resemble what they expect for a rock passing through the atmosphere. So that's maybe the most common type of meteor-wrong. Another one that gets us all confused is what called slag. So when you take a rock that has iron in it and you want to extract metal for industrial purposes, say you want to make steel, then you heat it up in an atmosphere, and then that leads to metal basically coming out of the rock, effectively. So these kinds of slag deposits can look like a very unusual type of meteorite that people often think of as the most common type of meteor. In fact, it's very rare, which we call stony-iron meteorites. So those artificial slag rocks have the metal that we often expect to be a sign of something coming from space. And again, they can be shiny and magnetic.

ERIC: So there are kind of some red herrings there that seem like they might be at least a unique rock on Earth, but actually came from, I mean, interesting processes, but nothing extraterrestrial.

BEN: That's right. So the most common type of meteorite is what we call an ordinary chondrite, and it's called that for a reason, because it's represents, like, more than 80% of the meteorites that we confirm are from space. So the meteor-rights in the capital R-I-G-H-T. And ordinary chondrites do tend to have little flecks of metal in them, because they're from bodies that never melted. So we think they're basically asteroids, small bodies, no more than a few hundred miles in diameter, say, and because they're so small, they never got hot when they formed, unlike the Earth, which got very, very hot. For large bodies like the Earth, because they get so hot when they form, they melt and then heavy stuff sinks to their centers. And the most abundant heavy element in the universe is iron. So planetary bodies tend to form these metallic iron-rich cores. That's what makes our magnetic field, for example, in the Earth. These small little bodies don't. And so their rocks basically have all of this extra iron in them, that rocks on the surface of planets like the Earth don't have. And that's a dead giveaway for what we call a chondrite, so a sample of a body never melted, you'll see little flecks of metal. They also look like sedimentary rocks, but cosmic sedimentary rocks. So like, if you went to a river bed, and you looked at the rock that's forming at the bottom of a river, you'd see little grains settling out of the water, and building up a kind of sedimentary layer below that, eventually turns into a rock. Chondrites are like that in space, it's just in this case, there's no water, it's just the vacuum of space. And these little grains settled onto their parent bodies, you know, 4.5 billion years ago, when they formed, they made a kind of cosmic sediment that never melted. And so if you pick up one of these rocks, you'll see little grains all kind of stuck together, never ever having reached any high temperatures after they formed.

ERIC: That's another one my favorite things about when you do have a meteorite: nothing's happened to them in billions of years. Like some of them look the same way they did when Earth wasn't even formed. Yet. There are no real young meteorites, right?

BEN:Yeah, the most dangerous part of a meteorite's life is after it lands on Earth. So it's been usually, in most cases, for the last four and a half billion years in the deep freeze left alone in space, you know, orbiting the sun on its lonely parent body with no geologic history. And then it comes into the Earth's atmosphere, which is a very dramatic process, it heats it up, it melts on the outside, then it lands. If it's lucky enough, someone will find it right away. But in most cases, it just sits there for forever until it rusts away from the action of water. Occasionally, someone will find one of these things, unless of course, it lands in the ocean, which is what happens in most cases.

ERIC: Most cases, yeah.

BEN:And then if someone does find it, then often they will stick a magnet on it, which by the way: don't do that, because that actually de-magnetizes, it destroys information about the early solar system.

ERIC: Good to know.

BEN:And then they'll cut it open and probe it with all their different instruments. And so actually, we're lucky that these meteorites have basically been in some kind of like a natural museum, shall we say, in space where they've been preserved for eons. And now they're very slowly being delivered to us by natural processes.

ERIC: So speaking of poking and prodding things, we have a potential meteorite here from our collections that we've been doing a couple of basic tests on, but I thought maybe you could take a look at it. And tell us what you think. Do you think it could be a meteorite? Give us your first impressions.

BEN:Alright, so I'm holding this thing. It's a brown, kind of elongate rock about the size of my hand, and it does have a kind of shiny outside. Definitely heavier than your average rock, maybe like one and a half times as heavy as a typical, say, a basalt from Hawaii that just erupted and crystallized. The museum was kind enough to cut it open. And I don't see any kind of texture that would tell me that it looked like a sediment. It looks like a homogenous interior. There's no grains stuck together, obviously. It just looks like a massive solid thing.

ERIC: It looks like all the same inside, no special detail?

BEN:Yeah, no little, like, grains stuck together or obvious crystal boundaries. There's no clear, shiny metal pieces. It looks just kind of brownish. And I'm told that the density of this is...

ERIC: It's about four and a half grams per cubic centimeter.

BEN: Okay, yeah. So that confirms that it is heavier than than a typical Earth meteorite. So I don't see any obvious fusion crust. And that's another important criteria. So I don't see any evidence that the exterior of it has been melted from passage through the atmosphere. Now that's not required. Because if this was sitting on the earth for say, 10,000 years before someone picked it up, often those fusion crusts have disappeared by weathering. If this was a meteor-wrong, the most likely case is that it would be a concretion of some kind. And I would say, so far, it fits all the characteristics of a concretion. Concretions are brown, they're heavier, they tend to be between four and five grams per cubic centimeter, they won't have any fusion crust, naturally, because it didn't come through the atmosphere. Often, when you rub them on paper, they'll leave a red streak, because that's a signature of one of the most common minerals that form when water deposits iron concretions on Earth, we call that hematite. It's an iron oxide. When you see the red rocks in the Grand Canyon, all that red, that's all hematite. Okay, so I'm going to try scratching it on a piece of paper here. (scratching noises) And there's not really a strong red color, although I do see some brownish stuff coming off. So there's probably a little bit of hematite here. But I wouldn't say it's overwhelming. It may not have lots of hematite in it. But the other mineral that's very common in a concretion is something we call magnetite. And that's a famous mineral, because magnetite is also an iron oxide, but it's very magnetic. Without sticking a magnet on it, I can actually use a little device here called a susceptibility meter to see how magnetic it is. (beeping noises) Okay, the results are that this is very magnetic.

ERIC: Interesting.

BEN: This is as magnetic as the most magnetic meteorites that we know of actually. Okay, so that actually further makes me suspicious that this is an iron concretion made out of mostly magnetite. Almost certainly a meteor-wrong, I would say.

ERIC: So if someone does a lot of the tests we've talked about, and it doesn't go down the line of concretion, if it does look like it might be a meteorite, what can that person do? What's the best way to go about maybe having a real meteorite?

BEN: You know, if you can cut it open, and you can look at the texture, that can tell you a lot, and just taking a photo of that texture. And if you see what looks like a sedimentary rock, but it's made up of little balls, which we call chondrules. That's what we named these little grains that formed chondrites, then you can send that picture to an expert, and they can look at it really quickly.

ERIC: And the highest level is sending our mystery rock to a lab for testing. Is that practical?

BEN: Even just measuring how much manganese is in there, or the iron abundance, it turns out, even looking at the relative amount of elements themselves, that's the kind of thing that can be done in almost any university analytical facility around the country. So there's maybe you know, a couple dozen places in Massachusetts alone, where you can make that kind of measurement.

ERIC: Have you ever found a meteorite?

BEN: I have never personally found a meteorite.

ERIC: Have you ever gone on one of the expeditions that goes to places where it's easier to find them, like Antarctica?

BEN: To my regret, I have not. I have been invited several times. And it always comes at an inconvenient time because I work at MIT. So I have to teach classes. And I have a good friend who basically leads annual trips to the Atacama Desert. That's a new location that's being explored now for meteorite discoveries. It turns out up there, you know, it rarely rains and the surface is ancient. And there are meteorites that are all over the place up there. But that trip always happens in the fall when I can't go. There's also opportunities for scientists to go down to Antarctica to join the ANSMET expedition. It's an NSF-funded expedition to find meteorites. That's a three month commitment.

ERIC: We'll maybe someday you'll get to go. Or maybe you've inspired one of our listeners to go on a meteorite hunting trip someday. Ben, I wish we ended up with a real example of a meteorite here but thanks for taking a look at it and giving our listeners some things to think about if they ever find something that they genuinely think might be from space.

BEN: Thanks very much. Happy to be here.

ERIC: On your next visit to the Museum of Science, check out Natural Beauty: The Harold Grinspoon collection, which includes a 1,700-pound iron meteorite. And while you're home, visit mos.org to sign up for Spacing Out, our new weekly newsletter covering the latest spaceflight and astronomy headlines. Until next time, keep asking questions.

If you liked this episode, be sure to check out:

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