A rock is a rock is a rock, right? Across the Solar System there are giant rocky asteroids and even rockier moons. What’s the difference between these two families of objects, and where did they come from?
PODCAST: Ep. 613: Pluto’s Demotion: 15 Years Later (Astronomy Cast)
Planetary differentiation (Academic Kids)
Ceres: The Other Former Planet Classic T-Shirt (Redbubble)
Moment of inertia (Hyperphysics)
All Mixed Up? Discover the Brazil Nut Effect (Scientific American)
Triple point (Wikipedia)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast, Episode 622, Rocky Moons and Giant Asteroids. Welcome to Astronomy Cast, your weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, Publisher of Universe Today, with me as always is Dr. Pamela Gay, a Senior Scientist for the Planetary Science Institute and the Director of CosmoQuest. Hey Pamela, how are you doing?
Dr. Gay: I am doing well. It is one of those blustery fall days that reminds us winter is coming. But at least where I am, it’s only slightly damp and only below freezing at night, so I can still enjoy winter coming. I understand you are still only slightly damp, but that could be changing.
Fraser: Yeah, I think we talked last week about the disaster that hit British Columbia, knocking out all of our roads and railways and killed now five people.
Dr. Gay: Oh, geez.
Fraser: But 20,000 pigs died and hundreds of heads of cattle. It’s been pretty bad.
Dr. Gay: Oh, god.
Fraser: And so now, there’s three more storms inbound. Actually, we had one yesterday, which wasn’t too bad. We’re supposed to get another one on the weekend, and then the bad one is gonna be on Tuesday. And that probably won’t be as bad as the one we had before, but the problem is that everything’s over capacity. Everything’s still flooded, and so it’s just gonna pump more water into the whole system. So yeah, we’ll see how this all plays out for us, but it’s definitely been a rough winter for British Columbia, for sure.
Dr. Gay: Yeah, I will take the dead leaves everywhere over that.
Fraser: Yeah. All right, but we’ve gotta do our show.
Dr. Gay: We do.
Fraser: So, a rock is a rock is a rock, right? Across the solar system, there are giant rocky asteroids and even giant-er rockier moons. What is the difference between these two families of objects, and where do they come from? All right, Pamela. So, you threw this into the list. What was your thinking? What is the heart of the conversation that you wanna have today about the difference between moons and asteroids?
Dr. Gay: Well, it wasn’t so much the fact that they’re different, as they’re the same. When we started this 15th season of ours, we were discussing that Pluto’s still not considered a planet, but what exactly is a planet? And as we started to get into some of the geophysics of it, it became clear that there are all of these worlds that in some cases are bigger than Mercury, that have differentiation, and we’ve never really explained what differentiation is. We’ve never really explained how common these types of things are.
So, as we’re chewing through the solar system, I figured if we’re gonna dedicate episodes to gas giants and ice giants, we might as well also dedicate one to rocky things that aren’t terrestrial planets because we’re lame and don’t allow them to be planets.
Fraser: All right. Give me sort of the example, the classic giant asteroid or rocky moon that you wanna think about today.
Dr. Gay: So, the classic giant asteroid is Ceres, the O.G. former planet. We actually have a t-shirt on Redbubble from CosmoQuest about Ceres, the other former planet. It does not get nearly enough attention. This is a massive world that has ice volcanos, has hills. It’s a world. It was called a planet until it developed too many friendships with other things in its orbit. And then as far as moons go, you have to look at Ganymede, which is bigger than Mercury.
Fraser: And the biggest moon in the solar system.
Dr. Gay: Exactly.
Fraser: All right, so let’s take that knife that we’ve been slicing planets open with and let’s slice Ceres in half and crack it open and take a look inside. What would we see as an internal composition of Ceres?
Dr. Gay: Well, it’s differentiated, which means that it has heavier materials deeper inside, lighter materials at the surface, and it’s believed to actually have pockets of water inside of it. Very salty water, briny water, and it’s those salts that make some of its hills, volcanos, look shiny because as it erupts, that material falls back down and sublimates away the water, leaving behind the salt to make everything look shiny.
Fraser: So, would you get a rocky, metallic core, like what we have with the Earth?
Dr. Gay: It’s not going to be as big, and I don’t know if we actually have enough data to be able to get at that level of detail. The way we get information like that is we measure what’s called the “moment of inertia” of a world. This tells us how dense it is as a function of radius based on how it rotates, and we can get at this information using spacecraft in orbit around it. But it can get fiddly to get down to just what is in the core. We occasionally can escape this inability to figure it out based on moment of inertia if there’s a magnetic field.
And this is where, with Ganymede, it has a magnetic field that lets us know that it has this iron core, this metallic core. And Ganymede is really cool, because like I said, it’s bigger than Mercury, it has a magnetic field. It’s actually a lower density than Mercury; Mercury is one of the densest things in our solar system. And so it has this more asteroid-like composition, but we know there’s that metal core.
Fraser: So, back to Ceres, I’m kind of imagining it’s got this – you say “differentiated,” but do you mean differentiated like it has the heaviest elements down near the middle – what causes differentiation in a world?
Dr. Gay: Melting in the original formation with the solar system. So, if you think about it, our solar system started out extremely hot. And we’re always talking about, “Well, will planets get formed through the combination of planetesimals, so you get small things that come together, you get bigger things that come together?” And the mental image that gets put together by this story is often one of snowballs coming together and getting bigger and bigger, or clods of dirt coming together and getting bigger and bigger, or clods of dirt and ice coming together and getting bigger and bigger.
Fraser: That sounds so gentle.
Dr. Gay: Right. It sounds gentle, and it ignores all the heat that was involved in this process. And it turns out that pretty much everything started as molten lumps, and molten lumps means that higher density things are going to sink, lower density things are going to float, and you’re going to get differentiation by density over time as they cool. And cooling starts at the surface, where the heat’s radiating out. So, that heat initially gets trapped inside, giving it more time to continue this differentiation because things – just like a pie, I know a whole lot of Americans have been eating pie this week. Just like a pie cools from the crust inwards, and you burn your mouth on the apples or cherries inside. Well, if you could bite into an early asteroid, 1.) I wouldn’t recommend that, ’cause they are giant rocks, but 2.) They also froze from the outside in.
Fraser: Right. I’m sort of imagining when you grab, I don’t know, a beaker full of beach sand and water and stuff like that, and then you shake it around, then you’ll get the bigger rocks down at the bottom and you get smaller rocks on top. And then you get this sort of fine silt that came out of the water, and then you’ll get the water sitting on top, essentially differentiating itself.
Dr. Gay: Yeah, it’s the Brazil nut problem.
Fraser: The Brazil – what? You have to explain this. I don’t know what you’re talking about.
Dr. Gay: If you get a canister of mixed nuts and you shake the nuts, all the Brazil nuts end up on the bottom, and then the lame things like just your half a peanut that has started to fall apart ends up on the surface.
Fraser: Got it, okay.
Dr. Gay: So, it’s the Brazil nut problem.
Fraser: Okay. All right, we’ll talk about this more in a second, but it’s time for another break.
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Fraser: And we’re back. All right, so we talked about differentiation, and you mentioned with Ceres that there are pockets of water, mineral-y water, inside Ceres. How does that happen?
Dr. Gay: One of the weird things about geology is water is one of these things that has habits of flowing and altering the chemistry around it, because as it flows, it carries different minerals. It’s a solvent, it dissolves things. Here on the planet Earth, the reason that we have things like iron in veins is because it can get transported by water. So, within Ceres, basically the water ends up pooling in areas that have the correct chemistry that it can eat out the surroundings. It carries the water around, and it’s through this combination of chemistry, density, solvent behavior, you end up with pockets of water instead of this smooth continuum.
At first thought, I was always deeply confused, and at one point I think I cornered Emily Lakdawalla to get an explanation for this. How is it that you go from a molten object that is constantly getting hit next, and why do you have worlds that aren’t a smooth mixture of iron everywhere, of lime everywhere. It’s because of the chemistry leading to things segregating out in different ways.
Fraser: And Ceres, for example, as an asteroid is so big that it has weird volcanism, has these mineral-y water cryovolcanism, salty cryovolcanism.
Dr. Gay: So, for reasons we’re still trying to figure out, throughout the history of Ceres we’ve been able to track down various helix of different amounts of peaked-ness. Over time, they’ll settle back down. When you have an active volcano, just like here on Earth, it’s going to be more peaked, and these are the places where the fluid is coming out through the surface. And once a given spot becomes inactive, it will slump down and flatten out over time. And we can see on the surface of Ceres this history of, in this case, icy volcanism. People will include Io as an ocean world, where the magma is the subsurface ocean.
In geology, water and molten lava are very similar, so we only differentiate between Io with its sulfur-rich, molten rock magma, and worlds like Ceres, with its briny water, who would call that magma. We differentiate between them because of language, but the physics is very much the same.
Fraser: It’s interesting you brought up Io. That would definitely fit within this category of objects that we’re talking about.
Dr. Gay: Exactly.
Fraser: If Io was a little farther away from Jupiter, it would be much cooler and rocky, probably with layers of water and ice and maybe some cryovolcanism, but it wouldn’t be this extreme, volcano-rich asteroid moon that it is. But it is rocky, right?
Dr. Gay: Yeah, it’s totally a rocky moon. It’s just that those rocks tend to be a little bit on the soft and melty side of things.
Fraser: Yeah, exactly.
Dr. Gay: And what’s happening here is just tidal forces, and our own planet Earth is dealing with tidal forces from the moon. This is not something unique to Io, it is just most impressive on Io.
Fraser: Yeah, it’s cool. If you asked me to make a list of all of the rocky moons, I probably would’ve forgotten about Io because the rock happens to be molten. But it still does classify. All right, so let’s go back to Ganymede. You were jumping around, I’m trying to rein you in here and kinda move through it a little more carefully. So, with Ganymede, again, take that knife, slice Ganymede open and what are the layers that we probably see inside that moon?
Dr. Gay: So, it definitely has that metallic core that allows it to have its strong magnetic field. Now, what’s interesting is as you move out from that metallic core, it’s not as dense as Mercury. It has a larger radius than Mercury, but it’s not as dense. So, there’s going to be lower density material, whether it be water pockets, which in this case may be ice pockets. We’re not entirely sure because again, it’s not like we’ve taken a core sample of these worlds, and measuring the moment of inertia only gets us so far. But it’s believed that this is another world that probably has subsurface oceans. It has a cratered surface.
It’s weird because it’s not like Europa or Enceladus, where we have clearly that ice surface that is coating a complete ocean underneath. It’s more in the category of Pluto, where we can see hints that there’s ices, in this case in the composition, but it has a whole lot of rock. So, this is one of these transition objects. It’s not an ocean world in so much as it’s a world that has rocks and metals and waters. It’s a full-fledged world.
Fraser: Okay. So, we’ve talked about Ceres and Ganymede, one of which is the king of all asteroids, the other which is, I guess, the king of all moons. And we talked a bit about Io. So, let’s talk a bit more about some other objects that are really giant rocky objects in the solar system that catch your fancy.
Dr. Gay: Titan. We have to mention Titan. Titan is a world that has an atmosphere that is so thick that a human being wearing wings on their arms could glide around. You’d need a spacesuit. The atmosphere is things that we don’t breathe so well, like methane.
Fraser: Well, you wouldn’t need a pressure suit though, which is amazing. You’d need a coat and a way to breathe.
Dr. Gay: Well, you’d need a pressure suit so you don’t get crushed so much. I suspect being bruised up by the atmosphere wouldn’t be fun.
Fraser: Well, it’s like 1 1/2 times the atmospheric density of Earth, so you could wear a coat – it’s minus 70 Celsius on a good day, so you would need more than a coat, and that’s what a spacesuit is starting to do. But you wouldn’t need that same kind of pressured – it would be a simpler apparatus. But you can’t breathe the atmosphere.
Dr. Gay: It would be simpler, you would definitely feel squeezed though.
Fraser: Hmm, interesting. Yeah, it would be weird to feel more atmosphere. I guess it would be like when you swim down in the water, when you’re experiencing more pressure. Yeah, that’s interesting.
Dr. Gay: Exactly. Right, and just drawing in air gets more difficult when you have that extra pressure on your body. But different worlds have their own balance of gravity creating a thick atmosphere, cold temperatures creating a thick atmosphere, and you have to find that balance of gravity holding onto the gases. Mars doesn’t have enough gravity to hold onto the gases. And a temperature – Mars is too warm. Now, Titan – much smaller than Mars – has sufficient gravity to hold onto heavy molecules, and those molecules can form because its atmosphere is so cold.
So, you have the triple point of methane and ethane, where these molecules can exist as gases, as liquids, and as ices. So, we’re finding, essentially, great lakes – small oceans, I don’t know how you wanna put it – that are methane/ethane mixtures. We see this thick methane atmosphere, and one of the things that’s perplexing, and I’m hoping at some point we’ll figure this one out, is methane is a molecule that breaks down in ultraviolet light. And our sun kind of creates a lot of ultraviolet light, which means that the methane on Titan is constantly getting destroyed and has to be getting replenished from somewhere.
And the kinds of somewheres that we’re looking at are both geologic, which would make it geologically active, or life, and life is always a good answer. And there’s been some research by folks like –
Fraser: Mm-hmm, that’d be great.
Dr. Gay: Yeah. There’s been research by folks, like Chris McKay, who look at all the different chemicals that exist in the atmosphere as determined by Hoigans and Cassini, and the chemistry isn’t in equilibrium the way we would expect. So, there’s something knocking it out of equilibrium, and that’s just awesome.
Fraser: So, what is the source of these objects? Do they come from the same originating source, do you think?
Dr. Gay: What we’re starting to understand from looking at other solar systems forming using the Atacama Large Millimeter Array and some of the other massive telescopes out there, is you can end up with massive worlds, like Saturn and Jupiter, that are capable of having their own disc around them that acts like a baby planetary nebula. These planet-forming discs can have within them massive planets with their own moon-forming disc around them. Now, this isn’t how you get moons consistently. This is how Jupiter and Saturn get all of their moons. Other worlds, including Uranus and Neptune, seem to largely steal their moons.
Our own Earth’s moon is a giant splash of lightweight materials. But as we’re out there looking at Ganymede, Callisto, Io – Europa’s icy, we’re ignoring it today – as we look out at these giant moons, no matter what their composition is, that are orbiting these giant worlds, they probably formed in situ. And everything else was either stolen or splashed up.
Fraser: And with the asteroids, they’re material that just was never able to form into anything bigger because of Jupiter’s gravity.
Dr. Gay: Right. So, with Jupiter there grabbing onto so much material, there just wasn’t enough stuff left in that band of the solar system where the asteroids are that anything big could form. And on top of that, we’re still figuring out the dynamics of how everything ended up where it is. And while it largely looks like most of the asteroids formed in that band between Mars and Jupiter, some of that stuff got there via other means, and then some of the stuff in there got stolen out. Jupiter is an equal-opportunity flinger of rocks. The more we learn, the more complicated it looks.
Fraser: Yeah. All of the near-Earth objects are actually fairly new. We have objects that go from the asteroid belt to the region around the Earth over the course of say, a million years, and they’re pushed there, they’re shoved out constantly by Jupiter. And then they drift into our neighborhood and smash into the Earth, or are kicked out into different orbits because of our interactions, or they smash into Venus, or whatever. This is happening non-stop, it’s like a conveyor belt of Jupiter slowly shoving this material out of the asteroid belt and into the inner solar system.
And people always ask me, “If you could turn all of the asteroids into one object, would you have another planet?” You would end up with something that is vastly smaller than the mass of the moon, so there just isn’t a lot of material, just kilogram for kilogram, in the asteroid belt. But still, all of those objects are important too, and that’s why we spent today talking about them. And now we’re done.
Dr. Gay: Okay, we are done. And this is the part of the show where we really have to just take a second to thank all the people that allow this show to happen. We are funded through all of the good folks who donate to us over on patreon.com/astronomycast.
And this week, I wanna thank Matthias Heyden, Randa, Gregory Singleton, Nial Bruce, The Lonely Sand Person, Jeff Willson, Omar Del Rivero, marco iarossi, Paul D. Disney, Tim McMackin, Nate Detwiler, Cooper, Eran Segev, Kenneth Ryan, Steven Shewalter, Alex Raine, Shannon Humber, Bongman McBluntsmoke, Dean McDaniel, Don Mundis, Janelle Duncan, Michelle Cullin, Jeremy Kerwin, Karthik Venkatraman,
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Thank you all so much. You allow us to pay Nancy, Beth, Richard, to keep our servers happy, to keep our software going. We wouldn’t be here without you. Thank you.
Fraser: Thanks everybody. We’ll see you next week.
Dr. Gay: Bye-bye.
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