Ep. 624: Small Rocky Bodies (Including Phobos and Deimos)

Dec 13, 2021 | Astronomy, Missions, Our Solar System, Planetary Science, podcast, Science, Solar System | 0 comments

We’ve talked about the icy objects of the Solar System, today let’s talk about space rocks. There’s a surprising variety of rocky material in the Solar System, and each object has a story to tell about the history and formation of the planets, moons and other rocky bodies.

Download MP3 | Show Notes | Transcript

Show Notes

PODCAST: Ep. 622: Rocky Moons and Giant Asteroids (Astronomy Cast)

PODCAST: Ep. 623: NEOS: Concern or Nah? (Astronomy Cast)

Mars’ Moons: Facts About Phobos & Deimos (Space.com)

243 Ida (NASA)

433 Eros (NASA)

222 Lucia (Wikipedia)

25143 Itokawa (NASA)

Touching the asteroid Ryugu (EarthSky)

Why Bennu? (OSIRIS-REx)

Oblate Spheroid (Wolfram Mathworld)

Hydrostatic Equilibrium (Swinburne University)

M-type asteroid (Wikipedia)

Vesta: Facts About the Brightest Asteroid (Space.com)

Were the Mars moons Phobos and Deimos born from another shattered moon? (Space.com)

Martian Moons eXploration (MMX) (JAXA)

Did an asteroid collision cause abrupt Earth cooling? (EarthSky)

Back to Top


Transcriptions provided by GMR Transcription Services

Fraser:                         Astronomy Cast, Episode 624: Small Rocky Bodies. Welcome to Astronomy Cast, your weekly fact-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 you doing?

Dr. Gay:                      I’m doing well. It is rainy and cold, but that is superior to so many ways the weather could be right now.

Fraser:                         It’s funny how, as the weather turns, you’re like, “Oh, I just wish it was warm and rainy,” and then later on you’re like, “I wish it was at least cold and rainy but not wet snow.” So, every time as the weather turns into winter here, you long for the previous months’ weather until we get a round of spring.

Dr. Gay:                      And the irony we have here, and there’s science in this, I promise, is it’s the super-sunny, crystal clear, winter days that are the worst because that layer of clouds is thermal insulation. So, on a super cloudy day, it’s only gonna get so cold because the infrared radiation is bouncing around between the clouds. But those crystal clear days are brutally cold, and those are the days I sit in a sunbeam and wish my windows were better.

Fraser:                         We have the same situation, where when it’s clear and sunny is when it’s the coldest, for sure. All right. So, we’ve talked about icy objects of the solar system. Today, let’s talk about space rocks. There’s a surprising variety of rocky material in the solar system and each object has a story to tell about the history and formation of the planets’ moons and the other rocky bodies. All right, Pamela. You cued up this topic. Where do you want to go today?

Dr. Gay:                      Well, so we talked about the massive objects that are differentiated and could be called worlds. So, I think today is a good day to look at the weird, small stuff that you can still call a “world” because, really, you can all anything in space a “world,” except for man-made objects. And then, you can try. I would just argue with that one.

Fraser:                         Not yet. Make instructions. We’re not there yet.

Dr. Gay:                      Right.

Fraser:                         But last week, we talked about the near-earth objects, the asteroids, the potential planet killers or the bad-day-makers. But today, we’re gonna talk about large objects. So, give me some examples of some of the kinds of objects that you would classify as small, rocky bodies.

Dr. Gay:                      So, you have the classic sort of Phobos and Deimos, these two potato-shaped moons of Mars. You have Ida and Dactyl and Eros and Lucia and Itokawa and Ryugu and Bennu, and basically all of these objects that are, at most, 100 kilometers on one side but probably tens of kilometers on another, and are anything but round if you take a good look at their shape.

Fraser:                         So, that not-roundness, where is that coming from?

Dr. Gay:                      So, as objects get bigger and bigger and bigger, once they go past a certain crossover in size, gravity is able to basically say, “Nope, that hill, we’re gonna squish it down. Nope, that hill, we’re gonna squish that one down too.” And gravity is responsible for making things round. The smaller the object, the bigger the mountain you can have but overall, you’re gonna end up with a round object once you’re big enough. Now, if you’re small, gravity may not be the dominant thing holding you together in all cases. Gravity is there, but you also have to look at electrostatic forces, the forces that cause the molecules and the minerals to bond and build.

                                    A rock out by your driveway isn’t held together by gravity, and an asteroid is a lot more like that driveway rock than the asteroid Ceres.

Fraser:                         And we’re here on the surface of the earth, and from our perspective, you see a lot of hills and mountains and depressions and valleys and all kinds of landforms. But if you zoom way out and look at it from, say, the surface of the Moon, then the earth looks like a perfect sphere. And for all intents and purposes, it actually is very spherical although it’s a blatant spheroid. But it’s a very smooth sphere from the large scale, when you consider the entire size of the earth. And that’s gravity pulling it all together.

Dr. Gay:                      A slightly flattened basketball with the texture of a basketball would be far lumpier, if scaled up to the size of the earth, than the earth actually is and

Fraser:                         That’s amazing.

Dr. Gay:                      – that’s just something that gets me every time.

Fraser:                         A basketball would have bigger mountains than the earth does. That’s incredible. So, we’ve got these smaller objects there. They don’t have enough gravity to pull themselves into a sphere and so they can be kinda any shape. Where do they come from?

Dr. Gay:                      They have a bunch of different formation mechanisms. On one hand, some of them are just things that failed to get much larger in the early formation of the solar system. Some dust hits some dust.  They got together. They pulled in some more dust and they stopped. And so, you can end up with these chemically pristine rocks that are weathered by sunlight on the outside, but otherwise are just pure representations of what our early solar system was made of. In other cases, you also end up with objects that were shattered.

                                    There’s some thinking that metallic asteroids actually come from originally larger objects that got hit, had the earthy parts knocked off the outer shell, leaving behind just this metallic core. And then, of course, you have things that are the leftovers of massive collisions that just left rocks.

Fraser:                         Right.

Dr. Gay:                      It’s thought that Ryugu could be the remains of multiple objects that smashed one another to smithereens.

Fraser:                         It’s interesting, like Vesta, for example, which is one of the largest asteroids in the asteroid belt and one of the targets of the Dawn mission, astronomers have traced a bunch of objects which are probably in a similar class as Vesta. And, in fact, many of the meteorites that have been found on earth that come from another world have come from Vesta. And so, at some point, Vesta got smacked hard and debris is making its way out into the solar system. So, there’s forces pulling these larger objects apart, but also forces bringing these smaller objects together.

Dr. Gay:                      We live in a dynamic and evolving solar system, and what we see today tells us that, in the past, things were exceedingly more violent. I’m actually gonna talk about a little asteroid that’s doing a really good job of masquerading as a moon, and that is the moon, Phobos. Phobos has a series of cracks that were originally thought to be related to a massive crater that’s on its leading edge. As it goes around, it’s tidally locked so that crater’s basically like a bulldozer scoop in front of it. But it’s not perfectly lined up in the path that this little moon is taking around Mars, and it turns out that these cracks aren’t aligned with the crater.

                                    They’re aligned with the orbit and it’s thought that, on a regular basis, as massive objects hit Mars, create massive new craters, chunks of Mars will get sent up at escape velocities, or at least velocities that put them in orbit. And this little moon plows into that and those objects form these cracks on its surface.

Fraser:                         The actual source of Phobos is a bit of a controversy, isn’t it?

Dr. Gay:                      Yeah.

Fraser:                         Where it came from.

Dr. Gay:                      There are those who believe it is a captured asteroid and there are those who believe that, just like our moon, it was created through some sort of an impact event. And then, making things even more interesting is, with the moons of Mars, the inner one is actually working its way around Mars faster than Mars is rotating. And that inner one is Phobos, which is much more boring-looking, and as Phobos gets closer and closer and closer, it’s eventually going to tidally tear itself apart. And the reason it gets closer while our own moon moves outwards is that fact that it’s going around faster than Mars is rotating.

                                    So, angular momentum does bad things if you orbit too fast. If you orbit faster than a world is rotating, you move inward. These

Fraser:                         Right.  

Dr. Gay:                      – are the rules. If you go around a world faster than that world is rotating, your orbit moves inward. This is the opposite of what we see with our own moon, where as it goes around, we’re rotating much faster, so it’s able to move outward. With Phobos moving inwards, this gets folks thinking that maybe these could be rocks that regenerate themselves by migrating where they don’t belong, shattering, and then coming back together and perhaps mass has been lost over the time. There’s crazy computer simulation showing how this can be possible in special circumstances.

Fraser:                         Wow.

Dr. Gay:                      I’m not sure we’re ever gonna know how these moons formed, but we’re gonna do the best we can to figure this out. There’s actually a Japanese mission in its way to go check out these moons, and you probably know more about this mission than I do.

Fraser:                         The Japanese JAXA’s MMX Phobos mission and it’s going to visit at least Phobos, maybe Deimos as well, and possibly drop landers and hoppers and things similar to what happened –

Dr. Gay:                      Hayabusa2.

Fraser:                         – with Hayabusa 2 and Ryugu, and maybe even drill a sample a little bit under the surface. And the goal is to try to answer once and for all, “Was Phobos and Deimos broken off from Mars through some enormous impact the past or were they captured asteroids?” I think, my reading right now, the consensus is definitely leaning towards that they are shrapnel from a giant impact in the past. All right, enough time spent on Phobos. Let’s think of another characteristic example of a small rocky body that fits the bill. Where else do you wanna talk about?

Dr. Gay:                      Well, we have Ryugu, Bennu, and all the other rubble-pile asteroids that are out there. And there’s a whole lot of those.

Fraser:                         We saw Itokawa, which was the asteroid that was visited by the first high boost emission.

Dr. Gay:                      The great cashew.

Fraser:                         And so, it’s really more like a rock, less like a rubble pile. But then, we see with Ryugu and Bennu, these things are rotating, 10-sided dice. They’re rubble piles. So, what’s the cause of these?

Dr. Gay:                      Some objects are less lucky than others is the best way to put it. In a universe that may be mostly empty, things still find a way to collide and the degree to which this happens is something that we’re only now, I think, really coming to fully appreciate. Once upon a time we taught people “No, stars never collide.” Stars do collide, folks. They do collide. We used to say, “The asteroid belt is so empty, the collisions between asteroids are rare.” And now, we go looking at Ryugu with Hayabusa2 and discover there’s bits of Vesta in there. There’s bits of regular chondrite in there. It is a mess of stuff from at least three different sources.

Fraser:                         Vesta really got around?

Dr. Gay:                      Yeah. Yeah. And so, you can just imagine two rocks minding their own business and they’re just on intersecting orbits and these intersecting orbits caused them to collide and form a cloud of debris. And that cloud, gravitationally, everything is acting on the center of mass and it’s gonna collapse in on that center of mass and apparently end up shaped like an eight-sided, 10-sided, or 20-sided dice, depending on your opinion.

Fraser:                         Right, right. Ten-sided, that’s the right one. That’s what they look like. The fact that we’ve two of these now, do we have a sense of how much of the solar system are made up of these guys?

Dr. Gay:                      So, we’ve only seen two in high enough resolution to confirm that these things are essentially giant ball pits where some of the balls just happen to be boulder-sized. But we have observations of enough other objects to understand there are actually a lot of things that are either two giant pieces held together by a bridge of debris, that there are things that are collections of larger rocks. There’s a lot of things now that we’re willing to say, “This is a rubble pile.” So, in terms of exactly how many, we’re not there yet. But my feeling is that it’s probably gonna be at least 10%. This is a gut feeling based on current observations. I couldn’t find a paper to answer this question for me. I did try.

Fraser:                         Right, right.

Dr. Gay:                      But the more we look, the more of these things we find. It’s just, luckily, there are the metal ones out there going, “Hi, we’re 40% and we aren’t rubble,” so…

Fraser:                         All right, so we’ve talked about asteroids posing as moons or maybe not, asteroids smashing into each other and turning into just piles of rubble. What other kinds of rocky objects would you find in a survey of the solar system?

Dr. Gay:                      Well, in general, you have the ones that are M-type. Those are the ones that have nickel iron cores and may, it turns out, have a volcanically-formed crust.

Fraser:                         What?

Dr. Gay:                      Yeah. So, in prepping for this episode, I came across a 2019 paper where they talked about the problem of how asteroids cool. So, if you start out with a asteroid that is big enough that it has started to differentiate, so you have a earthy, carbonaceous, chondroid-y crust around a nickel iron core. The outer part will solidify first, leaving you with a still squishy, molten iron nickel core. Now, if something comes along and smashes that and causes the earthy parts to get knocked off, that crust to get knocked off, leaving behind the higher density nickel iron, well, you’ve just added more energy, you’ve added more heat into the system.

                                    So, you still probably have a mushy, gushy nickel iron core and that’s going to harden on the outside first, but you’ve now trapped the heat on the inside and it’s going to want to escape because that’s what heat does. And so, the idea is that it’s possible that when the mission gets to Psyche, it’s going to find evidence of past volcanoes where it was basically molten metal erupting up out of these volcanoes. And that’s just a lovely idea in a very dark and twisted, where’s the Magic: The Gathering card for this?” kind of way.

Fraser:                         For metal volcanoes?

Dr. Gay:                      Yeah, yeah. So, we will have cryovolcanoes. We will have iron volcanoes. It’s awesome.

Fraser:                         Awesome. Rock volcanoes that we’re so familiar with today?

Dr. Gay:                      Yes, yes.

Fraser:                         And so, DART Mission has launched and we talked about a bit last week. But do we know anything about the targets, about Didymos and Didymoon and how these compare?

Dr. Gay:                      So, these are small, rocky objects. They appear to be solid objects so far, but our images of them aren’t that great. These aren’t objects that are large enough, as far as we understand, to be segregated out into different layers, to be differentiated. It’s basically a rock in a smaller rock.

Fraser:                         Right, and I guess we’re gonna find out if DART will dart, slam into the surface of Didymoon, or will it punch a hole through and create a cloud that will then reform?

Dr. Gay:                      And the concern, as I mentioned last week, is the moon could be something that is knocked off instead of captured by the larger object, the parent object. And knocked-off stuff could either be rubble or just a chunk knocked off. So, we won’t know until we hit it or to a degree –


Fraser:                         Right. Right.

Dr. Gay:                      – when we see it.

Fraser:                         But that’s the whole point. That’s the whole point of this mission is to whack into it and learn what happens, not have any assumptions. “We’ll find out through experiment,” and chances are it will shatter all of our assumptions about what’s gonna happen. So, with the number of asteroids that are currently in the solar system, are there significantly less today than there were back in the early history of the solar system?

Dr. Gay:                      I’m not sure I would say that there’s significantly less. There are so many, but in the past, you had much larger objects that have now knocked themselves into smaller objects. There are chunks of Vesta out there. There are entire families out in the asteroid belt where we can go, “This section of asteroids all has the same chemical properties and probably came from one object. There’s a whole section over here, have the same chemistry, probably came from the same parent object.” So, yes, asteroids are striking planets, striking moons, striking things and ceasing to exist. But they’re also striking each other and reproducing by breaking into many different pieces.

Fraser:                         Right, so you’re getting more but smaller pieces as they continue to crash into each other. It’s interesting. I did a bunch of research into this as well; just that Jupiter is constantly shoving asteroids out. There’s a constant train of asteroids leaving the asteroid belt and making their way into the near-earth object realm that they are a renewable resource. You could remove all the asteroids that were potentially dangerous to the earth, give Jupiter a few hundred years and it will have more lined up for us.

Dr. Gay:                      And Jupiter’s an equal opportunity rock thrower. As a lot of models done by one of our friends, Kevin Grazier has shown, it is perfectly capable of flinging things inwards and outwards, and so we also get to watch as some of the Trojan objects migrate in, the Centaurs. Centaurs are not stable. They are going to migrate in. It’s kind of awesome.

Fraser:                         Very cool. It’s kinda hard in one episode to give a survey of what could be hundreds, thousands of different examples of these kinds of objects. But it’s not to get a sorta high-level survey. So, thank you, Pamela.

Dr. Gay:                      My pleasure.

Fraser:                         All right. Do you have some names for us?

Dr. Gay:                      We are, as always, here thanks to the generous contributions of folks like you. It is our community over on patreon.com/astronomycast that makes it possible for us to have Allie and Rich out there for me to constantly apologize to as I make mistakes in these episodes. It’s what allows us to have Nancy Graziano running herd over the two of us cats, and well, Beth Johnson out there promoting us on social media. You give these people jobs, keep us sane, and allow this show to happen.

                                    This week I would like to thank Bruce Amazeen, NinjaNick, Arthur Latz-Hall, Thomas Sepstrup, Mountain Goat, Burry Gowen, Jordan Young, Stephen Veit, Kevin Lyle, Jeanette Wink, Andrew Poelstra, Venkatesh Chary, Brian Cagle, David Truog, TheGiantNothing, Aurora Lipper, Joe Hook, David, Gerhard Schwarzer, Will Hamilton, Ronald McCoy, Jean-François Rajotte, cacoseraph, Bill Hamilton, Robert Palsma, Laura Kittleson, Jack Mudge, Joshua Pierson, Les Howard, Joe Hollstein, Sean Martz, Gordon Dewis, Frank Tippin, Alexis, Neuterdude, Helge Bjørkhaug, Adam Annis-Brown, Ben Lieberman, William Baker, WandererM101, William Andrews, Jeff Collins, Travis Dalziel, Harald Bardenhagen, Nicky Lynch, marco iarossi, David Gates, Randa, Brian P. Cox, Matthew Horstman, Phillip Walker.

                                    Thank you all. Thank you for making what we do possible.

Fraser:                         Thank you everyone and we’ll see you next week.

Dr. Gay:                      Bye-bye. Astronomy Cast is a joint product of Universe Today and the Planetary Science Institute. Astronomy Cast is released under a Creative Commons attribution license. So, love it, share it, and remix it but please credit it to our hosts, Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website: astronomycast.com. This episode was brought to you thanks to our generous patrons on Patreon. If you want to help keep the show going, please consider joining our community at patreon.com/astronomycast. Not only do you help us pay our producers a fair wage, you will also get special access to content right in your inbox and invites to online events.

                                    We are so grateful to all of you who have joined our Patreon community already. Anyways, keep looking up. This has been Astronomy Cast.

Back to Top


Submit a Comment

Your email address will not be published. Required fields are marked *