We begin a miniseries on Mars. How many episodes will we do? Who knows? But we start today with a discussion of the two Mars moons, Phobos and Deimos.
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Show Notes


Transcription services provided by: GMR Transcription

Fraser Cain: Astronomy Cast, Episode 428, Moons 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 doing?
Dr. Pamela Gay: I’m doing well. How are you doing, Fraser?
Fraser: Very well. I believe that by the time a person is listening to this, you are probably on the west coast somewhere, some kind of meet up happening, maybe?
Pamela: By the time they listen to this, not quite yet. On December 14th, which is a Wednesday, we are going to do a meet up in San Francisco at my favorite random dive food place in San Francisco. It’s 2832 Mission Street, and it’s called Rosumende’s. It’s a sausage and craft brew place, because really who doesn’t want sausages and craft brew on a plate filled with random things like sauerkraut. It’s a fabulous place. They have big, wooden tables where we can sprawl, and talk, and eat, and drink, and enjoy life. So, yeah, go sign up. We have it up on universe.com/events. Just search for San Francisco, and CosmoQuest, and hey, we will see you in San Francisco.
Fraser: I think we’re all sold out of the Eclipse Trip, right?
Pamela: We are totally sold out, and there is an impressive wait list.
Fraser: Okay. We kept nagging you to get on it quickly, and now we’re all sold out. So, thanks to everyone who is joining us, and we’ll try to figure something out, maybe. I don’t know.
Pamela: There’ll be other events in the future.
Fraser: Yeah. So, we begin our miniseries on Mars. How many episodes will we do? Who knows? Who cares? But, today we start with a discussion of the two Martian moons: Phobos and Deimos. Okay, great, Pamela. Good choice. I couldn’t agree more with these moons. Where shall we begin? What are they?
Pamela: A good question – we don’t totally know the answer to. They are two lumpy, potato shaped, low density, probably piles of rubble that are orbiting in suspiciously circular orbits around Mars, with compositions that look a lot like a condrous chondrite, carbonaceous chondrite – to get all of the syllables in the word.
Fraser: And, for those who aren’t perhaps steeped in meteorite lore, carbonaceous chondrite is –
Pamela: They’re rocks. They’re not metal.
Fraser: They’re rocks, not metal. Okay. Great. I really appreciate the precise scientific terminology, and then we can follow that up with something a little more down to Earth.
Pamela: This is how I keep them straight in my head: it is, at the end of the day, your average, everyday rock, were it sent through space, and allowed to come back down toward Earth, it would resemble a carbonaceous chondrite, but with planetary – each object is a different distance from the sun, and has slightly different ratios of the stuff that goes into it. So, carbonaceous chondrites – they’re things that have carbon in them. They have organic compounds, silicate, oxides, sulfurs. They’re rocks. Sorry. They’re rocks.
Fraser: Well, but the part that’s kind of amazing is that they are so similar to the asteroids that you find across the solar system, and yet you said they have a suspiciously circular orbit.
Pamela: Yeah. I don’t know about you, but when I was little what I was taught in school – which so much of what we were taught was wrong – but, what I was taught was Phobos and Deimos were most likely captured asteroids, and that made for a great story. Lumpy, potato shaped, small – these things are only a couple dozen-ish kilometers. So, it seemed like a good story. But, the problem is if they were like an everyday captured asteroid, they would have been an object that was happily in orbit around the sun, going along in a nice friendly orbit, minding its own business, and then Mars’ gravity said, “Hi. I like you,” and sucked it over, and didn’t let go.
And, in the process of doing this, because they would have started with two different orbits; they would have started with different velocities – the moons – what are now moons Phobos and Deimos – would have ended up in these highly elliptical orbits. And, it’s possible to imagine that over time things and stuff happened that interacted gravitationally with the two objects, and caused them to, over time, get more and more circularized orbits, but as near as we can tell, not enough time has passed for that to have happened as thoroughly as it currently appears to be circularized. So, this is a bit frustrating if you’re trying to figure out, “Where did these things come from?”
Fraser: And, a little weird that they both have a very circular orbit. If one had a fairly circular orbit, and the other one was highly elliptical, then you could think that one was captured, and one was born in place. Okay. So, the source was it was originally thought that they were captured asteroids, and now people are leaning toward they were formed in place. Science is still out.
Pamela: Yeah. There are a bunch of different theories. The one that you run across most often, I think, is that at some point in the past Mars had a big old ring around it; formed in place. Or, I’ve also seen arguments of it once had a bigger moon. That bigger moon ended up – through tidal forces – crashing into the surface of Mars, or getting broken apart, or something catastrophic happened that ended up with things in orbit that ended up as Phobos and Deimos. So, there are a whole lot of violent ideas.
One of the ones that I find particularly interesting just to think about – but, that doesn’t mean it’s high-probability, it just means I think it’s cool to think about – is once upon a time, Mars had a swarm of asteroid-ish moons, and through many, many interactions over time, it was like who is the weakest link. And, they threw out of orbit all but these two that ended up in circularized orbits.
Fraser: What about just a crazy collision, right? Because with the Earth, we had this Thea object that collided with the Earth way back in the day, and created the Moon in place, and it has a roughly circular orbit. It’s not so bad.
Pamela: And, this is where you see theories that range from either there was a big moon in the past – and, by big I mean in comparison to the size of Mars – that crashed into the surface of Mars, or got tidally shredded; or there was a completely external object that collided, and you ended up with Phobos and Deimos getting hurled up – or a cloud of stuff that formed Phobos and Deimos actually is what would have happened. And, this actually seems to kind of align with the idea that perhaps the lowlands on Mars are actually a giant crater.
There are all sorts of interesting things that you can get at through mathematical models, but the truth is at the end of the day, we need to go pick up rocks just about everywhere, compare their compositions, and sort this out compositionally.
Fraser: So, the only way we’re going to know is if we actually send a spacecraft.
Pamela: Send lots of spacecrafts, and pick up lots of rocks. I’m a fan of picking up rocks.
Fraser: And, pick up lots of rocks, and study them. That makes sense. Can you see these moons if you have a pretty good telescope? What does it take to be able to actually spot them, and how were they first discovered?
Pamela: So, what’s cool is first of all I love where they were found. They were actually found at Foggy Bottom, which is now – I believe it’s an FBI training facility out on the east coast. And, they were discovered in 1877 at the U.S. Naval Observatory facility, which has since moved. It’s not in the same place anymore. And, they were found with, I believe it was a 20-something-inch telescope. I’m flipping through my notes trying to find the exact size of this telescope. It was the kind of thing where when you look at what modern telescopes are capable of doing, you could totally do it with a modern, backyard 20-inch telescope today.
Fraser: A modern, backyard 20-inch telescope?
Pamela: Well, they’re a thing. You can get them – Orion.
Fraser: Yeah. Planewave, probably.
Pamela: Yeah, so go to Oceanside Photo and Telescopes… spend –
Fraser: Order the 20-inch telescope.
Pamela: Spend the amount of money that you would normally spend on a car on your telescope, and yeah, you can totally do this.
Fraser: Buy your own monster telescope. That sounds good. Okay. So, you have these two moons discovered. It was what – a couple hundred years ago? When were they discovered?
Pamela: Actually, it wasn’t that long ago. It was 1877. They were discovered just a couple of days apart. It was basically an August observing run. I’m not entirely sure how to say his name, because it’s an older style name. Asaph Hall, I think is how you say is name – discovered them. He thought he saw a moon on August 10th 1877, but the weather was kind of bad, so it wasn’t a confirmed discovery, but he was there with observing time to look for moons. This is why he was observing. And, sure enough, he was able to confirm moons.
On the 18th was when he found Phobos, and – sorry, I’m going to re-say this so that it sounds better on microphone. I took back notes – is what I’m currently learning. This is where you get to watch the sausage being made. Now I have my notes organized by order, not by –
Fraser: Did you get that, Chad? There was an edit there?
Pamela: Yes. Sorry, Chad. Okay. So, I’m going to apologize for how I pronounce this poor astronomers name. It’s an older style name. I believe it’s pronounced Asaph Hall. And, he was looking in August. The first one – he thought on August 10th that he’d seen a moon, but the weather was bad, so it wasn’t a confirmed discovery –
Fraser: Foggy Bottom.
Pamela: – but, he was using the telescope. – It’s the appropriate name. So, he kept looking. He was there to find moons. And, on the night of August 12th he saw Deimos. And, then on the night of August 18th – that was when Phobos became the second moon that we had discovered. And, what’s cool is these are objects that lots of people thought had to exist as a pair of objects for reasons that had nothing to do with science for many, many years.
Fraser: What reasons?
Pamela: So –
Fraser: Because they thought there were going to be canals on the surface of Mars?
Pamela: No. That came later. That was Percival Lowell. No, the – I kid you not – logic behind this was Mercury has no moons; Venus has no moons; Earth has one moon. And, at the time of discovery, Jupiter –
Fraser: Jupiter had four.
Pamela: Right.
Fraser: Right.
Pamela: So, it was a mathematical sequence: 1, 1, 2, 4. And, via that logic – and, there were things like in Gulliver’s Travels, Swift had two moons around Mars. Voltaire, in later writings that were considered to be inspired by Swift, had two moons of Mars. It was just kind of a given. Mars has two moons. I love how logic works sometimes.
Fraser: I’m not going to stop looking until I find both moons of Mars. Yeah.
Pamela: Yes.
Fraser: That’s awesome. So, now, I’m just trying to think about their modern orbits – what we kind of know now – they have very interesting orbits around the planet – Phobos, especially.
Pamela: Let’s start with Deimos which is somewhat simpler. Deimos is not quite in synchronous orbit. It’s close. If you want to build a space elevator, it’s not totally insane to imagine if you wanted to move a potentially unstable object – moving Deimos, and making a space elevator. The problem is Phobos would be in the way. So, actually, that’s a bad idea.
Fraser: Well, Phobos is going to take care of that problem in a couple of million years.
Pamela: Yeah. So, you have Deimos. It’s in an orbit that is only slightly longer than a Martian day. So, it’s just over 30 hours. Martian day is just over the length of our day. So, it actually seems like the moon is not all that far from staying in the exact same place over head as the planet rotates if you’re not paying a whole lot of attention. So, it actually goes a couple of days before it appears to set.
Fraser: Right. So, it slowly moves across the sky, bit by bit, and then sets, and a while later it rises again.
Pamela: Relative to your place on the surface of the planet. So, as you’re going night to day, night to day, it’s trying to basically tag along so that it takes half a day for you to get half way across the sky, so that rising. It’s crazy, but at least is doing the rise in the east, set in the west thing.
Fraser: Now, Phobos –
Pamela: It’s just crazy talk.
Fraser: Phobos is super weird.
Pamela: Phobos is orbiting faster than Mars is rotating. And, the side effect of the this is you can sort of imagine if you have a child on the most pathetic merry-go-round ever, that if you’re running around the merry-go-round, the child will perceive you to be rising and setting in the opposite direction as someone who’s just standing in place beside the merry-go-round.
Fraser: Right. But, I think the thing that’s interesting – right. So, you would see it –
Pamela: Rise in the west.
Fraser: It would rise in the wrong place. It would go quickly across the sky, and then –
Pamela: In the wrong direction.
Fraser: And, rise again in the wrong place because it is orbiting faster than the day.
Pamela: Yeah.
Fraser: Which is mind-bending.
Pamela: And, one of the side effects of this orbiting in the wrong direction – it’s orbiting in the correct direction. It appears to move across the sky in the wrong direction. It’s strictly an optical illusion. I swear to god, it is orbiting in the correct direction.
Fraser: Right.
Pamela: Now, one of the side effects of this is where our moon is happily moving away from the Earth at a few centimeters per year, it’s not quite such a happy story for Phobos. Phobos is actually getting about a meter closer to Mars every 50 years; 2 meters closer every 100 years; and, in 30 to 50 million years – about the time Earth becomes uninhabitable – it’s going to get tidally torn apart, and potentially do all kinds of badness to the orbital area around Mars by forming a ring.
Fraser: And, then smash into the planet, eventually.
Pamela: Yeah. So, this whole backup system of putting humans on Mars has a few problems.
Fraser: I love this idea with our moon, because the moon takes 28 days to go around the Earth, that orbit that takes longer than a single 24 hour period means that the moon is slowing down the Earth’s rotation speed, and it is slowly drifting away to compensate. And, wherever we see this in the solar system, that same mechanism will play out again and again – this tidal locking that happens. But, if you get a moon that is within the day period of it’s planet, then it’s the opposite cycle that it is speeding up the rotation of its host planet, and it is drifting closer to the planet to compensate, and in the end it must crash into the home planet. And, so Phobos is doomed.
And, when you think about – just to go back to that idea that Phobos has been around for 4.6 billion years, like the rest of the objects.
Pamela: Give or take.
Pamela: Well, our moon has only been around for less than 4, so we don’t know quite when –
Fraser: Sure, but billions of years, right that we are here for the final 30 to 50 million years of its life, which is just an amazing coincidence that all of the other moons – and, sorry. I know I’m going on a rant. I find this so interesting and fascinating that – I’ll go back to some questions in a second – but, that this probably happened across the solar system many other times, but they’re all gone now because they all crashed into their planets like they’re supposed to. And, Phobos is like the last one to sort of follow in with that crowd. I find this concept just mind-bending. I love it.
Pamela: There’s a certain amount of fabulous poetic justice going on because Mars is named after the god Aries, just instead of using the Greek version, they use the Roman version. And, in naming Phobos and Deimos, they went for basically the children of Mars. So, you have Phobos, which means panic and fear. So, the moon that is going to be shredded is the moon of panic and fear, and if I knew my destiny was to be gravitationally shredded by my father, which is mythologically what we’re talking about here – panic and fear would be entirely the right things to tie to my creation.
Fraser: Perfect. Now, before it gets destroyed, and takes out a good portion of the terraformed Martian future, Phobos is a nice place to visit. It’s been the target of space exploration – of failed space exploration, but it still has some really intriguing properties, right?
Pamela: The Soviet Union – Russia – did try – try being the optimal word – Mars curse. They did try to go there. Failed. Two spacecraft died, both really due to mistakes. So, it was human intervention caused space stuff. But, we have some pretty amazing pictures of Phobos and Deimos thanks to other orbiting spacecraft that orbited fairly close. There was just not successful landing on them. So, the thing with Phobos that makes it so tantalizing is that it is the larger of the two objects. It’s close enough to the surface of Mars that you can do the whole fly-by-wire thing with your robot.
And, since it’s zipping around so frequently, you can’t pay direct attention to one robot continuously for an entire day, but you’re going to keep coming back to that sucker. And, it’s dimension are 27 x 22 x 18 meters, when you sum up the fact that it’s –
Fraser: Kilometers.
Pamela: Kilometers, yes. Kilometers. 27 x 22x 18 kilometers. Pretend it’s a rectangle. It’s a weird shaped potato. This is big enough to be interesting to be able to imagine finding a fairly good landing site on. Now, this is a highly porous object, which means there are a lot of empty cavities inside, as near as we can tell. The surface doesn’t seem to have a lot of water from spectroscopic measurements, but that isn’t to say that there isn’t potentially frozen volatile stuff that would melt, gases, water, deeper down in Phobos. And, the gravity is super low. You would weigh less than 1,000 times what you weigh now.
So, let’s say you weighed 1,000 pounds on Earth. Neither of us weigh that. We’re quite grateful. But, say you did, your weight on Phobos would be about half a pound.
Fraser: Right.
Pamela: Which is awesome.
Fraser: Well, as we learned with the Rosetta mission, attempting to interact with that kind of micro-gravity is incredibly difficult, and the slightest movements will kick you off into space. It’s a bad scene.
Pamela: It’s not as bad as Rosetta. With Rosetta, your typical human being would weigh about what a sheet of paper weighs. You and I both weigh more than 2,000 sheets of paper. So, you do have added gravity that you’d have on that particular comet. So, yeah, it’s bad news in terms of you can easily hurl stuff off of the surface. The escape velocity is just eleven meters per second, so a good golf ball swing, and you’re gone.
Fraser: You wouldn’t be able to jump off, though. I don’t think anyone can jump eleven meters per second.
Pamela: Well, on Earth gravity, you can’t. I don’t know here. But, it’s kinetically messy; ergonomically messy – and, messy is good in this case. So, the nice thing is you go, you land, you maybe or maybe not have water deep down if you dig through the regolith, but even if you don’t you can imagine sending – just like people talk about doing with Mars – sending your supplies ahead of time. Land on Phobos. Do your remote reconnaissance. You have real-time ability to explore. You don’t have to worry about the germy bugs that human beings carry getting of you and killing things because Phobos is dead. We’re all pretty confident in this.
So, the plague upon Mars fear that we have with putting people on Mars isn’t a problem with Phobos. So, we can work to maintain the not actually pristine, but not terrible surface conditions from Mars life as invaded by human robots by putting the human beings close, but not actually there.
Fraser: Right. And, Phobos provides this wonderful halfway point to Mars in terms of just the amount of velocity that you’re going to need. You get to Phobos; go to the hollowed out space dock that Pamela has been planning for decades now. You get more landing fuel, whatever you need to do; visit with the future colonists, and then make your way down to the surface of Mars. When you want to leave Mars, fly up, dock at Phobos, pick up more fuel, head for home. It’s a really great – having these objects are very useful for space exploration in the future, and as you said, a great place to observe the surface of Mars without actually having to go down and stand on the surface of Mars.
Pamela: So, all in all, it seems like a kind of ideal option minus the whole, “We don’t know how to get there without dying,” problem. But, I think that’s where we’re going for the next couple of episodes.
Fraser: Yeah. That’s exactly it. We’re going to talk about getting to and from Mars, and so this is going to play into our future plans. Would Phobos work – I’m half-joking about this – but as one of your, “Let’s take an asteroid, hollow it out, spin it up.” Would Phobos do the trick? It’s pretty big, right?
Pamela: So, the real issue is it may be a rocky pile of rubble that is loosely held together gravitationally, and taking something that’s loosely held together, and spinning it up seems like a really bad idea. So, no, I would not spin it up until we knew a lot more about how well it was gravitationally held together. When you spin something, you add all sorts of extra forces that do not help it hold together.
Fraser: And, of course we have this problem of it getting lower and lower to Mars, and eventually crashing into the surface of the planet, which would be a very bad day.
Pamela: Yeah. But, what’s cool is when you look at the pictures of Phobos, it’s covered in these deep groves that can – if you trace them back – they’re traced back to probably four different events in time by looking at the directions of them. And, it’s thought that these grooves are created by stuff other than Phobos or Deimos hitting Mars, throwing debris up, and then poor Phobos had to plow through the debris, so Phobos has already had its life ruined by Mars many times. So, again, we’re looking at poetic justice here.
Fraser: And, one of your ideas, as well, is that there might be water under the surface of Phobos, so once again one of the rarest substances in the inner solar system is water. It very well be that you can find it under the surface on Phobos, which would make getting to and from Mars easier, like a refueling station right there.
Pamela: And, if you haven’t ever seen a good image of Phobos, Google Mars Reconnaissance Orbiter and Stickney Crater. Stickney Crater is the largest crater on Phobos, and it’s near this crater that all of these grooves that aren’t necessarily associated with the carter happen to exist, and these are some truly stunningly beautiful, “Wow, that object has had a bad life!” kind of images.
Fraser: I’m able to show an image here for the people watching us live, but yeah, you’re going to have to do this for yourself. There we go.
Pamela: And, while he’s pulling that up, as we’re running out of time, I do want to throw in a correction for a past episode. On the episode where we talked about naming telescopes, I incorrectly said based on an argument I heard scientists having that Hubble wasn’t named Hubble until after launch. And, some who actually works on – well, worked, past-tense, and is now a friendly, awesome, Ameritis person, reminded me, “No, actually it was named prior to launch,” which happened when I was mostly interested in marching band in high school. I’m very sorry for the mistake. I will from now on reference check people who have interesting arguments in front of me.
Fraser: Right. Okay. Well, like I said, this is the beginning of a multi-part episode series on Mars. How many are we going to do? Who knows? We do them until we run out of the topics that we want to talk about. We have some interesting ones coming up next week. We’re going to be talking about getting to Mars, I think.
Pamela: I think tomorrow is living on Mars.
Fraser: Living on Mars, that’s right.
Pamela: – Mars doesn’t have an answer. So, we’re going to talk about living on Mars, and getting back from Mars, because we don’t know how to get there safely without death.
Fraser: That’s right. And, other topics I’d like to talk about, so that sounds great. Alright. Thanks, Pamela.
Pamela: Bye-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 info@astronomycast.com, tweet us at @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 Earl, and the show was edited by Chad Weber.
[End of Audio]
Duration: 31 minutes

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  1. Robin Denke

    for some reason I cant get this the moons of mars. it just tells me to stand by. is this a problem with my computer, or something alas.

    • susie

      You should be able to see the video now – you’ll get the standby screen if we’ve scheduled the livestream but it hasn’t recorded yet.


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