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To really study something, you want to reach out and touch it. But what can you do if you’re separated by a huge distance? You reach out with electromagnetic or sound waves and watch how they bounce back. Thanks to radar, sonar and lidar.
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Pamela Gay: This episode of Astronomy Cast is brought to you by BarkBox. For a free extra month of BarkBox, visit barkbox.com/astronomy when you subscribe to a six or a 12-month plan.
Frasier Cain: Astronomy Cast, Episode 503; Gravity Maps. 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.
I’m Frasier Cain, publisher of “Universe Today,” with me as always, Dr. Pamela Gay, the director of Cosmo Quest. Hey Pamela, How ya doin’?
Pamela: I’m doing well, how are you doing, Frasier?
Frasier: Hello, from TwitchCon in San Jose. I hope you have a great time there. Before we get any further, I just wanna say, by the way, last week I said it was about to happen. Now, it has actually happened. Our book has hit the shelves. October 23rd came and went and now you can just go to a bookstore and buy a copy of our book.
Many of you I know ordered copies of it. The, sort of, first versions people are loving how it looks. All of the photography, but nobody is reviewing it on Amazon. So, we are the top-selling book – it’s the weirdest thing. We are the top-selling book in astronomy and astrophysics on Amazon – take that Neil deGrasse Tyson – But there is like no reviews of the book.
So, if you’ve ordered a copy, and you’ve got a copy in your hands. Please take a moment and go to Barnes & Noble, Amazon, wherever you bought the book from, and please write a review. It’s so weird to me. I don’t know; did we do a good job?
Pamela: They’re reviewing it on Twitter! I’ve seen the reviews on Twitter. So, go one link further.
Frasier: I know, I know, I know. But, there’s none on Amazon. So, it looks very strange. It looks bizarre to have this book that’s the top-selling book, and yet, nobody is reviewing it. So, anyway please I beg you; when you get a copy, go to wherever you bought it from, and please do review. It means the world to us.
Pamela: Five stars.
Frasier: What? Five what?
Pamela: Five stars. Leave five stars.
Frasier: Oh, yeah. Of course, yes, yes and five stars. Yes, by all means –post an honest review and honestly rate it the five stars that you know it deserves. All right, let’s move on.
So, the Earth looks like a perfect sphere, but down here, on the surface we see that there are mountains, rivers, oceans, glaciers, all kinds of features with different densities and shapes. Scientists can map this to produce a highly detailed gravity map of our planet. And it turns out; this is very useful for other worlds, too. Pamela, gravity maps. What is the technical term for a gravity map? Do you know that one?
Pamela: I don’t know how to pronounce it, geodesy, I believe.
Frasier: I didn’t know how to pronounce it either, so I wanted you to do it. But, of course, I should’ve remembered –Pamela can’t pronounce.
Pamela: I read too much.
Frasier: But, is it gee-oh, gee-oh, geodesy? Geodesic? Yeah, because you think about it; geodesy –
Pamela: This is where we need keeper of maps. Keeper of Maps can tell us how to pronounce it.
Frasier: Yeah, gee-oh-deezee? So, someone, please help us out with the actual pronunciation. And you’ve probably seen these images. They look like the Earth, but the Earth is covered in, like, red and yellow and green and blue blops, and that is this map.
First, what is this idea of a gravity map? What is a gravity map?
Pamela: I first learned about gravity maps when I learned about the asteroid that killed the dinosaurs –and what it is, a scientist walks around in the olden-days – nowadays we also use spacecraft to do this – and measures the fluctuation in how much the Earth is pulling on their instrumentation and them as a function of position.
Now, if you know how far you are from the center of the Earth and you know how much the Earth is pulling on you, you can calculate the amount of stuff between you and the center of the Earth. And as that amount of stuff changes, we will see the amount of gravity change. Now, what’s cool is you’d think, “Hey, I’m at the top of a mountain, there’s gonna be a whole lot more stuff between me and the Earth, and I work out the math, and yeah I’m higher up so I weigh less but there’s more stuff –” and that’s all true.
But the other thing is, you can be moving along a perfectly level surface and see the pull of gravity fluctuate, as you walk over shocked, compressed former asteroid impact sites, as you walk over oil wells, as you change what is beneath you, the pull of gravity changes. And this allows us to map out the structure of our planet from the surface of the planet.
Frasier: It’s pretty mind-bending that the instruments are sensitive enough that they can actually determine that difference in gravity. You know, like what kinds of gravity differences are they detecting?
Pamela: So, what kind of got me when I learned this – I’m an astrophysicist, this is not a measurement I have ever made, I know there’s super-sensitive instrumentation in laboratories, such that, when it rains you can measure the difference in gravitational pull of the mountains around you.
There is a gravity lab up in Seattle that actually sees changes in their results – when it rains. But, what I hadn’t realized is just how amazingly sensitive these instruments are in the field. People who are out there looking for oil have instruments that, if you put them at your feet, and then put them at your nose you can measure the difference in the pull of gravity as a function of that distance, seeing the numbers change between your toes and your nose for how much gravity is pulling on you. That broke my brain.
Frasier: Yeah, yeah. I mean, I think people just have no sense – I mean just – that idea is amazing. You know, you could walk past a mountain and you’re gonna be experiencing gravity from a mountain. This idea that you’re gonna be experiencing gravity from rainfall is crazy, it’s a really stunning technology. So, you are able to produce this incredibly precise map. What is it used for?
Pamela: People are out there, they’re looking for oil. But, beyond looking for oil, this is also an amazing way to map out things like past impacts into the planet Earth, where like, when the dinosaurs died large rock from space come in, large rock from space hits Earth, compresses the surface of the Earth. Over eons it fills in with material to a certain degree, and you’re left with not as noticeable a surface differentiation, but you can map out with gravity the rings and shockwaves in the planet from that prior impact.
Frasier: It’s funny. So, like one of the first pieces of reporting that I did was back, I think, 2001, 2000 it was this German experiment, it was on the Space Shuttle. Space Shuttle launched with, like a cable that it reeled out, and then it had this instrument at the end of it. And as the Space Shuttle and this fishing-rod, you know, went over the Earth’s surface the two parts were experiencing a difference of pull, right? And they were experiencing this force between the Space Shuttle and the probe that it had set out.
And they were able to produce this incredibly detailed map of the entire Earth’s surface. One of the most complicated ones that’s ever been done. But, there are now a bunch of instruments – a bunch of satellites – in space that are specifically doing this. One European space agency has the gravity field and steady-state ocean explorer, or GOCE, Gochay?
Pamela: And then, of course, there is the GRAIL mission that NASA launched to map out the moon, which is a weirdo world with an asymmetric core, and we’re able to map that out, and it’s this double mission that gives us this extra sensitivity like what you were mentioning with the fishing rod.
Frasier: Right, yeah. Again, like the Earth is the place that’s had its gravity map measured out the best but, as you said, the Moon has experienced this, as well. If you’ve got this kind of instrument.
Let’s talk a minute about GRAIL. So, it’s the Gravity Recovery and Interior Laboratory. Launched by NASA back in 2011. Is it still alive?
Pamela: No, it is not still alive. But, what was amazing about this mission is it’s these two spacecraft, and they were maintaining contact between them and with the planet Earth so they could very carefully, constantly update distance information, and they were measuring the slight fluctuations in orbital speed and how these spacecraft were orbiting, so that they could measure – Oh! A has gotten pulled ahead towards this mountain range.
Oh! Now, B is getting pulled ahead – and by looking at these slight differences caused by – Hey, there’s less gravity here, there’s more gravity there – we’re able to see a wavelength of light at a time differences in how gravity is causing these things to orbit.
And with the Moon, the thing though, I know, a lot of us were just like, we knew that the center-of-mass of the Moon was off-center, but being able to see how radically different the side of the Moon that we always see – the front Earth-facing side that’s tidally locked towards us – is from that backside, both with gravity and appearance, has led to changes in how we look at the formation of the Moon.
Where there’s new ideas that maybe, the back-side of the Moon looks so different because, it was splatted upon by a second smaller moon that failed to remain uniquely separate from what we have now.
Frasier: I just wanna take a second and recap that, too. Because, it is this really amazing – you know, if you see what the far side of the Moon looks like; looks totally different from the near side. You don’t get these beautiful seas and dark lunar mare regions. It’s just pounded by impacts, and one of the ideas is that it used to have another moon. There was a second moon around the Earth, and at some point they merged.
And that’s why the two sides of the Moon look so different, which is just amazing. Would’ve been a crazy sight to see it happen.
Pamela: What’s cool, though, is there’s this, essentially, primordial –that’s the wrong word cause that goes too far back in time. There is essentially this original moonlet core that’s in one location, towards the Earth, and then we now have this spherical object with an asymmetric core and trying to model that is all the scientific fun. And we can trace out the details with gravity.
Frasier: And not only Earth and the Moon have gotten the gravity map experience; I know that Mars has, as well.
Pamela: And this is where we’re starting to try and figure out where might the land have been crunched under an ancient ocean. How has its history been hidden as dust storms sweep across the surface, and just like here on Earth, we’re using new and amazing techniques to map out; well, Viking ships buried in cemeteries was a result that came out two weeks ago.
We’re not finding Viking ships on Mars, although that would be awesome. What we are finding instead, is these differences in how the soil has been crushed under oceans, how it has been crushed under glaciers and rebounded over time, helping us understand what is there that we can’t see that is structurally relevant to the history of Mars?
Frasier: So, with these gravity maps, they sort of serve as your first really big survey of a world, to try and figure out where the large features are that are really interesting that you’re gonna wanna explore. What are some places of unusual extra-density? Things like that. And then, you can follow up with a more detailed mission that will explore some of these features more specifically.
Pamela: And I can’t stress enough, how much this is the first step in what we’re now calling, multi-messenger astronomy. This is the idea that, while we spent the first, essentially 500 years of astronomy as a science looking at light, we now have the capacity to look at worlds, look at stars, look at –well, mergers of stars, using, first of all the light we can see with our telescopes.
We look at the Moon. We see the craters. We measure their heights using RADAR and LIDAR, reflecting light off the Moon. But, that still light. Now, we have this ability to also map things out with gravity and this gives us a completely different view that allows us to see things that we, otherwise, couldn’t get to, pretty much, any other way. Cause, who’s gonna take the Moon apart atom-by-atom to measure its density?
And then on top of that, we start adding all of the particle physics, where we’re doing, that’s revealing water entrapped in the minerals. So, we’ve entered this new era where light is just one tiny part of this well, weighty story with gravity.
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Frasier: While you were talking, I was showing off images of SIRI’s gravity mapped and were watching us live. Again, DAWN was equipped with a way to be able to sense the gravity of the dwarf planet as it was orbiting around it.
And so, one of the handy things about orbiting around a world is you can detect how its gravity is pulling and pushing, and twisting and turning you as you are going around to produce this gravity map. And again, super-fascinating for the geologists.
Pamela: And this also allows us in complicated ways that I’m not going to try and explain, because moment of inertia is mean. We can also start to get at the moment of inertia of different worlds, and that is a fancy way of labeling the math’s that says if you have an ice-skater with her arms out, she rotates slower. If you have an ice-skater with her arms in she rotates faster, and if you replace her bones with liquid, that changes how she rotates, as well.
Cause solid, liquid and gas all have slightly different ways of interacting while rotating. And that difference in how a can of soup versus a solid can of frozen soup, same mass, same density, different phase-state. That difference in how they rotate is reflected in how worlds rotate and helps us get at; is there a liquid ocean. And so, it’s moment of inertia information that allows us to start thinking; maybe even Ganymede has liquid oceans.
Frasier: Yeah, it would be nice to come back to some of those other places; to Europa, to Enceladus and perform very detailed gravity map measurements. Because, you have to be in orbit and nothing has ever gone into orbit around Europa, or Enceladus. And those are the big questions. We wanna know, is there a place where there are –is there liquid water close to the surface?
Although, I wonder, can you tell the difference between liquid water and solid water with a gravity map?
Pamela: No, but by combining information on how an object is rotating, which gets you the moment of inertia, with the gravity maps it allows you to start to piece together; okay, we have a map of this object’s density that – it only tells you the density between the center and the surface.
We also have a map of roughly how it slishes in different waves from the inertia. And you get hints, it’s not gonna give you a detailed 3D structure. And this is where you start wanting to do things like; we can measure more into the surface using sound waves reflected off, using RADAR reflected off.
And with Europa, we actually have the chance someday, with powerful enough instrumentation, to just blast in with RADAR, and start to get ideas of what’s beneath the surface.
Frasier: This is like, checking out all of these places with gravity maps. I’ve been able to find a gravity map of Mercury, as well. Thanks to Messenger. And of course, BepiColombo is going back to Mercury now. It has just launched, and so I’m sure will help add on to the gravity maps that were produced of Messenger.
One thing that I always love; whenever there is a big earthquake on Earth, the geologists will tell us how much the Earth’s rotation has changed, because of how chunks of land –
Pamela: The moment of inertia changed.
Frasier: Exactly, and so chunks of land have moved around. The actual shape of our planet has shifted, and that has changed the moment of inertia. And sometimes, the day length on Earth is slowing down every day. But, every now and then you get an earthquake, and it speeds up for a moment. And then, has to continue.
Pamela: It actually stays sped-up if things don’t settle back the way they were, so the big Nepalese earthquake, several years ago – which changed the height of Mount Everest – that massive earthquake changed forever the rotation of the Earth.
Now, we can also change our planet in ways that change the rotation period of the Earth. The massive dam that was built in China, on the Yangtze River that wiped out a ton of villages and archeological sites, also changed how our planet is rotating.
So, anytime you change the mass distribution, it changes the planet’s rotation. This also means that massive droughts, massive flooding changes the planet’s rotation. So, our planet is always at the tiny fraction of a second level, changing as we move the water around. As we move the land around.
So, the day is only 24 hours long, with an error, and that’s awesome.
Frasier: Well, and you get these situations where, for example, you mentioned drought, or you can have a situation where say, a glacier will recede, and the Earth’s surface will bounce back up, cause it was being pushed down by gigatons of ice. And again, geologists can measure thanks to doing the gravity mapping.
They can sense how much the Earth is responding to say, having the weight taken off on top of it. Again, just mind-bending levels of capability.
Is there any kind of analogy to this at all in astronomy? So, you know, when we look out into the universe, can we produce gravity maps of blobs of dark matter and things like that?
Pamela: Yeah. So here, this is where it gets down to using microlensing, instead. So, instead of looking at how a satellite’s orbit is changed as it flies by, or goes around a planet, what we instead look at is photons, which is still stuff. We look at how that photon, which doesn’t have mass, but does have inertia – how its path is changed as it goes past an object, and that change maps out the gravity of the object.
When we look at gravitational lenses, we can see this background object was twisted and turned in this way, which means there is this shape of mass in between. This other one was shredded in this other way; that means there’s this distribution of mass in between.
This is how the Cosmos project produced the very first three-dimensional map we’ve ever had of the distribution of dark matter, is they worked off of the assumption that if you look at a bunch of galaxies, and you average their shape, the average shape is a circle on the sky. And if the average is, instead, a teardrop, a dead ameba, some other squished, not circle shape, that shape is the reflection of the intervening dark matter.
And if you look at reasonably nearby galaxies and measure their distortion, that gives you one depth of dark matter. If you then look at slightly further away galaxies, that gives you the next depth of dark matter.
And they looked at galaxies at a whole variety of distances, mapped out their gravity, distortions from dark matter, and got us the shape of dark matter.
Frasier: And one other thing, although I know, it looks similar when you look into the Cosmic Microwave Background radiation, false-colored images that they are producing; you’re looking at these temperature variations, but don’t the temperature variations of the CMB map to density fluctuations, and wouldn’t that impact the gravity a bit?
Pamela: Somewhat. So, what you’re getting at is Sunyaev–Zel’dovich effect. And that one I actually know how to say.
Frasier: Nice!
Pamela: So, with the Sunyaev–Zel’dovich effect you have the Cosmic Microwave Background radiation is getting red-shifted as it passes through the massive gravity field of a galaxy cluster. And so, the most massive galaxy clusters appear as holes in the Cosmic Microwave Background because they are, essentially shifting all of that light. It’s a great way to find high-mass objects that aren’t giving off a whole lot of light by looking for the missing light of that background illumination.
Frasier: And you’re detecting it through its gravitational impact on the light that coming from the birth of the universe.
Pamela: Exactly. It’s shifting the color of the background universe.
Frasier: Aw –it’s so amazing. Pamela, again, thank you so much. Have a great time at Twitchcon. I realized when I started; I forgot to mention the name of our book. It’s The Universe Today – Ultimate Guide to Viewing the Cosmos – Everything You Need to Know to Become an Amateur Astronomer, and I know I mentioned it last week, but that’s the book. So, again, if you got a copy please, please, please do a review. That would be awesome.
Pamela: It’s a beautiful book.
Frasier: It is beautiful. Yeah, I’m really proud of it. Alright, we’ll see ya next week, Pamela.
Pamela: See you. Bye-bye.
Announcer: Thank you for listening to Astronomy Cast. A non-profit resource provided by Astrosphere New Media Association, Frasier Cain, and Dr. Pamela Gay.
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Duration: 26 minutes
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