Ep. 502: No Touching: Determining Composition of Worlds Remotely

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How do you know what something is made of if you can’t reach out and touch it? How do we know what planets lights years away have in their atmosphere? What about the rocks all around Curiosity? Or the geysers coming out of Europa and Enceladus? Scientists have a few handy tricks.
This episode was recorded on Thursday, Oct 18, 2018. We usually record Astronomy Cast every Friday at 3:00 pm EDT / 12:00 pm PDT / 19:00 UTC. You can watch us live on here on AstronomyCast.com, or the AstronomyCast YouTube page.

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This episode is sponsored by: Casper.

Show Notes

What is spectroscopy?
Studying spectrographs of stars and planets
How Does a Spectrograph Work? [Infographic]
Every color of the Sun’s rainbow: Why are there so many missing?
Spectrographs and Spectra (with images)
KMOS (K-Band Multi Object Spectrometer) at VLA
Spectrophotography With a GRISM Star Spectrograph
Mars Curiosity Rover and its equipment
Emission and Absorption lines
TESS Exoplanet Mission – (Transiting Exoplanet Surveying Satellite)


Transcription services provided by: GMR Transcription

Fraser Cain: Astronomy Cast, episode 502. How do we know what something is made of if we can’t touch it? 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 Fraser Cain, publisher of Universe Today. With me is Dr. Pamela Gay, the Director of Technology and Citizen Science at the Astronomical Society of the Pacific and … the director of CosmoQuest. Hey, Pamela. How are you doing?
Pamela Gay: I’m doing well. How are you doing, Fraser?
Fraser Cain: Doing great. I’m in Texas, attending my daughter-in-law’s wedding. I’m hoping for good weather tomorrow. Before we get into this week’s show, I wanted to make a quick mention about a conference that’s coming up that people might dig. It’s called ThinkerCon. It’s gonna be in Huntsville. It’s organized by Destin Sandlin from SmarterEveryDay. There’s gonna be a bunch of people there. It’s pretty cool. It’s like a dinner where they’re going to be having dinner underneath a Saturn V rocket in the Rocket and Space Museum in Huntsville?
Pamela Gay: Yeah. Space and Rocket Center.
Fraser Cain: Yeah. So November 17, 2018 from 6:00 to 10:00 p.m. And if you go to thinkercon.com, you can get some tickets. So it’s about a month from now, ish. So if that’s something that you wanna do – and you should check out the guests and the people who are gonna be there. It’s actually gonna be pretty exciting. We might be there.
Pamela Gay: We might. We’re still working to sort this out. At least, I am.
Fraser Cain: Still looking at the logistics, yeah. We just saw a Saturn V. How cool can they be? Really cool. All right. So how do you know what something is made of if you can’t reach out and touch it? How do we know what planets light-years away have in their atmospheres? What about the rocks all around Curiosity or the geysers coming out of Europa and Enceladus? Scientists have a few handy tricks. It does sound like a tough one, right? You see a planet that’s really far away and all you can do is look at it. How do you know what it’s made out of?
Pamela Gay: It’s tricky and it all comes down to light. At the end of the day, we know everything we know about non-touchy objects by how they reflect light, how they absorb light, and how they interact with gravity. And interacting with gravity sorta helps us understand what things are made of, but when it comes to the fine bits and pieces, it really is how does the light and the material play or not play well together.
Fraser Cain: So what is this technique called? And I wanna hear you say the word so that I don’t mess it up.
Pamela Gay: Spectroscopy.
Fraser Cain: Spectroscopy. And this is this method of being able to analyze the light coming from an object. And just by looking at the light alone, they can tell what something is made of. Explain how.
Pamela Gay: Well, every object has its own unique pattern of energy levels that electrons are allowed to occupy. And as a electron gets energized to a new level, it has to absorb some energy, usually in the form of light. Every time it cascades from a higher energy level down to a lower energy level, it has to shed energy, again, in the color of light.
When you look at a lit-up sign at your local store or restaurant or bar, the color is defined by what is the gas inside that tube. So you get distinctive colors that are indicative of a specific atom where electrons are going from a level they got bumped up to by the electricity. Then they cascade down, they give off a photon, we see a distinctive color. Well, we see a distinctive color that matches that thermodynamic electrical energy level thing and stuff and it is the brightest one that our eye is detecting. But atoms and molecules have an entire well-defined series of energies that are the only energies that our electrons are able to be in.
So when we look at a rainbow, continuum light of all temperatures coming off of a warm object, and we see on top of that continuum bright lights, that means that there is an excited atom that is giving off photons. That’s an emission spectra. And we can figure out what atom it belongs to by measuring out what is the pattern we see. If we see a bunch of dark lines on that continuum, that rainbow spectrum, we know that there is cool gas that is absorbing out the light from some background source. With our own sun, the inner parts of the sun emits a continuum, a rainbow of light, and the outer atmosphere of the sun absorbs out different elements, causing dark bands in that solar spectrum that we can observe.
Fraser Cain: When we think about that idea of a rainbow, we imagine it as this universal rainbow, that all rainbows will look like this rainbow because that’s what rainbows look like. But the reality is that’s just the rainbow that comes from the sun, our sun. And so that is a custom rainbow that we get that is kind of trying to tell us what the sun is made out of, isn’t it?
Pamela Gay: Well, that custom rainbow that we see with our eyeballs is just the continuum spectrum of the sun and it is consistent with any star that temperature.
Fraser Cain: All I’m saying is if it was a red giant star – If it was a blue star, we would see a different rainbow.
Pamela Gay: Exactly. And it all comes down to which colors give off the most light and which colors give off the least light and where in the colors the most energy is coming out is determined entirely by the temperature of an object. So a hot blue star is blue.
Fraser Cain: And that’s your baseline. That’s your custom rainbow that tells you the temperature of the star. But then if we take the light that comes from the sun and magnify the rainbow, we can see what you were talking about. We can see this custom fingerprint in the light itself.
Pamela Gay: That needs to be a new Skittles line. “Magnify the Rainbow.”
Fraser Cain: Magnify your custom rainbow.
Pamela Gay: Exactly. So, when we zoom in, when we take that rainbow and we spread it out so that instead of being a few thumb widths across the sky, it is instead 30 feet wide in some extravagant spectrograph. When we spread it out like that, we’re able to see exactly what’s going on at every fine-grained wavelength. This is like going from that pack of eight crayons to 24 to 128 to an [inaudible] [00:06:56] of 120,000. And with that high resolution, we’re able to see, “Oh, here are the lines of oxygen in our atmosphere absorbing out sunlight. Here are the lines of the hydrogen atoms in the sun’s atmosphere.” When we’re trying to understand what something’s made of, this actually makes things more complex.
When we look at the moon, if we look at its spectra in detail, we are seeing three different things. We are seeing the absorption in emission lines that are native to the sun. So these are things happening in the sun’s atmosphere that are reflected off the moon. We’re seeing the colors of light that are absorbed out by the moon’s surface. And we’re seeing the colors of light that are emitted and absorbed within our own atmosphere. This all layers together where you can end up with crazy things like the sodium lamp near your observatory makes it look like the sun has sodium. And we have to know how to correct for all of these things.
Fraser Cain: So when we look at the rainbow and we expand that rainbow, we’re seeing the light from the sun and we’re seeing the interaction with our atmosphere. And when we look at the moon, we’re seeing the light from the sun, we’re seeing the interaction from our atmosphere, and we’re seeing the special little bit of the recipe that the light bouncing off the moon gets added to this picture as well.
Pamela Gay: Exactly. So we have to correct for what the sun is doing, we have to correct for what our atmosphere is doing, and once we correct for those two things, there’s chunks that we know nothing about because the lines from the Earth’s atmosphere and the lines from the sun are interacting. And then we’re left with a stripy rainbow of what the moon is doing.
Fraser Cain: That’s really cool. Now, how did astronomers figure out that each individual kind of rainbow matched the chemical? Like, how do they know that, “Oh, if I see this in the rainbow, then I know that’s the presence of oxygen?”
Pamela Gay: It was a non-linear, highly-creative process. Spectrographs started to be understood. Prisms started to be understood. As long ago as Herschel, we knew you could send light through a prism and get a rainbow. But we didn’t understand the tie-in to individual atoms until people started doing laboratory chemistry and looking at a variety of different atoms and then noticed that if you take the patterns seen in the laboratory and compare it to the patterns seen in the stars, by golly, it’s the exact same pattern, just shifted around because things in space move.
And so when people first tried to understand the stellar spectrum, started to try and understand the dark and bright lines in the continuum of stars, they labeled the lines not-entirely useful things like A, B, C, and D instead of hydrogen alpha or the calcium two lines. It took time to figure out the atomic origins. And what was kind of cool was they saw the relationship between color of star, kind of star, and the spectral lines that could be seen before they fully understood how all of these pieces fit together.
This is why our stellar categories have crazy names like O stars, B stars. It’s because we didn’t quite know how to fully put them together when people started labeling the spectra. Once we got this combined understanding of how things vary as a function of temperature and a function of luminosity, that gave us the color of magnitude diagram, the Hertzsprung–Russell diagram.
But that was just the first step. That really helped us understand atoms, but when it comes to understanding rocky stuff, the story’s a little bit more complicated.
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Fraser Cain: So you can figure out what a gas is made of, but how do you figure out what something – how does Curiosity know that a rock is made out of something if it can’t just go over and… I don’t know, lick it?
Pamela Gay: Well, Curiosity does go on over and grind on the rocks.
Fraser Cain: It does sometimes, and look at it very closely, but it also has a laser.
Pamela Gay: It does. Curiosity has a variety of different techniques. It can cheat and pick up a sample and run it through a mass spectrometer and literally count up the atoms of what is inside, and the molecules, of what is inside a given rock. It has the ability to use lasers to heat them up. It will take spectrum.
And it’s that reflective light again that makes things interesting, but here, we do a couple things with mass spectrometry. When we look at those samples on Mars, it’s knowing, “Oh, if I pick up this rock and I shine light on it, it looks like this, and if I take that exact same rock and grind it up through my mass spectrometer, then I see these atoms. So again, it’s laboratory chemistry that tells us how to understand all of the reflected light.
Fraser Cain: It’s such a great idea. Scientists have been burning things and vaporizing things for hundreds of years, or 100 years, to get that chemical signature of every single chemical that they can think of. When Curiosity, on the surface of Mars, is zapping things with its laser, they’re looking at the signature of the light that’s coming back from these things that have been blasted by Curiosity, and it’s able to know what they’re made of.
Pamela Gay: And with asteroids, it gets even more interesting because here, what we’re relying on is, “Oh expletive, a rock fell out of the sky and knocked a hole in this roof. Oh cool, let’s take this rock over, shine light on it, measure the emission and absorption of the light as it reflects off of the rock, and then let’s grind up a piece it, run it through a mass spectrogram, and know we know that when we have a piece of space rock that looks in a given way, it is related to this thing that fell to the earth.”
What’s awesome is we actually have pieces of Vesta that fell out of the sky, we’ve been able to pick them up, and so we’ve ground up pieces of Vesta to measure its composition. And we know whenever we look at things that reflect light the same way that Vesta reflects light, this is what they’re made of. We’re violet.
Fraser Cain: That is really cool. So let’s talk about some of the other stuff. One of the big techniques that astronomers are gonna be trying to figure out over the next little while is: is there the presence of biosignatures in the atmospheres of other worlds? Is there evidence that there is life there on planets orbiting other stars? How will they do that?
Pamela Gay: Well, first of all, here, one of the things I need to remind everyone of, because we couldn’t do this otherwise, is when another star, another planet is moving relative to the earth, we see its spectrum shifted to the blue if it’s coming towards us, towards red if it’s going away from us.
The reason this is important is what we’re looking for is that planet. It’s gonna have a star, hopefully. That star’s light will periodically shine through the planet’s atmosphere. We measured the star light. We measured the star light going through the atmosphere. We subtract the star’s light. that leaves us with the emission and absorption caused by the planet’s atmosphere. So, just like we see when sunlight goes through our atmosphere, we see the oxygen lines.
Because that other planet and star’s light is shifted redward or blueward compared to ours, their oxygen lines don’t hide behind our atmosphere’s oxygen lines. And this allows us to look for oxygen, to look for other atoms that we have that hopefully these other worlds will have as well.
Fraser Cain: I love this idea. We talked again about how you look at the sun and you were able to subtract the effect from the earth and you were able to subtract the effect from the moon. So in this case, you look at the star and then you watch as the planet passes in front and then you were able to see how the signature changes. Then you were able to subtract out and what you’re left with is the chemicals that are in the atmosphere of this other planet.
Pamela Gay: That is entirely true. So much depends on our ability to accurately get rid of our own atmosphere. This is where spacecraft like TESS were designed, really, to function with the James Webb space telescope so that James Webb, out in its position beyond the moon, would have no Earth atmosphere to be looking through. Unfortunately, JWST is currently slated for the end of March 2021. We’ll see if that actually happens. I am a pessimist.
Fraser Cain: We just interviewed someone from James Webb last night. He felt pretty confident, but we’ll see. He was pretty confident two years ago when we interviewed him before. Understandably, these things take longer.
Pamela Gay: It’s dead to me until it returns data. We’re just gonna go with that. It’s dead to me until it returns data. So as we collect all this data, we have to constantly correct for, well, if you’re taking observations from Mount Wilson, which is at a low altitude, instead of Atacama, which is in the high desert, you’re gonna have more water lines in your observations because you’re observing more water in the atmosphere.
This is where space telescopes are so important when studying planetary atmospheres because otherwise, our own atmosphere gets in the way. With space telescopes, we can go into wavelengths that most molecules are really happy to do all of their happy spectral absorption and emission in infrared and millimeter wavelengths, so they’re already in the red, either in wavelengths that we can’t consistently see from the surface of the planet except in small windows that telescopes like ALMA really take advantage of. But in space, no atmosphere. No dark places in the rainbow.
Fraser Cain: There are wavelengths that we are incapable of seeing. As you said, some stuff in the infrared, things in the x-rays, gamma rays, that we just have no way to even see from down here on Earth. So once you get up into space, you can see them at all and then try to analyze the spectra from there.
Pamela Gay: This is literally a problem with our atmospheric composition going, “Oh, I like this color,” and absorbing it out completely as the atoms or molecules become excited.
Fraser Cain: Let’s talk about some of the limitations then, because although it’s a wonderful technique, I’m sure there are things that it’s not great for. What are the limitations of this technique of spectroscopy?
Pamela Gay: One of the most frustrating limitations is you are limited in how much you can expand the rainbow, just how fine a detail you can see, by how big your equipment is and how bright the object you’re looking at is. If I’m using, as I did in graduate school, 107-inch telescope, to try to get spectra the faint galaxy that is 0.3 billion light years away, I can’t get really detailed spectra. I can figure out what its red shift is or certain key lines. I can see, but I can’t get in there and see all the little details.
When I look at extraordinarily bright stars, the kinds of things that you can see with your eyeball, because they’re bright, I receive enough photons that I can spread the light out vast amounts. Not only can I see all the atoms, but I can start to see the difference between one isotope of an atom, one number of neutrons in its core, and another number of neutrons in its core. When you’re studying another planet’s atmosphere, what this means is you’re going to be limited in how fine a detail you can resolve by how bright that host star is. This is where TESS, which is set to observe nearby stars looking for planets, is seen as the flagship mission for identifying objects that we can measure the atmospheres.
Fraser Cain: I kind of imagined the situation where you’re looking at a star field of a bunch of stars and a bunch of faint galaxies, maybe, and for the bright stars, you can get very crisp, very bright spectra and for the dim stars, you get very faint spectra and you just can’t get the detail, the resolution in that spectra to be able to figure out, with a satisfying answer, what something is made of. And then imagine then how difficult it’s gonna be to find a planet orbiting another star.
Pamela Gay: This is something that anyone who’s ever used a zoom lens in low-light conditions or anyone who’s done astro imaging has encountered. If you try and zoom in on that dark field, you’re gonna get nothing but noise. But if you have a larger area hitting each pixel, you have more photons per pixel and you get a better image. So you can do wide-field photography fairly easy in low-light situations, but you can’t use your zoom lens if you like yourself or if you like your images to look good. Now, in astro imaging, this is where we start referring to it as binning your image, combining light across multiple pixels where you have lower resolution, but you’re able to see fainter objects.
Fraser Cain: Which I always turn off because I don’t like the way the pictures look. “Nope, too low resolution. I want the higher resolution.” And then I’ll just let it collect more light when I’m doing astrophotography.
So the other thing that I know that is a bit of a problem is although you can tell that there is the presence of certain chemicals, you can’t really tell how much of it there is, right?
Pamela Gay: Yeah. There’s limits. Each spectral line has – we call it a depth that it can have. It can absorb a tiny amount of light, it can absorb all the light in a given color. Once an absorption line is saturated, this is where all the light in that color is absorbed, you can tell nothing else.
So to a certain degree, looking at stellar atmospheres, we’re able to say what a star’s composition is by looking at the ratios of the depths of the lines at different wavelengths, looking to see how as a function of temperature do these lines change. We’ve built up an understanding of what the ratios of the lines should be. It gets very harried and there’s lots of tracing of points and spectra involved and thank God there’s now software for this.
But we can do it up to a point. Once something’s saturated, you can’t tell anything else. It’s like, “I absorbed all the light. I have more of me. I can absorb more light if you had it. You don’t.” So once you hit that saturation point, it’s like filling a bucket. The hose may still be on, but you can’t tell how much water came out because the bucket’s full and now it’s overflowing.
Fraser Cain: But still, I think it’s safe to say that this technique is – modern astronomy wouldn’t be the same without it.
Pamela Gay: Oh no, this is the most powerful tool we have in our astronomical tool kit. Photometry, which is what I do, which is measuring the total amount of light that comes from an individual object, it’s powerful. You can do lots of different things with it. But it’s just a tool. The spectroscopy allows you to do a vast variety of different kinds of things. Measuring the temperature, measuring the composition, measuring the velocity. And when you start looking at alien worlds, it allows you to say, “Yes, there is life there and potentially, thanks to pollution, there is intelligent life there.”
Fraser Cain: So we don’t know how much, but at least we know that they’re there. It’s an amazing technique. Whenever I hear that a mass spectrometer’s being added to a mission, I’m like, “Yes.” It’s so great. Thanks, Pamela, and we’ll see you next week.
Pamela Gay: Sounds great, Fraser. See you later.
Announcer: Thank you for listening to Astronomy Cast, a nonprofit 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 @astronomycast, like us on Facebook, or circle us on Google Plus.
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[End of Audio]
Duration: 27 minutes

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