Date: October 30, 2009
Title: Sky and Earth: Together at Last! Part 1
Podcaster: Alan Perkins
Organization: Department of Geosciences at San Francisco State University.
Description: The disciplines of astronomy and geology, which used to be separate, are working together in the new field of planetary science. The episode also discusses how discoveries in astronomy and geology are useful in the study of the other field. Examples might be how geology assists in the understanding of other celestial objects, such as Titan. On the other hand, knowledge obtained through the study of astrophysics, for example cosmic rays, is useful to geologists in dating geologic processes on earth. The content is conveyed through an interview of Dr. Leonard Sklar of the Department of Geosciences at San Francisco State University.
Bio: Alan Perkins is an amateur astronomy/planetary science enthusiast. Dr. Leonard Sklar of the Department of Geosciences at San Francisco State University.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Joseph Brimbacombe.
Alan Perkins: Welcome to the “365 Days of Astronomy” podcast. I’m Alan Perkins, your host for today. With me is Dr. Leonard Sklar. He’s an Assistant Professor of Geology in the Department of Geosciences at San Francisco State University.
One of Dr. Sklar’s areas of emphasis is the study of geomorphology, which is the scientific study of land forms and the processes that shape them. Thank you for joining the podcast, Dr. Sklar.
Leonard Sklar: Thank you, very much, Alan. It’s great to be here.
Alan Perkins: Dr. Sklar, in the past many people assumed that Astronomy and Geology were completely separate fields of study. One involves studying things beyond the earth; the other was the study of the earth. They seemed totally different.
Now scientists seem to begin a more detailed study of other bodies in the solar system and beyond. It seems that the two fields of study are beginning to work together.
Some universities now have department names such as Planetary Science or Geoscience. What is Planetary Science and why are universities using these new names for their departments?
Leonard Sklar: Well, you’re right, and it’s a very interesting change. My home department at UC-Berkeley where, University of California at Berkeley, where I got my PhD recently did change the name of the department from Department of Geology and Geophysics to Earth and Planetary Science. Note the singular there.
And I think this reflects the degree to which the study of the earth is very interdisciplinary and the earth is a planet. It’s the planet we know best. But to understand the earth well it’s very helpful to look at other planets and other planetary type bodies.
We learn a lot about the earth from studying planets such as Mars and also moons such as the moon of Saturn called Titan.
Alan Perkins: So it sounds like knowing geology is now an important part of studying other celestial bodies. I know that the Mars rovers, of course, are a well known example of that. And you also mentioned Titan. Have you been involved in the study of other bodies in the solar system personally?
Leonard Sklar: I have, yeah. I’m involved in what’s a real exciting and fun project. My – one of the areas of expertise of mine is the study of rivers and how rivers cut canyons. And so I’ve studied how that happens on earth and recently a probe – the Cassini probe and its daughter probe, the Huygens probe, traveled to Saturn and the Huygens probe actually went and landed on Titan, the largest moon around Saturn.
We weren’t able to see the surface of Titan with telescopes previously. Titan has a thick atmosphere, thicker than earth’s, but the Huygens probe went down, parachuted through the atmosphere and successfully landed on this icy moon and sent back astonishing pictures. Pictures of a landscape that looked so much like earth, myself and many, many other scientists and people around the world, when we saw these pictures we fell out of our chairs. It was so exciting to see.
And what we saw were hills and valleys and canyons and rivers and lakes and a topography that could be many, many places on earth. And so immediately people started asking, “Well, how can it be that there are river channels on this moon that’s at about a 90 degrees Kelvin, or negative 180 degrees Centigrade?”
The bedrock on Titan is water ice, we think. And the – so it can’t be water that’s flowing in those river channels. Instead it’s methane, liquid methane. And that methane forms – there’s enough methane in the atmosphere of Titan to form clouds and even convective thunderstorms.
And so there are potential torrential downpours of liquid methane falling on the hill slopes around these river channels. And that liquid flows into the channels and somehow cuts these canyons and channels.
So in particular what I’m working on, together with my students and collaborators at Wheaton College, Jeff Collins, a planetary geologist who encouraged me to get into this business.
The question of how does the ice actually get broken off? The particles of ice get eroded form the river bed in order to create the canyons. So as the rivers cut down they have to detach this – what we would call bedrock material.
And from my dissertation research and subsequent research I developed a theoretical model to explain how this happens on earth. And this was fortunately a model that’s explicit enough in the physics to be transferable to another planet, a place where the gravity’s different, where the material is different, where the fluid has a different density, different viscosity.
And this is an example of what we were talking about just a moment ago about the interdisciplinary nature of study of planets, that we need to be able to use physics and insights that come from engineering and many other disciplines in understanding the earth.
And so with this model we’re able to make predictions about how old the surface is on Titan, how rapidly those rivers are being cut, and get a picture of how that planetary body works.
Alan Perkins: In fact, I’ve heard that based on some of the Huygens probe results the Cassini was essentially rerouted or redirected to make further passes, and that some of the radar photographs at the higher latitudes have shown things that at least appear to resemble lakes. Is that true?
Leonard Sklar: Yeah, that’s right. We have fantastic images now being sent by – back by the Cassini probe using radar which can see through this thick atmosphere. And we’re seeing features that I think people have confirmed now must be liquid bodies, lakes, presumably of liquid methane.
And where the river channels come into those lakes is some of the most remarkably earthlike topography with wide meandering bends in the rivers, with what look like flood plains. It’s a setting that must be active now.
One of the challenges when we look at other planetary surfaces is the – is trying to understand the age. And, you know, when we look at Mars, much of what we see are features that may have been created billions of years ago that are fossilized and preserved there.
And one way that we can measure how old a surface is, is by looking at the frequency of cratering. And when we look at Titan we see very few craters, even though we know that there are objects whizzing around in the orbit of Saturn that are likely to impact Titan and create craters.
So the absence of craters and the very earthlike topography of the river valleys and the lakes makes us think that the processes that are shaping that surface on Titan are active today.
Alan Perkins: Wow, that’s really interesting. On a slightly different topic, last year the journal Nature had a feature on the work of Bill Dietrich and Taylor Perron on looking for a topographic signature of life.
What did they mean by that, and is that another example of how geomorphology can be used in the study of other celestial bodies?
Leonard Sklar: Yeah, this is a really interesting paper that has sparked a lot of work. In fact, I just a few months ago was at a conference that was motivated by this paper about life and its landscape.
And in much of what we have done so far in geomorphology and in terrestrial geology to understand our surface – the surface of our planet, we haven’t looked carefully at the effects of life.
But we know that life influences processes that shape land forms and that move, for example, that break rocks apart and move them down slope. The microbes that break apart the rock, the gophers and the trees on the hill slopes that move the soil down slope.
But we haven’t incorporated those processes into our quantitative understanding of why the mountains are as tall as they are and how long it takes to wear away a mountain range.
And so what Dietrich and Perron did was to step back and say, “Well, how would we know if we look at the topography of earth that there’s life here?”
Or you could pose the question, “Could we look at another planet and see a telltale signature of the influence of life just by looking at the topography, just by looking at what we can see with telescopes or we can see with probes without having to visit and test for the presence of microbes or larger life forms that might potentially exist there?”
And so they’ve found that when we look at earth’s topography we can see what we can interpret as the effects of life almost everywhere. However, what we can’t see is a kind of land form or a pattern that couldn’t form without the influence of life.
I’ll give you a simple example from rivers. Plants that grow along riverbanks strengthen the banks of the rivers, and all else equal we’ll get a narrower channel if we’ve got more riparian vegetation.
But we also get narrow channels when the banks are made out of rock or are clay rich. And so a narrow channel isn’t a surefire indicator that there’s life there.
They – Dietrich and Perron looked at the frequency of occurrence of different land forms, the shapes of hill slopes, the steepness of hill slopes, and came to the conclusion that life does impart on average a statistical tendency towards the kind of land forms that we see on earth. For example, smoother hill slopes and less – not as jagged or as rocky as they would be without the presence of life.
But we also see those same land forms on, for example, Mars, and the Mars rovers have sent back pictures, for example, of the Columbia hills that look remarkably earthlike.
So while their question, “Do we see a topographic signature of life?” doesn’t have a definitive answer, “Yes, we can point to this as a fingerprint of life” it has sparked a very, very fertile strain of research that many, many people are pursuing. And we may yet be able to look at another planet and say, “We see features that are very unlikely to have been produced without the agency of life.”
Alan Perkins: I’ve heard the synergy of geology and astronomy is really a two-way street. Has the study of astronomy and astrophysics provided some useful techniques for the study of the earth?
Leonard Sklar: Oh, yes, for sure. I mean, one of the reasons that we go to study other planets is so that we understand our own better. But, for example, testing the model that I’ve developed for river incision into bedrock on Titan confronts us with the limitations and forces us to understand better the physics here on earth when we’re trying to explain what happens on some other planet.
But in terms of techniques there are many, many ways that astrophysics and the study, trying to develop both techniques and insight for how things work outside of earth, helps us here on earth.
One example is a very powerful dating technique called cosmogenic radionuclide dating that has revolutionized our understanding of the rate at which earth’s surface processes shape land forms.
And what this is, is an understanding from astrophysics of the flux of high energy particles that come in from other stars and even from other galaxies and there’s a rain of these particles that enter into our atmosphere and actually pass through our bodies even. When these particles, these charged particles, hit the atmosphere it actually creates a kind of complicated cascade of interactions in the atmosphere that makes a rain of these minute particles.
But the key thing that happens is when they strike the nucleus of an atom, for example, in a quartz grain, in a rock, they cause a reaction that forms an isotope that’s unstable. And that unstable isotope will decay at some known rate.
And so what we can do is we can go and take a sample of rock and dissolve and do quite extensive work on it in the lab. But eventually put it in a big machine where we can count the number of atoms that are produced by this process of the impact of the cosmogenic rays striking the nuclei and from that estimate how long the rock has been at the surface of the earth, because the influence of these reactions only happens right at the surface. It’s about a meter deep. The rate decays to a very negligible rate.
So you can think of this as a kind of a cosmic sunburn that the rocks receive and by how burned they are, or how unburned they are, if you like, we can work out how long those rocks have been sitting at the surface.
And from that we can work out how rapidly the surface is eroding. And that’s a super useful thing to know to try to infer how landscapes evolve and how they respond to the forcing from tectonics or from climate or how different rock types – at the rates that rock types erode. And as I said, this has really revolutionized our study of land forms on earth.
Alan Perkins: One question I have is if you don’t know how fast or how much or – volume, I guess, of cosmic rays are coming, how do you know that technique is accurate?
Leonard Sklar: Well, people have studied this a great deal. And so we now have a good idea of if you were outside the earth’s atmosphere what the rate of cosmic rays would be that you would experience. And the key thing is to know how that’s attenuated by where your place of interest is on the earth.
And so the thickness of the atmosphere matters and so elevation matters. So if you’re on top of a mountain you’ll be feeling – you’re more likely to get this cosmic sunburn than if you’re down at sea level.
And also the earth is in a plane orbiting around the sun which is also similar to the plane of our galaxy. And so the orientation of where you are with latitude, if you’re close to the pole versus being close to the equator also influences the flux.
And so when you take a measurement, you take a rock back to the lab, in the field at your site where you’ve sampled, you need to measure – you need to know your elevation, your latitude, but also you need to measure how much topographic shielding there is.
So if you’re in the shadow of a big mountain that’s going to reduce the flux of the cosmic rays. But there are well worked out techniques to do this.
Alan Perkins: Wow. Well, thank you. So in the future both astronomy and geology seem to be integral parts of the study of other celestial bodies. Does that mean that some fields of geology give a student a chance to use one set of skills to study both the earth and other bodies?
Leonard Sklar: Oh, very much so. It’s such an exciting time to be a geologist because of the influence of astronomy and the growth of the field of planetary geology.
So many problems that we are trying to understand on earth have analogs in trying to understand other planets. You know, for example, the question of how do you get valleys that have a sort of amphitheater shape to them has been a subject of a lot of recent study, and I’ve taken students on field trips in Southern Utah to go look at these features.
They’re very beautiful and compelling cliffs of red rock that have topographic analogs on other planets, for example, on Mars. And the classic interpretation of Mars, you know, people looking through telescopes at Mars was that these must be formed by water seeping out of the base of the cliff.
But we – here on earth we have examples of the same features that people made the assumption were formed by this seepage process, but when we’ve gone deeper to look at “Well, what are the exact mechanisms by which the particles are detached due to seepage and what are the alternative explanations that might be out there?” we find that more conventional river flow, water spilling over the edge of the cliff, is capable of creating these land forms as well.
So in Utah, there’s another place in Idaho, where students have gone to look at these box canyons, they stand there and are confronted not only with the question of, “How do things work on earth?” but “Could this be the way things work on Mars as well?”
Alan Perkins: What might be a typical curriculum in the U.S. for a university undergraduate who wanted to major in geology leading towards further study of planetary science?
Leonard Sklar: Well, there’s many ways you could go. Of course, the more math and physics a student has the more readily available the techniques and the whole fields of disciplines in geophysics and in astrophysics and so forth become available.
But I tell students who are interested in this that they should follow their curiosity and follow their passion. And so, you know, the study of sedimentology, for example, has application on planets. You know, for example, the Mars rovers have been sending back images from craters where we can see sedimentary structures in the crater walls.
Geomorphology, you know, my own discipline, is an excellent place to pursue one’s curiosity and passions for both understanding the earth and other planets.
So a conventional geology curriculum or one that has a stronger quantitative element of mathematics and physics and chemistry as well, either way could be a good path to further study and a career in planetary science.
Alan Perkins: Do you have any recommendations for websites or other references for people who wants to learn more about pursuing a career in planetary science?
Leonard Sklar: Oh, there’s a wealth of information on the web, so many great places to go. Of course, NASA has many excellent websites. I’ve certainly been inspired by the astronomy picture of the day site which has everyday a different image with many, many links. And some of those will lead back to planetary science.
There’s the International Association of Geomorphologists has a division of planetary geomorphology. They’re at the American Geophysical Union meetings, there’s a section on planetary geology where – so the American Geophysical Union or agu.org website is a good place to go to find more scholarly abstracts and so forth.
There are many researchers and good groups – research groups to pursue at Cal Tech, at Arizona, at Brown University. There’s another fantastic website has been created by a geomorphologist named Alan Howard at the University of Virginia.
Alan does numerical modeling, among other things, of the evolution of earth’s surface and planetary surfaces, and has some fantastic animations of calculated evolution of Mars and of the moon and of other planetary bodies.
Certainly the web is a fantastic place to find good connections. But I also would encourage people who are interested to directly contact other scientists, scientists such as myself or people researching in this field. You’ll find that geologists and planetary geologists are eager to talk about their field and delighted to share what they know with anybody who’s interested.
Alan Perkins: Well, Dr. Sklar, you pointed out that many professional scientists like you are extraordinary giving of their time to help people learn more about science. We appreciate your giving your time today to help the International Year of Astronomy. Thank you again for joining us.
This concludes today’s episode. We hope you look forward to future editions of the “365 Days of Astronomy” podcast.
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365 Days of Astronomy
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