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To geomorphologist Taylor Perron, the answers to some of the biggest questions about our solar system are found in the way a landscape changes over time.
Studying a range of topics that at first glance might appear unrelated – from the terrain of Mars to human prehistory – Perron’s research group at the Massachusetts Institute of Technology uses landscape to glean clues about the past and future. “You’ve got one person studying archeology, and another looking at spacecraft data,” Perron said. “But rivers run through it all.”
Rivers are powerful forces that help shape the landscapes they wind through, as well as provide crucial resources to communities that dwell on or near their banks. As the “circulatory system” of our planet’s surface, rivers are heavily influenced by climate.
Geomorphologist Taylor Perron. Photo by the John D. and Catherine T. MacArthur Foundation
Understanding how landscapes have shifted in the past, and what forces underpinned those changes, helps us get a clearer understanding of how current and future climatic shifts may reshape parts of the planet. That inquiry is particularly relevant in the context of human-caused climate change, given the myriad ways the warming world is already and will continue to change our terrain — like by drying up rivers that have flowed for millennia. Understanding how humans have more directly shaped Earth’s landscapes through engineering interventions like dams and modified coasts is an equally important venture, Perron said.
Perron and his team combine their observations of the natural world with mathematical models and computer simulations in order to make specific predictions.
“We try to look for things in nature that either contradict our expectations, or that form some kind of a pattern that we can’t explain,” Perron said.
His research group’s interests run the gambit. He noted that a handful of affiliated academics are currently examining how humans managed to thrive across the Amazon, creating their own rich soils within a largely nutrient poor, flood-prone landscape and developing agriculture despite those obstacles.
They’ve also studied the planet Mars and one of Saturn’s many moons, Titan, because they are the only other places in the solar system that are known to have had or still have rivers, Perron said. These celestial bodies are vastly different from the Earth we know, but some aspects of their unique makeups might offer helpful insights about our own planet’s early days or how a world driven by very different forces than ours — like a methane cycle rather than a water cycle — plays out hundreds millions of miles away.
An image with a river network on Mars, the Nile River on Earth, and a river network on Titan (not all the same scale). Image courtesy Taylor Perron
Perron’s winding career path began in the study of prehistoric archeology, but he soon became fascinated with how Earth’s physical environment and climate may have “steered the course” of our collective history. In September, he was named a MacArthur fellow in recognition of his sprawling, intersectional work examining how landscapes evolve and for developing tools to predict changes we might expect in the context of climate change.
Here’s a closer look at his research, why finding “experiments” in nature gives us key insight into the joint forces of climate and land and how Titan offers a glimpse into an alien landscape with unexpected similarities to our own.
This interview has been condensed for length and clarity.
The answers to some of the biggest questions about the solar system are written in landscapes. How does the planet regulate its own climate? Why did Mars start off with water running over its surface and rivers and lakes, but end up a cold desert? When could it have supported life? Why do some areas on Earth’s surface have much higher biodiversity than other places? How did Earth’s surface environments steer the course of human history?
The challenge there is figuring out how to read the landscape and find the answers to these questions.
[Another] reason that this is a very important field is that we live on Earth’s surface, right? We evolved here. We depend on landscapes for water, food, mineral resources and much more. And sometimes landscapes threaten us by creating natural hazards like landslides and floods. So where’s the connection between some of the very large scale, long-term things we study and shorter-term events that influence humans on the timescale of their lives?
One of the reasons we study landscapes over the longer term is analogous to being able to measure how much rain falls over the course of several years and then take an average. Because we want to determine, by observing over the long term, how things like climate interact with geological processes and life to make landscapes. And if we can do that by studying how the landscapes we see now have evolved over a long period of time, then that will give us a much better basis for anticipating what might happen if there are similar changes in the future. That includes understanding the potential repercussions of our own actions and the way we have modified both landscapes by doing things like building dams, modifying coasts, and also the way we’ve changed the climate.
You can think about Earth’s surface topography — the high elevations, the low elevations — as emerging from a big ongoing battle between plate tectonics, which tend to scrunch up Earth’s crust and deform it and create mountain ranges, and other mechanisms that are driven by gravity and by climate — meaning wind, water, ice — that wear down those mountains and transport all the material they’ve eroded back out to the oceans.
So rivers are really important in all of this because they are one of the mechanisms that drives a lot of that erosion. And then they also take the products of that erosion — all the little broken up pieces of rock and soil — and move it all the way across the continents to the oceans. So rivers are kind of like the circulatory system of Earth’s surface. They’re as important [to it] as our arteries and veins are to our bodies.
Anything that can remove rock from the landscape and move it away is erosion. That can include a river flowing over rock and cutting down at a fraction of a millimeter per year. Or it could include a landslide that maybe only happens every few thousand years but suddenly removes a lot of material in one place. Both kinds of mechanisms, the gradual and the sudden, can play very important roles. And they’re both part of the systems that we study and we try to come up with mathematical equations that describe how both the slow and sudden processes work.
Can you give an example of a “natural experiment”?
A natural experiment is a situation in which nature has held a few key aspects of a landscape constant… like the kind of rock that a landscape is made of, or the rate at which plate tectonics is lifting up rocks to build mountains, but then changed another key process that influences erosion and how the landscape changes. In our case, we often look for natural experiments in which nature has varied the climate.
Volcanic islands are terrific natural experiments. First of all, there are lots of them in different parts of the oceans with different plate tectonic contexts and different climates. But a lot of these volcanic islands are made of very similar rock. They’re made of basalt, which is the same kind of rock that the Hawaiian Islands are made of.
But there are some big differences on islands in climate. We’ve worked on the Hawaiian island of Kaua’i, which is the northernmost of the main Hawaiian Islands, and one of the oldest. And because the trade winds on the ocean usually blow from a very consistent direction — from the northeast — the side of the island that faces into the wind gets a lot more rainfall because there’s this moist air from the ocean being carried towards the island, and then it rains on the side that faces the wind. And then on the side that faces away from the wind, it’s very dry because most of that rainfall already fell out of the air on the other side of the island.
Steep rivers meet the ocean along the Na Pali coast on the Hawaiian island of Kaua’i. A stark difference in rainfall between opposite sides of volcanic islands create a “natural experiment.” Photo by Taylor Perron
So if you look at a picture in Google Earth of Kaua’i, you can see that the northeast side is really green and verdant and the southwest side is brown. It looks more like a desert.
That’s a terrific natural experiment because [the island] started off with the same topography, the same steepness, the same type of rock. It’s uplifted and then sunk back down at about the same rate everywhere. But there is this huge difference in climate. It’s almost like nature designed this experiment specifically for geomorphologists to look at. And there are these islands all around the world that have a lot of the similar characteristics, but with different ages [and] with a bunch of different climates. And so that allows us to look not just at a single island, but also look at groups of islands around the world to try to understand how different climates and differences in plate tectonics have shaped landscapes differently and created different rates of erosion.
What does the term “tectonics” refer to, and how does it contribute to landscape formation?
Tectonics just means building up landforms, building up mountains and high elevation, deforming the crust of a planet. And one way that can happen is plate tectonics, where we have these big, mostly rigid plates that move all over the surface of the Earth.
We think that plate tectonics may have played a critical role in regulating the climate of Earth’s surface for billions of years. So it may have made it a really nice place for life to evolve and then continue to survive. As far as we can tell, there’s no clear evidence that Mars had plate tectonics like we have on Earth. And so far, the same is true for Titan. But it has probably had other forms of tectonics, meaning processes that build mountains and make some parts of the surface much higher than others.
That is one question that we are trying to answer in our research in my group: What made the landscapes of Titan? What made the mountains and the lake basins, instead of just having a totally flat surface?
One possibility is that Titan has experienced strong tides because of the gravitational influence of Saturn, which Titan is orbiting. When you hear tides, you think of the ocean going up and down at the beach, right? But actually, the solid part of the Earth also experiences tides. We just can’t feel it — those tides are much smaller than the tides in the ocean. On Earth, it’s a response to the gravitational attraction of both the sun and the moon. On Titan, it’s really far away from the sun, and so mostly is a consequence of the gravitational attraction of Saturn. But it’s possible that the tides, as they repeatedly deformed the rigid outer shell of Titan, may have contributed to building mountains and other topographic features.
What’s the value in studying extraterrestrial places like Mars and Titan?
I think of Titan as the ultimate natural experiment in geomorphology. Because it has taken a system that’s kind of like the one we have on Earth, where there’s weather — meaning there are lakes, evaporation, clouds, rainfall, runoff and rivers, there are mountains, there are valleys, there are big lake basins. But a few really important things have changed. Unfortunately, it’s not a perfect natural experiment, because a bunch of things changed [on Titan] at once. But you have different gravity. You have, as far as we know right now, no life. Instead of a water cycle, there’s a methane cycle, and methane is also called natural gas. You have a surface that is made not of rock, like it is on Earth, but of ice mixed in with some organic particles that have fallen out of the atmosphere.
So what is a world like where the weather is based on a methane cycle instead of a water cycle? Could you actually support life in a place like that? And what are the differences in the landscape between Earth and the rivers and mountains and coasts that we have here, and Titan, where you have a lot of the same types of landforms, but lots and lots of differences? What happens to a river when you reduce gravity and fill it with liquid natural gas? And then make it flow over ice instead of rock? That’s Titan.
One of the reasons Titan is interesting is that many scientists think that there could be analogies between the chemistry that occurs on Titan and the chemistry that may have occurred on early Earth when life was originating. So it provides an opportunity to study a world that is currently geologically active, but has very different chemistry of the atmosphere and surface than Earth currently does.
Isabella Isaacs-Thomas is a digital reporter on the PBS NewsHour's science desk.
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