Mapping the Unknown
Two hundred million years ago, Earth looked very different. Its landmass was pushed together into one giant continent. Today scientists refer to it as Pangaea. Over time, the rocky plates that make up Earth’s crust split this mega-continent apart. The plates later ripped those new continents apart, too, moving them around and smooshing some of them back together again. What resulted was the world we see on maps today.
But what did our planet look like while all of this was happening? That’s what Sabin Zahirovic is working to find out. “It’s a journey of discovery, and sometimes undiscovery, that’s exciting,” he says.
Today, GPS systems in our phones can tell us exactly where we are and how to get where we want to go. It seems like every inch of our planet has been explored. Still, some places — and times — remain unmapped.
“Earth has changed so much that it’s really a privilege to be able to look into the past. It’s the closest thing we have to time travel,” Zahirovic says.
From the ocean floor, to the mysterious force of dark energy, to the movement of continents, here are three scientists who are venturing into uncharted territory.
Putting tectonic plates on rewind
It’s hard to imagine something that moves slower than our continents. They steadily creep along at around 5 centimeters (2 inches) a year. At least, that’s what geoscientists had thought.
But when scientists at the University of Sydney in Australia studied how Earth’s crust had changed over the past 200 million years, they got a surprise. Slabs that make up Earth’s outer layer did spend most of their time crawling along at that slow pace. But every once in a while, these tectonic plates sped up in short bursts. Sometimes they sprinted at up to 18 centimeters (7 inches) a year. And they might keep up that pace for 10 million years or so.
Scientists had long known that Earth’s tectonic plates are constantly on the move. Their activity constantly reshapes the planet’s surface. Using clues in rock, scientists reconstructed what the continents must have looked like at various times in the past. But these were only brief snapshots from every 20 million years or so. What they wanted were details of how the plates moved over much shorter timescales.
Zahirovic belongs to the EarthByte Group in Australia. Its scientists input large troves of geological data into a computer, and then use them to model how the Earth works. The goal of this computer model has been to simulate real-world events, such as the movement of tectonic plates and the continents they carried.
Zahirovic and the other scientists started by combining data from many different sources. They used data from rocks and fossils. They added satellite observations of gravity, magnetism and the shape of surface features — Earth’s topography (Tuh-PAAHG-ruh-fee). But mapping the changing boundaries of tectonic plates was trickier. Why? Many rest at the bottom of the sea.
Studying magnetic stripes
To get that type of information, the team used data on magnetic anomalies that research ships had gathered from the ocean’s floor. Scientists knew that Earth’s magnetic poles (points near the axis around which the planet rotates, and where magnetic fields point downward) switch sides. It doesn’t happen often. Just every few million years or so. But when this happens, tiny magnetic particles in the rocks near areas where the seafloor is spreading will change direction to line up with the new pole. (It’s like when iron filings are drawn toward a magnet.)
This flip flop in the magnetic poles forms magnetic “stripes” on the ocean’s floor. Those stripes provide clues to the past, similar to tree rings. Scientists can use the stripes to figure out how fast Earth’s crust pulled apart. Where this happened quickly, the lines are far apart. Where it happened slowly, they are closer together.
“Over time, you build up this magnetic [recording] of what the plates have done,” Zahirovic says. His job is to write computer programs to analyze the data. When the scientists combined all of their data, they were able to create a model that showed how the plates had moved, one million years at a time. In a sense, Zahirovic says, “We can ‘rewind’ back to when Pangaea formed — and before and after.”
His team shared its model with scientists. They made it available for free to anyone who wanted it. That should aid scientists who are curious about what Earth looked like millions of years ago. Their model also might help scientists understand other, more current issues — such as climate change.
The movement of tectonic plates is an important factor in climate change. In the past, volcanic eruptions caused by changes at plate boundaries have warmed the atmosphere by pumping out greenhouse gases. And when continents cover the north and south poles, inland ice sheets form, lowering sea levels.
“By understanding the past, you can help understand the present,” Zahirovic explains. “And also the future.”
Supernovas help probe
Gravity is supposed to hold things together. That’s why scientists long believed one of Albert Einstein’s now debunked theories. It said that the combined gravity of all stars, planets and galaxies in the universe should act like brakes to slow the expansion of the universe. So it came as a surprise when astronomers discovered in 1998 that the universe is expanding faster and faster, not ever more slowly.
Scientists called the strange force pushing things apart “dark energy.” They didn’t know what it was. But they found it made up about 70 percent of all the matter and energy in the universe.
“Dark energy is a mysterious something that seems to be making our universe expand at an accelerated rate,” says Rachel C. Wolf. She’s a cosmologist, someone who studies the origins of the universe, at the University of Pennsylvania in Philadelphia. Wolf is one of around 320 scientists around the world working on a project known as the Dark Energy Survey.
It uses a powerful telescope in Chile to study large patches of the sky. Over five years, researchers will use this telescope to survey more than 300 million distant galaxies and detect thousands of new supernovas — exploding stars. Wolf got involved in the project because she studies supernovas. They are a tool for “probing” dark energy. “I use exploding stars to learn about how our universe has evolved over time,” she explains.
When a star explodes into a supernova, it emits a flash of light that can appear brighter than a galaxy. Then it fades away over weeks to months. Different supernovas behave in different ways. But the ones Wolf studies, type 1a supernovas, are very consistent. Scientists discovered that how brightly they explode is related to how long it takes their light to fade away. This information makes type 1a supernovas useful markers for measuring distances across the universe. In a sense, their light becomes a type of cosmic “yardstick.”
By comparing the brightness of different supernovas, scientists can figure out how far they are from each other. That’s because they know how fast light travels (it’s always the same speed). Scientists can analyze light from the exploding stars to learn how space was expanding at the time the supernova occurred. The faster the supernova was moving away, the more its light waves were “stretched” as they traveled toward Earth. These data can tell scientists whether the universe is expanding faster or more slowly than expected.
Although Wolf studies the sky, most of her job is spent not in front of a telescope but at a computer. She takes data from hundreds of supernovas and writes computer programs that pull out useful information. She might be asked to write a computer code that will use data about a supernova to determine how far away it is. Other programs might look for patterns in the data.
“A lot of what I do is like taking a big puzzle and trying to find the links between these different pieces,” she explains.
So far, she has tested her programs using hundreds of observations. “By the end of the survey, we’re going to have thousands of different supernovae we can look at, to learn more about dark energy,” she says.
Satellites eye the seafloor
Humans have studied nearly every inch of the seven continents. But more than 70 percent of our planet still remains largely unexplored. That’s because it’s at the bottom of the sea. Only about 11 percent of the ocean floor has been mapped in detail. Completing those maps in the past has involved sounding. This method uses sonar to bounce sound pulses off the seafloor. But a research ship would have to spend the next 200 years sailing the world’s oceans to map the entire seafloor this way.
Scientists have figured out a faster way to do this — and without getting wet. They use satellites to precisely measure the topography of the ocean’s surface.
The ocean may look flat, but it is actually covered with bumps and dips. That’s because the gravitational attraction from a massive undersea mountain range causes water to pile up in a broad sea-surface bump several feet high. A deep valley leaves a slight dip in the ocean surface.
“The highs and lows [on the satellite maps] represent highs and lows on the ocean floor,” says David Sandwell. A geophysicist, he works at the Scripps Institution of Oceanography in San Diego, Calif.
By creating specialized maps of the ocean’s surface, known as marine gravity maps, Sandwell and his co-workers have discovered undersea mountain ranges and chasms (KAZ-ums). They’ve also turned up “lots and lots of seamounts” (extinct undersea volcanoes). They used this information to maps the ocean floor in detail. “Every day you see something new,” he says. “That’s the exciting part.”
“Big, long cracks” at the bottom of the sea in the South Pacific were among his team’s more surprising and mysterious discoveries. They run along seamount ridges. “We still don’t know how they form, and we don’t know how old they are,” Sandwell says. “We can see them, but we still don’t understand them.”
Radar to the rescue
To create their maps, he and his colleagues use data from satellites equipped with altimeters. These instruments shoot a radar beam at the surface of the ocean and measure the time it takes to bounce back. That measurement tells scientists the distance a beam traveled — and the precise height of the ocean at that location. Those height measurements are accurate to within “a centimeter [0.39 inch],” Sandwell says.
The altimeter shoots around 2,000 radar signals each second to collect many measurements. When the scientists find something that looks interesting, they might plan an expedition to study the area by ship. By taking soundings from a ship and making other measurements, they are able to get a much more detailed picture of what’s below. “You run the ship back and forth, just like mowing the lawn, building up a complete map of the sea floor,” Sandwell says.
So ship soundings can offer more detail of features first spied by satellite, he notes.
We’ll never have to hike or drive across the seafloor, so why do we need maps of it? Aside from human curiosity, military agencies use the information for navigation. And much of the money for research comes from energy companies, which search for places to drill for oil at shallow sites on the seafloor.
But for scientists, learning about the ocean’s floor can give a new understanding of Earth and how it works. The planet’s surface is constantly being recycled as tectonic plates slide underneath one another. It’s a process called subduction. As a result, the ocean’s floor is much younger geologically than will be the continental landmasses. It’s also not affected by some of the forces that have shaped the land above sea level, such as erosion. That intrigues scientists like Sandwell. “It’s like a different planet down there,” he says.
Humans can never visit long-gone Pangaea nor the distant edges of the universe. Most will never travel to the ocean’s depths. So scientists have found different ways to explore these places: using satellites, telescopes and clues from Earth itself. By mapping the unknown, they are helping answer many questions about our mysterious world.