Five outstanding questions in earth science
Copyright Mary Caperton Morton
Copyright Matt Martyniuk, Creative Commons Attribution-Share Alike 3.0 Unported
Even 15 years after the release of “Good Will Hunting,” there remains something appealing about watching the title character, a mathematically inclined janitor at MIT, scribble the solution to an unsolved mathematics problem on a hallway blackboard. In reality, there are a number of unsolved problems in mathematics, seven of which were designated in 2000 by the Clay Mathematics Institute as “Millennium Prize Problems,” each with a purse of $1,000,000. To date, only one has been solved.
Earth science, too, has put forth a long stream of unsolved questions. In fact, solving one question typically floods the field with new lines of investigation. The discovery, for example, in the mid-19th century that carbon dioxide traps heat in Earth’s atmosphere led scientists to engage in lengthy studies — many that continue today — on the mechanics of the greenhouse effect, emission sources and impacts on global climate.
So, what are today’s biggest unanswered questions in earth science? EARTH Associate Editor Kathryn Hansen posed this question to experts across a variety of earth science disciplines, from atmospheric and space science to glaciology. The experts weighed in with their favorite questions and discussions of the questions’ history and relevance, along with the current approaches to finding the answers.
Read on for paraphrased versions of the experts’ responses. There’s no million-dollar prize on the line, but if you’re up for the challenge, grab a pencil or laptop and get ready to think. You never know how or where the next paradigm-changing solution will arise.
1. Where are the big magma chambers that produce huge super-eruptions?
— John Eichelberger, volcanologist, U.S. Geological Survey
This is a problem that people began talking about at least by the mid-20th century, when it was recognized that large eruptions produced caldera collapse, with the roof of a shallow magma chamber falling into the chamber as the magma erupted. Nearly all eruptions of 10 cubic kilometers or more involve the formation of large craters — calderas — by surface subsidence. Crater Lake in Oregon is a modest example, but these calderas can be up to 100 kilometers across with eruptions of 1,000 cubic kilometers or more of magma. The big ones, like in Yellowstone, have become popularly known as supervolcanoes.
The search for giant magma chambers that could feed supervolcanoes began in earnest in the 1980s in hopes of finding large sources of geothermal energy. One target of investigation was Long Valley Caldera in California, which had a super-eruption about 760,000 years ago and in the 1980s was undergoing “unrest” expressed by strong earthquakes and surface uplift approaching a meter. Scientists have since concluded that little or nothing remains of the huge magma body that must have once been present beneath this area.
Although success in prospecting for geothermal energy doesn’t depend upon finding a huge magma body — dead but still hot magma will serve the purpose — it would be nice to know where the next super-eruption might occur. These eruptions are so rare however, perhaps one per 100,000 years on average, that they pose little risk to humans now.
Finding them is challenging and must be a multidisciplinary task. Volcanologists can determine where super-eruptions have occurred in the past, and how big and when. Geophysicists can, in principle, find magma as zones of slow seismic wave velocity and high electrical conductivity. Scientists can also detect, particularly from satellites with radar instruments, large areas of uplift or subsidence that might be due to movement of large magma bodies. Anomalous zones under many volcanoes, including Yellowstone, have been found that are best explained as containing some liquid rock — but perhaps only as the liquid part of a mostly crystalline slush — not what gets erupted as mostly liquid magma. Rock slush could also cause uplift and subsidence but so can other things like hydrothermal fluids. A huge, mostly liquid body of magma remains to be definitively located.
Why haven’t we found one yet? There are several possibilities and the reasons are not mutually exclusive. We might not have looked in the right place yet. Maybe by looking where there were big eruptions in the past, we are looking at places where the magma chambers have completely emptied. Or maybe they develop very fast and quickly erupt. If so, at any given point in time they are rare, like a short-lived adult insect from a long-lived larva. Or maybe our indirect geophysical techniques aren’t yet good enough. For example, scientists were recently quite surprised when they were drilling in a caldera in Iceland and encountered mostly liquid magma in a place where they expected a hydrothermal system at half the temperature; they were using state-of-the-art geophysics.
We may have some surprises coming, as we don’t know exactly where these huge magma bodies are or what they are like. With more geothermal drilling going deeper and hotter, we may have more discoveries “by accident.”
It is often said that the Lake Toba eruption about 74,000 years ago, the most recent super-eruption, may have nearly wiped out our ancestors, leaving a small band of survivors comprising a sort of Adam and Eve. This, together with the prospect that it might happen again, is interesting to contemplate. Whenever unrest is reported at Yellowstone, the U.S. Geological Survey receives a flood of inquiries about whether doom is near. Of course it’s possible, but so is another asteroid impact, all-out nuclear war, or the rise of a new pandemic-causing microbe, all of which pale before the risk of walking across the street. Danger, however unlikely, is fascinating.
2. How stable is the West Antarctic Ice Sheet and what does it mean for global sea levels?
— Richard Alley, glaciologist, Penn State University
This is a question that’s been asked for decades. There is always some danger in picking who first asked it, but glaciologist John Mercer in 1968 is a strong candidate. Mass changes in the ice sheet translate into changes in sea level, and a lot of people live close enough to sea level to be displaced if the ice sheet were to be lost, while many more enjoy the beaches and ports that would be affected.
Many things affect sea level. For example, more snow on an ice sheet — as we expect on the Antarctic ice sheet and central parts of Greenland in a warming world — tends to lower sea level by taking water that evaporated from the ocean and storing it on top of an ice sheet. But more melt on an ice sheet — as we expect in parts of Greenland with warming — takes water from the ice sheet and puts it back in the ocean, raising sea level. Melting of mountain glaciers has a similar effect as melting the ice sheet, as does pumping of water out of the ground to irrigate crops or for other uses, because most of that water ends up in the ocean rather than back in the ground. Warming the ocean causes expansion of the water and raises sea level. All of these are interesting and important influences, with notable uncertainties, but we don’t think that those uncertainties are huge.
Another way to raise sea level is for ice sheets to spread more rapidly under their own weight, taking ice from above sea level and delivering it to the ocean to make icebergs. This influence on sea level is complicated, and is where various uncertainties arise. Many factors control how rapidly ice flows, and thus how rapidly ice sheets can transfer land ice to the ocean to raise sea level.
Today, around most of Antarctica and parts of Greenland, the ice reaching the ocean does not immediately break off to make icebergs; instead it remains attached while spreading over the ocean, forming an ice shelf. The ice shelves almost all exist in bays or fjords, and thus have friction with their sides; the undersides of many ice shelves also hit local high spots in the seafloor, generating additional friction. Furthermore, the undersides of the ice shelves, where they are in contact with the ocean, are at their melting point. Warming ocean water tends to thin the shelves — a warming of 1 degree Celsius increases melting by about 10 meters per year — reducing the friction, and thus allowing faster flow of the ice that feeds the shelves, raising sea level.
Various features of the geologic record, modern observations, and investigations with models point to the importance of “threshold” behavior. Increasing the ocean temperature increases the ice’s speed, with the potential that at some high-enough temperature, the speed will jump rather abruptly and irreversibly. Such behavior is especially interesting and important, but also difficult to predict. You can undoubtedly think of many questions: What happens if the water temperature stays the same, but the rate of ocean circulation changes? What about warming adding meltwater into crevasses that could wedge them open and remove the friction that way? If an ice shelf is thinned, by how much does the flow speed increase? What are the thresholds?
A large and vigorous community of scientists in the field, remote-sensing experts, and modelers is working to measure, understand, project, and test the projections. And, we’re doing so with some urgency — we want to get answers in time to provide useful guidance to people making decisions about energy and the environment.
3. Is there life on any other planetary body in our solar system?
— Alfred McEwen, planetary geologist, Arizona State University
NASA and others have been trying to address this question for decades, but it’s very difficult. Some scientists think that sample return is the only way to get a definitive result, although it might require several attempts to get the right samples and it is very expensive (about $10 billion for Mars). And even positive results might still be controversial. In-situ analysis is a less expensive approach, but still on the order of $1 billion to $2 billion per mission, and results would likely be controversial.
Maybe we’ll get lucky and find credible evidence in a Martian meteorite, but then the possibility of terrestrial contamination is very difficult to rule out. It was previously announced that signs of life were found in a Martian meteorite, but the strong consensus of the scientific community is now against this conclusion.
Icy moons in the outer solar system, such as Saturn’s Enceladus, could harbor life, but we know much less about these worlds than about Mars and need more basic robotic exploration. The outer solar system is even more expensive to explore than is Mars.
The good news is that efforts to find life result in new knowledge about the physical and chemical processes and geologic history of these interesting worlds. For example, the Viking Orbiters (which carried the Mars landers to search for life in the 1970s) revolutionized our understanding of Mars’ global geology, soil and atmosphere. Comparison of the atmospheric composition to gases trapped in some meteorites led to recognition that these rocks came from Mars, which in turn led to many more new results.
4. How will climate change affect forests and dry-land vegetation and, in turn, affect atmospheric composition?
— Steven Wofsy, atmospheric scientist, Harvard University
This question has been around for decades and it requires truly interdisciplinary efforts to answer.
The major ecosystems of the planet are critically important for humans and for all creatures, and they are currently subject to very strong pressure from climate change and from human-induced ecological disturbances, such as agriculture and invasive species. Ecosystems change both slowly and abruptly in apparent response to extreme events that may be embedded in long-term change. Ecosystems have strong feedback effects on the atmospheric budgets of heat, moisture, greenhouse gases and aerosols, and, therefore, on climate.
To find out how climate change will affect forests and dry-land vegetation, and how those changes will affect atmospheric composition, we need more observations. Observations need to be on timescales of decades or longer to reflect processes in the atmosphere and of the major ecosystems. They also need to span local, regional and global spatial scales. Finally, we need ecosystem models integrated with atmospheric models that work like real systems.
In the United States, there are efforts to study parts of this problem and to make the requisite observations, supported by the National Science Foundation, the Department of Energy, NASA, the U.S. Forest Service and NOAA. What remains to be done is to coordinate and invigorate these efforts, and to commit to solving the problem on the relevant time and space scales. We have certainly seen a lot of progress in the last few years, but it has mostly served to point to the serious the gaps in our knowledge.
Ecosystem health goes hand-in-hand with human health and economic vitality. Atmosphere-biosphere interactions are powerful and have vast implications.
5. So we know a lot about dinosaur fossils, what about dinosaur biology?
— Jack Horner, paleontologist, Montana State University
I don’t think there are many big questions left in dinosaur paleontology, but we have a lot of details to fill in concerning their biology. Although sauropods are among my least favorite dinosaurs, I think they are the key to understanding dinosaurs as living animals.
I don’t think many people realize the importance of sauropods to figuring out dinosaur paleobiology. We need to figure out how it is that these gigantic animals with their tiny heads and long necks turned out to be so very successful. The tallest-known sauropod species measured about 18 meters tall and the longest measured almost 35 meters long. Sauropods are cumbersome and very hard to work on because they are so gigantic.
It’s good that some questions remain; answers to all of the questions about dinosaurs might well take away the very mystery that surrounds them, and it’s the mystery that charges children’s imaginations.