Description from Sound Cloud:
With all the planets out there in the galaxy and Universe, it’s only a matter of time and data until we find another one with life on it. (Probably.) But while most of the searches have focused on finding the next Earth, sometimes called Earth 2.0, that’s very likely an overly restrictive way to look for life. Biosignatures, or more conservatively, bio-hints, might not only be plentiful on worlds very different from our own, but around Solar Systems other than our own. Earth-like worlds, in fact, might not even be the most ubiquitous places for life to arise in the Universe.
I’m happy to welcome scientist Adrian Lenardic onto the Starts With A Bang podcast, and explore what just might be out there if we look for life beyond our idea of Earth 2.0!
(Image credit: JPL-Caltech/NASA.)
You can read Ethan Siegel’s digest of the podcast on Forbes:
JUNE 12, 2019
Old ice and snow yields tracer of preindustrial ozone
Ancient air bubbles answer question about ozone levels after Industrial Revolution
HOUSTON — (June 12, 2019) — Using rare oxygen molecules trapped in air bubbles in old ice and snow, U.S. and French scientists have answered a long-standing question: How much have “bad” ozone levels increased since the start of the Industrial Revolution?
“We’ve been able to track how much ozone there was in the ancient atmosphere,” said Rice University geochemist Laurence Yeung, the lead author of a study published online today in Nature. “This hasn’t been done before, and it’s remarkable that we can do it at all.”
Researchers used the new data in combination with state-of-the-art atmospheric chemistry models to establish that ozone levels in the lower atmosphere, or troposphere, have increased by an upper limit of 40% since 1850.
“These results show that today’s best models simulate ancient tropospheric ozone levels well,” said Yeung. “That bolsters our confidence in their ability to predict how tropospheric ozone levels will change in the future.”
The Rice-led research team includes investigators from the University of Rochester in New York, the French National Center for Scientific Research’s (CNRS) Institute of Environmental Geosciences at Université Grenoble Alpes (UGA), CNRS’s Grenoble Images Speech Signal and Control Laboratory at UGA and the French Climate and Environmental Sciences Laboratory of both CNRS and the French Alternative Energies and Atomic Energy Commission (CEA) at the Université Versailles-St Quentin.
“These measurements constrain the amount of warming caused by anthropogenic ozone,” Yeung said. For example, he said the most recent report from the Intergovernmental Panel on Climate Change (IPCC) estimated that ozone in Earth’s lower atmosphere today is contributing 0.4 watts per square meter of radiative forcing to the planet’s climate, but the margin of error for that prediction was 50%, or 0.2 watts per square meter.
“That’s a really big error bar,” Yeung said. “Having better preindustrial ozone estimates can significantly reduce those uncertainties.
“It’s like guessing how heavy your suitcase is when there’s a fee for bags over 50 pounds,” he said. “With the old error bars, you’d be saying, ‘I think my bag is between 20 and 60 pounds.’ That’s not good enough if you can’t afford to pay the penalty.”
Ozone is a molecule that contains three oxygen atoms. Produced in chemical reactions involving sunlight, it is highly reactive, in part because of its tendency to give up one of its atoms to form a more stable oxygen molecule. The majority of Earth’s ozone is in the stratosphere, which is more than five miles above the planet’s surface. Stratospheric ozone is sometimes called “good” ozone because it blocks most of the sun’s ultraviolet radiation, and is thus essential for life on Earth.
The rest of Earth’s ozone lies in the troposphere, closer to the surface. Here, ozone’s reactivity can be harmful to plants, animals and people. That’s why tropospheric ozone is sometimes called “bad” ozone. For example, ozone is a primary component of urban smog, which forms near ground level in sunlit-driven reactions between oxygen and pollutants from motor vehicle exhaust. The Environmental Protection Agency considers exposure to ozone levels greater than 70 parts per billion for eight hours or longer to be unhealthy.
“The thing about ozone is that scientists have only been studying it in detail for a few decades,” said Yeung, an assistant professor of Earth, environmental and planetary sciences. “We didn’t know why ozone was so abundant in air pollution until the 1970s. That’s when we started to recognize how air pollution was changing atmospheric chemistry. Cars were driving up ground-level ozone.”
While the earliest measurements of tropospheric ozone date to the late 19th century, Yeung said those data conflict with the best estimates from today’s state-of-the-art atmospheric chemistry models.
“Most of those older data are from starch-paper tests where the paper changes colors after reacting with ozone,” he said. “The tests are not the most reliable — the color change depends on relative humidity, for example — but they suggest, nevertheless, that ground-level ozone could have increased up to 300% over the past century. In contrast, today’s best computer models suggest a more moderate increase of 25-50%. That’s a huge difference.
“There’s just no other data out there, so it’s hard to know which is right, or if both are right and those particular measurements are not a good benchmark for the whole troposphere,” Yeung said. “The community has struggled with this question for a long time. We wanted to find new data that could make headway on this unsolved problem.”
Finding new data, however, is not straightforward. “Ozone is too reactive, by itself, to be preserved in ice or snow,” he said. “So, we look for ozone’s wake, the traces it leaves behind in oxygen molecules.
“When the sun is shining, ozone and oxygen molecules are constantly being made and broken in the atmosphere by the same chemistry,” Yeung said. “Our work over the past several years has found a naturally occurring ‘tag’ for that chemistry: the number of rare isotopes that are clumped together.”
Yeung’s lab specializes in both measuring and explaining the occurrence of these clumped isotopes in the atmosphere. They are molecules that have the usual number of atoms — two for molecular oxygen — but they have rare isotopes of those atoms substituted in place of the common ones. For example, more than 99.5% of all oxygen atoms in nature have eight protons and eight neutrons, for a total atomic mass number of 16. Only two of every 1,000 oxygen atoms are the heavier isotope oxygen-18, which contains two additional neutrons. A pair of these oxygen-18 atoms is called an isotope clump.
The vast majority of oxygen molecules in any air sample will contain two oxygen-16s. A few rare exceptions will contain one of the rare oxygen-18 atoms, and rarer still will be the pairs of oxygen-18s.
Yeung’s lab is one of the few in the world that can measure exactly how many of these oxygen-18 pairs are in a given sample of air. He said these isotope clumps in molecular oxygen vary in abundance depending on where ozone and oxygen chemistry occurs. Because the lower stratosphere is very cold, the odds that an oxygen-18 pair will form from ozone/oxygen chemistry increase slightly and predictably compared to the same reaction in the troposphere. In the troposphere, where it is warmer, ozone/oxygen chemistry yields slightly fewer oxygen-18 pairs.
With the onset of industrialization and the burning of fossil fuels around 1850, humans began adding more ozone to the lower atmosphere. Yeung and colleagues reasoned that this increase in the proportion of tropospheric ozone should have left a recognizable trace — a decrease in the number of oxygen-18 pairs in the troposphere.
Using ice cores and firn (compressed snow that has not yet formed ice) from Antarctica and Greenland, the researchers constructed a record of oxygen-18 pairs in molecular oxygen from preindustrial times to the present. The evidence confirmed both the increase in tropospheric ozone and the magnitude of the increase that had been predicted by recent atmospheric models.
“We constrain the increase to less than 40%, and the most comprehensive chemical model predicts right around 30%,” Yeung said.
“One of the most exciting aspects was how well the ice-core record matched model predictions,” he said. “This was a case where we made a measurement, and independently, a model produced something that was in very close agreement with the experimental evidence. I think it shows how far atmospheric and climate scientists have come in being able to accurately predict how humans are changing Earth’s atmosphere — particularly its chemistry.”
Study co-authors include Asmita Banerjee and Huanting Hu, both of Rice; Lee Murray of the University of Rochester; Patricia Martinerie, Jérôme Chappellaz and Emmanuel Witrant, all of CNRS and Université Grenoble Alpes; and Anaïs Orsi from CEA at the Laboratoire des Sciences du Climat et de l’Environnement. This research was supported by the David and Lucile Packard Foundation, the European Research Council, CNRS and the French Polar Institute IPEV.
The DOI of the Nature paper is: 10.1038/s41586-019-1277-1
A copy of the paper is available at: http://dx.doi.org/10.1038/s41586-019-1277-1
Yeung lab: yeunglab.org
Rice Department of Earth, Environmental and Planetary Sciences: earthscience.rice.edu
Wiess School of Natural Sciences: naturalsciences.rice.edu
VIDEO is available at:
High-resolution IMAGES are available for download at:
CAPTION: Rice University researchers and collaborators used ice cores, like the one shown here from Antarctica, in combination with atmospheric chemistry models to establish an upper limit for the increase in ozone levels in the lower atmosphere since 1850. (Photo by Jeff Fitlow/Rice University)
CAPTION: Rice University geochemists Laurence Yeung and Asmita Banerjee studied the increase in tropospheric ozone from preindustrial time to present by constructing a record of oxygen-18 “clumped isotope” pairs from tiny bubbles of atmospheric gas that were trapped in ice and snow in Antarctica and Greenland. (Photo by Jeff Fitlow/Rice University)
By Kendall Schoemann (staff writer in Rice University’s Office of Public Affairs)
Two Rice University faculty members have been selected as 2019 Alfred P. Sloan Research Fellows.
Mark Torres, assistant professor of Earth, environmental and planetary sciences, was honored for his work in the field of ocean sciences, and Ming Yi, assistant professor of physics and astronomy, was awarded for physics. The two-year, $70,000 fellowships seek to stimulate fundamental research by early-career scientists and scholars while recognizing their distinguished performance and unique potential to make substantial contributions to their fields.
Torres and Yi are among 126 U.S. and Canadian researchers to receive a 2019 fellowship.
“This fellowship gives me freedom to pursue interesting ideas and, in a sense, validates the work I’ve started to do at Rice,” Torres said. “At this point in my career, these are my first steps on my own, and it’s nice to know those steps are headed in the right direction.”
Torres’ high school was attached to a museum of paleontology, which sparked his passion for studying Earth. Encouraged to turn this passion into a career, he took geology classes and found work at the interface between geology and chemistry to be the most captivating. His research concerns how concentrations of carbon dioxide and oxygen in the atmosphere are regulated over geologic time and what makes planets habitable.
“The chemistry of the ocean is really important,” Torres said. “The amount of carbon dioxide in the atmosphere is very dependent on the chemistry of the ocean. Most of my research so far has focused on how rivers, by delivering different chemical elements to the ocean, influence ocean chemistry and atmospheric carbon dioxide. On the other hand, the way in which chemical elements get removed from ocean water should be just as important and will be the focus of my work supported by this fellowship.”
Yi’s research lab, an experimental condensed matter physics group, aims to advance the fundamental understanding of exotic properties in materials using spectroscopy tools such as angle-resolved photoemission spectroscopy and X-ray scattering.
“Materials that have exotic properties, such as superconductivity, can revolutionize our future,” Yi said. “My job is to figure out, from a fundamental physics point of view, why these materials have these amazing properties.”
New to Rice, Yi has been on campus since January. “I’m very excited to be at Rice, and I’m honored to receive this award,” she said. “In certain ways, I think it shows that the Rice community is very supportive of young professors, and it’s an encouragement for me to start my own group.”
With her fellowship support, Yi will focus on ways to control and tune exotic material properties. “As physicists, we like to figure out patterns, or overarching frameworks that describe a phenomenon,” she said. “Our goal is to figure out rules to describe and unify the things we see in nature.”
Torres earned a bachelor’s degree in geology from Pitzer College in 2010 and a Ph.D. in geochemistry from the University of Southern California in 2015. He joined Rice in 2017 from the California Institute of Technology.
Yi earned a bachelor’s degree in physics from the Massachusetts Institute of Technology in 2007 and a Ph.D. in physics from Stanford University in 2014. She joined Rice in 2018 from the University of California, Berkeley.
The Alfred P. Sloan Foundation is a philanthropic, not-for-profit, grant-making institution based in New York. Established in 1934 by Alfred Pritchard Sloan Jr., then-president and chief executive officer of General Motors Co., the foundation makes grants in support of original research and education in science, technology, engineering, mathematics and economics.
A Research Spotlight highlighting their paper, “Pacific Plate Apparent Polar Wander, Hot Spot Fixity, and True Polar Wander During the Formation of the Hawaiian Island and Seamount Chain From an Analysis of the Skewness of Magnetic Anomaly 20r (44 Ma)” has been published on Eos.org.
text and images by Linda Welzenbach
After 24 hours of travel from Houston, Texas to Punta Arenas, Chile, I arrived exhausted yet excited (and perhaps just a wee bit anxious of the unknown) to be part of something very unique. My name is Linda Welzenbach and my role on this expedition is to share the scientific discoveries that will come from our journey to Antarctica on board the RVIB (research vessel/ice breaker) Nathaniel B. Palmer (NBP). Akin to a spacecraft exploring new frontiers beyond Earth, the Nathaniel B. Palmer and her marine operations specialists will navigate safe passage through Antarctica’s icy waters for the 26 ITGC scientists and the wide array of science activities they have planned over the next two months.
This first post will bring you aboard to see what she has to offer, and how scientists become seafaring explorers. NBP’s origins and history will be highlighted in a later post.
The NBP’s vitals can be found on the USAP website HERE
Upon arriving Sunday evening, we discover that the ship would be leaving a day early. The NBP was thirsty and if we wanted to stay on schedule, we would need to leave for the “gas station” Monday evening.
Prior to departure, we head to the U.S. Antarctic Program warehouse for orientation and distribution of the necessary Extreme Cold Weather (ECW) gear.
We are on board for just a couple hours, yet the first task will be to participate in a simulated exit. As part of that exercise, we must be able to put on flotation suits, which are required for situations that call for abandoning ship (below). During the cruise we will practice several more drills of this nature, to build muscle memory in the event of a real emergency.
In anticipation of an evening departure, we met in the laboratory to unpack and sort through all the core processing supplies, stowing them securely in anticipation of the rough seas that characterize Drake’s passage. To see live weather, and forecasts, click HERE.
A bit after 7pm, #NBP1902 left Punta Arenas to refuel. As of this posting, we are back in Punta Arenas, and remain here until maintenance activities have been completed.
Linda is the science communications specialist in the Department of Earth, Environmental and Planetary Sciences at Rice University. She is a geologist and veteran of two expeditions to collect meteorites from Antarctica. Her role for THOR is public outreach, and so she will communicate science activities during the first scientific cruise in 2019-NBP19-02 and serves as the project webmaster.
– JANUARY 23, 2019
Study: Planetary delivery explains enigmatic features of Earth’s carbon and nitrogen
Most of Earth’s essential elements for life — including most of the carbon and nitrogen in you — probably came from another planet.
Earth most likely received the bulk of its carbon, nitrogen and other life-essential volatile elements from the planetary collision that created the moon more than 4.4 billion years ago, according to a new study by Rice University petrologists in the journal Science Advances.
The evidence was compiled from a combination of high-temperature, high-pressure experiments in Dasgupta’s lab, which specializes in studying geochemical reactions that take place deep within a planet under intense heat and pressure.“From the study of primitive meteorites, scientists have long known that Earth and other rocky planets in the inner solar system are volatile-depleted,” said study co-author Rajdeep Dasgupta. “But the timing and mechanism of volatile delivery has been hotly debated. Ours is the first scenario that can explain the timing and delivery in a way that is consistent with all of the geochemical evidence.”
In a series of experiments, study lead author and graduate student Damanveer Grewal gathered evidence to test a long-standing theory that Earth’s volatiles arrived from a collision with an embryonic planet that had a sulfur-rich core.
The sulfur content of the donor planet’s core matters because of the puzzling array of experimental evidence about the carbon, nitrogen and sulfur that exist in all parts of the Earth other than the core.
“The core doesn’t interact with the rest of Earth, but everything above it, the mantle, the crust, the hydrosphere and the atmosphere, are all connected,” Grewal said. “Material cycles between them.”
One long-standing idea about how Earth received its volatiles was the “late veneer” theory that volatile-rich meteorites, leftover chunks of primordial matter from the outer solar system, arrived after Earth’s core formed. And while the isotopic signatures of Earth’s volatiles match these primordial objects, known as carbonaceous chondrites, the elemental ratio of carbon to nitrogen is off. Earth’s non-core material, which geologists call the bulk silicate Earth, has about 40 parts carbon to each part nitrogen, approximately twice the 20-1 ratio seen in carbonaceous chondrites.
“Nitrogen was largely unaffected,” he said. “It remained soluble in the alloys relative to silicates, and only began to be excluded from the core under the highest sulfur concentration.”Grewal’s experiments, which simulated the high pressures and temperatures during core formation, tested the idea that a sulfur-rich planetary core might exclude carbon or nitrogen, or both, leaving much larger fractions of those elements in the bulk silicate as compared to Earth. In a series of tests at a range of temperatures and pressure, Grewal examined how much carbon and nitrogen made it into the core in three scenarios: no sulfur, 10 percent sulfur and 25 percent sulfur.
Carbon, by contrast, was considerably less soluble in alloys with intermediate sulfur concentrations, and sulfur-rich alloys took up about 10 times less carbon by weight than sulfur-free alloys.
Using this information, along with the known ratios and concentrations of elements both on Earth and in non-terrestrial bodies, Dasgupta, Grewal and Rice postdoctoral researcher Chenguang Sun designed a computer simulation to find the most likely scenario that produced Earth’s volatiles. Finding the answer involved varying the starting conditions, running approximately 1 billion scenarios and comparing them against the known conditions in the solar system today.
“What we found is that all the evidence — isotopic signatures, the carbon-nitrogen ratio and the overall amounts of carbon, nitrogen and sulfur in the bulk silicate Earth — are consistent with a moon-forming impact involving a volatile-bearing, Mars-sized planet with a sulfur-rich core,” Grewal said.
Dasgupta, the principal investigator on a NASA-funded effort called CLEVER Planets that is exploring how life-essential elements might come together on distant rocky planets, said better understanding the origin of Earth’s life-essential elements has implications beyond our solar system.
“This study suggests that a rocky, Earth-like planet gets more chances to acquire life-essential elements if it forms and grows from giant impacts with planets that have sampled different building blocks, perhaps from different parts of a protoplanetary disk,” Dasgupta said.
“This removes some boundary conditions,” he said. “It shows that life-essential volatiles can arrive at the surface layers of a planet, even if they were produced on planetary bodies that underwent core formation under very different conditions.”
Dasgupta said it does not appear that Earth’s bulk silicate, on its own, could have attained the life-essential volatile budgets that produced our biosphere, atmosphere and hydrosphere.
“That means we can broaden our search for pathways that lead to volatile elements coming together on a planet to support life as we know it.”
Additional co-authors on the Science Advances study are Kyusei Tsuno and Gelu Costin, both of Rice. The research was supported by NASA, the Deep Carbon Observatory and the David and Lucile Packard Foundation.
CLEVER Planets is part of the Nexus for Exoplanet System Science, or NExSS, a NASA astrobiology research coordination network that is dedicated to the study of planetary habitability. CLEVER Planets involves more than a dozen research groups from Rice, NASA’s Johnson Space Center, UCLA, the University of Colorado Boulder and the University of California, Davis. More information is available at cleverplanets.org.
MIKE WILLIAMS – JANUARY 11, 2019
Study shows how government promotes agricultural, environmental benefits – and could do more
Even though every dollar spent on soil improvement can save much more in environmental costs down the road, startup costs can sometimes make it hard for farmers to implement best environmental practices. A team of researchers from Rice and North Dakota State universities argues that this is especially true for using biochar, but that the problem can be addressed through well-designed policy.
Biochar is a porous, charcoal-like material produced via pyrolysis, the high-temperature decomposition of biomass. Studies show biochar improves soil water properties and enhances agricultural production by an average of 15 percent. It also reduces nutrient leaching, increases nitrogen available to plants and reduces the release of nitrogenous gases, which can improve local air quality.
Charcoal-like biochar improves soil hydration and enhances agricultural production while it curtails nutrient leaching, increases nitrogen available to plants and reduces the release of gas pollutants. A new study by researchers at Rice University and North Dakota State University gathers current and potential sources of government support to promote the production and use of biochar.
A policy study led by Ghasideh Pourhashem, a former postdoctoral fellow at Rice’s Baker Institute for Public Policy and now an assistant professor at North Dakota State, surveys existing government programs and details how they support the use of biochar to enhance agricultural productivity, sequester carbon and preserve valuable soil. The federal government already has 35 programs that do – or could – promote the deployment of biochar to improve agricultural preservation and head off future environmental woes, according to Pourhashem and her team.
For the open-access study in Global Change Biology Bioenergy, Pourhashem worked with Rice mentors Caroline Masiello, a professor of Earth, Environmental and Planetary Sciences, and Kenneth Medlock, the James A. Baker, III, and Susan G. Baker Fellow in Energy and Resource Economics.
“Our previous research has shown that wide-scale application of biochar across the United States’ agricultural soils can save millions of dollars in health costs by improving regional air quality,” said Pourhashem, who joined North Dakota’s Department of Coatings and Polymeric Materials and Center for Sustainable Materials Science in 2017 and is a member of the US Biochar Initiative advisory board.
But despite the accumulating evidence, she said biochar adoption has been slow.
“Our new study shows how policy frameworks can support biochar as a resource-saving, crop-boosting and health care-improving material,” Pourhashem said.
The researchers note the United States has long invested in biofuels like ethanol, and they argue that biochar deserves similar long-term support to improve the nation’s soil and food security strategy.
Masiello, a biogeochemist and expert on biochar, noted the financial disconnect between those who bear the cost of land-management changes and those who benefit.
“There are plenty of farmers who want to do the right thing, but the benefits of biochar don’t accrue entirely on-farm,” she said, citing examples like cleaner water and air. “This paper is about how policy can connect the costs borne by farmers with the benefits to us all.”
The only active loan guarantee program the researchers identified for biochar production is the Department of Agriculture’s Biorefinery, Renewable Chemical and Bio-based Product Manufacturing Assistance Program, which provides up to $250 million per project. But other programs run by the Department of Energy, the Department of Agriculture and initiatives in Iowa, Oregon, Colorado and Minnesota provide nearly $250 million more in grants, matching or production payments and tax credits. The researchers also identified eight programs that offer a total of nearly $30 million in research and development funding that implicitly support biochar.
The researchers determined that government’s emphasis on biochar production is inadequate and tilts heavily toward biofuels. They also noted the need for more upfront investment in large biochar-focused production rather than small-scale facilities. They suggested investment could be encouraged by the development of a broadly accepted set of product standards for biochar.
“The potential long-term impacts of biochar are enormous,” Medlock said. “Much of the recent support for biochar has focused on its use for carbon dioxide sequestration, but the benefits are much broader. The potential effects on local air quality and water quality have implications that extend far beyond the application site, which makes policy supportive of investment in biochar production and market development something that should be seriously considered.”
Shih Yu “Elsie” Hung, an energy forum research analyst at the Baker Institute, is co-author of the paper. Rice’s Shell Center for Sustainability supported the research.
JADE BOYD – NOVEMBER 19, 2018
Rice U. scientists use Hawaiian hot spot to study movement of Earth’s poles
Earth’s latest ice age may have been caused by changes deep inside the planet. Based on evidence from the Pacific Ocean, including the position of the Hawaiian Islands, Rice University geophysicists have determined Earth shifted relative to its spin axis within the past 12 million years, which caused Greenland to move far enough toward the north pole to kick off the ice age that began about 3.2 million years ago.
Their study in the journal Geophysical Research Letters is based on an analysis of fossil signatures from deep ocean sediments, the magnetic signature of oceanic crust and the position of the mantle “hot spot” that created the Hawaiian Islands. Co-authors Richard Gordon and Daniel Woodworth said the evidence suggests Earth spun steadily for millions of years before shifting relative to its spin axis, an effect geophysicists refer to as “true polar wander.”
“The Hawaiian hot spot was fixed, relative to the spin axis, from about 48 million years ago to about 12 million years ago, but it was fixed at a latitude farther north than we find it today,” said Woodworth, a graduate student in Rice’s Department of Earth, Environmental and Planetary Sciences. “By comparing the Hawaiian hot spot to the rest of the Earth, we can see that that shift in location was reflected in the rest of the Earth and is superimposed on the motion of tectonic plates. That tells us that the entire Earth moved, relative to the spin axis, which we interpret to be true polar wander.”
By volume, Earth is mostly mantle, a thick layer of solid rock that flows under intense pressure and heat. The mantle is covered by an interlocking puzzle of rocky tectonic plates that ride atop it, bumping and slipping against one another at seismically active boundaries. Hot spots, like the one beneath Hawaii, are plumes of hot solid rock that rise from deep within the mantle.
Gordon, the W.M. Keck Professor of Earth, Environmental and Planetary Science, said the new findings build on two 2017 studies: one from his lab that showed how to use hot spots as a global frame of reference for tracking the movement of tectonic plates and another from Harvard University that first tied true polar wander to the onset of the ice age.
“We’re taking these hot spots as marked trackers of plumes that come from the deep mantle, and we’re using that as our reference frame,” he said. “We think the whole global network of hotspots was fixed, relative to the Earth’s spin axis, for at least 36 million years before this shift.”
Like any spinning object, Earth is subject to centrifugal force, which tugs on the planet’s fluid interior. At the equator, where this force is strongest, Earth is more than 26 miles larger in diameter than at the poles. Gordon said true polar wander may occur when dense, highly viscous bumps of mantle build up at latitudes away from the equator.
“Imagine you have really, really cold syrup, and you’re putting it on hot pancakes,” Gordon said. “As you pour it, you temporarily have a little pile in the center, where it doesn’t instantly flatten out because of the viscosity of the cold syrup. We think the dense anomalies in the mantle are like that little temporary pile, only the viscosities are much higher in the lower mantle. Like the syrup, it will eventually deform, but it takes a really, really long time to do so.”
If the mantle anomalies are massive enough, they can unbalance the planet, and the equator will gradually shift to bring the excess mass closer to the equator. The planet still spins once every 24 hours and true polar wander does not affect the tilt of Earth’s spin axis relative to the sun. The redistribution of mass to a new equator does change Earth’s poles, the points on the planet’s surface where the spin axis emerges.
Woodworth said the hot spot data from Hawaii provides some of the best evidence that true polar wander was what caused Earth’s poles to start moving 12 million years ago. Islands chains like the Hawaiians are formed when a tectonic plate moves across a hot spot.
“True polar wander shouldn’t change hot spot tracks because the hot spot track is the record of the motion of the plate relative to the hot spot,” Woodworth said.
Gordon said, “It was only about a 3 degree shift, but it had the effect of taking the mantle under the tropical Pacific and moving it to the south, and at the same time, it was shifting Greenland and parts of Europe and North America to the north. That may have triggered what we call the ice age.”
Earth is still in an ice age that began about 3.2 million years ago. Earth’s poles have been covered with ice throughout the age, and thick ice sheets periodically grow and recede from poles in cycles that have occurred more than 100 times. During these glacial cycles, ice has extended as far south as New York and Yellowstone National Park. Earth today is in an interglacial period in which ice has receded toward the poles.
Gordon said true polar wander is not merely a change in the location of Earth’s magnetic poles. As the planet spins, it’s iron core produces a magnetic field with “north” and “south” poles near the spin axis. The polarity of this field flips several times every million years, and these changes in polarity are recorded in the magnetic signatures of rocks the world over. The paleomagnetic record, which is often used to study the movement of tectonic plates across Earth’s surface, contains many instances of “apparent polar wander,” which tracks the motion of the spin axis and which includes the effects of both plate motion and true polar wander, Gordon said.
He said Earth’s mantle is ever-changing as new material constantly cycles in and out from tectonic plates. The drawing down and recycling of plates via subduction provides a possible explanation for the highly viscous mantle anomalies that probably cause true polar wander.
“In class, I often demonstrate this with lead fishing weights and pliers,” Gordon said. “It’s easy to deform the lead with the pliers, and it’s not brittle. It doesn’t crack or fly apart when it fails. That’s a pretty good analogy for mantle flow because that’s the way silicate rock deforms under intense heat and pressure.”
He and Woodworth are working with colleagues to extend their analysis, both from 12 million years ago to the present as well as further into the past than the 48-million-year start date in the newly published study.
The National Science Foundation supported the research.
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