Duo develops framework for Earth’s interior

Earths' inner mantle is not necessarily one well mixed mass.
Earths’ inner mantle is not necessarily one well mixed mass.

A new model of inner Earth constructed by Arizona State University researchers pulls past information and hypotheses into a coherent story to clarify mantle motion.

“The past maybe two or three years there have been a lot of papers in Science and Nature about the deep mantle from seismologists and mineral physicists and it’s getting really confusing because there are contradictions amongst the different papers,” says Ed Garnero, seismologist and an associate professor in Arizona State University’s School of Earth and Space Exploration.

“But we’ve discovered that there is a single framework that is compatible with all these different findings,” he adds.

Garnero partnered with geodynamicist and assistant professor Allen McNamara, also in the School of Earth and Space Exploration in ASU’s College of Liberal Arts and Sciences, to synthesize the information for their paper to be published in the May 2 issue of Science.

“Our goal was to bring the latest seismological and dynamical results together to put some constraints on the different hypotheses we have for the mantle. If you Google ‘mantle’ you’ll see 20 different versions of what people are teaching,” explains McNamara.

According to the ASU scientists, all this recent research of the past few years fits into a single story. But what is that story? Is it a complicated and exceedingly idiosyncratic story or is it a straightforward simple framework?

“In my opinion,” explains Garnero, “it’s simple. It doesn’t really appeal to anything new; it just shows how all those things can fit together.”

The pair paints a story for a chemically complex inner earth, a model that sharply contrasts the heavily relied upon paradigm of the past few decades that the mantle is all one thing and well mixed. The original model was composed of simple concentric spheres representing the core, mantle and crust – but the inner Earth isn’t that simple.

What lies beneath

Earth is made up of several layers. Its skin, the crust, extends to a depth of about 40 kilometers (25 miles). Below the crust is the mantle area, which continues to roughly halfway to the center of Earth. The mantle is the thick layer of silicate rock surrounding the dense, iron-nickel core, and it is subdivided into the upper and lower mantle, extending to a depth of about 2,900 km (1,800 miles). The outer core is beneath that and extends to 5,150 km (3,200 mi) and the inner core to about 6,400 km (4,000 mi).

The inner Earth is not a static storage space of the geologic history of our planet – it is continuously churning and changing. How a mantle convects and how the plates move is very different depending on whether the mantle is isochemical (chemically homogenous made entirely of only one kind of material) or heterogeneous, composed of different kinds of compounds.

Garnero and McNamara’s framework is based upon the assumption that the Earth’s mantle is not isochemical. Garnero says new data supports a mantle that consists of more than one type of material.

“Imagine a pot of water boiling. That would be all one kind of composition. Now dump a jar of honey into that pot of water. The honey would be convecting on its own inside the water and that’s a much more complicated system,” McNamara explains.

Observations, modeling and predictions have shown that the deepest mantle is complex and significantly more anomalous than the rest of the lower mantle. To understand this region, seismologists analyze tomographic images constructed from seismic wave readings. For 25 years they have been detecting differences in the speeds of waves that go through the mantle.

This difference in wave speeds provides an “intangible map” of the major boundaries inside the mantle – where hot areas are, where cold areas are, where there are regions that might be a different composition, etc. The areas with sluggish wave speeds seem to be bounded rather abruptly by areas with wave speeds that are not sluggish or delayed. An abrupt change in wave speed means that something inside the mantle has changed.

If the mantle is all the same material, then researchers shouldn’t be observing the boundary between hot and cold in the mantle as a super sharp edge and the temperature anomalies should also be more spread out. The abrupt change in velocity was noticeable, yet they didn’t know what caused it.

Garnero and McNamara believe that the key aspect to this story is the existence of thermo-chemical piles. On each side of the Earth there are two big, chemically distinct, dense “piles” of material that are hundreds of kilometers thick – one beneath the Pacific and the other below the Atlantic and Africa. These piles are two nearly antipodal large low shear velocity provinces situated at the base of Earth’s mantle.

“You can picture these piles like peanut butter. It is solid rock but rock under very high pressures and temperatures become soft like peanut butter so any stresses will cause it to flow,” says McNamara.

Recently mineral physicists discovered that under high pressure the atoms in the rocks go through a phase transition, rearranging themselves into a tighter configuration.

In these thermo-chemical piles the layering is consistent with a new high pressure phase of the most abundant lower mantle mineral called post-perovskite, a material that exists specifically under high pressures that cause new arrangements of atoms to be formed.

Perovskite is a specific arrangement of silicon and magnesium and iron atoms.

“At a depth a few hundred kilometers above the core, the mineral physicists tell us that the rocks’ atoms can go into this new structure and it should happen abruptly and that’s consistent with the velocity discontinuities that the seismologists have been seeing for decades,” says Garnero.

These thick piles play a key role in the convection currents. Ultra-low velocity zones live closest to the edges of the piles because that’s the hottest regions of the mantle due to the currents that go against the pile walls as they bring the heat up from the core. Off their edges exist instability upwellings that turn out to be the plumes that feed hot spots such as Hawaii, Iceland and the Galapagos.

“We observe the motions of plate tectonics very well, but we can’t fully understand how the mantle is causing these motions until we better understand how the mantle itself is convecting,” says McNamara. “The piles dictate how the convective cycles happen, how the currents circulate. If you don’t have piles then convection will be completely different.”

An earthquake’s aftermath

An image in Tambo de Mora, Peru, which was hit hard by last year's magnitude 8 earthquake. Credit: Danielle Martin
An image in Tambo de Mora, Peru, which was hit hard by last year’s magnitude 8 earthquake. Credit: Danielle Martin

Last August, Peru was shaken by a devastating magnitude 8 earthquake. In the coastal town of Tambo de Mora, not far from the epicenter, about 90 percent of the houses were damaged or destroyed by the quake and very few have yet to be rebuilt.

Students and faculty from MIT’s CityScope class visited the town of 5,200 people during this year’s spring break to learn about the devastated city’s needs and how MIT ingenuity might be harnessed to help. They were looking at a whole range of infrastructure issues including restoring water supplies, sewerage and health-care delivery, as well as projects for rebuilding a sense of community.

At least 300 families are still living in tents on a nearby hillside, unable to return to their homes. Some houses were completely destroyed by the quake, but many others have been condemned by authorities and slated for demolition. In many cases, the owners dispute that assessment and say they want the chance to go back and make repairs.

“There are red Xs on most of the houses” signaling that they are to be demolished, said Kari Williams, a freshman in that class. “But most people are saying no, they can be repaired. It’s uncertain what their fate is.”

The people are still feeling devastated by their losses and many have told the local health service that they need psychological help to cope with it. But they gave the MIT class a warm reception. “They were all very friendly, very easy to talk with,” Williams said.

The trip was focused on getting to know how MIT people might best be able to help out, by making it clear that “we need to know what you need, how you want to proceed,” said Dorian Dargan, another freshman in the class.

As pressing as many of their needs are, the lost sense of community seems to weigh heavily. As in many Latin American towns, the central town square, or plaza, was a vibrant center of civic life where residents would socialize, sell their wares or produce, or just hang out. But the plaza’s concrete paving was deeply broken and buckled by the quake and more than half a year later it remains unrepaired. When the class held a group meeting for the residents, “The first thing they said was, ‘Fix that town square,'” Williams says. “It’s not a pleasant place to be anymore.”

Repairing the cracked concrete may not be something that the class can help with, but a variety of other needs were evident in their discussions with residents, and some graduate students from the class will be returning there this summer to carry out plans that they have been developing as a result of the on-site visit.

“There were a lot of pre-existing problems,” Dargan said, which were exacerbated by the earthquake damage. “There is a significant Afro-Peruvian population who felt marginalized.” Because their community was on the other side of the river from the rest of the town, and there was no working bridge, the CityScope group had a separate meeting with that community.

“We got some ideas of what they wanted,” Dargan said. Now, the MIT group is working in cooperation with the Universidad del Pacifico, a few hours away in Lima, “to bring these projects to fruition.”

Among the projects is developing a community garden, both in order to help meet the food needs of the impoverished town and to help built a sense of community. “They get most of their produce from other places, when they could very well be self-sufficient,” he says.

Water was less of a problem than they had anticipated, as the city sits near the mouth of a river. Water quality from wells in parts of the city and surroundings, however, might benefit from improved filtration, such as the systems pioneered by MIT senior lecturer Susan Murcott, Williams says.

Another project the class is looking into is a design for a pedal-powered washing machine, which could be built locally and provide not only better hygiene but a potential new business for local people.

Williams plans to major in mechanical engineering, but says this class has already influenced her thinking about what she wants to do with her life. She’s interested in product design and likes the idea of working with MIT’s D-Lab on products that have real social consequences.

“After this, I’m looking at some kind of social justice track, something that can really help,” she says of her experience in Peru. “It really made me want to do something positive for the Third World.”

Geochemist Challenges Key Theory Regarding Earth’s Formation

Working with colleagues from NASA, a Florida State University researcher has published a paper that calls into question three decades of conventional wisdom regarding some of the physical processes that helped shape the Earth as we know it today.

Munir Humayun, an associate professor in FSU’s Department of Geological Sciences and a researcher at the National High Magnetic Field Laboratory, co-authored a paper, “Partitioning of Palladium at High Pressures and Temperatures During Core Formation,” that was recently published in the peer-reviewed science journal Nature Geoscience. The paper provides a direct challenge to the popular “late veneer hypothesis,” a theory which suggests that all of our water, as well as several so-called “iron-loving” elements, were added to the Earth late in its formation by impacts with icy comets, meteorites and other passing objects.

“For 30 years, the late-veneer hypothesis has been the dominant paradigm for understanding Earth’s early history, and our ultimate origins,” Humayun said. “Now, with our latest research, we’re suggesting that the late-veneer hypothesis may not be the only way of explaining the presence of certain elements in the Earth’s crust and mantle.”

To illustrate his point, Humayun points to what is known about the Earth’s composition.

“We know that the Earth has an iron-rich core that accounts for about one-third of its total mass,” he said. “Surrounding this core is a rocky mantle that accounts for most of the remaining two-thirds,” with the thin crust of the Earth’s surface making up the rest.

“According to the late-veneer hypothesis, most of the original iron-loving, or siderophile, elements” — those elements such as gold, platinum, palladium and iridium that bond most readily with iron — “would have been drawn down to the core over tens of millions of years and thereby removed from the Earth’s crust and mantle. The amounts of siderophile elements that we see today, then, would have been supplied after the core was formed by later meteorite bombardment. This bombardment also would have brought in water, carbon and other materials essential for life, the oceans and the atmosphere.”

To test the hypothesis, Humayun and his NASA colleagues — Kevin Righter and Lisa Danielson — conducted experiments at Johnson Space Center in Houston and the National High Magnetic Field Laboratory in Tallahassee. At the Johnson Space Center, Righter and Danielson used a massive 880-ton press to expose samples of rock containing palladium — a metal commonly used in catalytic converters — to extremes of heat and temperature equal to those found more than 300 miles inside the Earth. The samples were then brought to the magnet lab, where Humayun used a highly sensitive analytical tool known as an inductively coupled plasma mass spectrometer, or ICP-MS, to measure the distribution of palladium within the sample.

“At the highest pressures and temperatures, our experiments found palladium in the same relative proportions between rock and metal as is observed in the natural world,” Humayun said. “Put another way, the distribution of palladium and other siderophile elements in the Earth’s mantle can be explained by means other than millions of years of meteorite bombardment.”

The potential ramifications of his team’s research are significant, Humayun said.

“This work will have important consequences for geologists’ thinking about core formation, the core’s present relation to the mantle, and the bombardment history of the early Earth,” he said. “It also could lead us to rethink the origins of life on our planet.”

Scientists Head to Warming Alaska on Ice Core Expedition

The Saint Elias Mountains from the window of a Twin Otter aircraft en route to an ice core drill site in May 2002. Scientists have been gathering a series of ice cores from around the Arctic to better understand the regional climate-change picture. Photo by Cameron Wake, UNH.
The Saint Elias Mountains from the window of a Twin Otter aircraft en route to an ice core drill site in May 2002. Scientists have been gathering a series of ice cores from around the Arctic to better understand the regional climate-change picture. Photo by Cameron Wake, UNH.

The state of Alaska has the dubious distinction of leading the lower 48 in the effects of a warming climate. Small villages are slipping into the sea due to coastal erosion, soggy permafrost is cracking buildings and trapping trucks.

In an effort to better understand how the Pacific Northwest fits into the larger climate-change picture, scientists from the University of New Hampshire and University of Maine are heading to Denali National Park on the second leg of a multi-year mission to recover ice cores from glaciers in the Alaska wilderness.

Cameron Wake of the UNH Institute for the Study of Earth, Oceans, and Space (EOS) and Karl Kreutz of the University of Maine Climate Change Institute are leading the expedition, which is funded by the National Science Foundation.

This year’s month-long reconnaissance mission will identify specific drill sites for surface-to-bedrock ice cores that will provide researchers with the best climate records going back some 2,000 years. The fieldwork is part of a decade-long goal to gather climate records from ice cores from around the entire Arctic region.

“Just as any one meteorological station can’t tell you about regional or hemispheric climate change, a series of ice cores is needed to understand the regional climate variability in the Arctic,” says Wake, research associate professor at UNH. “This effort is part of a broader strategy that will give us a fuller picture.”

Kreutz says the 2,000-year ice core record will provide a good window for determining how the climate system has been affected by volcanic activity, the variability of solar energy, changes in greenhouse gas concentrations and the dust and aerosols in the atmosphere that affect how much sunlight reaches the Earth.

“This is a joint effort in the truest sense,” says Kreutz, who has collaborated with Wake in both Arctic and Asian research for the better part of a decade. Kreutz’s UMaine team will consist of Erich Osterberg, who received his Ph.D. in December, second-year M.S. candidate Ben Gross, and Seth Campbell, an undergraduate majoring in Earth science.

Wake conducted an initial aerial survey of the Denali terrain two years ago but notes there have been “no boots on the ground.” Through May, Wake, his Ph.D. student Eric Kelsey, the UMaine team, and Canadian ice-core driller Mike Waszkiewicz will visit potential deep drilling sites and use a portable, ground-penetrating radar to determine the ice thickness and internal structure on specific glaciers. They will be looking for “layer-cake” ice with clear, well-defined annual stratigraphy.

A clear record from Denali will help round out the bigger paleoclimate picture by adding critical information gathered from ice cores recovered in the North Pacific, all of which can be compared to a wealth of climate data already gathered in the North Atlantic region.

According to Wake, scientists have long thought the North Atlantic drives global climate changes. However, there are now indications that a change in the North Pacific might happen first and be followed by a North Atlantic response. “We need to better understand the relationship in terms of the timing and magnitude of climate change between these two regions,” he says.

At the potential drill sites, the scientists will also collect samples for chemical analysis from 20-foot-deep snowpits and shallow ice cores, and install automatic weather stations at 7,800 feet and 14,000 feet. The chemical analyses, which will be carried out at both UNH and UMaine labs, are needed to decipher changes in temperature, atmospheric circulation, and environmental change such as the phenomenon known as “Arctic haze,” which has brought heavily polluted air masses to the region for decades from North America, Europe, and Asia.

Scientists Discover New Ocean Current

The North Pacific Gyre Oscillation explains changes in salinity, nutrients and chlorophyll seen in the Northeast Pacific.
The North Pacific Gyre Oscillation explains changes in salinity, nutrients and chlorophyll seen in the Northeast Pacific.

Scientists at the Georgia Institute of Technology have discovered a new climate pattern called the North Pacific Gyre Oscillation. This new pattern explains, for the first time, changes in the water that are important in helping commercial fishermen understand fluctuations in the fish stock. They’re also finding that as the temperature of the Earth is warming, large fluctuations in these factors could help climatologists predict how the oceans will respond in a warmer world. The research appears in the April 30 edition of the journal Geophysical Research Letters.

“We’ve been able to explain, for the first time, the changes in salinity, nutrients and chlorophyll that we see in the Northeast Pacific,” said Emanuele Di Lorenzo, assistant professor in Georgia Tech’s School of Earth and Atmospheric Sciences.

Since 1945, fishermen in the California current of the Pacific Ocean have been tracking temperature, salinity and nutrients, among other things, in the ocean to help them predict changes in fish populations like sardines and anchovies that are important for the industry. Studying this data, along with satellite images, Di Lorenzo discovered a pattern of current that he named the North Pacific Gyre Oscillation.

Recent satellite data suggest that this current is undergoing intensification as the temperature of the Earth has risen over the past few decades.

“Although the North Pacific Gyre Oscillation is part of a natural cycle of the climate system, we find evidence suggesting that its amplitude may increase as global warming progresses,” said Di Lorenzo.

If this is true, this newly found climate pattern mey help scientists predict how the ecosystem of the Pacific Ocean is likely to change if the world continues to warm, as predicted by the Intergovernmental Panel on Climate Change.

Mapping Earth’s soil moisture

Professor Dara Entekhabi will lead the science team for NASA's Soil Moisture Active-Passive (SMAP) satellite mission, scheduled to launch in December 2012. A 6-meter deployable mesh antenna on the satellite will gather soil moisture and freeze/thaw data across 1,000-kilometer swaths, creating ribbons of measurements around the globe and completing the cycle every few days. - Credit: NASA
Professor Dara Entekhabi will lead the science team for NASA’s Soil Moisture Active-Passive (SMAP) satellite mission, scheduled to launch in December 2012. A 6-meter deployable mesh antenna on the satellite will gather soil moisture and freeze/thaw data across 1,000-kilometer swaths, creating ribbons of measurements around the globe and completing the cycle every few days. – Credit: NASA

Professor Dara Entekhabi will lead the science team designing a NASA satellite mission to collect global soil moisture measurements and other data seen as key to improving weather, flood and drought forecasts and predictions of agricultural productivity and climate change.

At present, scientists have no network for gathering soil moisture data as they do for rainfall, winds, humidity and temperature. Instead, that data is gathered only at a few scattered points around the world. But NASA’s Soil Moisture Active-Passive mission (SMAP), scheduled to launch in December 2012, aims to change that.

“Soil moisture is the lynchpin of the water, energy and carbon cycles over land. It is the variable that links these three cycles through its control on evaporation and plant transpiration. Global monitoring of this variable will allow a new perspective on how these three cycles work and vary together in the Earth system,” said Entekhabi, Bacardi and Stockholm Water Foundations Professor.

“Additionally, because soil moisture is a state variable that controls both water and energy fluxes at the land surface, we anticipate that assimilation of the global observations will improve the skill in numerical weather prediction, especially for events that are influenced by these fluxes at the base of the atmosphere,” added Entekhabi, who holds joint appointments in MIT’s Department of Civil and Environmental Engineering and the Department of Earth, Atmospheric and Planetary Sciences and is also director of the Parsons Laboratory for Environmental Science and Engineering.

The SMAP mission is based on an earlier satellite project led by Entekhabi that had been selected by NASA from among 20 proposals and scheduled for a 2009 launch. However, the Hydrosphere State Mission (Hydros) was cancelled abruptly in 2005 when funding for NASA’s earth sciences missions was diverted. But in July 2007, the National Research Council recommended that NASA make the soil moisture measurement project a top priority and place it on a fast track for launch. The Jet Propulsion Laboratory in Pasadena, Calif., is the lead NASA center for the project.

SMAP’s launch in 2012 is feasible in part because Entekhabi and other scientists continued to develop the mission, even when NASA’s support was withdrawn in 2005.

The instruments that will be deployed in SMAP will gather both passive and active low-frequency microwave measurements on a continuous basis, essentially creating a map of global surface soil moisture. A 6-meter deployable mesh antenna on a satellite will gather data across a swath of 1,000 kilometers, creating ribbons of measurements around the globe and completing the cycle every few days.

In addition to measuring soil moisture, the satellite will detect if the surface moisture is frozen. In forests, the freeze/thaw state determines the length of the growing season and the balance between carbon assimilation into biomass and the loss of carbon due to vegetation respiration. The result of this balance can tell scientists if a forest is a net source or net sink of carbon.

One mission obstacle that Entekhabi and team solved last year was integrating the two types of measurements the satellite would gather: passive measurements collected by radiometer, and active collected by radar. The radiometer measurements provide highly accurate data at a coarse resolution of 40 kilometers. The radar measurements provide much higher resolution (3 kilometers), but with less sensitivity. The combination of the two measurements through algorithms designed by the SMAP science team will result in accurate mapping of global soil moisture at 10 kilometers.