Monday, May 18, 2015
Brown Gymnasium – Scott Brown Gymnasium

Jupiter’s Grand Attack

Tuesday, May 19, 2015
Guggenheim 101 (Lees-Kubota Lecture Hall) – Guggenheim Aeronautical Laboratory

Science in a Small World - Short Talks

Tuesday, May 19, 2015
Dabney Hall, Garden of the Associates – The Garden of the Associates

Science in a Small World - Poster Session #2

Tuesday, May 19, 2015
Dabney Hall, Garden of the Associates – The Garden of the Associates

Science in a Small World - Poster Session #1

Caltech Geologists Discover Ancient Buried Canyon in South Tibet

A team of researchers from Caltech and the China Earthquake Administration has discovered an ancient, deep canyon buried along the Yarlung Tsangpo River in south Tibet, north of the eastern end of the Himalayas. The geologists say that the ancient canyon—thousands of feet deep in places—effectively rules out a popular model used to explain how the massive and picturesque gorges of the Himalayas became so steep, so fast.

"I was extremely surprised when my colleagues, Jing Liu-Zeng and Dirk Scherler, showed me the evidence for this canyon in southern Tibet," says Jean-Philippe Avouac, the Earle C. Anthony Professor of Geology at Caltech. "When I first saw the data, I said, 'Wow!' It was amazing to see that the river once cut quite deeply into the Tibetan Plateau because it does not today. That was a big discovery, in my opinion." 

Geologists like Avouac and his colleagues, who are interested in tectonics—the study of the earth's surface and the way it changes—can use tools such as GPS and seismology to study crustal deformation that is taking place today. But if they are interested in studying changes that occurred millions of years ago, such tools are not useful because the activity has already happened. In those cases, rivers become a main source of information because they leave behind geomorphic signatures that geologists can interrogate to learn about the way those rivers once interacted with the land—helping them to pin down when the land changed and by how much, for example.

"In tectonics, we are always trying to use rivers to say something about uplift," Avouac says. "In this case, we used a paleocanyon that was carved by a river. It's a nice example where by recovering the geometry of the bottom of the canyon, we were able to say how much the range has moved up and when it started moving."

The team reports its findings in the current issue of Science.

Last year, civil engineers from the China Earthquake Administration collected cores by drilling into the valley floor at five locations along the Yarlung Tsangpo River. Shortly after, former Caltech graduate student Jing Liu-Zeng, who now works for that administration, returned to Caltech as a visiting associate and shared the core data with Avouac and Dirk Scherler, then a postdoc in Avouac's group. Scherler had previously worked in the far western Himalayas, where the Indus River has cut deeply into the Tibetan Plateau, and immediately recognized that the new data suggested the presence of a paleocanyon.

Liu-Zeng and Scherler analyzed the core data and found that at several locations there were sedimentary conglomerates, rounded gravel and larger rocks cemented together, that are associated with flowing rivers, until a depth of 800 meters or so, at which point the record clearly indicated bedrock. This suggested that the river once carved deeply into the plateau.

To establish when the river switched from incising bedrock to depositing sediments, they measured two isotopes, beryllium-10 and aluminum-26, in the lowest sediment layer. The isotopes are produced when rocks and sediment are exposed to cosmic rays at the surface and decay at different rates once buried, and so allowed the geologists to determine that the paleocanyon started to fill with sediment about 2.5 million years ago.

The researchers' reconstruction of the former valley floor showed that the slope of the river once increased gradually from the Gangetic Plain to the Tibetan Plateau, with no sudden changes, or knickpoints. Today, the river, like most others in the area, has a steep knickpoint where it meets the Himalayas, at a place known as the Namche Barwa massif. There, the uplift of the mountains is extremely rapid (on the order of 1 centimeter per year, whereas in other areas 5 millimeters per year is more typical) and the river drops by 2 kilometers in elevation as it flows through the famous Tsangpo Gorge, known by some as the Yarlung Tsangpo Grand Canyon because it is so deep and long.

Combining the depth and age of the paleocanyon with the geometry of the valley, the geologists surmised that the river existed in this location prior to about 3 million years ago, but at that time, it was not affected by the Himalayas. However, as the Indian and Eurasian plates continued to collide and the mountain range pushed northward, it began impinging on the river. Suddenly, about 2.5 million years ago, a rapidly uplifting section of the mountain range got in the river's way, damming it, and the canyon subsequently filled with sediment.

"This is the time when the Namche Barwa massif started to rise, and the gorge developed," says Scherler, one of two lead authors on the paper and now at the GFZ German Research Center for Geosciences in Potsdam, Germany.

That picture of the river and the Tibetan Plateau, which involves the river incising deeply into the plateau millions of years ago, differs quite a bit from the typically accepted geologic vision. Typically, geologists believe that when rivers start to incise into a plateau, they eat at the edges, slowly making their way into the plateau over time. However, the rivers flowing across the Himalayas all have strong knickpoints and have not incised much at all into the Tibetan Plateau. Therefore, the thought has been that the rapid uplift of the Himalayas has pushed the rivers back, effectively pinning them, so that they have not been able to make their way into the plateau. But that explanation does not work with the newly discovered paleocanyon.

The team's new hypothesis also rules out a model that has been around for about 15 years, called tectonic aneurysm, which suggests that the rapid uplift seen at the Namche Barwa massif was triggered by intense river incision. In tectonic aneurysm, a river cuts down through the earth's crust so fast that it causes the crust to heat up, making a nearby mountain range weaker and facilitating uplift.

The model is popular among geologists, and indeed Avouac himself published a modeling paper in 1996 that showed the viability of the mechanism. "But now we have discovered that the river was able to cut into the plateau way before the uplift happened," Avouac says, "and this shows that the tectonic aneurysm model was actually not at work here. The rapid uplift is not a response to river incision."

The other lead author on the paper, "Tectonic control of Yarlung Tsangpo Gorge revealed by a buried canyon in Southern Tibet," is Ping Wang of the State Key Laboratory of Earthquake Dynamics, in Beijing, China. Additional authors include Jürgen Mey, of the University of Potsdam, in Germany; and Yunda Zhang and Dingguo Shi of the Chengdu Engineering Corporation, in China. The work was supported by the National Natural Science Foundation of China, the State Key Laboratory for Earthquake Dynamics, and the Alexander von Humboldt Foundation. 

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Kimm Fesenmaier
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Photosynthesis: A Planetary Revolution

Watson Lecture Preview

Two and a half billion years ago, single-celled organisms called cyanobacteria harnessed sunlight to split water molecules, producing energy to power their cells and releasing oxygen into an atmosphere that had previously had none. These early environmental engineers are responsible for the life we see around us today, and much more besides.

At 8:00 p.m. on Wednesday, November 19, in Caltech's Beckman Auditorium, Professor of Geobiology Woodward "Woody" Fischer will describe how they transformed the planet. Admission is free.

 

Q: What do you do?

A: I'm a geobiologist of the historical variety. I'm trying to understand both how the earth works, and why it works that way. The whys are hard, because you can't redo this planetary experiment. You have to create clever ways to work backward from what you can observe to answer the question you've posed.

When you boil down the earth's history, there are maybe a half-dozen singularities—fundamental changes in how our planet and the life on it interact. Photosynthetic cyanobacteria reengineered the planet. Photosynthesis led to two more singularities—plants and animals appeared. The remaining singularities are mass extinctions as a result of something happening to the global environment, and photosynthesis likely caused one of those as well. Oxygen can be highly toxic because it's so reactive. It chews up your DNA, and it binds to the metal compounds that cells use to shuttle electrons around. Any microbes that couldn't cope with this new pollutant died off, or were forced to hide in oxygen-depleted environments.

Atmospheric oxygen resulted from a change to a microbe's metabolism that evolved once, at a specific time in the earth's history. We want to know why that happened. What were those bacteria doing beforehand? What forced them to develop this radically new way of making a living?

Bacteria don't leave fossils, per se, but they can leave behind metabolic signatures that sedimentary rocks preserve. They impact the rock's elemental composition, and they alter the ratios between heavier and lighter isotopes of certain elements as well. We can work backward from that information to deduce what the bacteria were doing on the ocean floor and in the seawater above it as those sediments were being laid down.

 

Q: If the earth has had breathable oxygen for billions of years, why should we care where it came from?

A: There are two really good reasons.

One has to do with meeting society's energy demands. There's a tremendous effort at Caltech and elsewhere to develop "solar fuels." Can we do better than green plants? If cyanobacteria did the best they could under tight constraints, maybe not. But if there are a variety of ways to do that chemistry, maybe we can clear the slate and do something entirely different.

The deeper reason is that atmospheric oxygen rewrote life's recipe book. Oxygen-based metabolism provides extra energy that can be invested in cellular specialization. A group of specialized cells can become a tissue, and eventually you have complex creatures with limbs. It's like agriculture—when you start growing crops, you have surplus food. Villages spring up. Craftsmen appear.

It gets to the Big Question—how rare are we? The earth is 4.5 billion years old, and the oldest evidence for life is about 3.5 billion years old. It took another billion years until photosynthesis, and two billion more for animals to develop. Is it possible to evolve advanced creatures under a different set of constraints leading to completely different metabolisms? If we're looking for life on worlds that play by different rules, will we recognize it?

 

Q: How did you get into this line of work?

A: As a small kid, I always loved science. That disappeared somewhere in middle school, so I went to Colorado College in Colorado Springs—a small, liberal-arts school with a really intense curriculum called the block plan. You take one class at a time for a month. You're completely immersed—lecture from nine to twelve, break for lunch, afternoon labs, evening homework. Lather, rinse, repeat. I took a geology class on a whim, because my grandfather had once taught paleontology there. The class vanished into the mountains for a month, and I was hooked.

In graduate school at Harvard, I worked with Andy Knoll, a Precambrian paleontologist who's trying to understand what the world looked like before animals. Andy's primary appointment is actually in the biology department, and I built on my sedimentary-geology background with a lot of biology classes—molecular biology, biochemistry, genomics, comparative biology, evolutionary biology. And then I came here as an Agouron Postdoctoral Scholar in Geobiology in 2007. I was fortunate that they invited me to stay.

 


Named for the late Caltech professor Earnest C. Watson, who founded the series in 1922, the Watson Lectures present Caltech and JPL researchers describing their work to the public. Many past Watson Lectures are available online at Caltech's iTunes U site.
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Watson Lecture: "Photosynthesis: A Planetary Revolution"
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Unexpected Findings Change the Picture of Sulfur on the Early Earth

Scientists believe that until about 2.4 billion years ago there was little oxygen in the atmosphere—an idea that has important implications for the evolution of life on Earth. Evidence in support of this hypothesis comes from studies of sulfur isotopes preserved in the rock record. But the sulfur isotope story has been uncertain because of the lack of key information that has now been provided by a new analytical technique developed by a team of Caltech geologists and geochemists. The story that new information reveals, however, is not what most scientists had expected.

"Our new technique is 1,000 times more sensitive for making sulfur isotope measurements," says Jess Adkins, professor of geochemistry and global environmental science at Caltech. "We used it to make measurements of sulfate groups dissolved in carbonate minerals deposited in the ocean more than 2.4 billion years ago, and those measurements show that we have been thinking about this part of the sulfur cycle and sulfur isotopes incorrectly."

The team describes their results in the November 7 issue of the journal Science. The lead author on the paper is Guillaume Paris, an assistant research scientist at Caltech.

Nearly 15 years ago, a team of geochemists led by researchers at UC San Diego discovered there was something peculiar about the sulfur isotope content of rocks from the Archean era, an interval that lasted from 3.8 billion to about 2.4 billion years ago. In those ancient rocks, the geologists were analyzing the abundances of stable isotopes of sulfur.

When sulfur is involved in a reaction—such as microbial sulfate reduction, a way for microbes to eat organic compounds in the absence of oxygen—its isotopes are usually fractionated, or separated, from one another in proportion to their differences in mass. That is, 34S gets fractionated from 32S about twice as much as 33S gets fractionated from 32S. This process is called mass-dependent fractionation, and, scientists have found that it dominates in virtually all sulfur processes operating on Earth's surface for the last 2.4 billion years.

However, in older rocks from the Archean era (i.e., older than 2.4 billion years), the relative abundances of sulfur isotopes do not follow the same mass-related pattern, but instead show relative enrichments or deficiencies of 33S relative to 34S. They are said to be the product of mass-independent fractionation (MIF).

The widely accepted explanation for the occurrence of MIF is as follows. Billions of years ago, volcanism was extremely active on Earth, and all those volcanoes spewed sulfur dioxide high into the atmosphere. At that time, oxygen existed at very low levels in the atmosphere, and therefore ozone, which is produced when ultraviolet radiation strikes oxygen, was also lacking. Today, ozone prevents ultraviolet light from reaching sulfur dioxide with the energy needed to fractionate sulfur, but on the early Earth, that was not the case, and MIF is the result. Researchers have been able to reproduce this effect in the lab by shining lasers onto sulfur dioxide and producing MIF.

Geologists have also measured the sulfur isotopic composition of sedimentary rocks dating to the Archean era, and found that sulfides—sulfur-bearing compounds such as pyrite (FeS2)—include more 33S than would be expected based on normal mass-dependent processes. But if those minerals are enriched in 33S, other minerals must be correspondingly lacking in the isotope. According to the leading hypothesis, those 33S-deficient minerals should be sulfates—oxidized sulfur-bearing compounds—that were deposited in the Archean ocean.

"That idea was put forward on the basis of experiment. To test the hypothesis, you'd need to check the isotope ratios in sulfate salts (minerals such as gypsum), but those don't really exist in the Archean rock record since there was very little oxygen around," explains Woody Fischer, professor of geobiology at Caltech and a coauthor on the new paper. "But there are trace amounts of sulfate that got trapped in carbonate minerals in seawater."

However, because those sulfates are present in such small amounts, no one has been able to measure well their isotopic composition. But using a device known as a multicollector inductively-coupled mass spectrometer to precisely measure multiple sulfur isotopes, Adkins and his colleague Alex Sessions, a professor of geobiology, developed a method that is sensitive enough to measure the isotopic composition of about 10 nanomoles of sulfate in just a few tens of milligrams of carbonate material.

The authors used the method to measure the sulfate content of carbonates from an ancient carbonate platform preserved in present-day South Africa, an ancient version of the depositional environments found in the Bahamas today. Analyzing the samples, which spanned 70 million years and a variety of marine environments, the researchers found exactly the opposite of what had been predicted: the sulfates were actually enriched by 33S rather than lacking in it.

"Now, finally, we're looking at this sulfur cycle and the sulfur isotopes correctly," Adkins says.

What does this mean for the atmospheric conditions of the early Earth? "Our findings underscore that the oxygen concentrations in the early atmosphere could have been incredibly low," Fischer says.

Knowledge of sulfate isotopes changes how we understand the role of biology in the sulfur cycle, he adds. Indeed, the fact that the sulfates from this time period have the same isotopic composition as sulfide minerals suggests that the sulfides may be the product of microbial processes that reduced seawater sulfate to sulfide (which later precipitated in sediments in the form of pyrite). Previously, scientists thought that all of the isotope fractionation could be explained by inorganic processes alone.

In a second paper also in the November 7 issue of Science, Paris, Adkins, Sessions, and colleagues from a number of institutions around the world report on related work in which they measured the sulfates in Indonesia's Lake Matano, a low-sulfate analog of the Archean ocean.

At about 100 meters depth, the bacterial communities in Lake Matano begin consuming sulfate rather than oxygen, as do most microbial communities, yielding sulfide. The researchers measured the sulfur isotopes within the sulfates and sulfides in the lake water and sediments and found that despite the low concentrations of sulfate, a lot of mass-dependent fractionation was taking place. The researchers used the data to build a model of the lake's sulfur cycle that could produce the measured fractionation, and when they applied their model to constrain the range of concentrations of sulfate in the Archean ocean, they found that the concentration was likely less than 2.5 micromolar, 10,000 times lower than the modern ocean.

"At such low concentration, all the isotopic variability starts to fit," says Adkins. "With these two papers, we were able to come at the same problem in two ways—by measuring the rocks dating from the Archean and by looking at a model system today that doesn't have much sulfate—and they point toward the same answer: the sulfate concentration was very low in the Archean ocean."

Samuel M. Webb of the Stanford Synchrotron Radiation Lightsource is also an author on the paper, "Neoarchean carbonate-associated sulfate records positive Δ33S anomalies." The work was supported by funding from the National Science Foundation's Division of Earth Sciences, the Henry and Camille Dreyfus Foundation's Postdoctoral Program in Environmental Chemistry, and the David and Lucile Packard Foundation.

Paris is also a co-lead author on the second paper, "Sulfate was a trace constituent of Archean seawater." Additional authors on that paper are Sean Crowe and CarriAyne Jones of the University of British Columbia and the University of Southern Denmark; Sergei Katsev of the University of Minnesota Duluth; Sang-Tae Kim of McMaster University; Aubrey Zerkle of the University of St. Andrews; Sulung Nomosatryo of the Indonesian Institute of Sciences; David Fowle of the University of Kansas; James Farquhar of the University of Maryland, College Park; and Donald Canfield of the University of Southern Denmark. Funding was provided by an Agouron Institute Geobiology Fellowship and a Natural Sciences and Engineering Research Council of Canada Postdoctoral Fellowship, as well as by the Danish National Research Foundation and the European Research Council.

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Rock-Dwelling Microbes Remove Methane from Deep Sea

Methane-breathing microbes that inhabit rocky mounds on the seafloor could be preventing large volumes of the potent greenhouse gas from entering the oceans and reaching the atmosphere, according to a new study by Caltech researchers.

The rock-dwelling microbes, which are detailed in the Oct. 14 issue of Nature Communications, represent a previously unrecognized biological sink for methane and as a result could reshape scientists' understanding of where this greenhouse gas is being consumed in subseafloor habitats, says Professor of Geobiology Victoria Orphan, who led the study.

"Methane is a much more powerful greenhouse gas than carbon dioxide, so tracing its flow through the environment is really a priority for climate models and for understanding the carbon cycle," Orphan says.

Orphan's team has been studying methane-breathing marine microorganisms for nearly 20 years. The microbes they focus on survive without oxygen, relying instead on sulfate ions present in seawater for their energy needs. Previous work by Orphan's team helped show that the methane-breathing system is actually made up of two different kinds of microorganisms that work closely with one another. One of the partners, dubbed "ANME" for "ANaerobic MEthanotrophs," belongs to a type of ancient single-celled creatures called the archaea.

Through a mechanism that is still unclear, ANME work closely with bacteria to consume methane using sulfate from seawater. "Without this biological process, much of that methane would enter the water column, and the escape rates into the atmosphere would probably be quite a bit higher," says study first author Jeffrey Marlow, a geobiology graduate student in Orphan's lab.

Until now, however, the activity of ANME and their bacterial partners had been primarily studied in sediments located in cold seeps, areas on the ocean bottom where methane is escaping from subseafloor sources into the water above. The new study marks the first time they have been observed to oxidize methane inside carbonate mounds, huge rocky outcroppings of calcium carbonate that can rise hundreds of feet above the seafloor.

If the microbes are living inside the mounds themselves, then the distribution of methane consumption is significantly different from what was previously thought. "Methane-derived carbonates represent a large volume within many seep systems, and finding active methane-consuming archaea and bacteria in the interior of these carbonate rocks extends the known habitat for methane-consuming microorganisms beyond the relatively thin layer of sediment that may overlay a carbonate mound," Marlow says.

Orphan and her team detected evidence of methane-breathing microbes in carbonate rocks collected from three cold seeps around the world: one at a tectonic plate boundary near Costa Rica; another in the Eel River basin off the coast of northwestern California; and at Hydrate Ridge, off the Oregon coast. The team used manned and robotic submersibles to collect the rock samples from depths ranging from 2,000 feet to nearly half a mile below the surface.

Marlow has vivid memories of being a passenger in the submersible Alvin during one of those rock-retrieval missions. "As you sink down, the water outside your window goes from bright blue surface water to darker turquoise and navy blue and all these shades of blue that you didn't know existed until it gets completely dark," Marlow recalls. "And then you start seeing flashes of light because the vehicle is perturbing the water column and exciting florescent organisms. When you finally get to the seafloor, Alvin's exterior lights turn on, and this crazy alien world is illuminated in front of you."

The carbonate mounds that the subs visited often serve as foundations for coral and sponges, and are home to rockfishes, clams, crabs, and other aquatic life. For their study, the team members gathered rock samples not only from carbonate mounds located within active cold seeps, where methane could be seen escaping from the seafloor into the water, but also from mounds that appeared to be dormant.

Once the carbonate rocks were collected, they were transported back to the surface and rushed into a cold room aboard a research ship. In the cold room, which was maintained at the temperature of the deep sea, the team cracked open the carbonates in order to gather material from their interiors. "We wanted to make sure we weren't just sampling material from the surface of the rocks," Marlow says.

Using a microscope, the team confirmed that ANME and sulfate-reducing bacterial cells were indeed present inside the carbonate rocks, and genetic analysis of their DNA showed that they were related to methanotrophs that had previously been characterized in seafloor sediment. The scientists also used a technique that involved radiolabeled 14C-methane tracer gas to quantify the rates of methane consumption in the carbonate rocks and sediments from both the actively seeping sites and the areas appearing to be inactive. They found that the rock-dwelling methanotrophs consumed methane at a slower rate than their sediment-dwelling cousins.

"The carbonate-based microbes breathed methane at roughly one-third the rate of those gathered from sediments near active seep sites," Marlow says. "However, because there are likely many more microbes living in carbonate mounds than in sediments, their contributions to methane removal from the environment may be more significant."

The rock samples that were harvested near supposedly dormant cold seeps also harbored microbial communities capable of consuming methane. "We were surprised to find that these marine microorganisms are still viable and, if exposed to methane, can continue to oxidize this greenhouse gas long after surface expressions of seepage have vanished." Orphan says.

Along with Orphan and Marlow, additional coauthors on the paper, "Carbonate-hosted methanotrophy represents an unrecognized methane sink in the deep sea," include former Caltech associate research scientist Joshua Steele, now at the Southern California Coastal Water Research Project; Wiebke Ziebis, an associate professor at the University of Southern California; Andrew Thurber, an assistant professor at Oregon State University; and Lisa Levin, a professor at the Scripps Institution of Oceanography. Funding for the study was provided by the National Science Foundation; NASA's Astrobiology Institute; the Gordon and Betty Moore Foundation Marine Microbiology Initiative grant; and the National Research Council of the National Academies. 

Written by Ker Than

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Textbook Theory Behind Volcanoes May Be Wrong

In the typical textbook picture, volcanoes, such as those that are forming the Hawaiian islands, erupt when magma gushes out as narrow jets from deep inside Earth. But that picture is wrong, according to a new study from researchers at Caltech and the University of Miami in Florida.

New seismology data are now confirming that such narrow jets don't actually exist, says Don Anderson, the Eleanor and John R. McMillian Professor of Geophysics, Emeritus, at Caltech. In fact, he adds, basic physics doesn't support the presence of these jets, called mantle plumes, and the new results corroborate those fundamental ideas.

"Mantle plumes have never had a sound physical or logical basis," Anderson says. "They are akin to Rudyard Kipling's 'Just So Stories' about how giraffes got their long necks."

Anderson and James Natland, a professor emeritus of marine geology and geophysics at the University of Miami, describe their analysis online in the September 8 issue of the Proceedings of the National Academy of Sciences.

According to current mantle-plume theory, Anderson explains, heat from Earth's core somehow generates narrow jets of hot magma that gush through the mantle and to the surface. The jets act as pipes that transfer heat from the core, and how exactly they're created isn't clear, he says. But they have been assumed to exist, originating near where the Earth's core meets the mantle, almost 3,000 kilometers underground—nearly halfway to the planet's center. The jets are theorized to be no more than about 300 kilometers wide, and when they reach the surface, they produce hot spots.  

While the top of the mantle is a sort of fluid sludge, the uppermost layer is rigid rock, broken up into plates that float on the magma-bearing layers. Magma from the mantle beneath the plates bursts through the plate to create volcanoes. As the plates drift across the hot spots, a chain of volcanoes forms—such as the island chains of Hawaii and Samoa.

"Much of solid-Earth science for the past 20 years—and large amounts of money—have been spent looking for elusive narrow mantle plumes that wind their way upward through the mantle," Anderson says.

To look for the hypothetical plumes, researchers analyze global seismic activity. Everything from big quakes to tiny tremors sends seismic waves echoing through Earth's interior. The type of material that the waves pass through influences the properties of those waves, such as their speeds. By measuring those waves using hundreds of seismic stations installed on the surface, near places such as Hawaii, Iceland, and Yellowstone National Park, researchers can deduce whether there are narrow mantle plumes or whether volcanoes are simply created from magma that's absorbed in the sponge-like shallower mantle.

No one has been able to detect the predicted narrow plumes, although the evidence has not been conclusive. The jets could have simply been too thin to be seen, Anderson says. Very broad features beneath the surface have been interpreted as plumes or super-plumes, but, still, they're far too wide to be considered narrow jets.

But now, thanks in part to more seismic stations spaced closer together and improved theory, analysis of the planet's seismology is good enough to confirm that there are no narrow mantle plumes, Anderson and Natland say. Instead, data reveal that there are large, slow, upward-moving chunks of mantle a thousand kilometers wide.

In the mantle-plume theory, Anderson explains, the heat that is transferred upward via jets is balanced by the slower downward motion of cooled, broad, uniform chunks of mantle. The behavior is similar to that of a lava lamp, in which blobs of wax are heated from below and then rise before cooling and falling. But a fundamental problem with this picture is that lava lamps require electricity, he says, and that is an outside energy source that an isolated planet like Earth does not have.  

The new measurements suggest that what is really happening is just the opposite: Instead of narrow jets, there are broad upwellings, which are balanced by narrow channels of sinking material called slabs. What is driving this motion is not heat from the core, but cooling at Earth's surface. In fact, Anderson says, the behavior is the regular mantle convection first proposed more than a century ago by Lord Kelvin. When material in the planet's crust cools, it sinks, displacing material deeper in the mantle and forcing it upward.

"What's new is incredibly simple: upwellings in the mantle are thousands of kilometers across," Anderson says. The formation of volcanoes then follows from plate tectonics—the theory of how Earth's plates move and behave. Magma, which is less dense than the surrounding mantle, rises until it reaches the bottom of the plates or fissures that run through them. Stresses in the plates, cracks, and other tectonic forces can squeeze the magma out, like how water is squeezed out of a sponge. That magma then erupts out of the surface as volcanoes. The magma comes from within the upper 200 kilometers of the mantle and not thousands of kilometers deep, as the mantle-plume theory suggests.

"This is a simple demonstration that volcanoes are the result of normal broad-scale convection and plate tectonics," Anderson says. He calls this theory "top-down tectonics," based on Kelvin's initial principles of mantle convection. In this picture, the engine behind Earth's interior processes is not heat from the core but cooling at the planet's surface. This cooling and plate tectonics drives mantle convection, the cooling of the core, and Earth's magnetic field. Volcanoes and cracks in the plate are simply side effects.

The results also have an important consequence for rock compositions—notably the ratios of certain isotopes, Natland says. According to the mantle-plume idea, the measured compositions derive from the mixing of material from reservoirs separated by thousands of kilometers in the upper and lower mantle. But if there are no mantle plumes, then all of that mixing must have happened within the upwellings and nearby mantle in Earth's top 1,000 kilometers.

The paper is titled "Mantle updrafts and mechanisms of oceanic volcanism."

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Looking Forward to 2020 . . . on Mars

A Q&A With Project Scientist Ken Farley

While the Curiosity rover continues to interrogate Gale Crater on Mars, planning is well under way for its successor—another rover that is currently referred to as Mars 2020. The new robotic explorer, scheduled to launch in 2020, will use much of the same technology (even some of the spare parts Curiosity left behind on Earth) to get to the Red Planet. Once there, it will pursue a new set of scientific objectives including the careful collection and storage (referred to as "caching") of compelling samples that might one day be returned to Earth by a future mission. Today, NASA announced the selection of seven scientific instruments that Mars 2020 will carry with it to Mars.

Ken Farley, Caltech's W.M. Keck Foundation Professor of Geochemistry and chair of the Division of Geological and Planetary Sciences, is serving as project scientist for Mars 2020. We recently sat down with him to talk about the mission and his new role.

 

Congratulations on being selected project scientist for this exciting mission. For those of us who do not know exactly what a project scientist does, can you give us a little overview of the job?

Sure. Conveniently, NASA has a definition, which says that the project scientist is responsible for the overall scientific success of the mission. That's a pretty concise explanation, but it encompasses a lot. My main duty thus far has been helping to define the science needs for equipment that we are going to send to Mars. So while we haven't actually done any science yet, we have had to make a lot of design decisions that are related to the science.

The easiest place to illustrate this is in the discussion of what is necessary, from the science point of view, in terms of the samples that we will cache. We have to consider things like how much mass we need to bring back, what kind of magnetic fields and temperatures the samples are going to be exposed to, and how much contamination of different chemical constituents we can allow. Every one of those questions drives a design decision in how you build the drilling system and the caching system. And if you get those wrong, there's nothing you can do. So there's a lot of thought that has to be put into that, and I convey a lot of that information to the engineers.

Now that we have a science team, I will be helping to facilitate all of its investigations and helping the members to work as a team. MSL [the Mars Science Laboratory, Curiosity's mission] is demonstrating how you have to operate when you have a complex tool (a rover) and a bunch of sensors, and every day you have to figure out what you're going to do to further science. The team has to pull together, pool all of its information, and come up with a plan, so an important part of my job will be figuring out how to manage the team dynamics to keep everybody moving forward and not fragmenting.

 

What aspects of the job were particularly appealing to you?

One of the parts of being a division chair that I have really enjoyed is being engaged with something that's bigger than my own research. And there's definitely a lot of that on 2020. It's a huge undertaking. There are not many science projects of this scale to be associated closely with, so this just seemed like a really good opportunity.

The kinds of questions that 2020 is going after—they're really big questions. You could never answer them on your own. The key objective is about life—is there or was there ever life on Mars, and more broadly what does its presence or absence mean about the frequency and evolution of life within the universe? There's no way you could answer these questions on Earth. The simple reason for that is that Mars is covered by rocks that are of the era in which, at least on our planet, we believe life was evolving. There are almost no rocks left of that age on the earth, and the ones that are left have been really badly beaten up. So Mars is a place where you really stand a chance of answering these questions in a way that you probably can't anywhere else.

It's not the kind of science I'm usually associated with, but the mission is trying to address truly profound scientific questions.

 

As you said, space has not been the focus of your research for most of your career. Can you talk a bit about how a terrestrial geochemist like yourself wound up in this role on a Mars mission?

Several years ago, I participated in a workshop about quantifying martian stratigraphy, which was hosted by the Keck Institute for Space Studies [KISS]. One of the topics that was discussed was geochronology—the dating of rocks and other materials—on other planetary bodies, like Mars. This is important for establishing the history of a planet and is particularly challenging because it requires such exacting measurements. After interacting with some people who are now my JPL collaborators at the workshop, it seemed like we might be able to do something special that would help solve this problem. And we got support from KISS to do a follow-on study.

As I was getting deeper and deeper into thinking about how we could do this on Mars, John Grotzinger (the Fletcher Jones Professor of Geology at Caltech and project scientist for MSL) was conducting the landing-site workshops for MSL. He would say things like, "Oh, it would be really great if we could date this." And we'd agree. Then there was a call for participating scientists on MSL. I had no background whatsoever in this, but I knew there was a mass spectrometer on Curiosity. That's one of the analytical instruments we need to make these dating measurements because it allows us to determine the relative abundances of various isotopes in a sample. Since those isotopes are produced at known rates, their abundances tell us something about the age of the sample. So I wrote a proposal basically saying let's see if we can make Curiosity's mass spectrometer work for this purpose. And it did.

 

What do you think led to your selection as project scientist?

Although I don't have a long track record in studying Mars, this mission is possibly the first step in bringing samples back to Earth. In order to do that, you have to answer a lot of questions related to geochemistry, which is my specialty. The geochemistry community is not ordinarily thinking about rocks coming back from Mars. I happen to have enough crossover between what I know about Mars from the work I just described and my background from working in geochemistry labs, especially those working with the type of very small samples we might get back from Mars, to be a good fit.

 

Given Curiosity's success on Mars, why is it important and exciting for us to be sending another rover to the Red Planet?

One thing to realize is that the surface of Mars is more or less equivalent in size to the entire continental surface area of the earth, and we've been to just a few points. It's naturally tempting to look at the few places we have been on Mars and draw grand conclusions from them, but you could imagine if you landed in the middle of the Sahara Desert and studied the earth, you would come up with different answers than if you landed in the Amazon, for example. So that's part of it.

But the big thing that distinguishes Mars 2020 is the fact that we are preparing this cache, which is the first step in a process that will hopefully bring samples back to Earth some day. It's very clear that from the science community's point of view, this is a critical motivation for this mission.

 

How has the experience been working on the mission thus far?

I enjoy it very much. It's extremely different to go from a lab group of two or three people to a project that, at the end of the day, is going to have spent $1.5 billion over the next seven or eight years. It's a completely different scale of operation.

I find it really fascinating to see how everything works. I've spent my entire career among scientists. Suddenly transitioning and working with engineers is interesting because their approach and style is completely different. But they're all extremely good at what they do.

It's a lot of fun to work with these people and to face completely new and unexpected challenges. You never know what new thing is going to pop up.

Writer: 
Kimm Fesenmaier
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