Caltech Scientists Predict Greater Longevity for Planets with Life

Billion-year life extension for Earth also doubles the odds that advanced life will be found elsewhere in the universe

PASADENA, Calif.- Roughly a billion years from now, the ever-increasing radiation from the sun will have heated Earth into uninhabitability; the carbon dioxide in the atmosphere that serves as food for plant life will disappear, pulled out by the weathering of rocks; the oceans will evaporate; and all living things will disappear.

Or maybe not quite so soon, say researchers from the California Institute of Technology (Caltech), who have come up with a mechanism that doubles the future lifespan of the biosphere—while also increasing the chance that advanced life will be found elsewhere in the universe.

A paper describing their hypothesis was published June 1 in the early online edition of the Proceedings of the National Academy of Science.

Earth maintains its surface temperatures through the greenhouse effect. Although the planet's greenhouse gases—chiefly water vapor, carbon dioxide, and methane-have become the villain in global warming scenarios, they're crucial for a habitable world, because they act as an insulating blanket in the atmosphere that absorbs and radiates thermal radiation, keeping the surface comfortably warm.

As the sun has matured over the past 4.5 billion years, it has become both brighter and hotter, increasing the amount of solar radiation received by Earth, along with surface temperatures. Earth has coped by reducing the amount of carbon dioxide in the atmosphere, thus reducing the warming effect. (Despite current concerns about rising carbon dioxide levels triggering detrimental climate change, the pressure of carbon dioxide in the atmosphere has dropped some 2,000-fold over the past 3.5 billion years; modern, man-made increases in atmospheric carbon dioxide offset a fraction of this overall decrease.)

The problem, says Joseph L. Kirschvink, the Nico and Marilyn Van Wingen Professor of Geobiology at Caltech and a coauthor of the PNAS paper, is that "we're nearing the point where there's not enough carbon dioxide left to regulate temperatures following the same procedures."

Kirschvink and his collaborators Yuk L. Yung, a Caltech professor of planetary science, and graduate students King-Fai Li and Kaveh Pahlevan, say that the solution is to reduce substantially the total pressure of the atmosphere itself, by removing massive amounts of molecular nitrogen, the largely nonreactive gas that makes up about 78 percent of the atmosphere. This would regulate the surface temperatures and allow carbon dioxide to remain in the atmosphere, to support life, and could tack an additional 1.3 billion years onto Earth's expected lifespan.

In the "blanket" analogy for greenhouse gases, carbon dioxide would be represented by the cotton fibers making up the blanket. "The cotton weave may have holes, which allow heat to leak out," explains Li, the lead author of the paper.

"The size of the holes is controlled by pressure," Yung says. "Squeeze the blanket," by increasing the atmospheric pressure, "and the holes become smaller, so less heat can escape. With less pressure, the holes become larger, and more heat can escape," he says, helping the planet to shed the extra heat generated by a more luminous sun.

Strikingly, no external influence would be necessary to take nitrogen out of the air, the scientists say. Instead, the biosphere itself would accomplish this, because nitrogen is incorporated into the cells of organisms as they grow, and is buried with them when they die.

In fact, "this reduction of nitrogen is something that may already be happening," says Pahlevan, and that has occurred over the course of Earth's history. This suggests that Earth's atmospheric pressure may be lower now than it was earlier in the planet's history.

Proof of this hypothesis may come from other research groups that are examining the gas bubbles formed in ancient lavas to determine past atmospheric pressure: the maximum size of a forming bubble is constrained by the amount of atmospheric pressure, with higher pressures producing smaller bubbles, and vice versa.

If true, the mechanism also would potentially occur on any extrasolar planet with an atmosphere and a biosphere.

"Hopefully, in the future we will not only detect earth-like planets around other stars but learn something about their atmospheres and the ambient pressures," Pahlevan says. "And if it turns out that older planets tend to have thinner atmospheres, it would be an indication that this process has some universality."

Adds Yung: "We can't wait for the experiment to occur on Earth. It would take too long. But if we study exoplanets, maybe we will see it. Maybe the experiment has already been done."

Increasing the lifespan of our biosphere—from roughly 1 billion to 2.3 billion years—has intriguing implications for the search for life elsewhere in the universe. The length of the existence of advanced life is a variable in the Drake equation, astronomer Frank Drake's famous formula for estimating the number of intelligent extraterrestrial civilizations in the galaxy. Doubling the duration of Earth's biosphere effectively doubles the odds that intelligent life will be found elsewhere in the galaxy.

"It didn't take very long to produce life on the planet, but it takes a very long time to develop advanced life," says Yung. On Earth, this process took four billion years. "Adding an additional billion years gives us more time to develop, and more time to encounter advanced civilizations, whose own existence might be prolonged by this mechanism. It gives us a chance to meet."

The work described in the paper, "Atmospheric Pressure as a Natural Regulator of the Climate of a Terrestrial Planet with Biosphere," was funded by NASA and the Virtual Planetary Laboratory at Caltech.

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Caltech Visiting Associate Champions the Study of Solar Eclipses in the Modern Era

PASADENA, Calif.-Championing the modern-day use of solar eclipses to solve a set of modern problems is the goal of a review article written by Jay Pasachoff, visiting associate at the California Institute of Technology (Caltech) and Field Memorial Professor of Astronomy at Williams College. The review is the cover story of the June 11 issue of Nature, as part of its coverage of the International Year of Astronomy.

Pasachoff's review article describes the history of eclipse discoveries, as well as current themes in eclipse research. "In the article, I try to show how there is still a vital role for eclipses in the range of observations we want to make of the sun," he says.

While space-based telescopes provide "wonderful" data for astronomers to examine, there are still observations that are "inaccessible from space," Pasachoff points out. "[I]t appears that for decades ground-based capabilities will still allow unique observations to be made from Earth rather than from space," he writes in his review.

Indeed, viewing an eclipse from the ground provides "the flexibility to use the latest equipment and to take advantage of new theoretical ideas to frame observations," he notes.

Despite the novelty of these approaches, Pasachoff says, "Many people still have an old-fashioned view of eclipses going back to the discovery of helium or the use of the eclipse 90 years ago this month for verifying Einstein's general theory of relativity. But those are old problems. These days there are a whole series of new questions and new methods that we can apply at eclipses."

Scientists will get their chance to ask those questions and use those methods next month, Pasachoff says, during what will be the longest solar eclipse in the 21st century. The upcoming total eclipse-which will be visible in China and India on July 22 for almost six minutes, "an unusually long time for a totality"-will allow Pasachoff's team, as well as many other teams of scientists, to make important observations that are expected to advance our understanding of the solar atmosphere.

Pasachoff will view the eclipse-his 49th such event-from a 3,000-foot-high mountain in Tianhuangping, China, along with a group of colleagues and students from Williams College. There, he will gather data to continue his research into the heating of the solar corona, which has a temperature of millions of degrees. "We'll be looking for waves in the corona," he says, "for vibrations in the corona that are a sign of these particular waves in the magnetic field that are heating the corona."

The study of eclipses, Pasachoff says, has been enhanced by advances in computer imaging that make it possible to "bring out" low-contrast features. Just such an image-computer-processed by Pasachoff's colleague, Miloslav Druckmüller of the Brno University in the Czech Republic-was chosen by Nature for the cover of the issue containing Pasachoff's review article. Pasachoff and Druckmüller have been collaborating on interpreting these images.

For these and many other reasons, Pasachoff says, the ground-based study of solar eclipses will continue to provide insights and observations of the sun that would otherwise be unobtainable. As he notes in his Nature review article, "At present the paired science and beauty of solar eclipses remain uniquely available to scientists and others in the path of totality."

Pasachoff's expedition to China will be supported by the National Geographic Society. His eclipse research has been supported by the Committee for Research and Exploration of the National Geographic Society, the National Science Foundation, NASA, and Williams College. NASA's Planetary Sciences Division has also provided the electronic cameras that Pasachoff's team uses both in his eclipse studies and in his studies of Pluto and other outer-solar-system objects, in which he has collaborated with Mike Brown, Caltech's Richard and Barbara Rosenberg Professor and professor of planetary astronomy.

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Caltech Scientists Lead Deep-Sea Discovery Voyage

Research mission uncovers several new species and thousands of fossilized coral samples.

PASADENA, Calif.--Scientists from the California Institute of Technology (Caltech) and an international team of collaborators have returned from a month-long deep-sea voyage to a marine reserve near Tasmania, Australia, that not only netted coral-reef samples likely to provide insight into the impact of climate change on the world's oceans, but also brought to light at least three never-before-seen species of sea life.

"It was truly one of those transcendent moments," says Caltech's Jess Adkins of the descents made by the remotely operated submersible Jason. Adkins was the cruise's lead scientist and is an associate professor of geochemistry and global environmental science at Caltech. "We were flying--literally flying--over these deep-sea structures that look like English gardens, but are actually filled with all of these carnivorous, Seuss-like creatures that no one else has ever seen."

The voyage on the research vessel RV Thompson explored the Tasman Fracture Commonwealth Marine Reserve, southwest of Tasmania. The voyage was funded by the National Science Foundation and was the second of two cruises taken by the team, which included researchers from the United States--including scientists from Caltech and the Woods Hole Oceanographic Institution in Massachusetts, which owns and operates the submersible Jason--and Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO). The first of those voyages was taken in January 2008, with this most recent one spanning 33 days from mid-December 2008 through mid-January 2009.

Up until now, the area of the reef the scientists were exploring--called the Tasman Fracture Zone--had only been explored to a depth of 1,800 meters (more than 5,900 feet). Using Jason, the researchers on this trip were able to reach as far down as 4,000 meters (well over 13,000 feet).

"We set out to search for life deeper than any previous voyage in Australian waters," notes scientist Ron Thresher from CSIRO's Climate Adaptation and Wealth from Oceans Flagships.

The cruise had two main goals, says Adkins. One was to try to use deep-sea corals to reconstruct the paleoclimate--with an emphasis on the changes in climate over the last 100,000 years--and to understand the fluctuations in CO2 found in the ice-core records. Investigators also wanted to look at changes in the ocean over a much smaller slice of time--the past few hundred to one thousand or so years. "We want to see what's happened to the corals over the Industrial Revolution timescale," says Adkins. "And we want to see if we can document those changes."

The second goal? "Simply to document what's down there," says Adkins.

"In one sense, the deep ocean is less explored than Mars," he adds. "So every time you go to look down there you see new things, magical things."

Among the "magical things" seen on this trip were

  • a new species of carnivorous sea squirt that "looks and behaves like a Venus fly trap," says Adkins;
  • new species of barnacles (some of which Adkins says may even belong to an entirely new family); and
  • a new species of sea anemone that Adkins calls "the bane of our existence," because it looks just like the coral they were trying to collect.

The sea anemone was particularly vexing for the researchers, because they were hoping to find deep-sea (or abyssal) samples of the fossilized coral, but were unable to find the coral much below 2,400 meters (nearly 7,800 feet). The look-alike sea anemone, on the other hand, kept popping up all over the place on the deep-sea floor, raising--and then dashing--the scientists' hopes.

This carnivorous sea squirt was one of the new species seen during the voyage of the RV Thompson.
Credit: Advanced Imaging and Visualization Laboratory, WHOI/Jess Adkins, Caltech

"Not being able to find the coral down deeper was our single biggest disappointment on the trip," says Adkins.

Still, the 10,000-plus samples collected will help the researchers begin their work of deciphering just what has been happening to the ocean throughout the centuries of climate change, and during and between glacial cycles. First up: dating the fossils collected on this trip in order to determine which slice of history they came from.

"The deep ocean is part and parcel of these rapid climate changes," says Adkins. "These corals will be our window into what their impact is on climate, and how they have that impact. The info is there; now we just have to unpack it."

Further funding for the research came from CSIRO, the Commonwealth Environmental Research Facilities' Marine Biodiversity Hub, and the Australian Department of the Environment, Water, Heritage and the Arts.

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Caltech Researchers Find Ancient Climate Cycles Recorded in Mars Rocks

PASADENA, Calif.-- Researchers at the California Institute of Technology (Caltech) and their colleagues have found evidence of ancient climate change on Mars caused by regular variation in the planet's tilt, or obliquity. On Earth, similar "astronomical forcing" of climate drives ice-age cycles.

Using stereo topographic maps obtained by processing data from the high-resolution camera onboard NASA's Mars Reconnaissance Orbiter, the Caltech scientists, led by graduate student Kevin Lewis and Oded Aharonson, associate professor of planetary science, along with John Grotzinger, the Fletcher Jones Professor of Geology, identified and measured layered rock outcrops within four craters in the planet's Arabia Terra region. The layering in different outcrops occurs at scales ranging from a few meters to tens of meters, but at each location the layers all have similar thicknesses and exhibit similar features.

Based on a pattern of layers within layers measured at one location, known as Becquerel crater, the scientists propose that each layer was formed over a period of about 100,000 years and that these layers were produced by the same cyclical climate changes.

In addition, every 10 layers were bundled together into larger units, which were laid down over an approximately one-million-year period; in the Becquerel crater, the 10-layer pattern is repeated at least 10 times. This one-million-year cycle corresponds to a known pattern of change in Mars's obliquity caused by the dynamics of the solar system.

"Due to the scale of the layers, small variations in Mars's orbit are the best candidate for the implied climate changes. These are the very same changes that have been shown to set the pacing of ice ages on the Earth and can also lead to cyclic layering of sediments," says Lewis, the first author of a paper about the work published in this week's issue of Science.

Sequences of cyclic sedimentary rock layers exposed in an unnamed crater in Arabia Terra, Mars.
Credit: Topograpy, Caltech; HiRISE Images, NASA/JPL/University of Arizona

The tilt of Earth on its axis varies between 22.1 and 24.5 degrees over a 41,000-year period. The tilt itself is responsible for seasonal variation in climate, because the portion of the Earth that is tipped toward the sun--and that receives more sunlight hours during a day--gradually changes throughout the year. During phases of lower obliquity, polar regions are less subject to seasonal variations, leading to periods of glaciation.

Mars's tilt varies by tens of degrees over a 100,000-year cycle, producing even more dramatic variation. When the obliquity is low, the poles are the coldest places on the planet, while the sun is located near the equator all the time. This could cause volatiles in the atmosphere, like water and carbon dioxide, to migrate poleward, where they'd be locked up as ice.

When the obliquity is higher, the poles get relatively more sunlight, and those materials would migrate away. "That affects the volatiles budget. If you move carbon dioxide away from the poles, the atmospheric pressure would increase, which may cause a difference in the ability of winds to transport and deposit sand," Aharonson says. This is one effect that could change the rate of deposition of layers such as those seen by the researchers in the four craters.

Another effect of the changing tilt would be a change in the stability of surface water, which alters the ability of sand grains to stick together and cement in order to form the rock layers.

"The whole climate system would be different," Aharonson says.

However, such large changes in climate would influence a variety of geologic processes on the surface. While the researchers cannot tie the formation of the rhythmic bedding on Mars to any particular geologic process, "a strength of the paper is that we can draw conclusions without having to specify the precise depositional process," Aharonson says.

"This study gives us a hint of how the ancient climate of Mars operated, and shows a much more predictable and regular environment than you would guess from other geologic features that indicate catastrophic floods, volcanic eruptions, and impact events," Lewis adds. "More work will be required to understand the full extent of the information contained within these natural geologic archives," he says.

"One of the fun things about this project for me is that we were able to use techniques on Mars that are the bread and butter of studies of stratigraphy on Earth," says Aharonson. "We substituted a high-resolution camera in orbit around Mars and stereo processing for a geologist's Brunton Compass and mapboard, and were able to derive the same quantitative information on the same scale. This enabled conclusions that have qualitative meaning similar to those we chase on Earth."

The paper, "Quasi-Periodic Bedding in the Sedimentary Rock Record of Mars," will be published in the December 5 issue of Science. The work was supported by NASA's Mars Data Analysis Program and the NASA Earth and Space Science Fellowship program.

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Potential for Large Earthquake Off the Coast of Sumatra Remains Large, Says Caltech-Led Team of Scientists

Recent seismic activity not enough to release strain in the area's subduction zone, the researchers report in the journal Nature

PASADENA, Calif.--The subduction zone that brought us the 2004 Sumatra-Andaman earthquake and tsunami is ripe for yet another large event, despite a sequence of quakes that occurred in the Mentawai Islands area in 2007, according to a group of earthquake researchers led by scientists from the Tectonics Observatory at the California Institute of Technology (Caltech).

"From what we saw," says geologist Jean-Philippe Avouac, director of the Tectonics Observatory and one of the paper's lead authors, "we can say with some confidence that we're probably not done with large earthquakes in Sumatra."

Their findings were published in a letter in the December 4 issue of the journal Nature.

The devastating magnitude 9.2 earthquake that occurred off the western coast of Sumatra on December 26, 2004-the earthquake that spawned a lethal tsunami throughout the Indian Ocean-took place in a subduction zone, an area where one tectonic plate dips under another, forming a quake-prone region.

It is that subduction zone that drew the interest of the Caltech-led team. Seismic activity has continued in the region since the 2004 event, they knew. But have the most recent earthquakes been able to relieve the previous centuries of built-up seismic stress?

Yes . . . and no. Take, for instance, an area just south of the 2004 quake, where a magnitude 8.6 earthquake hit in 2005. (That same area had also been the site of a major earthquake in 1861.) The 2005 quake, says Avouac, did a good job of "unzipping" the stuck area in that patch of the zone, effectively relieving the stresses that had built up since 1861. This means that it should be a few centuries before another large quake in that area would be likely.

The same cannot be said, however, of the area even further south along that same subduction zone, near the Mentawai Islands, a chain of about 70 islands off the western coasts of Sumatra and Indonesia. This area, too, has been hit by giant earthquakes in the past (an 8.8 in 1797 and a 9.0 in 1833). More recently, on September 12, 2007, it experienced two earthquakes just 12 hours apart: first a magnitude 8.4 quake and then a magnitude 7.9.

These earthquakes did not come as a surprise to the Caltech researchers. Caltech geologist and paper coauthor Kerry Sieh, who is now at the Nanyang Technological University in Singapore, had long been using coral growth rings to quantify the pattern of slow uplift and subsidence in the Mentawai Islands area; that pattern, he and his colleagues knew, is the result of stress build-up on the plate interface, which should eventually be released by future large earthquakes.

But was all that accumulated stress released in 2007? In the work described in the Nature letter, the researchers analyzed seismological records, remote sensing (inSAR) data, field measurements, and, most importantly, data gathered by an array of continuously recording GPS stations called SuGAr (for Sumatra Geodetic Array) to find out.

Their answer? The quakes hadn't even come close to doing their stress-reduction job. "In fact," says Ali Ozgun Konca, a Caltech scientist and the paper's first author, who did this work as a graduate student, "we saw release of only a quarter of the moment needed to make up for the accumulated deficit over the past two centuries." (Moment is a measure of earthquake size that takes into account how much the fault slips and over how much area.)

"The 2007 quakes occurred in the right place at the right time," adds Avouac. "They were not a surprise. What was a surprise was that those earthquakes were way smaller than we expected."

"The quake north of this region, in 2005, ruptured completely," says Konca. "But the 2007 sequence of quakes was more complicated. The slippage of the plates was patchy, and it didn't release all the strain that had accumulated."

"It was what we call a partial rupture," adds Avouac. "There's still enough strain to create another major earthquake in that region. We may have to wait a long time, but there's no reason to think it's over."

Other authors on the paper include Anthony Sladen, Aron J. Meltzner, John Galetzka, Jeff Genrich, and Don V. Helmberger from Caltech; Danny H. Natawidjaja from the Indonesian Institute of Science (LIPI); Peng Fang and Yehuda Bock from the Scripps Institution of Oceanography in La Jolla; Zhenhong Li from the University of Glasgow in Scotland; Mohamed Chlieh from the Université de Nice Sophia-Antipolis in France; Eric J. Fielding from the Jet Propulsion Laboratory; and Chen Ji from the University of California, Santa Barbara. The work detailed in the paper, "Partial rupture of a locked patch of the Sumatra megathrust during the 2007 earthquake sequence," was supported by funding from the National Science Foundation and the Gordon and Betty Moore Foundation.

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Caltech Geobiologists Discover Unique "Magnetic Death Star" Fossil

Fossil and other new forms date to ancient period of global warming

PASADENA, Calif.-- An international team of scientists has discovered microscopic, magnetic fossils resembling spears and spindles, unlike anything previously seen, among sediment layers deposited during an ancient global-warming event along the Atlantic coastal plain of the United States.

The researchers, led by geobiologists from the California Institute of Technology (Caltech) and McGill University, describe the findings in a paper published online this week in the Proceedings of the National Academy of Sciences (PNAS).

Fifty-five million years ago, Earth warmed by more than 9 degrees Fahrenheit after huge amounts of carbon entered the atmosphere over a period of just a few thousand years. Although this ancient global-warming episode, known as the Paleocene-Eocene Thermal Maximum (PETM), remains incompletely explained, it might offer analogies for possible global warming in the future.

Perhaps in response to the environmental stress of the PETM, many land mammals in North America became dwarfed. Almost half of the common sea bottom-dwelling microorganisms known as foraminifera became extinct in newly warmer waters that were incapable of carrying the levels of dissolved oxygen for which they were adapted.

"Imagine our surprise to discover not only a fossil bloom of bacteria that make iron-oxide magnets within their cells, but also an entirely unknown set of organisms that grew magnetic crystals to giant sizes," said Caltech postdoctoral scholar Timothy Raub, who collected the samples from an International Ocean Drilling Program drill-core storehouse at Rutgers University in New Jersey.

A typical "giant" spearhead-shaped crystal is only about four microns long, which means that hundreds would fit on the period at the end of this sentence. However, the crystals found recently are eight times larger than the previous world record for the largest bacterial iron-oxide crystal.

According to Dirk Schumann, a geologist and electron microscopist at McGill University and lead author of the study, "It was easy to focus on the thousands of other bacterial fossils, but these single, unusual crystals kept appearing in the background. It soon became evident that they were everywhere."

In addition to their unusually large sizes, the magnetic crystals occur in a surprising array of shapes. For example, the spearhead-like crystals have a six-sided "stalk" at one end, a bulbous middle, and a sharp, tapered tip at the other end. Once reaching a certain size, spearhead crystals grow longer but not wider, a directed growth pattern that is characteristic of most higher biological organisms.

The spearhead magnetic crystals compose a minor fraction of all of the iron-oxide crystals in the PETM clay layer. Most of the crystals have smaller sizes and special shapes, which indicate that they are fossils of magnetotactic bacteria. This group of microorganisms, long studied at Caltech by study coauthor Joseph Kirschvink, the Nico and Marilyn Van Wingen Professor of Geobiology, use magnets to orient themselves within Earth's magnetic field, and proliferate in oxygen-poor water.

Spearheads are not, however, the rarest fossil type in the deposit. That honor belongs to a spherical cluster of spearheads informally dubbed the "Magnetic Death Star" by the researchers. The Magnetic Death Star may have preserved the crystals as they occurred in their original biological structure.

The researchers could not find a similar-shaped organism anywhere in the paleontological annals. They hypothesize that it may have been a single-celled eukaryote that evolved for the first time during the PETM and was outcompeted once the strange climate conditions of that time diminished. Alternatively, it may still exist today in a currently undiscovered location.

"The continental shelf of the mid-Atlantic states during the PETM must have been very iron-rich, much like the Amazon shelf today," notes study coauthor Robert Kopp of Princeton University, who first started working on the project while a graduate student at Caltech. "These fossils may be telling a story of radical environmental transformation: imagine a river like the Amazon flowing at least occasionally where the Potomac is today."

The paper, "Gigantism in unique biogenic magnetite at the Paleocene-Eocene Thermal Maximum," will appear in the early online issue of PNAS the week of October 20. The Caltech work was supported by the NASA Exobiology program.

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Caltech Scientists Offer New Explanation for Monsoon Development

PASADENA, Calif.--Geoscientists at the California Institute of Technology have come up with a new explanation for the formation of monsoons, proposing an overhaul of a theory about the cause of the seasonal pattern of heavy winds and rainfall that essentially had held firm for more than 300 years.

The traditional idea of monsoon formation was developed in 1686 by English astronomer and mathematician Edmond Halley, namesake of Halley's Comet. In Halley's model, monsoons are viewed as giant sea-breeze circulations, driven by the differences in heat capacities between land and ocean surfaces that, upon heating by sunlight, lead to temperature differences between warmer land and cooler ocean surfaces--for example, between the Indian subcontinent and the oceans surrounding it.

"These circulations form overturning cells, with air flowing across the equator toward the warmer land surface in the summer hemisphere, rising there, flowing back toward and across the equator aloft, and sinking in the winter hemisphere," explains Tapio Schneider, associate professor of environmental science and engineering at Caltech.

A different explanation is offered by Schneider and Simona Bordoni of the National Center for Atmospheric Research in Boulder, Colorado. The duo used a computer-generated, water-covered, hypothetical earth (an "aquaplanet") to simulate monsoon formation and found that differences in heat capacities between land and sea were not necessary. Bordoni was a Moore Postdoctoral Scholar at Caltech and will return to Caltech as an assistant professor in 2009.

Monsoons arise instead because of an interaction between the tropical circulation and large-scale turbulent eddies generated in the atmosphere in middle latitudes. These eddies, which can span more than 300 miles across, form the familiar systems that govern the weather in middle latitudes.

The eddies, Schneider says, are "basically large waves, which crash into the tropical circulation. They 'break,' much like water waves on the beach, and modify the circulation as a result of the breaking. There are feedbacks between the circulation, the wind pattern associated with it in the upper atmosphere, and the propagation characteristics of the waves, which make it possible for the circulation to change rapidly." This can quickly generate the characteristic high surface winds and heavy rainfall of the monsoon.

Bordoni adds: "These feedbacks provide one possible explanation for the rapidity of monsoon onset, which had been a long-standing conundrum in the traditional view of monsoons," because substantial differences between land and sea temperatures can only develop slowly through heating by sunlight.

Although the results won't immediately produce better forecasts of impending monsoons, Schneider says, "in the long run, a better understanding of monsoons may lead to better predictions with semi-empirical models, but much more work needs to be done."

The paper, "Monsoons as eddy-mediated regime transitions of the tropical overturning circulation," appears in the advance online edition of Nature Geosciences. The work was supported by the Davidow Discovery Fund, a David and Lucile Packard Fellowship, a Moore Postdoctoral Fellowship, and the National Science Foundation. The National Center for Atmospheric Research is sponsored by the National Science Foundation. Any opinions, findings and conclusions or recommendations expressed in this publication are those of the author and do not necessarily reflect the views of the National Science Foundation.

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Giant Impact Explains Mars Dichotomy

PASADENA, Calif.--The surface landscape of Mars, divided into lowlands in the north and highlands in the south, has long perplexed planetary scientists. Was it sculpted by several small impacts, via mantle convection in the planet's interior, or by one giant impact? Now scientists at the California Institute of Technology have shown through computer modeling that the Mars dichotomy, as the divided terrain has been termed, can indeed be explained by one giant impact early in the planet's history.

"The dichotomy is arguably the oldest feature on Mars," notes Oded Aharonson, associate professor of planetary science at Caltech and an author of the study. The feature arose more than four billion years ago, before the rest of the planet's complex geologic history was superimposed.

Scientists had previously discounted the idea that a single, giant impactor could have created the lower elevations and thinner crust of Mars's northern region, says Margarita Marinova, a graduate student in Caltech's Division of Geological and Planetary Sciences (GPS) and lead author of the study, which appears June 26 in the journal Nature. This special issue of the journal features a trio of papers on the Mars dichotomy.

For one thing, Marinova explains, it was thought that a single impact would leave a circular footprint, but the outline of the northern lowlands region is elliptical. There is also a distinct lack of a crater rim: topography increases smoothly from the lowlands to the highlands without a lip of concentrated material in between, as is the case in small craters. Finally, it was believed that a giant impactor would obliterate the record of its own occurrence by melting a large fraction of the planet and forming a magma ocean.

"We set out to show that it's possible to make a big hole without melting the majority of the surface of Mars," Aharonson says. The team modeled a range of projectile parameters that could yield a cavity the size and ellipticity of the Mars lowlands without melting the whole planet or making a crater rim.

After cranking 500 simulations combining various energies, velocities, and impact angles through the GPS division's Beowulf-class computer cluster CITerra, the researchers narrowed in on a "sweet spot"--a range of single-impact parameters that would make exactly the type of crater found on Mars. Although a large impact had been suggested (and discounted) in the past, Aharonson says, computers weren't fast enough to run the models. "The ability to search for parameters that allow an impact compatible with observations is enabled by the dedicated machine at Caltech," he adds.

The favored simulation conditions outlined by the sweet spot suggest an impact energy of around 10 to the 29 joules, which is equivalent to 100 billion gigatons of TNT. The impactor would have hit Mars at an angle between 30 and 60 degrees while traveling at 6 to 10 kilometers per second. By combining these factors, Marinova calculated that the projectile was roughly 1,600 to 2,700 kilometers across.

Estimates of the energy of the Mars impact place it squarely between the impact that is thought to have led to the extinction of dinosaurs on Earth 65 million years ago and the one believed to have extruded our planet's moon four billion years ago.

Indeed, the timing of formation of our moon and the Mars dichotomy is not coincidental, Marinova notes. "This size range of impacts only occurred early in solar system history," she says. The results of this study are also applicable to understanding large impact events on other heavenly bodies, like the Aitken Basin on the moon and the Caloris Basin on Mercury.

The Caltech study comes at a time of renewed interest in the ancient crustal feature on Mars, Aharonson notes. Also in this issue of Nature, Jeffrey Andrews-Hanna and Maria Zuber of MIT and Bruce Banerdt of JPL examine the gravitational and topographic signature of the dichotomy with information from the Mars orbiters. Another accompanying report, from a group at UC Santa Cruz led by Francis Nimmo, explores the expected consequences of mega-impacts.

The other author on this study is Erik Asphaug, a professor of earth and planetary sciences at UC Santa Cruz. 

Writer: 
Elisabeth Nadin
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Stress Buildup Precedes Large Sumatra Quakes

PASADENA, Calif.--The island of Sumatra, Indonesia, has shaken many times with powerful earthquakes since the one that wrought the infamous 2004 Indian Ocean tsunami. Now, scientists from the California Institute of Technology and the Indonesian Institute of Sciences are harnessing information from these and earlier quakes to determine where the next ones will likely occur, and how big they will be.

Mohamed Chlieh, the lead author of a new report, looked at the region during his postdoctoral studies at Caltech with Jean-Philippe Avouac, professor of geology and director of Caltech's Tectonics Observatory (TO) and Kerry Sieh, Sharp Professor of Geology. They found that in the time between great earthquakes, some portions of the fault zone locked up while others crept along steadily, and the portions that were locked in the past few decades coincided with portions that rupture to produce large-magnitude quakes. The correlation was especially strong for two temblors of magnitude 8.7 that struck the region in 1861 and again in 2005.

The study also reveals which part of the Sumatra megathrust is storing strain that will be released during future large earthquakes.

Earthquakes in Sumatra are the manifestation of a sudden release of strain that constantly builds as the plates beneath the Indian Ocean creeps steadily toward southeast Asia and dive into the subduction zone under the island. If the total tectonic plate motion in the region is not taken up by fault slip during earthquakes, then a deficit builds until the next earthquake rupture. The patch of the fault where slip is greatest during an earthquake and releases the most pent-up strain, known as an asperity, also gets stuck between quakes. The scientists were interested in what was happening at the land surface, above these asperities, between big earthquakes.

Investigations by Caltech scientists in the region began when Sieh and his students started documenting the history of subsidence and emergence of the islands offshore Sumatra using the record provided by coral heads. Later on, a network of geodetic stations was deployed by the TO. To measure how strain built up in the calm interseismic period between earthquakes, Chlieh and his colleagues analyzed GPS measurements collected since 1991 and annual banding in corals from the past 50 years. Coral growth bands indicate vertical land motion because as the seafloor on which corals live shifts down or up, the creatures either grow to chase sunlight from below water or die back when elevated above water. Both the bands and the GPS data record small land-position shifts in interseismic periods. In contrast, they show drastic shifts during an earthquake, as the corals typically die when they are thrust high enough above or sunk too deep below sea level to survive.

The data provide a record of unevenly distributed deformation of the land surface directly above the subduction zone during the interseismic period. Modeling further indicates that this results from the asperities along the plate interface, while other parts remain smoothly slipping. These interseismic asperities are 10 times as wide--up to 175 kilometers--in the region where great earthquakes have occurred in the past.

"Our model shows asperities exactly at the same places that the 2005 Nias and the 1797 and 1833 earthquakes in the Mentawai islands occurred, indicating that aperities seem to be persistent features from one seismic cycle to another," Chlieh remarks. Avouac adds, "This is clear indication that the characteristics of large earthquakes are somewhat determined by properties of the plate interface that can be gauged in advance from measuring interseismic deformation.

"A priori, large earthquakes should not be expected where the plate interface is creeping, but are inescapable where it is locked. So it seems that we can, with interseismic observations, see these asperities before the earthquake occurs," he says. "The question now is, 'How well are we able to estimate the characteristics of the earthquakes that these asperities could produce?'"

The implications of the study are major, according to Chlieh. "Using the asperity locations, we may be able to construct some more realistic earthquake and tsunami models following different scenarios. Then we will have a good idea of the risk induced by these locked fault zones."

The study appears in the May issue of the Journal of Geophysical Research. Other authors on the paper are Danny Natawidjaja, a former Caltech grad student who is now at the Indonesian Institute of Sciences, and John Galetzka, staff geodesist with the TO.

Abstract: http://www.agu.org/pubs/crossref/2008/2007JB004981.shtml 

Writer: 
Elisabeth Nadin
Writer: 

Partnerships of Deep-Sea Methane Scavengers Revealed

PASADENA, Calif.--The sea floor off the coast of Eureka, California, is home to a diverse assemblage of microbes that scavenge methane from cold deep-sea vents. Researchers at the California Institute of Technology have developed a technique to directly capture these cells, lending insight into the diverse symbiotic partnerships that evolved among different species in an extreme environment.

The community's interconnected metabolism sheds light on how the anaerobic microbes, which consume nearly 80 percent of the methane leaked from marine sediments, limit oceanic emissions of this potent greenhouse gas.

"Ninety-nine percent of what's out there we can't grow in the lab, including these methane-oxidizing organisms," says Victoria Orphan, an assistant professor of geobiology at Caltech in whose lab the cell sampling technique was developed.

"We know from ribosomal RNA studies that there is a lot of microbial diversity in nature, but we don't know what the vast majority of microbes are doing," Orphan adds. "We needed a method for separating specific organisms out of complex environments."

Metagenomic analysis, in which the genetic material of all microorganisms swept from their homes in a sample is sequenced wholesale, yields a plenitude of general information. Annelie Pernthaler, a former Caltech postdoc who is now a research scientist at the Centre for Environmental Research in Leipzig, Germany, and Orphan devised a technique to tease out individuals from the diverse microbial community of the deep-sea sediment. Their aim: to simplify the genomic sequencing to target only the organisms they were interested in.

They began with descents in the manned submersible Alvin, collecting cores of sea-floor sediment from areas where methane migrates from below. Back in the lab, the team used enzyme-tagged short DNA probes to specifically bind the ribosomal RNA in the methane-consuming microbes of the sediment. A second reaction used the enzyme to deposit fluorescent molecules within and around the cell, a method known as CARD-FISH, for "catalyzed reporter deposition fluorescence in situ hybridization."

The fluorescing cells and attached microorganisms were captured using microbeads that are both paramagnetic--a form of magnetism occurring only in the presence of an externally applied magnetic field--and coated with an antibody to the fluorescent molecule. This Caltech-patented technique, called "magneto-FISH," bypasses the need to grow the microorganisms in culture because it targets the fluorescing molecules around the cell instead of a specific molecule within the cell.

The cells separated by magneto-FISHing reveal who's partnered up with whom, and provides a fresh look at microbial symbiosis in nature, Orphan says. The main player near the methane vents is a methane-metabolizing member of the Archaea, a prokaryotic domain of life distinct from both bacteria and eukaryotes. Piggybacked on the archaeal cells are some members from among four different species of bacteria--three more than were previously known to be associated with these particular archaea--whose exact roles in the system can now be addressed.

The methane-vent partnership between archaea that consume methane and bacteria that reduce sulfate is believed to be a form of cometabolism or syntrophy, meaning "feeding together," where one species lives off the metabolic products of others. Using the information obtained from the metagenome of these partnerships, says Orphan, biologists can develop specific experiments to directly test the physiological and nutritional relationships between these organisms, as well as the ecological strategies used to successfully colonize deep-sea environments.

One example of such an experiment is highlighted in the group's study, published May 8 in the early online edition of the journal PNAS. The researchers discovered that the organisms possess genes for nitrogen fixation, a process that converts nitrogen gas into nourishing compounds like ammonia. "We were surprised to see these genes in the captured cells," says Anne Dekas, a geobiology graduate student at Caltech, "because we thought these organisms were relatively energy-starved, and nitrogen fixation takes a lot of energy."

Orphan and Dekas were able to show that the organisms are not just equipped for the task, they are actually carrying it out. "Showing nitrogen fixation is a great finding in itself," Dekas comments, "but it is also just one example of the hypothesis testing that can follow magneto-FISH coupled to metagenomic analysis."

Other authors on the study are Caltech's C. Titus Brown, a postdoc in biology; Shana Goffredi, a senior research fellow in environmental science and engineering; and Tsegereda Embaye, a technician in the division of geological and planetary sciences.

Writer: 
Elisabeth Nadin
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