Scientists Find a Martian Igneous Rock that is Surprisingly Earth-like

During the nearly 14 months that it has spent on the red planet, Curiosity, the Mars Science Laboratory (MSL) rover, has scooped soil, drilled rocks, and analyzed samples by exposing them to laser beams, X-rays, and alpha particles using the most sophisticated suite of scientific instruments ever deployed on another planet. One result of this effort was evidence reported last March that ancient Mars could have supported microbial life.

But Curiosity is far more than a one-trick rover, and in a paper published today in the journal Science, a team of MSL scientists reports its analysis of a surprisingly Earth-like martian rock that offers new insight into the history of Mars's interior and suggests parts of the red planet may be more like our own than we ever knew.  

The paper—whose lead author is Edward Stolper, Caltech's William E. Leonhard Professor of Geology, provost, and interim president—is one of five appearing in the journal with results from the analysis of data and observations obtained during Curiosity's first 100 martian days (sols). The other papers include an evaluation of fine- and coarse-grained soil samples and detailed analyses of the composition and formation process of a windblown drift of sand and dust.

"The results presented go beyond the question of habitability," says John Grotzinger, MSL project scientist and Caltech's Fletcher Jones Professor of Geology. "Mars Science Laboratory also has a major mission objective to explore and characterize the geological environment at all scales and also the atmosphere. In doing this we learn about the fundamental physical and chemical properties that distinguish the terrestrial planets from each other and also what they share in common."

The paper by Stolper and his colleagues—including Caltech senior research scientist Michael Baker and graduate student Megan Newcombe—examines in detail a 50-centimeter-tall pyramid-shaped rock named "Jake_M" (after MSL surface operations systems chief engineer Jacob "Jake" Matijevic, who passed away two weeks after Curiosity's landing).

The rock was encountered by Curiosity a few weeks after it landed, during its slow drive across Gale Crater on the way toward the crater's central peak, Mount Sharp. Visual inspection of the dark gray rock suggested that it was probably a fine-grained basaltic igneous rock formed by the crystallization of magma near the planet's surface. The absence of obvious mineral grains on its essentially dust-free surface further suggested that it would have a relatively uniform (i.e., homogeneous) chemical composition.

For that reason, MSL's scientists decided it would be a good test case for comparing the results obtained by two of the rover's scientific instruments, the Alpha Particle X-ray Spectrometer (APXS) and ChemCam, both of which are used to measure the chemical compositions of rocks, sediments, and minerals.

The APXS analyses, however, produced some unanticipated results. Far from being similar in its chemical composition to the many martian igneous rocks analyzed by the Spirit and Opportunity rovers on the surface of Mars or to martian meteorites found on Earth, Jake_M is highly enriched in sodium and potassium, making it chemically alkaline.

Although Jake_M is very different from known martian rocks, Stolper and colleagues realized that it is very similar in its chemical composition to a relatively rare type of terrestrial igneous rock, known as a mugearite, which is typically found on ocean islands and in continental rift zones.

"We realized right away that although nothing like it had ever been found on Mars, Jake_M is similar in composition to terrestrial mugearites, which although uncommon are very well known to igneous petrologists who study volcanic rocks on Earth," Stolper says. "In fact, if this rock were found on Earth, we would be hard pressed, based on its elemental composition, to tell it was not an Earth rock." However, he notes, "such rocks are so uncommon on Earth that it would be highly unlikely that, if you landed a spacecraft on Earth in a random location, the first rock you encountered within a few hundred meters of your landing site would be an alkaline rock like Jake_M."

On both Earth and Mars, basaltic liquids form by partial melting of rocks deep inside the planet. By analogy with terrestrial mugearites, Jake_M probably evolved from such a partial melt that cooled as it ascended toward the surface from the martian interior; as it cooled, crystals formed, and the chemical composition of the remaining liquid changed (just as, in the making of rock candy, a sugar-water solution becomes less sweet as it cools and sugar crystallizes from it).

"The minerals that crystallize have different elemental compositions than the melt and are either more dense or less dense than the liquid and thus tend to physically separate, that is, to settle to the bottom of the magma chamber or float to the top, causing the chemical composition of the remaining liquid to change," Baker explains.  

The MSL team then modeled the conditions required to produce a residual liquid similar in composition to Jake_M by crystallization of plausible partial melts. From those results, they inferred that the cooling and crystallization that eventually produced Jake_M probably occurred at pressures of several kilobars, the equivalent of the pressure at a depth of a few tens of kilometers beneath the martian surface. The modeling also suggested—particularly by analogy with terrestrial mugearites—that the martian magmas were relatively rich in dissolved water.

According to Stolper, Baker, and their colleagues, Jake_M probably originated via the melting of a relatively alkali- and water-rich martian mantle that was different from the sources of other known martian basalts. Because the primitive martian mantle is believed to have been as much as two times richer in sodium and potassium than Earth's mantle, the researchers say that, in hindsight, it might not be surprising if alkaline magmas, which are so uncommon on Earth, are more common on Mars.

Moreover, Stolper adds, "there are many hypotheses for origin of alkaline magmas on Earth that are similar to Jake_M. Perhaps the most plausible is that regions deep in the mantle become enriched in alkalis by a process known as metasomatism, in which the chemical compositions of rocks are altered by the flow of water- and carbon-dioxide-rich fluids. The existence of Jake_M may be evidence that such processes also occur in the interior of Mars."

Intriguingly, the potassium-rich nature of many of the sedimentary rocks that have been analyzed by the MSL mission may turn out to reflect the presence of such a region enriched in alkalis in the mantle underlying Gale Crater.

However, he says, "with only one rock having this odd chemical composition, we don't want to get carried away. Is it a one-off, or is it a representative of an important class of igneous rocks from the Gale Crater region? Determining the answer to this will be an important goal for the ongoing MSL mission."

"The paper by Stolper et al. shows that the internal composition of Mars is more similar to Earth than we had thought and illustrates how even a single rock can provide insight into the evolution of the planet as a whole," Grotzinger says.

The work in the paper, "The Petrochemistry of Jake_M: A Martian Mugearite," was supported by grants from the National Science Foundation, the National Aeronautics and Space Administration, the Canadian Space Agency, and the Centre National d'Études Spatiales.

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New Gut Bacterium Discovered in Termite's Digestion of Wood

Caltech researchers find new species of microbe responsible for acetogenesis, an important process in termite nutrition.

When termites munch on wood, the small bits are delivered to feed a community of unique microbes living in their guts, and in a complex process involving multiple steps, these microbes turn the hard, fibrous material into a nutritious meal for the termite host. One key step uses hydrogen to convert carbon dioxide into organic carbon—a process called acetogenesis—but little is known about which gut bacteria play specific roles in the process. Utilizing a variety of experimental techniques, researchers from the California Institute of Technology (Caltech) have now discovered a previously unidentified bacterium—living on the surface of a larger microorganism in the termite gut—that may be responsible for most gut acetogenesis.

"In the termite gut, you have several hundred different species of microbes that live within a millimeter of one another. We know certain microbes are present in the gut, and we know microbes are responsible for certain functions, but until now, we didn't have a good way of knowing which microbes are doing what," says Jared Leadbetter, professor of environmental microbiology at Caltech, in whose laboratory much of the research was performed. He is also an author of a paper about the work published the week of September 16 in the online issue of the Proceedings of the National Academy of Sciences (PNAS).

Acetogenesis is the production of acetate (a source of nutrition for termites) from the carbon dioxide and hydrogen generated by gut protozoa as they break down decaying wood. In their study of "who is doing what and where," Leadbetter and his colleagues searched the entire pool of termite gut microbes to identify specific genes from organisms responsible for acetogenesis.

The researchers began by sifting through the microbes' RNA—genetic information that can provide a snapshot of the genes active at a certain point in time. Using RNA from the total pool of termite gut microbes, they searched for actively transcribed formate dehydrogenase (FDH) genes, known to encode a protein necessary for acetogenesis. Next, using a method called multiplex microfluidic digital polymerase chain reaction (digital PCR), the researchers sequestered the previously unstudied individual microbes into tiny compartments to identify the actual microbial species carrying each of the FDH genes. Some of the FDH genes were found in types of bacteria known as spirochetes—a previously predicted source of acetogenesis. Yet it appeared that these spirochetes alone could not account for all of the acetate produced in the termite gut.

Initially, the Caltech researchers were unable to identify the microorganism expressing the single most active FDH gene in the gut. However, the first authors on the study, Adam Rosenthal, a postdoctoral scholar in biology at Caltech, and Xinning Zhang (PhD '10, Environmental Science and Engineering), noticed that this gene was more abundant in the portion of the gut extract containing wood chunks and larger microbes, like protozoans. After analyzing the chunkier gut extract, they discovered that the single most active FDH gene was encoded by a previously unstudied species from a group of microbes known as the deltaproteobacteria. This was the first evidence that a substantial amount of acetate in the gut may be produced by a non-spirochete.

Because the genes from this deltaproteobacterium were found in the chunky particulate matter of the termite gut, the researchers thought that perhaps the newly identified microbe attaches to the surface of one of the chunks. To test this hypothesis, the researchers used a color-coded visualization method called hybridization chain reaction-fluorescent in situ hybridization, or HCR-FISH.

The technique—developed in the laboratory of Niles Pierce, professor of applied and computational mathematics and bioengineering at Caltech, and a coauthor on the PNAS study—allowed the researchers to simultaneously "paint" cells expressing both the active FDH gene and a gene identifying the deltoproteobacterium with different fluorescent colors simultaneously. "The microfluidics experiment suggested that the two colors should be expressed in the same location and in the same tiny cell," Leadbetter says. And, indeed, they were. "Through this approach, we were able to actually see where the new deltaproteobacterium resided. As it turns out, the cells live on the surface of a very particular hydrogen-producing protozoan."

This association between the two organisms makes sense based on what is known about the complex food web of the termite gut, Leadbetter says. "Here you have a large eukaryotic single cell—a protozoan—which is making hydrogen as it degrades wood, and you have these much smaller hydrogen-consuming deltaproteobacteria attached to its surface," he says. "So, this new acetogenic bacterium is snuggled up to its source of hydrogen just as close as it can get."

This intimate relationship, Leadbetter says, might never have been discovered relying on phylogenetic inference—the standard method for matching a function to a specific organism. "Using phylogenetic inference, we say, 'We know a lot about this hypothetical organism's relatives, so without ever seeing the organism, we're going to make guesses about who it is related to," he says. "But with the techniques in this study, we found that our initial prediction was wrong. Importantly, we have been able to determine the specific organism responsible and a location of the mystery organism, both of which appear to be extremely important in the consumption of hydrogen and turning it into a product the insect can use." These results not only identify a new source for acetogenesis in the termite gut—they also reveal the limitations of making predictions based exclusively on phylogenetic relationships.

Other Caltech coauthors on the paper titled "Localizing transcripts to single cells suggests an important role of uncultured deltaproteobacteria in the termite gut hydrogen economy," are graduate student Kaitlyn S. Lucey (environmental science and engineering), Elizabeth A. Ottesen (PhD '08, biology), graduate student Vikas Trivedi (bioengineering), and research scientist Harry M. T. Choi (PhD '10, bioengineering). This work was funded by the U.S. Department of Energy, the National Science Foundation, the National Institutes of Health, the Programmable Molecular Technology Center within the Beckman Institute at Caltech, a Donna and Benjamin M. Rosen Center Bioengineering scholarship, and the Center for Environmental Microbial Interactions at Caltech.

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Bruce Murray

1931 – 2013

Bruce Churchill Murray, Caltech Professor of Planetary Science, Emeritus, and former head of NASA's Jet Propulsion Laboratory (JPL), succumbed to complications of Alzheimer's disease on August 29, 2013. He was 81.

A founder of planetary science, Murray transformed our understanding of the solar system by applying his geologic training to the study of other worlds. He played a key role in equipping JPL's first Mars missions with cameras, an idea often dismissed at the time as a public relations gimmick; and as JPL's director during the Carter and Reagan administrations, he rescued the Galileo mission to Jupiter and Voyager 2's flybys of Uranus and Neptune from the budget ax.

Murray was born on November 30, 1931, in New York City. His parents moved to California soon after, and he graduated from Santa Monica High in 1949. Upon being refused admission to Caltech—"I got a D in physics my senior year," he explained in his oral history—he enrolled at MIT, emerging in 1955 with a PhD in geology. He married his first wife, Joan O'Brien, in 1954.

After graduating, Murray logged offshore drilling cores for the Standard Oil Company until 1958, when he entered the Air Force to fulfill his ROTC commitment. Assigned to Hanscom Field in Bedford, Massachusetts, Murray helped make the high-precision calculations of the geoid—Earth's gravitational field—needed to aim our ballistic missiles accurately.

When Murray's hitch ended in 1960, Caltech geochemist Harrison Brown hired him as a research fellow to study meteorites. Murray's interests were broader; in 1961, he, Kenneth Watson (MS '59, PhD '64), and Brown showed that significant water might remain on the dust-dry moon, preserved in the permanently shadowed areas near the poles—a prediction not confirmed until 2009. "It was a revolutionary idea," says David Stevenson, Caltech's Marvin J. Goldberger Professor of Planetary Science. "Bruce was the first person to think about the role of ice in the solar system, which is now absolutely fundamental to our understanding of, for example, the moons of Jupiter and Saturn."

A casual conversation would set the course of Murray's career. Soon after arriving on campus, he flew out to Dallas to give his final paper on the geoid. His seatmate led the "special projects" branch out at China Lake, a rocket lab that Caltech and the Navy had set up during World War II, and the special project was a top-secret, highly sensitive infrared detector for the heat-seeking Sidewinder missile. (Infrared light has longer wavelengths than visible light and is emitted by all warm objects.) Murray mentioned a desire to make infrared observations of the moon, "and he said, 'Why don't you come up and see us sometime?' I still had [my security] clearance . . . so I did."

Murray took master instrument builder Jim Westphal (then a senior engineer in geology; later a professor of planetary science) with him. Westphal had no security clearance, so they couldn't discuss the detector in any way. Instead, the Caltech duo specified the hardware and electronics needed to mate a detector to a telescope, and the missile men used those specs to build a "black box" containing the detector—on the condition, of course, that the box remained unopened.

The box blew the socks off civilian technology, and in December 1962, Murray, Westphal, and postdoc Robert Wildey (BS '57, MS '58, PhD '62) mounted it on the 200-inch Hale Telescope at Palomar Observatory to look at Venus while JPL's Mariner 2 made the first-ever flyby of another planet. Venus's cloud-covered face is a blank disk, and the spacecraft carried no camera. Instead, an infrared radiometer would construct a temperature profile of the Venusian atmosphere by scanning perpendicularly across the planet's edge. The plan was simply to check the radiometer data; however, the Hale is designed to peer into deepest space, and Venus is so close that a tiny patch of its surface filled the field of view. Unsure of the radiometer's exact aim, Murray and company methodically scanned the entire planet, recording their readings on a roll of chart paper. Discovering that the scans weren't uniform, they cut them up and laid them side by side across a circle drawn to represent Venus. The resulting temperature map had two cold spots on opposite edges of the circle—Venus's north and south poles—revealing the planet's rotational axis for the first time.

JPL's Mariner 4 did carry a camera on the first-ever flyby of Mars in July 1965. Caltech astronomer Robert Leighton (BS '41, MS '44, PhD '47) led the so-called television experiment team, which included Robert Sharp (BS '34, MS '35), chair of the geology division and an expert on landforms, and Murray, by now Caltech's first associate professor of planetary science. Creating a single image takes an awful lot of pixels, and the camera had to share downlink time with the "real" data—the numbers being returned by the other six instruments. The engineers wanted to minimize the transmission load by encoding the images at 3 bits per pixel, or eight shades of gray. Murray insisted on 8 bits (256 shades)—not exactly high-def, but enough to show geologically interesting details. The resulting pictures of a heavily cratered moonscape destroyed the notion of Mars as a habitable, Earthlike world.

Murray and Leighton then used Mariner's atmospheric data to calculate the regional heat exchange between the surface of Mars and its atmosphere. They found that the poles got cold enough each winter to freeze the carbon-dioxide atmosphere into seasonal caps of dry ice, another revolutionary idea that has since proven true—Mars's atmospheric density varies by some 25 percent over the course of its year.

Murray continued exploring Mars as a TV team member on the Mariner 6 and 7 flybys of 1969 and on Mariner 9, which in 1971 became the first spacecraft to orbit another planet, photographing the entire globe in fine detail—eventually, that is. This Mariner arrived at Mars during the thickest dust storm ever seen, as Murray recounted in his oral history. In the weeks that followed, "there was a gradual clearing, like a stage scene, and three dark spots showed up." As the dust continued to settle, the spots developed into huge craters and slowly became the summits of volcanoes—each one some 10 miles tall and as wide as the state of Missouri. Other photos revealed a rift system, the Valles Marineris, long enough to stretch from San Francisco to Baltimore.

Murray revisited the inner solar system in 1974 as the TV team leader for Mariner 10, which flew by Venus and hitherto unexplored Mercury. By then, JPL could make "color" images by compositing pictures taken through various filters mounted on a wheel in front of the camera. Recent ultraviolet observations of Venus from Earth had found a faint, fast-moving feature resembling a sideways 'Y' that seemed to circle the planet every four days, so Murray persuaded JPL to add an ultraviolet segment to Mariner 10's filter wheel. These ultraviolet images—thousands of them—documented an exquisitely complex collection of markings that traced the circulation patterns in Venus's upper atmosphere.

From April 1, 1976 to June 30, 1982, Murray served as director of JPL—a tenure that balanced the highs of the Viking landings on Mars and the Voyagers' flights by Jupiter and Saturn against the constant battles with Washington to keep the Lab afloat. "He was like Lyndon Johnson. He pushed people hard, and he expressed his opinions forcefully," says Professor of Planetary Science Andrew Ingersoll, the "weatherman" on the Voyagers' atmospheric-science team. "But he never held a grudge; after a disagreement he would just forge ahead. And that's the key: he did forge."

Murray mobilized public support for JPL by cofounding The Planetary Society with Carl Sagan and Louis Friedman in 1980, serving variously as vice president, president, and chairman. "Nowadays, NASA expects every mission to do public outreach. They set aside money for it in the budget, and you have to do it," says David Stevenson. "Back then, it was not required, but Bruce felt very strongly that it was the right thing to do."

"Bruce was really dedicated to exploring the frontiers," concurs Ed Stone, the David Morrisroe Professor of Physics, director of JPL from 1991 to 2001 and Voyager's project scientist. "He recognized the power of scientific imaging, and the opportunity it provided to engage the public."

Murray was a fellow of the American Academy of Arts & Sciences and the American Association for the Advancement of Science, and a member of the American Astronomical Society and the American Geophysical Union. One of his books, Journey Into Space, won two awards for science writing. His other honors include NASA's Exceptional Scientific Achievement Medal, NASA's Distinguished Public Service Medal, and two NASA Distinguished Service Medals.

He is survived by Suzanne, his wife of 41 years; five children; and 10 grandchildren.

A public memorial service will be held at 2:00 p.m. on Sunday, November 10, at the Caltech Athenaeum. 


Douglas Smith
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Friday, October 4, 2013

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Monday, August 12, 2013
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Magnetic Fields: A Window to a Planet's Interior and Habitability

Seeing Snow in Space

Caltech helps capture the first image of a frosty planetary-disk region

Although it might seem counterintuitive, if you get far enough away from a smoldering young star, you can actually find snow lines—frosty regions where gases are able to freeze and coat dust grains. Astronomers believe that these snow lines are critical to the process of planet formation.

Now an international team of researchers, including Caltech's Geoffrey Blake, has used the Atacama Large Millimeter/submillimeter Array (ALMA) to capture the first image of a snow line around a Sun-like star. The findings appear in the current issue of Science Express.

"This first direct imaging of such internal chemical structures in an analog of the young solar nebula was made possible by the extraordinary sensitivity and resolution of the not-yet-completed ALMA and builds on decades of pioneering research in millimeter-wave interferometry at the Caltech Owens Valley Radio Observatory, by universities now part of the Combined Array for Research in Millimeter-wave Astronomy, and by the Harvard-Smithsonian Submillimeter Array," says Blake, a professor of cosmochemistry and planetary science and professor of chemistry at Caltech. "The role of these facilities, in research, in technology development, and in education, along the road to ALMA cannot be overstated."

Since different gases freeze at different distances from the star, snow lines are thought to exist as concentric rings of grains encased in the various frozen gases—a ring of grains coated with water ice, a ring of grains coated with carbon dioxide, and so on. They might speed up planet formation by providing a source of solid material and by coating and protecting dust grains that would normally collide with one another and break apart.

Earlier this year, Blake and his group used spectrometers onboard the Spitzer Space Telescope and Herschel Space Observatory to constrain the location of the water snow line in a star known as TW Hydrae. The star is of particular interest because it is the nearest example of a gas- and dust-rich protoplanetary disk that may show similarities to our own solar system at an age of only 10 million years.

Snow lines have escaped direct imaging up until this point because of the obscuring effect of the hot gases that exist above and below them. But thanks to work at the Harvard-Smithsonian Submillimeter Array and at Caltech, the team had a good idea of where to begin looking. Additionally, the lead authors of the new paper, Chunhua "Charlie" Qi (PhD '01), now of the Harvard-Smithsonian Center for Astrophysics, and Karin Öberg (BS '05), currently at Harvard University, figured out a novel way to trace the presence of frozen carbon monoxide—a trick that enabled them to use ALMA to chemically highlight TW Hydrae's carbon monoxide snow line.

"The images from ALMA spectacularly confirm the presence of snow lines in disks," Blake says. "We are eagerly looking forward to additional studies with the full ALMA telescope—especially those targeting less volatile species such as water and organics that are critical to habitability."

The paper is titled "Imaging of the CO snow line in a solar nebula analog." A full press release about the work can be found here.

Kimm Fesenmaier
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Evidence for a Martian Ocean

Researchers at the California Institute of Technology (Caltech) have discovered evidence for an ancient delta on Mars where a river might once have emptied into a vast ocean.

This ocean, if it existed, could have covered much of Mars's northern hemisphere—stretching over as much as a third of the planet.

"Scientists have long hypothesized that the northern lowlands of Mars are a dried-up ocean bottom, but no one yet has found the smoking gun," says Mike Lamb, an assistant professor of geology at Caltech and a coauthor of the paper describing the results. The paper was published online in the July 12 issue of the Journal of Geophysical Research.

Although the new findings are far from proof of the existence of an ancient ocean, they provide some of the strongest support yet, says Roman DiBiase, a postdoctoral scholar at Caltech and lead author of the paper.

Most of the northern hemisphere of Mars is flat and at a lower elevation than the southern hemisphere, and thus appears similar to the ocean basins found on Earth. The border between the lowlands and the highlands would have been the coastline for the hypothetical ocean.

The Caltech team used new high-resolution images from the Mars Reconnaissance Orbiter (MRO) to study a 100-square-kilometer area that sits right on this possible former coastline. Previous satellite images have shown that this area—part of a larger region called Aeolis Dorsa, which is about 1,000 kilometers away from Gale Crater where the Curiosity rover is now roaming—is covered in ridge-like features called inverted channels.

These inverted channels form when coarse materials like large gravel and cobbles are carried along rivers and deposited at their bottoms, building up over time. After the river dries up, the finer material—such as smaller grains of clay, silt, and sand—around the river erodes away, leaving behind the coarser stuff. This remaining sediment appears as today's ridge-like features, tracing the former river system.

When looked at from above, the inverted channels appear to fan out, a configuration that suggests one of three possible origins: the channels could have once been a drainage system in which streams and creeks flowed down a mountain and converged to form a larger river; the water could have flowed in the other direction, creating an alluvial fan, in which a single river channel branches into multiple smaller streams and creeks; or the channels are actually part of a delta, which is similar to an alluvial fan except that the smaller streams and creeks empty into a larger body of water such as an ocean.

To figure out which of these scenarios was most likely, the researchers turned to satellite images taken by the HiRISE camera on MRO. By taking pictures from different points in its orbit, the spacecraft was able to make stereo images that have allowed scientists to determine the topography of the martian surface. The HiRISE camera can pick out features as tiny as 25 centimeters long on the surface and the topographic data can distinguish changes in elevation at a resolution of 1 meter.

Using this data, the Caltech researchers analyzed the stratigraphic layers of the inverted channels, piecing together the history of how sediments were deposited along these ancient rivers and streams. The team was able to determine the slopes of the channels back when water was still coursing through them. Such slope measurements can reveal the direction of water flow—in this case, showing that the water was spreading out instead of converging, meaning the channels were part of an alluvial fan or a delta.

But the researchers also found evidence for an abrupt increase in slope of the sedimentary beds near the downstream end of the channels. That sort of steep slope is most common when a stream empties into a large body of water—suggesting that the channels are part of a delta and not an alluvial fan.

Scientists have discovered martian deltas before, but most are inside a geological boundary, like a crater. Water therefore would have most likely flowed into a lake enclosed by such a boundary and so did not provide evidence for an ocean.

But the newly discovered delta is not inside a crater or other confining boundary, suggesting that the water likely emptied into a large body of water like an ocean. "This is probably one of the most convincing pieces of evidence of a delta in an unconfined region—and a delta points to the existence of a large body of water in the northern hemisphere of Mars," DiBiase says. This large body of water could be the ocean that has been hypothesized to have covered a third of the planet. At the very least, the researchers say, the water would have covered the entire Aerolis Dorsa region, which spans about 100,000 square kilometers.

Of course, there are still other possible explanations. It is plausible, for instance, that at one time there was a confining boundary—such as a large crater—that was later erased, Lamb adds. But that would require a rather substantial geological process and would mean that the martian surface was more geologically active than has been previously thought.

The next step, the researchers say, is to continue exploring the boundary between the southern highlands and northern lowlands—the hypothetical ocean coastline—and analyze other sedimentary deposits to see if they yield more evidence for an ocean. 

"In our work and that of others—including the Curiosity rover—scientists are finding a rich sedimentary record on Mars that is revealing its past environments, which include rain, flowing water, rivers, deltas, and potentially oceans," Lamb says. "Both the ancient environments on Mars and the planet's sedimentary archive of these environments are turning out to be surprisingly Earth-like."

The title of the Journal of Geophysical Research paper is "Deltaic deposits at Aeolis Dorsa: Sedimentary evidence for a standing body of water on the northern plains of Mars." In addition to DiBiase and Lamb, the other authors of the paper are graduate students Ajay Limaye and Joel Scheingross, and Woodward Fischer, assistant professor of geobiology. This research was supported by the National Science Foundation, NASA, and Caltech.

Marcus Woo
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Thursday, September 26, 2013

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A Stepping-Stone for Oxygen on Earth

Caltech researchers find evidence of an early manganese-oxidizing photosystem

For most terrestrial life on Earth, oxygen is necessary for survival. But the planet's atmosphere did not always contain this life-sustaining substance, and one of science's greatest mysteries is how and when oxygenic photosynthesis—the process responsible for producing oxygen on Earth through the splitting of water molecules—first began. Now, a team led by geobiologists at the California Institute of Technology (Caltech) has found evidence of a precursor photosystem involving manganese that predates cyanobacteria, the first group of organisms to release oxygen into the environment via photosynthesis.  

The findings, outlined in the June 24 early edition of the Proceedings of the National Academy of Sciences (PNAS), strongly support the idea that manganese oxidation—which, despite the name, is a chemical reaction that does not have to involve oxygen—provided an evolutionary stepping-stone for the development of water-oxidizing photosynthesis in cyanobacteria.

"Water-oxidizing or water-splitting photosynthesis was invented by cyanobacteria approximately 2.4 billion years ago and then borrowed by other groups of organisms thereafter," explains Woodward Fischer, assistant professor of geobiology at Caltech and a coauthor of the study. "Algae borrowed this photosynthetic system from cyanobacteria, and plants are just a group of algae that took photosynthesis on land, so we think with this finding we're looking at the inception of the molecular machinery that would give rise to oxygen."

Photosynthesis is the process by which energy from the sun is used by plants and other organisms to split water and carbon dioxide molecules to make carbohydrates and oxygen. Manganese is required for water splitting to work, so when scientists began to wonder what evolutionary steps may have led up to an oxygenated atmosphere on Earth, they started to look for evidence of manganese-oxidizing photosynthesis prior to cyanobacteria. Since oxidation simply involves the transfer of electrons to increase the charge on an atom—and this can be accomplished using light or O2—it could have occurred before the rise of oxygen on this planet.

"Manganese plays an essential role in modern biological water splitting as a necessary catalyst in the process, so manganese-oxidizing photosynthesis makes sense as a potential transitional photosystem," says Jena Johnson, a graduate student in Fischer's laboratory at Caltech and lead author of the study.

To test the hypothesis that manganese-based photosynthesis occurred prior to the evolution of oxygenic cyanobacteria, the researchers examined drill cores (newly obtained by the Agouron Institute) from 2.415 billion-year-old South African marine sedimentary rocks with large deposits of manganese.

Manganese is soluble in seawater. Indeed, if there are no strong oxidants around to accept electrons from the manganese, it will remain aqueous, Fischer explains, but the second it is oxidized, or loses electrons, manganese precipitates, forming a solid that can become concentrated within seafloor sediments.

"Just the observation of these large enrichments—16 percent manganese in some samples—provided a strong implication that the manganese had been oxidized, but this required confirmation," he says.

To prove that the manganese was originally part of the South African rock and not deposited there later by hydrothermal fluids or some other phenomena, Johnson and colleagues developed and employed techniques that allowed the team to assess the abundance and oxidation state of manganese-bearing minerals at a very tiny scale of 2 microns.

"And it's warranted—these rocks are complicated at a micron scale!" Fischer says. "And yet, the rocks occupy hundreds of meters of stratigraphy across hundreds of square kilometers of ocean basin, so you need to be able to work between many scales—very detailed ones, but also across the whole deposit to understand the ancient environmental processes at work."

Using these multiscale approaches, Johnson and colleagues demonstrated that the manganese was original to the rocks and first deposited in sediments as manganese oxides, and that manganese oxidation occurred over a broad swath of the ancient marine basin during the entire timescale captured by the drill cores.

"It's really amazing to be able to use X-ray techniques to look back into the rock record and use the chemical observations on the microscale to shed light on some of the fundamental processes and mechanisms that occurred billions of years ago," says Samuel Webb, coauthor on the paper and beam line scientist at the SLAC National Accelerator Laboratory at Stanford University, where many of the study's experiments took place. "Questions regarding the evolution of the photosynthetic pathway and the subsequent rise of oxygen in the atmosphere are critical for understanding not only the history of our own planet, but also the basics of how biology has perfected the process of photosynthesis."

Once the team confirmed that the manganese had been deposited as an oxide phase when the rock was first forming, they checked to see if these manganese oxides were actually formed before water-splitting photosynthesis or if they formed after as a result of reactions with oxygen. They used two different techniques to check whether oxygen was present. It was not—proving that water-splitting photosynthesis had not yet evolved at that point in time. The manganese in the deposits had indeed been oxidized and deposited before the appearance of water-splitting cyanobacteria. This implies, the researchers say, that manganese-oxidizing photosynthesis was a stepping-stone for oxygen-producing, water-splitting photosynthesis.

"I think that there will be a number of additional experiments that people will now attempt to try and reverse engineer a manganese photosynthetic photosystem or cell," Fischer says. "Once you know that this happened, it all of a sudden gives you reason to take more seriously an experimental program aimed at asking, 'Can we make a photosystem that's able to oxidize manganese but doesn't then go on to split water? How does it behave, and what is its chemistry?' Even though we know what modern water splitting is and what it looks like, we still don't know exactly how it works. There is still a major discovery to be made to find out exactly how the catalysis works, and now knowing where this machinery comes from may open new perspectives into its function—an understanding that could help target technologies for energy production from artificial photosynthesis. "

Next up in Fischer's lab, Johnson plans to work with others to try and mutate a cyanobacteria to "go backwards" and perform manganese-oxidizing photosynthesis. The team also plans to investigate a set of rocks from western Australia that are similar in age to the samples used in the current study and may also contain beds of manganese. If their current study results are truly an indication of manganese-oxidizing photosynthesis, they say, there should be evidence of the same processes in other parts of the world.

"Oxygen is the backdrop on which this story is playing out on, but really, this is a tale of the evolution of this very intense metabolism that happened once—an evolutionary singularity that transformed the planet," Fischer says. "We've provided insight into how the evolution of one of these remarkable molecular machines led up to the oxidation of our planet's atmosphere, and now we're going to follow up on all angles of our findings."

Funding for the research outlined in the PNAS paper, titled "Manganese-oxidizing photosynthesis before the rise of cyanobacteria," was provided by the Agouron Institute, NASA's Exobiology Branch, the David and Lucile Packard Foundation, and the National Science Foundation Graduate Research Fellowship program. Joseph Kirschvink, Nico and Marilyn Van Wingen Professor of Geobiology at Caltech, also contributed to the study along with Katherine Thomas and Shuhei Ono from the Massachusetts Institute of Technology.

Katie Neith
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