Wednesday, October 29, 2014
Avery Courtyard

Fall Family Festival

Getting To Know Super-Earths

"If you have a coin and flip it just once, what does that tell you about the odds of heads versus tails?" asks Heather Knutson, assistant professor of planetary science at Caltech. "It tells you almost nothing. It's the same with planetary systems," she says.

For as long as astronomers have been looking to the skies, we have had just one planetary system—our own—to study in depth. That means we have only gotten to know a handful of possible outcomes of the planet formation process, and we cannot say much about whether the features observed in our solar system are common or rare when compared to planetary systems orbiting other stars.

That is beginning to change. NASA's Kepler spacecraft, which launched on a planet-hunting mission in 2009, searched one small patch of the sky and identified more than 4,000 candidate exoplanets—worlds orbiting stars other than our own sun. It was the first survey to provide a definitive look at the relative frequency of planets as a function of size. That is, to ask, 'How common are gas giant planets, like Jupiter, compared to planets that look a lot more like Earth?'

Kepler's results suggest that small planets are much more common than big ones. Interestingly, the most common planets are those that are just a bit larger than Earth but smaller than Neptune—the so-called super-Earths.

However, despite being common in our local corner of the galaxy, there are no examples of super-Earths in our own solar system. Our current observations tell us something about the sizes and orbits of these newly discovered worlds, but we have very little insight into their compositions.

"We are left with this situation where super-Earths appear to be the most common kind of exoplanet in the galaxy, but we don't know what they're made of," says Knutson.

There are a number of possibilities. A super-Earth could be just that: a bigger version of Earth—mostly rocky, with an atmosphere. Then again, it could be a mini-Neptune, with a large rock-ice core encapsulated in a thick envelope of hydrogen and helium. Or it could be a water world—a rocky core enveloped in a blanket of water and perhaps an atmosphere composed of steam (depending on the temperature of the planet).

"It's really interesting to think about these planets because they could have so many different compositions, and knowing their composition will tell us a lot about how planets form," Knutson says. For example, because planets in this size range acquire most of their mass by pulling in and incorporating solid material, water worlds initially must have formed far away from their parent stars, where temperatures were cold enough for water to freeze. Most of the super-Earths known today orbit very close to their host stars. If water-dominated super-Earths turn out to be common, it would indicate that most of these worlds did not form in their present locations but instead migrated in from more distant orbits.

In addition to thinking about exoplanets, Knutson and her students use space-based observatories like the Hubble and Spitzer Space Telescopes to learn more about the distant worlds. For example, the researchers analyze the starlight that filters through a planet's atmosphere as it passes in front of its star to learn about the composition of the atmosphere. Molecular species present in the planet's atmosphere absorb light at particular wavelengths. Therefore, by using Hubble and Spitzer to view the planet and its atmosphere at a number of different wavelengths, the researchers can determine which chemical compounds are present.

To date, nearly two dozen planets have been characterized with this technique. These observations have shown that the enormous gas giant exoplanets known as hot-Jupiters have water, carbon monoxide, hydrogen, helium—and potentially carbon dioxide and methane—in their atmospheres.

However, right now super-Earths are the hot topic. Unfortunately, although hundreds of super-Earths have been found, only a few are close enough and orbiting bright enough stars for astronomers to study in this way using currently available telescopes.

The first super-Earth that the astronomical community targeted for atmospheric studies was GJ 1214b, in the constellation Ophiuchus. Based on its average density (determined from its mass and radius), it was clear from the start that the planet was not entirely rocky. However, its density could be equally well matched by either a primarily water composition or a Neptune-like composition with a rocky core surrounded by a thick gas envelope. Information about the atmosphere could help astronomers determine which one it was: a mini-Neptune's atmosphere should contain lots of molecular hydrogen, while a water world's atmosphere should be water dominated.

GJ 1214b has been a popular target for the Hubble Space Telescope since its discovery in 2009. Disappointingly, after a first Hubble campaign led by researchers at the Harvard-Smithsonian Center for Astrophysics, the spectrum came back featureless—there were no chemical signatures in the atmosphere. After a second set of more sensitive observations led by researchers at the University of Chicago returned the same result, it became clear that a high cloud deck must be masking the signature of absorption from the planet's atmosphere.

"It's exciting to know that there are clouds on the planet, but the clouds are getting in the way of what we actually wanted to know, which is what is this super-Earth made of?" explains Knutson.

Now Knutson's team has studied a second super-Earth: HD 97658b, in the constellation Leo. They report their findings in the current issue of The Astrophysical Journal. The researchers used Hubble to measure the decrease in light when the planet passed in front of its parent star over a range of infrared wavelengths in order to detect small changes caused by water vapor in the planet's atmosphere.

However, again the data came back featureless. One explanation is that HD 97658b is also enveloped in clouds. However, Knutson says, it is also possible that the planet has an atmosphere that is lacking hydrogen. Because such an atmosphere could be very compact, it would make the telltale fingerprints of water vapor and other molecules very small and hard to detect. "Our data are not precise enough to tell whether it's clouds or the absence of hydrogen in the atmosphere that's causing the spectrum to be flat," she says. "This was just a quick first look to give us a rough idea of what the atmosphere looked like. Over the next year, we will use Hubble to observe this planet again in more detail. We hope those observations will provide a clear answer to the current mystery."

It appears that clouds are going to continue to pose a real challenge in studies of super-Earths, so Knutson and other researchers are working to understand the composition of the clouds around these planets and the conditions under which they form. The hope is that they will get to the point where they can predict which worlds will be shrouded in clouds. "If we can then target planets that we think should be cloud-free, that will help us make optimal use of Hubble's time," she says.

Looking to the future, Knutson says there is only one more known super-Earth that can be targeted for atmospheric studies with current telescopes. But new surveys, such as NASA's extended Kepler K2 mission and the Transiting Exoplanet Survey Satellite (TESS), slated for launch in 2017, should identify a large sample of new targets.

Of course, she says, astronomers would love to study exoplanets the size of Earth, but these worlds are just a bit too small and too difficult to observe with Hubble and Spitzer. NASA's James Webb Space Telescope, which is scheduled for launch in 2018, will provide the first opportunity to study more Earth-like worlds. "Super-Earths are at the edge of what we can study right now," Knutson says. "But super-Earths are a good consolation prize—they're interesting in their own right, and they give us a chance to explore new kinds of worlds with no analog in our own solar system."

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Kimm Fesenmaier
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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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Ker Than
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Friday, October 17, 2014
Center for Student Services 360 (Workshop Space)

TA Training: fall make-up session

Tuesday, October 7, 2014
Red Door Cafe

Samba and Salsa Exhibition

Tuesday, October 7, 2014
Center for Student Services 360 (Workshop Space)

Thirty Meter Telescope Groundbreaking and Blessing

Tuesday, October 7, 2014
Center for Student Services 360 (Workshop Space)

Caltech Peer Tutor Training

Wednesday, September 24, 2014
Annenberg Lecture Hall

A chance to meet Pasadena Unified School District Leadership

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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Seeing Protein Synthesis in the Field

Caltech researchers have developed a novel way to visualize proteins generated by microorganisms in their natural environment—including the murky waters of Caltech's lily pond, as in this image created by Professor of Geobiology Victoria Orphan and her colleagues. The method could give scientists insights to how uncultured microbes (organisms that may not easily be grown in the lab) react and adapt to environmental stimuli over space and time.

The visualization technique, dubbed BONCAT (for "bioorthogonal non-canonical amino-acid tagging"), was developed by David Tirrell, Caltech's Ross McCollum–William H. Corcoran Professor and professor of chemistry and chemical engineering. BONCAT uses "non-canonical" amino acids—synthetic molecules that do not normally occur in proteins found in nature and that carry particular chemical tags that can attach (or "click") onto a fluorescent dye. When these artificial amino acids are incubated with environmental samples, like lily-pond water, they are taken up by microorganisms and incorporated into newly formed proteins. Adding the fluorescent dye to the mix allows these proteins to be visualized within the cell.

For example, in the image, the entire microbial community in the pond water is stained blue with a DNA dye; freshwater gammaproteobacteria are labeled with a fluorescently tagged short-chain ribosomal RNA probe, in red; and newly created proteins are dyed green by BONCAT. The cells colored green and orange in the composite image, then, show those bacteria—gammaproteobacteria and other rod-shaped cells—that are actively making proteins.

"You could apply BONCAT to almost any type of sample," Orphan says. "When you have an environmental sample, you don't know which microorganisms are active. So, assume you're interested in looking at organisms that respond to methane. You could take a sample, provide methane, add the synthetic amino acid, and ask which cells over time showed activity—made new proteins—in the presence of methane relative to samples without methane. Then you can start to sort those organisms out, and possibly use this to determine protein turnover times. These questions are not typically tractable with uncultured organisms in the environment." Orphan's lab is also now using BONCAT on samples of deep-sea sediment in which mixed groups of bacteria and archaea catalyze the anaerobic oxidation of methane.

Why sample the Caltech lily pond? Roland Hatzenpichler, a postdoctoral scholar in Orphan's lab, explains: "When I started applying BONCAT on environmental samples, I wanted to try this new approach on samples that are both interesting from a microbiological standpoint, as well as easily accessible. Samples from the lily pond fit those criteria." Hatzenpichler is lead author of a study describing BONCAT that appeared as the cover story of the August issue of the journal Environmental Microbiology.

The work is supported by the Gordon and Betty Moore Foundation Marine Microbiology Initiative.

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