Tracking Down the "Missing" Carbon From the Martian Atmosphere

Caltech and JPL scientists suggest the fingerprints of early photochemistry provide a solution to the long-standing mystery

Mars is blanketed by a thin, mostly carbon dioxide atmosphere—one that is far too thin to prevent large amounts of water on the surface of the planet from subliming or evaporating. But many researchers have suggested that the planet was once shrouded in an atmosphere many times thicker than Earth's. For decades that left the question, "Where did all the carbon go?"

Now a team of scientists from Caltech and JPL thinks they have a possible answer. The researchers suggest that 3.8 billion years ago, Mars might have had only a moderately dense atmosphere. They have identified a photochemical process that could have helped such an early atmosphere evolve into the current thin one without creating the problem of "missing" carbon and in a way that is consistent with existing carbon isotopic measurements.

The scientists describe their findings in a paper that appears in the November 24 issue of the journal Nature Communications.

"With this new mechanism, everything that we know about the martian atmosphere can now be pieced together into a consistent picture of its evolution," says Renyu Hu, a postdoctoral scholar at JPL, a visitor in planetary science at Caltech, and lead author on the paper.

When considering how the early martian atmosphere might have transitioned to its current state, there are two possible mechanisms for the removal of excess carbon dioxide (CO2). Either the CO2 was incorporated into minerals in rocks called carbonates or it was lost to space.

A separate recent study coauthored by Bethany Ehlmann, assistant professor of planetary science and a research scientist at JPL, used data from several Mars-orbiting satellites to inventory carbonate rocks, showing that there are not enough carbonates in the upper kilometer of crust to contain the missing carbon from a very thick early atmosphere that might have existed about 3.8 billion years ago.

To study the escape-to-space scenario, scientists examine the ratio of carbon-12 and carbon-13, two stable isotopes of the element carbon that have the same number of protons in their nuclei but different numbers of neutrons, and thus different masses. Because various processes can change the relative amounts of those two isotopes in the atmosphere, "we can use these measurements of the ratio at different points in time as a fingerprint to infer exactly what happened to the martian atmosphere in the past," says Hu.

To establish a starting point, the researchers used measurements of the carbon isotope ratio in martian meteorites that contain gases that originated deep in the planet's mantle. Because atmospheres are produced by outgassing of the mantle through volcanic activity, these measurements provide insight into the isotopic ratio of the original martian atmosphere.

The scientists then compared those values to isotopic measurements of the current martian atmosphere recently collected by NASA's Curiosity rover. Those measurements show the atmosphere to be unusually enriched in carbon-13.

Previously, researchers thought the main way that martian carbon would be ejected into space was through a process called sputtering, which involves interactions between the solar wind and the upper atmosphere. Sputtering causes some particles—slightly more of the lighter carbon-12 than the heavier carbon-13—to escape entirely from Mars, but this effect is small. So there had to be some other process at work.

That is where the new mechanism comes in. In the study, the researchers describe a process that begins with a particle of ultraviolet light from the sun striking a molecule of CO2 in the upper atmosphere. That molecule absorbs the photon's energy and divides into carbon monoxide (CO) and oxygen. Then another ultraviolet particle hits the CO, causing it to dissociate into atomic carbon (C) and oxygen. Some carbon atoms produced in this way have enough energy to escape the atmosphere, and the new study shows that carbon-12 is far more likely to escape than carbon-13.

Modeling the long-term effects of this ultraviolet photodissociation mechanism coupled with volcanic gas release, loss via sputtering, and loss to carbonate rock formation, the researchers found that it was very efficient in terms of enriching carbon-13 in the atmosphere. Using the isotopic constraints, they were then able to calculate that the atmosphere 3.8 billion years ago might have had the pressure of Earth's or less under most scenarios.

"The efficiency of this new mechanism shows that there is in fact no discrepancy between Curiosity's measurements of the modern enriched value for carbon in the atmosphere and the amount of carbonate rock found on the surface of Mars," says Ehlmann, also a coauthor on the new study. "With this mechanism, we can describe an evolutionary scenario for Mars that makes sense of the apparent carbon budget, with no missing processes or reservoirs."

The authors conclude their work by pointing out several tests and refinements for the model. For example, future data from the ongoing Mars Atmosphere and Volatile EvolutioN (MAVEN) mission could provide the isotope fractionation of presently ongoing atmospheric loss to space and improve the extrapolation to early Mars.

Hu emphasizes that the work is an excellent example of multidisciplinary effort. On the one hand, he says, the team looked at the atmospheric chemistry—the isotopic signature, the escape processes, and the enrichment mechanism. On the other, they used geological evidence and remote sensing of the martian surface. "By putting these together, we were able to come up with a summary of evolutionary scenarios," says Hu. "I feel that Caltech/JPL is a unique place where we have the multidisciplinary capability and experience to make this happen."

Additional authors on the paper, "Tracing the Fate of Carbon and the Atmospheric Evolution of Mars," are Yuk Yung, the Smits Family Professor of Planetary Science at Caltech and a senior research scientist at JPL, and David Kass, a research scientist at JPL. The work was supported by funding from NASA.

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The Atmospheric Evolution of Mars
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Caltech and JPL researchers identify a process that helps explain how a moderately dense early martian atmosphere could have evolved into the current thin one.

New Jenkins Leadership Chair Is a Tribute to Exploration

Ted (BS '65, MS '66) and Ginger Jenkins—longtime Caltech supporters and early employees in the semiconductor industry—have established a leadership chair for the Division of Geological and Planetary Sciences (GPS). The new leadership chair is one of a set of discretionary endowments being established for Caltech's president, provost, and the leaders of each academic division.

"This new endowment will support some of the most exploratory work in the division and help secure our future," says John Grotzinger, the Fletcher Jones Professor of Geology and inaugural holder of the Ted and Ginger Jenkins Leadership Chair. "The completely flexible funding it provides—a rarity in geological and planetary sciences—will give me and future division chairs greater freedom to support highly creative and timely projects. By its nature our science involves complex interdisciplinary work for which flexible funding is ideal."

Ted Jenkins feels confident that this type of agility will give Caltech a competitive edge. "Possibilities come up that are not sufficiently well known or forecastable to pass the hurdles for a grant," he explains. "On the other hand, they could be worth a little bit of exploratory money. By applying extra resources to things that look exciting but uncertain, Caltech has come up with more than its fair share of interesting discoveries.

"These kinds of endowments for division chairs allow the whole Institute to go out and do this aggressive science. This, together with the other resources we have, puts us way, way ahead."

The story of the Jenkins gift can be traced back half a century to a moment of curiosity—and, admittedly, some jealousy—during Ted Jenkins's days as an engineering student at Caltech.

"The geology students were always going camping and taking field trips," he says. "They had great stories." So he jumped at the chance to take Ge 1, a class taught by the late Robert Sharp, whom he describes as an iconic geologist and leader.

Ted Jenkins's memory of that course inspired him and Ginger to sign up for a geology-focused Caltech Alumni Association (CAA) program in Alaska in 1997. Soon after, the couple's retirement freed them to travel often with the CAA and the Caltech Associates.

The more they learned about what was happening at Caltech, the more Ted and Ginger Jenkins got involved. Ted served as president of both the CAA and the Associates, and he became a Caltech trustee in 2006. He was also a founding member and chairman of the GPS chair's council, a volunteer leadership board.

"Ted and Ginger Jenkins have seen and supported Caltech from many angles," says Caltech President Thomas F. Rosenbaum, holder of the Sonja and William Davidow Presidential Chair and professor of physics. "The Institute has benefited enormously from their decades of thoughtful counsel and generosity. Their extraordinary new commitment goes even farther to position us as the destination of choice for the most imaginative scholars."

With past gifts to Caltech, the couple has endowed a professorship in information science and technology, and supported diverse initiatives across campus: two astrophysical observatories, the Community Seismic Network project, schizophrenia research, a graduate fellowship and travel funds for GPS students, an annual staff prize, improvements to the Athenaeum, and the Carver Mead New Adventures Fund, which provides early support for novel projects in information science and technology.

Ted and Ginger Jenkins see giving as a way to do their part—both for the world, through the discoveries and educational experiences they enable, and for Caltech itself.

Ted Jenkins's career, and even his marriage, hinged on a single conversation that took place in a Caltech conference room. In 1966, Gordon Moore (PhD '54) paid a visit to Jenkins's adviser Carver Mead, now Caltech's Gordon and Betty Moore Professor of Engineering and Applied Science, Emeritus. Mead introduced Moore to his students, and Moore recruited Jenkins to Fairchild Semiconductor International during that same visit. During his time at Fairchild, Ted Jenkins met the woman who would become his wife. Two years later, Moore hired him again, this time as employee number 22 at the start-up that would become Intel¾where Ted Jenkins built a three-decade career and rose to the rank of vice president.

"My Caltech experience and connections have been a huge part of the means that I was able to assemble," Ted Jenkins says. "Giving back so that the activity continues is really what I like to do."

In addition to the Jenkins chair, Caltech has received funding for the Sonja and William Davidow Presidential Chair, the Carl and Shirley Larson Provostial Chair, the William K. Bowes Jr. Leadership Chair in the Division of Biology and Biological Engineering, the Otis Booth Leadership Chair in the Division of Engineering and Applied Science, and the Kent and Joyce Kresa Leadership Chair in the Division of Physics, Mathematics and Astronomy.

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Ted (BS ’65, MS ’66) and Ginger Jenkins have established a leadership chair for the Division of Geological and Planetary Sciences.

Schools Help Researchers Understand Quakes

In the event of a major earthquake in Los Angeles, first responders ideally would immediately have a map of the most intense shaking around the city—allowing them to send help to the hardest-hit areas first.

A new collaboration between Caltech researchers and schools of the Los Angeles Unified School District (LAUSD) provides a crucial step in the creation of such damage maps by vastly broadening the scope of a dense network of seismic sensors in the Los Angeles Basin.

To create an accurate shaking-intensity map, seismologists need to measure ground motion—which can vary from kilometer to kilometer because of differences in soil and earth structure—at many locations across the region. In 2011, Professor of Geophysics Rob Clayton and his colleagues, Professor of Engineering Seismology Tom Heaton and Simon Ramo Professor of Computer Science, Emeritus, K. Mani Chandy, began creating a web of such sensors via the Community Seismic Network (CSN), a program funded by the Gordon and Betty Moore Foundation.

The CSN consists of hundreds of small, inexpensive accelerometers—instruments that detect ground movements before, during, and after a seismic event—installed initially in the homes of volunteers in the greater Pasadena area. Since 2011, each device has been actively collecting and feeding seismic information to the CSN via its host house's Internet connection, allowing Clayton and his colleagues to create high-resolution maps of seismic activity in the western San Gabriel Valley. But the Caltech team wanted to find a way to expand the reach of the network throughout the earthquake-prone broader Los Angeles Basin. Their inventive solution? Integrate accelerometers into the infrastructure of L.A.'s public schools.

Through the efforts of Richard Guy, CSN project manager, sensors already have been installed in 100 LAUSD schools, covering an area ranging from northeast Los Angeles to downtown. CSN is now working to expand the project to include all of the district's more than 1,000 schools.

The new collaboration has the potential to help millions of people in Southern California when a big quake strikes. For example, data from the new network could be incorporated into the ShakeAlert early-warning system that is currently under development. Although no sensor can predict an earthquake, the accelerometers can detect an earthquake in one area of the L.A. Basin so quickly that an alert or warning could be sent to people in adjacent areas of the LA Basin before strong shaking arrives—potentially giving them enough time to find a safe spot.

The new dense network of sensors will also provide an improved map of shaking intensity for the whole region. The U.S. Geological Survey already provides a similar service called ShakeMap, which relies on sensors that are located several miles from one another and hence cannot provide a block-by-block resolution of shaking and possible damage. The new dense network of sensors has the potential to provide ShakeMap with a more accurate assessment of damage for response and recovery efforts.

"You can imagine a fire chief stepping out and saying, 'Wow. That was a big one. Now where do I go to help the community?' Obviously they want their focus to be where the maximum damage and danger is. They have other things to worry about too, but the best proxy for damage that we have is the level of shaking—and our dense network of sensors can provide that information," Clayton says.

But the new sensors do more than feed information into the network—they also provide valuable information to individual schools. "Principals have a particularly difficult problem in the event of an earthquake," Clayton says. "The first thing during a quake, of course, is to tell everyone to get under their desk. When the shaking stops, all of the kids are evacuated out of the school and into the schoolyard. And then what do principals do? At that point, they have to decide if it's safe enough to go back into the school, or if they should just send the kids home. But they do not know how badly the school is damaged."

The new school sensors could help inform this judgment call, Clayton says. Although they work in much the same way as those that were previously placed in volunteers' homes—recording ground accelerations and transmitting those data back to the researchers via an Internet connection—the sensors also contain an onboard computer that compares the event to a so-called fragility curve. Fragility curves provide predictions of the damage that a particular building would sustain under the shaking measured.

"Coupled with the fragility curve, the sensors could allow a school official to decide whether or not it is safe to reenter the school," Clayton says.

The Community Seismic Network's LAUSD collaboration was funded by the Gordon and Betty Moore Foundation. The network is a collaboration between Caltech's seismology, earthquake-engineering, and computer-science departments.

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A new collaboration between seismologists at Caltech and local public schools helps Los Angeles prepare for the "big one."
Monday, November 30, 2015

Microbial diners, drive-ins, and dives: deep-sea edition

Elachi to Retire as JPL Director

Charles Elachi (MS '69, PhD '71) has announced his intention to retire as director of the Jet Propulsion Laboratory on June 30, 2016, and move to campus as professor emeritus. A national search is underway to identify his successor.

"A frequently consulted national and international expert on space science, Charles is known for his broad expertise, boundless energy, conceptual acuity, and deep devotion to JPL, campus, and NASA," said Caltech president Thomas F. Rosenbaum in a statement to the Caltech community. "Over the course of his 45-year career at JPL, Charles has tirelessly pursued new opportunities, enhanced the Laboratory, and demonstrated expert and nimble leadership. Under Charles' leadership over the last 15 years, JPL has become a prized performer in the NASA system and is widely regarded as a model for conceiving and implementing robotic space science missions."

With Elachi at JPL's helm, an array of missions has provided new understanding of our planet, our moon, our sun, our solar system, and the larger universe. The GRAIL mission mapped the moon's gravity; the Genesis space probe returned to Earth samples of the solar wind; Deep Impact intentionally collided with a comet; Dawn pioneered the use of ion propulsion to visit the asteroids Ceres and Vesta; and Voyager became the first human-made object to reach interstellar space. A suite of missions to Mars, from orbiters to the rovers Spirit, Opportunity, and Curiosity, has provided exquisite detail of the red planet; Cassini continues its exploration of Saturn and its moons; and the Juno spacecraft, en route to a July 2016 rendezvous, promises to provide new insights about Jupiter. Missions such as the Galaxy Evolution Explorer, the Spitzer Space Telescope, Kepler, WISE, and NuSTAR have revolutionized our understanding of our place in the universe.

Future JPL missions developed under Elachi's guidance include Mars 2020, Europa Clipper, the Asteroid Redirect Mission, Jason 3, Aquarius, OCO-2, SWOT, and NISAR.

Elachi joined JPL in 1970 as a student intern and was appointed director and Caltech vice president in 2001. During his more than four decades at JPL, he led a team that pioneered the use of space-based radar imaging of the Earth and the planets, served as principal investigator on a number of NASA-sponsored studies and flight projects, authored more than 230 publications in the fields of active microwave remote sensing and electromagnetic theory, received several patents, and became the director for space and earth science missions and instruments. At Caltech, he taught a course on the physics of remote sensing for nearly 20 years

Born in Lebanon, Elachi received his B.Sc. ('68) in physics from University of Grenoble, France and the Dipl. Ing. ('68) in engineering from the Polytechnic Institute, Grenoble. In addition to his MS and PhD degrees in electrical science from Caltech, he also holds an MBA from the University of Southern California and a master's degree in geology from UCLA.

Elachi was elected to the National Academy of Engineering in 1989 and is the recipient of numerous other awards including an honorary doctorate from the American University of Beirut (2013), the National Academy of Engineering Arthur M. Bueche Award (2011), the Chevalier de la Légion d'Honneur from the French Republic (2011), the American Institute of Aeronautics and Astronautics Carl Sagan Award (2011), the Royal Society of London Massey Award (2006), the Lebanon Order of Cedars (2006 and 2012), the International von Kármán Wings Award (2007), the American Astronautical Society Space Flight Award (2005), the NASA Outstanding Leadership Medal (2004, 2002, 1994), and the NASA Distinguished Service Medal (1999).

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He will move to campus as professor emeritus. A national search is underway to identify his successor.
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Probing the Mysteries of Europa, Jupiter's Cracked and Crinkled Moon

New research identifies possible sites of frozen, watery deposits.

Jupiter's moon Europa is believed to possess a large salty ocean beneath its icy exterior, and that ocean, scientists say, has the potential to harbor life. Indeed, a mission recently suggested by NASA would visit the icy moon's surface to search for compounds that might be indicative of life. But where is the best place to look? New research by Caltech graduate student Patrick Fischer; Mike Brown, the Richard and Barbara Rosenberg Professor and Professor of Planetary Astronomy; and Kevin Hand, an astrobiologist and planetary scientist at JPL, suggests that it might be within the scarred, jumbled areas that make up Europa's so-called "chaos terrain."

A paper about the work has been accepted to The Astronomical Journal.

"We have known for a long time that Europa's fresh icy surface, which is covered with cracks and ridges and transform faults, is the external signature of a vast internal salty ocean," Brown says. The areas of chaos terrain show signatures of vast ice plates that have broken apart, shifted position, and been refrozen. These regions are of particular interest, because water from the oceans below may have risen to the surface through the cracks and left deposits there.

"Directly sampling Europa's ocean represents a major technological challenge and is likely far in the future," Fischer says. "But if we can sample deposits left behind in the chaos areas, it could reveal much about the composition and dynamics of the ocean below." That ocean is thought to be as deep as 100 kilometers.

"This could tell us much about activity at the boundary of the rocky core and the ocean," Brown adds.

In a search for such deposits, the researchers took a new look at data from observations made in 2011 at the W. M. Keck Observatory in Hawaii using the OSIRIS spectrograph. Spectrographs break down light into its component parts and then measure their frequencies. Each chemical element has unique light-absorbing characteristics, called spectral or absorption bands. The spectral patterns resulting from light absorption at particular wavelengths can be used to identify the chemical composition of Europa's surface minerals by observing reflected sunlight.

The OSIRIS instrument measures spectra in infrared wavelengths. "The minerals we expected to find on Europa have very distinct spectral fingerprints in infrared light," Fischer says. "Combine this with the extraordinary abilities of the adaptive optics in the Keck telescope, and you have a very powerful tool." Adaptive optics mechanisms reduce blurring caused by turbulence in the earth's atmosphere by measuring the image distortion of a bright star or laser and mechanically correcting it.

The OSIRIS observations produced spectra from 1600 individual spots on Europa's surface. To make sense of this collection of data, Fischer developed a new technique to sort and identify major groupings of spectral signatures.

"Patrick developed a very clever new mathematical tool that allows you to take a collection of spectra and automatically, and with no preconceived human biases, classify them into a number of distinct spectra," Brown says. The software was then able to correlate these groups of readings with a surface map of Europa from NASA's Galileo mission, which mapped the Jovian moon beginning in the late 1990s. The resulting composite provided a visual guide to the composition of the regions the team was interested in.

Three compositionally distinct categories of spectra emerged from the analysis. The first was water ice, which dominates Europa's surface. The second category includes chemicals formed when ionized sulfur and oxygen­­—thought to originate from volcanic activity on the neighboring moon Io­­—bombard the surface of Europa and react with the native ices. These findings were consistent with results of previous work done by Brown, Hand and others in identifying Europa's surface chemistry.

But the third grouping of chemical indicators was more puzzling. It did not match either set of ice or sulfur groupings, nor was it an easily identified set of salt minerals such as they might have expected from previous knowledge of Europa. Magnesium is thought to reside on the surface but has a weak spectral signature, and this third set of readings did not match that either. "In fact, it was not consistent with any of the salt materials previously associated with Europa," Brown says.

When this third group was mapped to the surface, it overlaid the chaos terrain. "I was looking at the maps of the third grouping of spectra, and I noticed that it generally matched the chaos regions mapped with images from Galileo. It was a stunning moment," Fischer says. "The most important result of this research was understanding that these materials are native to Europa, because they are clearly related to areas with recent geological activity."

The composition of the deposits is still unclear. "Unique identification has been difficult," Brown says. "We think we might be looking at salts left over after a large amount of ocean water flowed out onto the surface and then evaporated away. He compares these regions to their earthly cousins. "They may be like the large salt flats in the desert regions of the world, in which the chemical composition of the salt reflects whatever materials were dissolved in the water before it evaporated."

Similar deposits on Europa could provide a view into the oceans below, according to Brown. "If you had to suggest an area on Europa where ocean water had recently melted through and dumped its chemicals on the surface, this would be it. If we can someday sample and catalog the chemistry found there, we may learn something of what's happening on the ocean floor of Europa and maybe even find organic compounds, and that would be very exciting." 

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New Research on Europa
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Researchers have mapped what may be salt deposits from the ocean below the ice onto the Jovian moon's surface.
Friday, October 30, 2015
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Cassini Begins Its Final Act: A Conversation with Charles Elachi

In September, the NASA/JPL Cassini mission began the last two years of the Solstice Mission, the final stretch of its explorations of Saturn, its rings, and its moons—including the giant Titan, a haze-enshrouded satellite with Earth-like features and complicated organic chemistry, and small, icy, and surprisingly active Enceladus. 

Launched in October 1997, after a decade and a half of planning, design, and construction, the Cassini spacecraft may be one of JPL's missions that is most well remembered, says JPL director Charles Elachi (MS '69, PhD '71). Elachi, who is also a Caltech professor of electrical engineering and planetary science, may be forgiven a small amount of bias toward Cassini, as he has served since the mid-1980s as the team leader for the spacecraft's radar experiment—the instrument responsible for mapping the previously hidden surface of Titan.

We recently spoke with Elachi to gain his unique perspective on Cassini's achievements—and what will come next.

How did you first get involved in the Cassini mission?

When the mission concept was being developed in the early 1980s, JPL worked with the science community to define the mission, and one of the key instruments that the community wanted was radar. At that time, all that we knew about Titan, Saturn's largest satellite, was that it is a ball. It is completely haze covered, and you cannot see the surface. Radar was an ideal instrument because of its capability to see through the haze to map the surface.

At the time, I was the lead scientist at JPL in radar activity. I was involved in some of the earlier radar missions like Seasat, one of the first orbiting radar satellites. I was the principal investigator of a series of shuttle imaging radar missions (SIR-A, SIR-B, SIR-C). I was a member of the Venus radar team for NASA's Magellan mission. So I decided to propose that type of instrument for Cassini. It was selected, and I was selected as the team leader. Caltech professor of planetary science Duane (Dewey) Muhleman, who is now retired, joined me on the radar team.

Other Caltech faculty and alumni include Andrew Ingersoll, professor of planetary science, who was a member of the Cassini imaging team, as was Torrence Johnson (PhD '70); imaging team leader Carolyn Porco (PhD '83), who is also the director of the Cassini Imaging Central Laboratory for Operations (CICLOPS); and Dennis Matson (PhD '72), who was the Cassini project scientist from the beginning of the mission through early part of the orbiting phase.

Do you recall what your hopes were for the mission when it first started?

At the time, some people had the theory that Titan is a water ball—with an ocean across the whole satellite. Other people were saying there is no ocean. It would be too arid. We had no idea what to expect. For me, the most exciting thing was that it was going to be a complete surprise. It was different from the case with Mars, for example, where we had some idea of what the planet looked like from ground telescopes.

My goal was to map as much of Titan as possible. The radar instrument maps Titan by serial flybys, where, with every flyby, we image a wide strip of the surface. My hope was to map at least 50 percent of Titan. So far, we have mapped almost 60 percent.

What was the biggest surprise?

In my mind, the two biggest surprises were, first, that it has lakes with rivers coming into them—all made of hydrocarbons. The lakes are roughly the same size as the Great Lakes in the U.S. It looks like Earth to some extent. The other big surprise was the sand dunes. We did not expect that there would be fields of sand dunes.

What are the sand dunes made of?

We don't know. They could be made of hydrocarbon particles or frozen grains of snow or ice. We cannot tell what their composition is from the radar or other instruments. That's for the next mission.

We know they are extensive. All around the equatorial region on Titan, you see sand dunes of different sizes and with different structures.

They look very similar to the sand dunes in Namibia and Saudi Arabia. The phenomenology is very similar, with the wind blowing particles around hills and mountains to create the patterns.

Have you seen changes in the radar imaging over the last 11 years?

We see changes in a couple of places, and we are very puzzled about the reason for those changes. In the lakes, some small islands have appeared a couple of times. When you see things like this you debate, is it some anomaly in the instrument or is it real? It's perfectly conceivable it could be real, that the level of the liquid could be moving up and down, like what happens in the winter or the summer in lakes here on Earth.

Is there anything that you still hope to learn over the next two years?

I'm always ready for some surprises. A few passes that we will be doing will cover some new areas. One thing we have been looking for is if there are lakes on the other side of Titan, the other pole, now that we know there are lakes in the north. If we detect lakes over areas that we haven't covered, that might give us some hint about why the lakes are there. Also, because Saturn's seasons are progressing, but very slowly, we keep looking for changes as the spacecraft goes over the same areas—changes in the lakes or changes of volcanic flow, changes in sand dune patterns, anything that gives us an indication that something dynamic is happening.

How will the Cassini mission end?

It is going to be a dramatic end. We are planning, on purpose, to have the spacecraft enter Saturn's atmosphere and burn up before it completely depletes its fuel. In order to do that, we have to do a number of orbits that come very close to the rings. In fact, we'll be going through the gap between the rings and the planet, so we might find something new that we haven't seen before now.

Now, you may say, "Why are you crashing into Saturn?" The reason is there are rules for planetary protection. In the long term, once we lose control of the spacecraft, we want to ensure that it doesn't end crashing into Titan, or crashing into Enceladus, to keep the satellites pristine.

How did the team pick Cassini's final day, September 15, 2017?

I think it came from the orbital dynamics. We needed to do it before we completely use the control fuel. The orbit guys came up with a number of scenarios, and then the science team collectively sat down and decided on one of the scenarios.

Have you thought about how you are going to feel on September 16?

On one hand, we will be thinking, "Gee, we are losing one of our great missions."

But this mission has been so amazing. It made so many discoveries—finding the lakes and the sand dunes on Titan; the geysers on Enceladus; details of the hexagonal hurricane in the northern hemisphere of Saturn, which is allowing us to understand the planet's atmospheric dynamics. Cassini's discoveries have completely changed our thinking about the whole Saturnian system. It is changing the textbooks.

The way I think about it is that Voyager gave us snapshots of all the outer planets—Jupiter, Saturn, Uranus, and Neptune. It triggered our curiosity about them. Cassini gave us an in-depth understanding of the whole Saturnian system, which is almost like a mini solar system.

It's like this with every scientific exploration. You answer a certain question, and it raises new questions. What are the sand dunes made of? Do they change? What is the liquid made of? How deep are the different lakes? We're starting to think about the next mission for Titan. Some people are looking at possibly dropping boats in the lakes. Some people are looking at rovers. That will be a different technological challenge at that very low temperature.

We also are looking at possibly landing on Enceladus or sending a spacecraft to fly through its plume, capture samples, and either analyze them in a mass spectrometer on the spacecraft or bring a capsule all the way back to Earth.

After Cassini's mission ends, how much longer will you still be analyzing the data?

The mission is funded at least through 2018. Beyond that, I think people will be analyzing the data at least for a decade, if not longer.

Time brings new perspective. As you learn more about how planets form and about the tectonic activities or the atmosphere, you come up with new ideas. Then you go and look at the data and see. Does that fit with the result of the measurements that were made using a different instrument? Plus, people now are getting much more knowledgeable about analyzing data from multiple instruments that complement each other.

Will you be involved in another mission?

If I'm still alive, maybe. The next big mission is to Jupiter's moon, Europa. I'm a member of the team doing the radar sounder to measure the thickness of the ice.

Did you ever consider stepping away from the science as your other responsibilities increased after becoming director of JPL in 2001?

I always keep a finger on the science. I have found that an important thing for the director at JPL is to stay involved in the science at the team level, so we understand what the institution is doing and understand the issues of the community. It's part of my management style.

For the Europa mission, I felt we needed a younger person to be the team leader. I am only a team member. We structured the team so that half the team is … let me call them "mature." More experienced. People like me. The other half is relatively younger people. We teamed each senior person with a young person. A person from the University of Texas who is in his early 30s is kind of my understudy, if you want. He will be working with me, so I will transfer my experience.

In addition, I am a team member on a very exciting Discovery Venus radar mission, which will be a major advance beyond Magellan.

Is it bittersweet, handing off the reins like this?

I tell people I'm envious of them, because there will be so many discoveries happening in the next 30 years. I wish I were 20 years younger so I could see those discoveries. I'm sure when I was young, people who were older were envious of us.

Hopefully, if I live long enough, I will see some of these results. It's an incentive to stay in good health.

In our business, you have to be patient. It takes a long time, particularly for the outer planets. With Cassini, we had seven years to sell it to Congress, seven years to build it, seven years to get to Saturn. That's even before we started getting the data. But, once you start getting the data, the excitement is worth every bit of that patience. I would say my career was worth it.

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Cassini Begins Its Final Act: A Conversation with Charles Elachi
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Taking Dinosaur Temperatures with Eggshells

Researchers know dinosaurs once ruled the earth, but they know very little about how these animals performed the basic task of balancing their energy intake and output—how their metabolisms worked. Now, a team of Caltech researchers that has measured the body temperatures of a wide range of dinosaurs has provided insight into how the animals may have regulated their internal heat.

The study was led by John Eiler, the Robert P. Sharp Professor of Geology and professor of geochemistry, and Rob Eagle, a former Caltech postdoctoral scholar now at UCLA. A paper describing the research appears in the October 13 issue of the journal Nature Communications.

The current study examined eggshells from the sauropods, a group that includes some of the biggest dinosaurs ever to live, called Titanosaurs, as well as eggshells of birdlike and approximately human-sized oviraptorid dinosaurs. The eggshells were analyzed to determine the extent to which carbon-13 and oxygen-18—rare, naturally occurring isotopes (variant forms of elements that differ in number of neutrons)—group together in the mineral structure. This "clumping" of rare isotopes previously has been shown to depend on mineral growth temperature. The eggshell data were compared with the results of a previous study by this same group that used similar techniques to examine the growth temperatures of the sauropod dinosaurs, including the giraffe-like Giraffatitan and a giant herbivore known as Camarasaurus.

A large clutch of titanosaur eggs that has been cleaned for research. Credit: Luis Chiappe, LA County Natural History Museum

The isotopic composition of the eggshells showed that smaller oviraptorid dinosaurs had body temperatures of 32 degrees Celsius—decidedly cooler than modern mammals and birds. The body temperatures of the larger Titanosaur dinosaurs were 38 degrees Celsius, indistinguishable from a previous finding for Giraffatitan teeth and similar to modern mammals. This finding—that larger dinosaurs maintained body temperatures like ours whereas smaller ones more closely resembled modern reptiles—has implications for our understanding of dinosaur physiology.

Modern mammals are described as warm blooded if they regulate their own temperature, as if tweaking an internal thermostat. In a process called endothermy, warm-blooded mammals utilize the heat generated by their own internal functions instead of drawing ambient heat from the environment, which is what a cold-blooded snake or lizard does by basking in the sun. Endothermy is relatively similar across many different sizes of mammals, from mice to humans to whales.

"Measuring cooler temperatures in small dinosaurs is the first evidence to suggest that at least some of them had lower basal metabolisms than most modern mammals and birds, and therefore the emergence of modern mechanisms of endothermy hadn't occurred in these dinosaurs," Eiler says.

The picture is not so clear for the larger dinosaurs that were studied. Although Eiler and his colleagues found that they had warm body temperatures similar to modern mammals, it is not known if the animals actually had endothermic metabolisms or if they were warm simply because of their enormous sizes—a phenomenon known as gigantothermy. Gigantotherms have small surface areas relative to their large volumes and thus have less area through which they can lose heat. Therefore, the heat is trapped internally. "If you weigh 80 tons, your problem is not staying warm—it's trying not to burst into flames," Eiler says.

The wide range of warm temperatures discovered among the various dinosaur species examined in the study suggests that "either they had a range of different metabolic strategies, or they all had low basal metabolisms, and the large ones were only warm due to gigantothermy," Eiler says.

The technique used to determine these animal body temperatures was first conceived and used by Eiler's group in 2011 on dinosaur tooth fossils and is related to methods they previously developed for nonbiological minerals and molecules. The method, called the clumped-isotope technique, relies on measurements of rare isotopes in bioapatite, or biologically grown calcium carbonate, a mineral present in bones, teeth, eggshells, and other fossils. In 2006, Eiler's lab quantified the degree to which carbon-13 and carbon-18 clump together to varying degrees in a biomineral, depending on the temperature at the time the mineral formed; this relationship subsequently was examined for many mineral types by Eiler's group at Caltech and at other laboratories.

"There's this cool idea that if I had a fossil skeleton, I could map the body temperature of the entire creature and come up with a physiological model of how it redistributed heat within its body," Eiler says. "There's no reason you couldn't do that, except that bone isn't very well preserved."

The team's next step is to compare fossils from the same species across stages of maturation. "It may be that some dinosaurs have a different metabolic strategy at different phases of life," Eiler says.

Lori Dajose
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Understanding Dinosaurs
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Researchers who have measured dinosaurs' body temperature using eggshells are providing insight into how the animals may have regulated their internal heat.
Wednesday, November 11, 2015
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