When Rocks Roll: How Sediment Transport Shapes Planetary Surfaces

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On Wednesday, March 19, Professor of Geology Michael Lamb will describe how flowing water and grains of sand create Earth's dramatic landscapes. Mars and Saturn's moon Titan show signs of similar processes. Lamb's work on the mechanics of landscape evolution may change how we think about debris flows in the San Gabriel Mountains, the effects of wildfires on erosion, and water on Mars.

The talk begins at 8:00 p.m. in Caltech's Beckman Auditorium. Admission is free.


Q: What do you do?

A: I'm a geologist. I study the basic mechanics of how sediment moves—how rivers erode rock and form canyons, how sediment builds deltas like the Mississippi's, how landslides work, and so on.

In the U.S., my field dates back to John Wesley Powell, who led a survey party by boat down the Grand Canyon a few years after the Civil War. For decades, geomorphology consisted of classifying landforms—identifying different types of mountain ranges, or hills, or canyons. Now, we're quantifying the processes that operate at Earth's surface to shape those landforms.

One challenge, which is true of much of the Earth sciences, is to link these process studies with the evolution of Earth's surface over geologic time. So in addition to field work, my research group has a lab where we make indoor rivers and landslides, and 'speed up time' to observe their dynamics. For example, the Mississippi River jumps to a new course from time to time, and each jump builds a new lobe of the delta. These jumps happen about once every 1,000 years, but by scaling things properly we can speed up the clock and build deltas much faster in the laboratory. We also study rare, catastrophic events, such as megafloods, which would be difficult or dangerous to measure in nature.

It's an exciting time to be a geomorphologist because we have the opportunity to apply lessons learned on Earth to Mars and Titan—two other planetary bodies with river networks. Titan's surface appears to be active, with rainfall feeding into lakes and rivers with enough force to move gravel and even cobbles, which are roughly tennis- to basketball-sized. Titan is so cold that the "rain" is liquid methane and the "rocks" are water ice, but otherwise the system seems very similar to the water cycle on Earth. On Mars, the river canyons are now dry, probably long dry. In many ways, Mars represents the ultimate inversion problem. With little to go on except for images and topographic data, what can we deduce about the water flows and climate on Mars that led to the formation of these ancient features?


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

A: I started college at the University of Minnesota in civil engineering. I always was fond of mechanics, and thought that I would likely become an engineer. But I also was fond of the outdoors—natural landscapes and national parks—so I had a desire to connect both interests. I took elective classes in geology and found them to be much more interesting than my engineering classes, so I switched majors. I also had an opportunity as an undergrad to work in the St. Anthony Falls Hydraulics Laboratory at the University of Minnesota, which introduced me to the indoor-analogue river experiments I now do.

Flume experiments have been used in civil engineering for many decades—for example, if you dam a river, what will the effects be? In fact, what we're doing today is similar in some ways to techniques that were pioneered here at Caltech by Professors Vito Vanoni [BS '26, MS '32, PhD '40], Norm Brooks [PhD '54], and others—first in the Sediment Lab, which was built in 1936, and later in the W. M. Keck Laboratory of Hydraulics and Water Resources. But now we're addressing how the natural world works over geologic time, rather than how the engineered world works over human time.


Q: In light of our recent mudslides, do you have any take-home lessons for the general public?

A: I have been working on the connection between the wildfires, floods, and debris flows that tend to plague not only Los Angeles but other areas in the southwestern United States, and I will focus my lecture on this topic. For now I'll just say that what you hear in the news about hillsides giving way and causing landslides after wildfires is probably not accurate in many landscapes. Our work shows that there might be a different way to think about the fire-flood problem, especially in very steep and rapidly eroding landscapes like the San Gabriel Mountains.


Named for the late Caltech professor Earnest C. Watson, who founded the series in 1922, the Watson Lectures present Caltech and JPL researchers describing their work to the public. Many past Watson Lectures are available online at Caltech's iTunes U site.

Douglas Smith
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Watson Lecture: "When Rocks Roll: How Sediment Transport Shapes Planetary Surfaces"
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Airborne Over Iceland: Charting Glacier Dynamics

Mark Simons, professor of geophysics at Caltech, along with graduate student Brent Minchew, recently logged over 40 hours of flight time mapping the surface of Iceland's glaciers. Flying over two comparatively small ice caps, Hofsjökull and Langjökull, they traveled with NASA pilots and engineers in a retrofitted Gulfstream III business jet, crisscrossing the glaciers numerous times. Using a radar instrument designed at the Jet Propulsion Laboratory (JPL) and mounted on the underbelly of the plane, they imaged the surface of the glaciers, obtaining precise data on the velocity at which these rivers of ice flow downstream.

Following a set of test flights in Iceland in 2009, Simons and Minchew went to Iceland in June 2012 to systematically image the two ice caps at the beginning of the summer melt season. They have just returned from a February 2014 expedition aimed at setting a baseline for glacier velocity—during the winter freeze, meltwater should not play as significant a role in glacier dynamics. They sat down recently to discuss the science and the adventure of monitoring Iceland's glaciers.

Why go to Iceland to study glaciers?

Mark Simons: Iceland is an ideal natural laboratory. The glaciers there are small enough that you can do detailed measurements of them, and afterward you can process the data and analyze each ice cap in its entirety without needing overwhelming computer resources. This manageable scale lets us explore a wide range of models. Glaciers in Greenland or Antarctica are far too big for that. Logistics are also a lot easier in Iceland. We can drive up to the glaciers in just a few hours from downtown Reykjavik.

Most importantly, the Icelanders have a long history of studying these ice caps. In particular, they have nearly complete maps of the ice-bedrock interface. We can complement this information with continuous maps of the daily movement or strain of the glacier surface as well as maps of the topography of the glacier surface. These data are then combined to constrain models of glacier dynamics.

How can you map bedrock that is under hundreds of feet of ice?

Brent Minchew: Our collaborators at the University of Iceland have been doing this work for decades. Helgi Björnsson and Finnur Pálsson mapped the subglacial bedrock by dragging long radar antennas behind snowmobiles driven over the glaciers. They use long-wavelength radar that penetrates through the ice to the underlying bedrock. By looking at the reflection of the radar signals, they can estimate where the interface is between ice and bedrock. They are expert at studying the cryosphere—the earth's frozen regions, including ice caps, glaciers, and sea ice—as you might expect given their location so far north of the equator.

Is this similar to the radar you use in your airplane flights over Iceland's glaciers?

Simons: It's a similar principle. Radar is an active imaging system, so unlike optical observations, where you're just looking at the reflected light from the sun, we're actually illuminating the surface like a flashlight, but using radar instead.

Was this radar technology developed specifically for imaging glaciers?

Minchew: No. The technique we use, InSAR [Interferometric Synthetic Aperture Radar], has been available since the mid-1990s. It has revolutionized a number of disciplines in the earth sciences, including glaciology. The system we are using in Iceland is truly state-of-the-art. It enables complete control over where and when we collect data, and it returns images with millions of independent pixels. It's a very rich data source.

Simons: Actually, the exact same airplane we use in Iceland to study glaciers is also used to measure motion above restless volcanoes due to changes in magma pressure or along major seismically active faults such as the San Andreas fault. Repeated radar imaging can show us the parts of the fault that are stuck—those are the places that will generate earthquakes every so often—and the other parts that are steadily creeping year after year. Basically, we're bringing our experience from earthquake physics, both in terms of observation and modeling, to see if it can help us address important problems in glaciology.

Are there other methods besides radar for studying glacier dynamics?

Minchew: We can drill to the bed and take direct measurements, but a lot of effort is involved in this. Compared to Greenland, where the ice is close to a mile thick, or Antarctica, where it is even thicker, Iceland's glaciers are relatively thin. But they're still on average 300 meters thick. That's a long way to drill down for one data point.

Simons: Traditionally people measured velocities of glaciers by putting stakes in the glacier, and then returning to see how far downstream those stakes had moved by the end of the melting season. This approach can give an average velocity over the season. We still utilize this principle by installing GPS units at various spots on the glacier. These GPS units also help us calibrate our radar-based measurements and confirm that our velocity estimates are accurate.

What advantages does radar have over these other methods?

Simons: One of the wonderful things about radar imaging, unlike optical imaging, is that we can "see" the glacier whether it's day or night, whether it's cloudy or clear.

Minchew: Right. Another major advantage of radar technology is that we don't just see the average velocity for the season; we can detect short-term dynamics and variability over the entire glacier if the imaging is done sufficiently often.

How exactly does radar work to image the ice cap?

Simons: Radar images are usually taken at oblique angles to the surface of the earth, not straight down in a perpendicular line. Given two radar images taken from nearly identical positions but at different times, we can combine them in such a way as to measure changes in ground position that occurred in the intervening period along the oblique direction of the transmitted energy. We quantify these displacements in terms of fractions of a radar wavelength. This process is called repeat pass interferometry. We design the plane's flight path to make several interferometric measurements from different viewing angles, in order that the surface of the glacier is imaged at least three times and often as many as six times. We then combine these different perspectives to create accurate 3-D maps of the surface velocity of the glaciers, detecting its underlying east, north, and up components.

How can you be so precise in your measurements from that high up in the air?

Simons: The altitude itself isn't a problem. The trick is making certain the plane is at the same absolute position over consecutive flights. We owe this precision to engineers at NASA/JPL; it has nothing to do with us down here at Caltech. They have developed the technology to fly this plane at 40,000 feet, at 450 miles per hour, and then to come back an hour later, a day later, or a year later, and fly that exact same path in coordinates relative to the ground. Essentially they are flying in a "virtual tube" in the air that's less than 10 meters in diameter. That's how accurate it is.

Minchew: Of course even within this virtual tube, the plane moves around; that's what aircraft do. But aircraft motion has a characteristic appearance in the data, and it's possible for us to remove this effect. It never ceases to amaze me that we can get centimeter-scale, even millimeter-scale accuracy from an airplane. But we can do it, and it works beautifully.

What was the motivation for JPL and NASA to develop this radar technology in the first place?

Simons: Part of what NASA has been doing with airborne radar technology is prototyping what they want to do with radar from satellites, and to understand the characteristics of this kind of measurement for different scientific targets. The instrument is called UAVSAR, for Uninhabited Aerial Vehicle Synthetic Aperture Radar. Right now it's clearly not uninhabited because the radar is on a plane with pilots and engineers on board. But the idea is that eventually we could do these radar measurements from a drone that would stay aloft making observations for a day or a day and a half at a stretch. We can also use satellites to make the same type of measurements.

Minchew: In ways, satellites are an easier platform for radar measurements. In space, there aren't a whole lot of dramatic perturbations to their motions; they fly a very steady path. But one advantage of an airborne platform is that we can collect data more frequently. We can sample the glacier surface every 24 hours if we wish. Satellites typically sample on the order of once a week to every several weeks.

What do you hope to learn from observing glacier dynamics in Iceland?

Simons: We want to use measurements of the ice cap to explore what is happening at the bottom of the glacier. We already know from the previous campaign in 2012 that over half of the movement measured in the early summer is associated with sliding at the bed rather than deformation of the ice. In the early part of the melt season, water gets down to the bottom of the glacier and doesn't have anywhere to go, so it increases the pressure at the bottom. It ends up reducing the friction so the glacier can flow faster over the bedrock. At some point there's so much water flow that it starts to make tunnels in the ice, and then the glacier drains more efficiently. But then the tunnels will collapse on themselves, and the whole glacier settles back down, compacting on itself. The glacier actually slides faster in the early part of the melt season than later in the melt season.

Minchew: The thing that propels glaciers is simply gravity. Ice is a viscous fluid, like honey. Very cold honey. Once it warms up and begins to melt slightly, the dynamics change tremendously. That's something we can observe in Iceland—unlike in Antarctica—where temperatures regularly go above the freezing point in summer. In Iceland, we think almost all the meltwater at the bed comes from surface melting. Geothermal heating from the earth and frictional heating from the sliding itself can also contribute to melting in Iceland's glaciers. These are the main sources of melting in Antarctica. But geothermal and frictional heating don't have anything to do with climate change nor should they vary with the seasons in the way that meltwater does.

Is climate change the major reason why you're studying glaciers?

Minchew: No, I just like cold and inhospitable places. Seriously, I was drawn to the field work aspect of geophysics, the opportunity to go to places in the world that are for the most part the way nature intends them to be. I'm also drawn to glaciers because they are fascinating and surprisingly complex physical systems. A number of fundamental problems in glaciology remain unsolved, so there is tremendous potential for discovery in this field. But helping to understand the potential effects of climate change is an obvious application of our work. People are much more interested in glaciers now as a result of climate change. One of the glaciologists at the University of Iceland likes to say, "We've turned a very cold subject into a hot one."

Simons: Iceland is actually a very good place to learn about how glaciers will react to climate change. We can watch these glaciers on a seasonal basis and see how they respond to temperature variation rather than trying to compare the behavior of those glaciers in Antarctica that have yet to experience surface melting to what we think their behavior might be 50 years from now. But for me, glaciology has always been interesting in itself. My job is to study the mechanics of the earth and how it deforms. And the cryosphere is just as much a part of that as the crust.


Simons's initial exploratory campaign on Iceland's glaciers was partially supported by the Terrestrial Hazard Observation and Reporting (THOR) Center at Caltech, funded by an endowed gift from Foster and Coco Stanback. Current efforts are supported by NASA.

Cynthia Eller
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Caltech Appoints Diana Jergovic to Newly Created Position of Vice President for Strategy Implementation

Caltech has named Diana Jergovic as its vice president for strategy implementation. In the newly created position, Jergovic will collaborate closely with the president and provost, and with the division chairs, faculty, and senior leadership on campus and at the Jet Propulsion Laboratory, to execute and integrate Caltech's strategic initiatives and projects and ensure that they complement and support the overall education and research missions of the campus and JPL. This appointment returns the number of vice presidents at the Institute to six.

"Supporting the faculty is Caltech's highest priority," says Edward Stolper, provost and interim president, "and as we pursue complex interdisciplinary and institutional initiatives, we do so with the expectation that they will evolve over a long time horizon. The VP for strategy implementation will help the Institute ensure long-term success for our most important new activities."

In her present role as associate provost for academic and budgetary initiatives at the University of Chicago, Jergovic serves as a liaison between the Office of the Provost and the other academic and administrative offices on campus, and advances campus-wide strategic initiatives. She engages in efforts spanning every university function, including development, major construction, and budgeting, as well as with faculty governance and stewardship matters. Jergovic also serves as chief of staff to University of Chicago provost Thomas F. Rosenbaum, Caltech's president-elect.

"In order to continue Caltech's leadership role and to define new areas of eminence, we will inevitably have to forge new partnerships and collaborations—some internal, some external, some both," Rosenbaum says. "The VP for strategy implementation is intended to provide support for the faculty and faculty leaders in realizing their goals for the most ambitious projects and collaborations, implementing ideas and helping create the structures that make them possible. I was looking for a person who had experience in delivering large-scale projects, understood deeply the culture of a top-tier research university, and could think creatively about a national treasure like JPL."

"My career has evolved in an environment where faculty governance is paramount," Jergovic says. "Over the years, I have cultivated a collaborative approach working alongside a very dedicated faculty leadership. My hope is to bring this experience to Caltech and to integrate it into the existing leadership team in a manner that simultaneously leverages my strengths and allows us together to ensure that the Institute continues to flourish, to retain its position as the world's leading research university, and to retain its recognition as such."

Prior to her position as associate provost, Jergovic was the University of Chicago's assistant vice president for research and education, responsible for the financial management and oversight of all administrative aspects of the Office of the Vice President for Research and Argonne National Laboratory. She engaged in research-related programmatic planning with a special emphasis on the interface between the university and Argonne National Laboratory. This ranged from the development of the university's Science and Technology Outreach and Mentoring Program (STOMP), a weekly outreach program administered by university faculty, staff, and students in low-income neighborhood schools on the South Side of Chicago, to extensive responsibilities with the university's successful bid to retain management of Argonne National Laboratory.

From 1994 to 2001, Jergovic was a research scientist with the university-affiliated National Opinion Research Center (NORC) and, in 2001, served as project director for NORC's Florida Ballot Project, an initiative that examined, classified, and created an archive of the markings on Florida's 175,000 uncertified ballots from its contested 2000 presidential election.

Jergovic earned a BS in psychology and an MA and PhD in developmental psychology, all from Loyola University Chicago, and an MBA from the Booth School of Business at the University of Chicago.

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Michael Gurnis Receives Geology Award

The American Association of Petroleum Geologists (AAPG) named Michael Gurnis, Caltech's John E. and Hazel S. Smits Professor of Geophysics, a 2014 recipient of the Wallace E. Pratt Memorial Award.

The award honors the authors of the best original article published in the AAPG Bulletin each year. Gurnis shares the $1,500 cash award with coauthor Sonja Spasojevic (MS '07, PhD, '11) for their 2012 article "Sea level and vertical motion of continents from dynamic earth model since the Late Cretaceous." The article was a key part in Spasojevic's 2010 Caltech doctoral thesis.

"I am very grateful to the American Association of Petroleum Geologists for the recognition of our work linking long-term sea level change to the dynamics of plate tectonics and the earth's deep interior," says Gurnis. "I would also like to acknowledge the generous support of Statoil, the Norwegian oil company, which provided the primary support for this work."

Gurnis' research at Caltech centers on local, regional, and global geological and geophysical observations to better understand the processes related to plate tectonics and deep mantle dynamics. His lab develops sophisticated computational models to study geological and geophysical properties. In addition, Gurnis is director of the Seismological Laboratory at Caltech, an internationally recognized center for the study of earthquakes, seismic monitoring, and fundamental science that underpins geophysics and seismology.

The Wallace E. Pratt Memorial Award is named in honor of a founding member and fourth president of the AAPG.

Brian Bell
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Detection of Water Vapor in the Atmosphere of a Hot Jupiter

Caltech researchers develop a new technique to find water vapor on extrasolar planets

Although liquid water covers a majority of Earth's surface, scientists are still searching for planets outside of our solar system that contain water. Researchers at Caltech and several other institutions have used a new technique to analyze the gaseous atmospheres of such extrasolar planets and have made the first detection of water in the atmosphere of the Jupiter-mass planet orbiting the nearby star tau Boötis. With further development and more sensitive instruments, this technique could help researchers learn about how many planets with water—like Earth—exist within our galaxy.

The new results are described in the February 24 online version of The Astrophysical Journal Letters.

Scientists have previously detected water vapor on a handful of other planets, but these detections could only take place under very specific circumstances, says graduate student Alexandra Lockwood, the first author of the study. "When a planet transits—or passes in orbit in front of—its host star, we can use information from this event to detect water vapor and other atmospheric compounds," she says. "Alternatively, if the planet is sufficiently far away from its host star, we can also learn about a planet's atmosphere by imaging it."

However, significant portions of the population of extrasolar planets do not fit either of these criteria, and there was not really a way to find information about the atmospheres of these planets. Looking to resolve this problem, Lockwood and her adviser Geoffrey Blake, professor of cosmochemistry and planetary sciences and professor of chemistry, applied a novel technique for finding water in a planetary atmosphere. Other researchers had used similar approaches previously to detect carbon monoxide in tau Boötis b.

The method utilized the radial velocity (RV) technique—a technique commonly used in the visible region of the spectrum to which our eyes are sensitive—for discovering non-transiting exoplanets. Using the Doppler effect, RV detection traditionally determines the motion of a star due to the gravitational pull of a companion planet; the star moves opposite that of the orbital motion of the planet, and the stellar features shift in wavelength. A large planet or a planet closer to its host star provides a larger shift.

Lockwood, Blake, and their colleagues expanded the RV technique into the infrared to determine the orbit of tau Boötis b around its star, and added further analysis of the light shifts via spectroscopy—an analysis of the light's spectrum. Since every compound emits a different wavelength of light, this unique light signature allows the researchers to analyze molecules that make up the planet's atmosphere. Using data of tau Boötis b from the Near Infrared Echelle Spectrograph (NIRSPEC) at the W. M. Keck Observatory in Hawaii, the researchers were able to compare the molecular signature of water to the light spectrum emitted by the planet, confirming that the atmosphere did indeed include water vapor.

"The information we get from the spectrograph is like listening to an orchestra performance; you hear all of the music together, but if you listen carefully, you can pick out a trumpet or a violin or a cello, and you know that those instruments are present," she says. "With the telescope, you see all of the light together, but the spectrograph allows you to pick out different pieces; like this wavelength of light means that there is sodium, or this one means that there's water."

In addition to using the spectrographic technique to study the planet's atmospheric composition, the method also provides a way for researchers to analyze the mass of planets. "They're actually two separate findings, but they're both very exciting," says Lockwood. "When you're doing calculations to look for the atmospheric signature—which tells you that there's water present—you also determine the 3-D motion of the star and the planet in the system. With this information, if you also know the mass of the star, you can determine the mass of the planet," she says.

Previous RV methods for measuring a planet's mass could only determine the planet's indicative mass—an estimation of its minimum mass, which might be much less than its actual mass. This new technique provides a way to measure the true mass of a planet since both light from the star and the planet are detected, which is critical for understanding how planets and planetary systems form and evolve.

Although the technique promises to augment how planetary scientists analyze the properties of extrasolar planets, it has limitations, the researchers say. For example, the technique is presently limited to so-called "hot Jupiter" gas giant planets like tau Boötis b—those that are large and orbit very close to their host star.

"The technique is limited by the light-collecting power and wavelength range of the telescope, and even with the incredible collecting area of the Keck Observatory mirror on the high, dry summit of Mauna Kea we can basically only analyze hot planets that are orbiting bright stars, but that could be expanded in the future as telescopes and infrared spectrographs improve," Lockwood says. In the future, in addition to analyzing cooler planets and dimmer stars, the researchers plan to continue looking for and analyzing the abundance of other molecules that might be present in the atmosphere of tau Boötis b.

"While the current state of the technique cannot detect earthlike planets around stars like the Sun, with Keck it should soon be possible to study the atmospheres of the so-called 'super-Earth' planets being discovered around nearby low-mass stars, many of which do not transit," Blake says. "Future telescopes such as the James Webb Space Telescope and the Thirty Meter Telescope (TMT) will enable us to examine much cooler planets that are more distant from their host stars and where liquid water is more likely to exist."

The findings appear in the The Astrophysical Journal Letters in a paper titled "Near-IR Direct Detection of Water Vapor in tau Boötis b." Other coauthors include collaborators from Pennsylvania State University, the Naval Research Laboratory, the University of Arizona, and the Harvard-Smithsonian Center for Astrophysics. The work was funded by the National Science Foundation Graduate Research Fellowship Program, the David and Lucile Packard and Alfred P. Sloan foundations, and the Penn State Center for Exoplanets and Habitable Worlds.

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Monday, April 7, 2014
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The First 50 Years of Planetary Science

If you ask Andy Ingersoll how Caltech has contributed to our understanding of the universe, he will tell you, "Caltech invented planetary science!" Indeed, the existence of planetary science as a scientific discipline springs largely from the vision of the late Robert P. Sharp (BS'34, MS'35), chair of Caltech's Division of Geological Sciences from 1952 to 1968, who sought a new frontier for geology. And since the field's origins just fifty years ago, Caltech has become one of the top centers of planetary science research in the world.

In the early 1960s, several Caltech faculty members were advocating deep-ocean studies of the sea floor, which would have meant acquiring a deep-ocean research vessel—or partnering with an institution that owned one. Instead, as Sharp explained in a video on the program's history, "We chose to go into space science because the bus, in a sense, was right at our back door." Indeed, NASA's Jet Propulsion Laboratory, which Caltech had founded as a rocket research facility during World War II, had claimed unmanned spaceflight as its turf by putting America's first successful satellite, Explorer 1, into orbit a mere five years earlier. "We had a greater chance for uniqueness by allying with the space program," he continued. Sharp; Bruce Murray, Caltech's first planetary science professor; and Caltech physicist Robert Leighton were key members of the science team for JPL's Mariner 4, the first spacecraft to reach Mars. The 5.2 million bits of data it returned—including 21 pictures of the Martian surface—allowed us to begin comparing Earth and its brethren firsthand. Today, planetary science has grown to encompass the study of solar systems beyond our own.

At a symposium held on Thursday, February 6, to celebrate planetary science at Caltech, a panel of speakers exemplified this expansion while illustrating the Institute's continued diversity of approaches to the field. Planetary scientists John Grotzinger and Bethany Ehlmann gave the latest updates from the rover-based exploration of Mars. Planetary scientist David Stevenson talked about the gas giants of the outer solar system, all of which have been visited by at least one JPL spacecraft. Planetary astronomer Mike Brown, who got Pluto demoted to a minor planet, spoke of the hordes of objects at the outer limits of the solar system that he expects to discover in April using a Japanese telescope on the summit of Hawaii's Mauna Kea. Physics professor Ed Stone, the project scientist for JPL's Voyager mission, described the journey of the twin spacecraft into interstellar space; while astronomer Gregg Hallinan and planetary scientist Heather Knutson outlined different approaches to finding habitable worlds circling nearby suns.

Says Stevenson, "Bob Sharp was always interested in doing something different, something new. We're now roaming Mars, finding new worlds in the outer solar system and starting to understand planets around other stars. He would be delighted with how it turned out."

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