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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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Researchers have mapped what may be salt deposits from the ocean below the ice onto the Jovian moon's surface.
Friday, October 30, 2015
Beckman Institute Auditorium – Beckman Institute

Teaching Statement Workshop

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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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.

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Wednesday, November 11, 2015
Center for Student Services 360 (Workshop Space) – Center for Student Services

Communication Strategies for Tutoring and Office Hours

Friday, October 23, 2015
Winnett Lounge – Winnett Student Center

TeachWeek Caltech Capstone Panel

Friday, October 16, 2015
Center for Student Services 360 (Workshop Space) – Center for Student Services

Course Ombudsperson Training, Fall 2015

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Wet Paleoclimate of Mars Revealed by Ancient Lakes at Gale Crater

A paper published on Oct. 9 in Science by members of the Mars Science Laboratory team describes ancient water flows and lakes on Mars, and what this might mean about the ancient climate.

We have heard the Mars exploration mantra for more than a decade: follow the water. In a new paper published October 9, 2015, in the journal Science, the Mars Science Laboratory (MSL) team presents recent results of its quest to not just follow the water but to understand where it came from, and how long it lasted on the surface of Mars so long ago.

The story that has unfolded is a wet one: Mars appears to have had a more massive atmosphere billions of years ago than it does today, with an active hydrosphere capable of storing water in long-lived lakes. The MSL team has concluded that this water helped to fill Gale Crater, the MSL rover Curiosity's landing site, with sediment deposited as layers that formed the foundation for the mountain found in the middle of the crater today.

Curiosity has been exploring Gale Crater, which is estimated to be between 3.8 billion and 3.6 billion years old, since August 2012. In mid-September 2014, the rover reached the foothills of Aeolis Mons, a three-mile-high layered mountain nicknamed "Mount Sharp" in honor of the late Caltech geologist Robert Sharp. Curiosity has been exploring the base of the mountain since then.

"Observations from the rover suggest that a series of long-lived streams and lakes existed at some point between 3.8 billion to 3.3 billion years ago, delivering sediment that slowly built up the lower layers of Mount Sharp," says Ashwin Vasavada (PhD '98), MSL project scientist. "However, this series of long-lived lakes is not predicted by existing models of the ancient climate of Mars, which struggle to get temperatures above freezing," he says.

This mismatch between the predictions of Mars's ancient climate that arise from models developed by paleoclimatologists and indications of the planet's watery past, as interpreted by geologists, bears similarities to a century-old scientific conundrum—in this case, about Earth's ancient past.

At the time, geologists first began to recognize that the shapes of the continents matched each other, almost like scattered puzzle pieces, explains John Grotzinger, Caltech's Fletcher Jones Professor of Geology, chair of the Division of Planetary and Geological Sciences, and lead author of the paper. "Aside from the shapes of the continents, geologists had paleontological evidence that fossil plants and animals in Africa and South America were closely related, as well as unique volcanic rocks suggestive of a common spatial origin. The problem was that the broad community of earth scientists could not come up with a physical mechanism to explain how the continents could plow their way through Earth's mantle and drift apart. It seemed impossible. The missing component was plate tectonics," he says. "In a possibly similar way, we are missing something important about Mars."

As Curiosity has trekked across Gale Crater, it has stopped to examine numerous areas of interest. All targets are imaged, and soil samples have been scooped from some; the rocks in a select few places have been drilled for samples. These samples are deposited into the rover's onboard laboratories. Using data from these instruments, as well as visual imaging from the onboard cameras and spectroscopic analyses, MSL scientists have pieced together an increasingly coherent and compelling story about the evolution of this region of Mars.

Before Curiosity landed on Mars, scientists proposed that Gale Crater had filled with layers of sediments. Some hypotheses were "dry," implying that the sediments accumulated from wind-blown dust and sand, whereas others focused on the possibility that sediment layers were deposited in ancient streams and lakes. The latest results from Curiosity indicate that these wetter scenarios were correct for the lower portions of Mount Sharp. Based on the new analysis, the filling of at least the bottom layers of the mountain occurred mostly by ancient rivers and lakes.

"During the traverse of Gale, we have noticed patterns in the geology where we saw evidence of ancient fast-moving streams with coarser gravel as well as places where streams appear to have emptied out into bodies of standing water," Vasavada says. "The prediction was that we should start seeing water-deposited, fine-grained rocks closer to Mount Sharp. Now that we've arrived, we're seeing finely laminated mudstones in abundance." These silty layers in the strata are interpreted as ancient lake deposits.

"These finely laminated mudstones are very similar to those we see on Earth," says Woody Fischer, professor of geobiology at Caltech and coauthor of the paper. "The scale of lamination—which occurs both at millimeter and centimeter scale—represents the settling of plumes of fine sediment through a standing body of water. This is exactly what we see in rocks that represent ancient lakes on Earth." The mudstone indicates the presence of bodies of standing water in the form of lakes that remained for long periods of time, possibly repeatedly expanding and contracting during hundreds to millions of years. These lakes deposited the sediment that eventually formed the lower portion of the mountain.

"Paradoxically, where there is a mountain today there was once a basin, and it was sometimes filled with water," says Grotzinger. "Curiosity has measured about 75 meters of sedimentary fill, but based on mapping data from NASA's Mars Reconnaissance Orbiter and images from Curiosity's cameras, it appears that the water-transported sedimentary deposition could have extended at least 150–200 meters above the crater floor, and this equates to a duration of millions of years in which lakes could have been intermittently present within the Gale Crater basin," Grotzinger says. Furthermore, the total thickness of sedimentary deposits in Gale Crater that indicate interaction with water could extend higher still—up to perhaps 800 meters above the crater floor, and possibly representing tens of millions of years.

But layers deposited above that level do not require water as an agent of deposition or alteration. "Above 800 meters, Mount Sharp shows no evidence of hydrated strata, and that is the bulk of what forms Mount Sharp. We see another 4,000 meters of nothing but dry strata," Grotzinger says. He suggests that perhaps this segment of the crater's history may have been dominated by eolian, or wind-driven, deposition, as was once imagined for the lower part explored by Curiosity. This occurred after the wet period that built up the base of the mountain.

A lingering question surrounds the original source of the water that carried sediment into the crater. For flowing water to have existed on the surface, Mars must have had a thicker atmosphere and warmer climate than has been theorized for the time frame bookending the intense geological activity in Gale Crater. Evidence for this ancient, wetter climate exists in the rock record. However, current models of this paleoclimate—factoring in estimates of the early atmosphere's mass, composition, and the amount of energy it received from the sun—come up, quite literally, dry. Those models indicate that the atmosphere of Mars could not have sustained large quantities of liquid water.

Yet the rock record discovered at Gale Crater suggests a different scenario. "Whether it was snowfall or rain, you have geologic evidence for that moisture accumulating in the highlands of the Gale Crater rim," Grotzinger says. In the case of Gale Crater, at least some of the water was supplied by the highlands that form the crater rim, but groundwater discharge—a standard explanation to reconcile wet geologic observations with dry paleoclimatic predictions—is unlikely in this area. "Right on the other side of Gale's northern rim are the Northern Plains. Some have made the argument that there was a northern ocean sitting out there, and that's one way to get the moisture that you need to match what we are seeing in the rocks." Pinpointing the possible location of an ocean, however, does not help to explain how that water managed to exist as a liquid for extended periods of time on the surface.

As climatologists try to develop new atmospheric models, help should be coming from the continuing explorations by Curiosity. "There are still many kilometers of Mars history to explore," says Fischer. He thinks that some of the most exciting data yet may come in the next few years as Curiosity climbs higher on Mount Sharp. "The strata will reveal Gale's early history, its story. We know there are rocks that were deposited underwater, in the lake. What is the chemistry of these rocks? That lake represented an interface between the water and the atmosphere, and should tell us important things about the environment of the time."

"We have tended to think of Mars as being simple," adds Grotzinger. "We once thought of the earth as being simple, too. But the more you look into it, questions come up because you're beginning to fathom the real complexity of what we see on Mars. This is a good time to go back to reevaluate all our assumptions. Something is missing somewhere." 

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Alumnus Arthur McDonald Wins 2015 Nobel Prize in Physics

Arthur B. McDonald (PhD '70), director of the Sudbury Neutrino Observatory (SNO) in Ontario, Canada, and Takaaki Kajita, at the University of Tokyo, Kashiwa, Japan, have shared the 2015 Nobel Prize in Physics for the discovery that neutrinos can change their identities as they travel through space.

McDonald and Kajita lead two large research teams whose work has upended the standard model of particle physics and settled a debate that has raged since 1930, when the neutrino's existence was first proposed by physicist Wolfgang Pauli. Pauli initially devised the neutrino as a bookkeeping device—one to carry away surplus energy from nuclear reactions in stars and from radioactive decay processes on Earth. In order to make the math work, he gave it no charge, almost no mass, and only the weakest of interactions with ordinary matter. Billions of them are coursing through our bodies every second, and we are entirely unaware of them.

There are three types of neutrinos—electron, muon, and tau—and they were, for many years, assumed to be massless and immutable. The technology to detect electron neutrinos emerged in the 1950s, and it slowly became apparent that as few as one-third of the neutrinos the theorists said the sun should be emitting were actually being observed. Various theories were proposed to explain the deficit, including the possibility that the detectable electron neutrinos were somehow transmuting into their undetectable kin en route to Earth.

Solving the mystery of the missing neutrinos would require extremely large detectors in order to catch enough of the elusive particles to get accurate statistics. Such sensitive detectors also require enormous amounts of shielding to avoid false readings.

The University of Tokyo's Super-Kamiokande neutrino detector, which came online in 1996, was built 1,000 meters underground in a zinc mine. Its detector, which counts muon neutrinos and records their direction of travel, found fewer cosmic-ray neutrinos coming up through the Earth than from any other direction. Since they should not be affected in any way by traveling through the 12,742-kilometer diameter of our planet, Kajita and his colleagues concluded that the extra distance had given them a little extra time to change their identities.

McDonald's SNO, built 2,100 meters deep in a nickel mine, began taking data in 1999. It has two counting systems. One is exclusively sensitive to electron neutrinos, which are the type emitted by the sun; the other records all neutrinos but does not identify their types. The SNO also recorded only about one-third of the predicted number of solar electron-type neutrinos—but the aggregate of all three types measured by the other counting systems matched the theory.

The conclusion, for which McDonald and Kajita were awarded the Nobel Prize, was that neutrinos must have a nonzero mass. Quantum mechanics treats particles as waves, and the potentially differing masses associated with muons and taus gives them different wavelengths. The probability waves of the three particle types are aligned when the particle is formed, but as they propagate they get out of synch. Therefore, there is a one-third chance of seeing any particular neutrino in its electron form. Because these particles have this nonzero mass, their gravitational effects on the large-scale behavior of the universe must be taken into account—a profound implication for cosmology.

McDonald came to Caltech in 1965 to pursue a PhD in physics in the Kellogg Radiation Laboratory under the mentorship of the late Charles A. Barnes, professor of physics, emeritus, who passed away in August 2015. "Charlie Barnes was a great mentor who was very proud of his students," says Bradley W. Filippone, professor of physics and a postdoctoral researcher under Barnes. "It is a shame that Charlie didn't get to see Art receive this tremendous honor."

A native of Sydney, Canada, McDonald received his bachelor of science and master's degrees, both in physics, from Dalhousie University in Halifax, Nova Scotia, in 1964 and 1965, respectively. After receiving his doctorate, he worked for the Chalk River Laboratories in Ontario until 1982, when he became a professor of physics at Princeton University. He left Princeton in 1989 and became a professor at Queen's University in Kingston, Canada; the same year, he became the director of the SNO. In 2006, he became the holder of the Gordon and Patricia Gray Chair in Particle Astrophysics, a position he held until his retirement in 2013.

Among many other awards and honors, McDonald is a fellow of the American Physical Society, the Royal Society of Canada, and of Great Britain's Royal Society. He is the recipient of the Killam Prize in the Natural Sciences; the Henry Marshall Tory Medal from the Royal Society of Canada, its highest award for scientific achievement; and the European Physics Society HEP Division Giuseppe and Vanna Cocconi Prize for Particle Astrophysics.

To date, 34 Caltech alumni and faculty have won a total of 35 Nobel Prizes. Last year, alumnus Eric Betzig (BS '83) received the Nobel Prize in Chemistry.

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Alumnus Arthur McDonald Wins 2015 Nobel Prize in Physics
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Summer Interns Return with a World of Experiences

Caltech undergraduate students returned to campus this week, many after spending the summer working at companies in biotechnology, technology, and finance, among other fields. These students have had the opportunity to learn firsthand about the career opportunities and paths that may be available to them after graduation. They also had the chance to put Caltech's rigorous academic and problem-solving training to the test.

In the summer of 2015, nearly a third of returning sophomores, juniors, and seniors were placed in an internship position through Caltech's Summer Undergraduate Internship Program (SUIP). The program, run through the Institute's Career Development Center (CDC), helps connect current undergraduate students with a wide range of companies and businesses that can provide practical skills and work experiences that give the students an edge in the future job market.

Many undergraduates find paid summer internships through the CDC, says Lauren Stolper, the director of fellowships, advising, study abroad, and the CDC. The center organizes fall and winter career fairs and offers workshops related to finding internships; provides individual advising on internship options and conducting a job hunt for an internship; organizes interviews for students through its on-campus recruiting program; and provides web-based internship listings and company information through Techerlink, its online job-posting system.

Through the formal establishment of SUIP two years ago—thanks, in part, to the initiative of Craig SanPietro (BS '68, engineering; MS '69, mechanical engineering) and with seed money provided by him and three of his alumni friends and former Dabney House roommates, Peter Cross (BS '68, engineering), Eric Garen (BS '68, engineering), and Charles Zeller (BS '68, engineering)—the CDC has been able to dedicate even more time and attention to helping undergraduates secure these important positions, Stolper says.

"Through internships, students have the opportunity to learn more about the practical applications of their knowledge by contributing to ongoing projects under the guidance of professionals," says Aneesha Akram, a career counselor for internship development/advising, who oversees SUIP.

"Completing summer internships help undergraduates become competitive candidates for full-time positions," says Akram. "When it comes to recruiting for full-time positions, companies seek out candidates with previous internship experience. We have found that many large companies extend return offers and full-time conversions to students who previously interned with them."

The infographic at the above right provides a snapshot of Caltech undergraduate internships over this past summer. Students seeking internships for next summer can contact Akram or look at the CDC website for more information.

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A World of Experiences
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