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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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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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New Courses for the 2015–16 School Year

The start of the 2015–2016 school year brings not only new freshmen and faculty, but also new courses.

Several new classes have been added in the Division of the Humanities and Social Sciences. These include a course on Old English Literature, in which students will study literature written in the earliest form of the English language, commonly used in England from roughly 450 to 1100 AD. The new course will be taught by the Weisman Postdoctoral Instructor in Medieval British Literature, Benjamin Saltzman.

"When we speak and write in English, we rarely think about the paths the language took to get to where it is today with all its quirks and varieties: why is it that we say 'one mouse,' but 'two mice'? And if you can figure that out, then why do we say 'two houses'?" Saltzman says. "Once we take a closer look at this early stage of the language, we gain access to some extraordinary pieces of literature—from riddles to poems about war—and in the process we'll learn about some of the idiosyncrasies that have persisted in the modern form of the language."

The Division of Engineering and Applied Science is also introducing three new interdisciplinary mechanical engineering courses, one of which is the Mechanics of Soils. The class will be taught by Professor of Mechanical and Civil Engineering Domniki Asimaki, and will focus on the basic principles of stiffness, deformation, stress, and strength of soils, sands, clays, and silts.

"Soils are very heterogeneous materials. Some are plastic like soft clays, others are brittle like cemented sands, and others are purely frictional like granular media. More frequently we see some mix of these," Asimaki says. "The top few hundred meters of the earth's crust, where most of the infrastructure of modern cities is founded on, is roughly made of 'soils'. Thus, we want to make predictions about the deformation and failure of soils, such as consolidation from groundwater pumping, slope stability failures, foundation capacity of buildings, or liquefaction of sands—so called quick-sands." The class, she says, aims to provide an understanding of soil behavior from laboratory experiments and field observations, and to develop idealized predictive models that capture aspects of that behavior.

A new course in the medical engineering department, New Frontiers in Medical Technologies, will examine space technologies, instruments, and engineering techniques with respect to their current and potential applications in medicine. The course will allow students to interact with both Caltech researchers in medical engineering and scientists at the Jet Propulsion Laboratory (JPL).

"The history of space exploration and its many spinoffs have taught us that many space technologies are very useful for on-earth medicine," says Shouleh Nikzad (PhD '90), a visiting associate in astrophysics and senior research scientist at JPL. She will teach the new course in the spring term.

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

Flat Space, Deep Learning: A Workshop by Eric Mazur

Wednesday, October 21, 2015
Beckman Institute Auditorium – Beckman Institute

The Teaching and Learning Project, a National Photographic Essay on Higher Education Featuring Caltech

Tuesday, October 20, 2015 to Wednesday, October 21, 2015
Center for Student Services 360 (Workshop Space) – Center for Student Services

Guest Consultations on Teaching, with Chris Duffy

Tuesday, October 20, 2015
Dabney Hall, Lounge – Dabney Hall

Bringing Joy into Your Teaching: A Workshop by Chris Duffy

Monday, October 19, 2015
Guggenheim 101 (Lees-Kubota Lecture Hall) – Guggenheim Aeronautical Laboratory

The Future of Teaching and Learning at Caltech: An Innovation Showcase

Monday, October 19, 2015 to Friday, October 23, 2015

TeachWeek Caltech

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