Imprimir

INTERVIEW

Igor Pacca: In search of the secrets of inner Earth

Physicist Igor Pacca trailblazed geophysics research in Brazil and studied continental drift by investigating changes in Earth’s magnetic field

Léo Ramos ChavesThe one thing that cannot be said of an interview with Igor Pacca is that he is self-centered. Asked by interviewers about his own personal and professional life experience, Pacca, with the gentleness of a man of 88, spoke at length about the professors he admires at the Institute of Astronomy, Geophysics, and Atmospheric Sciences at the University of São Paulo (IAG-USP), where he was among the first professors in the 1970s, having also served as head of the Department of Geophysics for 10 nonconsecutive years and as director from 1993 to 1997. With gusto, he also spoke about the inner earth and its ever-shifting magnetic field, which forms the basis of his field of research, paleomagnetism—or the study of the record of geomagnetic field reversals in iron oxides and other ferromagnetic materials preserved in rock. Paleomagnetism provides an understanding of the geological history of the earth and of continental drift. Pacca and his teams have published research showing that the blocks of rock that now form the Amazon were once separated from Brazil’s Midwest and Northeast by a sea and all adjoined present-day North America in an area near the South Pole.

His wife, Jesuína Pacca, and his only son, Sérgio, are professors at USP—she at the Institute of Physics and he at the School of Arts, Sciences, and Humanities. Whenever possible, Pacca and his family spend time together at his country property in Serra Negra, São Paulo State, where he grows coffee as a hobby. The photos displayed on one of the cabinets in his office at IAG-USP, where he will be found almost daily, depict places he visited on his field trips, and the laboratory where he used to work at the former IAG-USP campus in Parque do Estado. Two photos are especially noteworthy, of him shaking hands with former presidents Fernando Henrique Cardoso and Luiz Inácio Lula da Silva. “I have no prejudices,” he says before explaining that he met Cardoso in 1998 when he was dubbed Commander of the National Order of Scientific Merit, and Lula in 2007, when he received a Grand Cross of the same order.

Age 88
Field
Geophysics
Training
Undergraduate degree (1959) and PhD (1969) in physics from USP
Institution
University of São Paulo (USP)
Publications
Approximately 41 scientific articles and 6 book chapters

Why did you first take an interest in exploring the inner earth?
Because it’s fascinating, there’s lots to see and, of course, much that cannot be seen. The deepest boreholes have reached only 13 kilometers [km] before the diamond bits used to drill them softened, broke or bent. Measurements are generally made indirectly. When drilling for oil, you can get a lot of information about the earth’s interior. Other important sources of information include the seismic waves generated by earthquakes, which travel at different speeds depending on the density of the inner layers of the earth. The elastic parameters of rocks and gravitational attraction help to provide a picture of the distribution of mass in the earth’s interior. There is still much to learn. In 1910 Croatian geophysicist Andrija Mohorovičić [1857–1936] first proposed that a boundary, called the Moho discontinuity, separates the earth’s crust from its mantle at a point about 10 km beneath the oceans and 40 km beneath the continents. Another discontinuity occurs at a depth of about 3,000 km, separating the mantle from the core. This separation is incredible in the way it is drastic. There is an abrupt change not only in pressure and density, but also in materials, from solid rock in the mantle—consisting mainly of iron and magnesium silicates—to liquid iron in the core. The iron is in liquid form because it has reached melting temperature, which is dependent on pressure. On the earth’s surface, at sea level, the melting temperature of iron is 1,500 degrees Celsius [ºC]. The three states of matter are also called states of aggregation in allusion to the way atoms are banded much closer together in solids than in gases. An implication of this is that in mega-atmospheres, pressure can cancel out the action of temperature in driving atoms apart into a state of disorder. Indeed, in the inner core the pressure is so high that it overcomes the action of temperature. In the core there is also a small amount of some chemical element lighter than iron. It could be hydrogen, carbon, potassium—we don’t know what it is for sure. If it were only iron, the mass of Earth’s core would be much larger.

Where does Earth’s magnetic field—the principal subject of your research—come from?
It comes from the core. The notion that Earth is one huge bipolar magnet is quite old, dating back to the thirteenth century, but to this day the processes by which the geomagnetic field is generated are not completely understood. The energy needed to generate the geomagnetic field may come from convection driven by temperature gradients in the inner layers of the earth. This can be compared to the movement of boiling water in a pot: hotter water rises and cooler water sinks, forming convection cells. But in the earth’s interior the process is more complex as convection depends not only on temperature gradients but also on the composition of materials. Earth’s rotation also plays an important role. Theoretical models of the geomagnetic field are called dynamo models in reference to electric dynamos, which use a magnet and a coil to convert mechanical energy into electric energy. Within the earth, a large number dynamos, some stronger and some weaker, transform mechanical energy from rotation into electromagnetic energy. That is why Earth’s magnetic field is always wandering through its interior and exterior.

What has paleomagnetism shown about the history of the globe?
Paleomagnetism allows us to track the movement, collisions, and joining of continents. These movements, which require a large amount of energy, can also provide an understanding of the processes that occur in the earth’s interior. One of my studies showed that the continental block that today forms the Amazon was previously separated from Goiás and the Northeast by seas, and was closer to what is now the southern portion of Brazil and nearly joined to present-day North America. Another study led by three geophysicists here at IAG—Wilbor Poletti, Gelvam Hartmann, and Ricardo Trindade—based on fragments of ruins in southern Brazil, found evidence of how the earth’s magnetic field varied about 350 years ago (see Pesquisa FAPESP issue nos. 75 and 244). One of the most significant discoveries in this field is that the polarity of the earth’s magnetic field is always changing. About 700,000 years ago Earth’s magnetic poles were reversed: the north pole was south, and the south pole was north. Nobody knows why. Years ago the magnetic field in the Northern Hemisphere was strongest at the North Pole, but not anymore. There is a location in Siberia where the field is now strongest.

The duration of a year is always constant, but days vary in length. 400 million years ago, a year had 400 days

What are your current research interests?
I have been interested in the relationship between Earth’s rotation and the frequency of geomagnetic field reversals, which are affected by movements in the outer core. The rotation of the earth is closely linked to the geomagnetic field and its reversals, which have followed an irregular pattern over the last millions of years. The problem is that Earth rotates at varying speeds for reasons such as tidal friction due to interaction with the Moon. In addition, the earth is neither rigid nor elastic, which means it takes time to deform. The Moon opposes Earth’s rotation and its gravitational attraction causes deformation here and there, but maximum deformation will only occur after the earth has rotated further, precisely because it is not elastic. As a result of this play of forces, the duration of a year is always constant but days vary in length. An easy figure to commit to memory is that 400 million years ago, a year had 400 days. Today the earth is turning at a slower rate and therefore a year has fewer days than before. The Moon is also drifting further from Earth. There are other factors affecting the earth’s rotation. Earth’s mass distribution can change when a tectonic plate sinks into the mantle. It can also change when glaciers melt and sea levels rise. It is like a figure skater, who spins slower when she stretches her arms out and faster when she tucks them in, because of the moment of inertia and the conservation of angular momentum. Unfortunately, very limited data is available on sea level variation over time scales of millions of years, but we can derive a notion of ​​historical variation and temperatures from the ratio of two oxygen isotopes, O16 and O18. Sea water has both isotopes, but the lighter of the two, O16, evaporates more easily when temperatures increase. Conversely, O18 is more prevalent when temperatures drop and the oceans cool. By measuring the ratio of the two in ancient shells, we can determine whether temperatures rose or fell, or whether ice was built up at the poles.

Does the magnetic field affect our daily life?
The magnetic field is much weaker in an area known as the South Atlantic Anomaly, whose drift we have helped to track here at IAG. Space station crews have to protect themselves from cosmic rays with heavy shielding when they pass through the anomaly. The magnetic field wards off the cosmic rays coming from the Sun or from outside the galaxy and also affects the operation of communications satellites.

You’re a geophysicist but started out as a physicist, didn’t you?
I completed my PhD in cosmic-radiation physics under César Lattes [1924–2005]. I was officially under Celso Orsini [1929–2014] but in practice worked with Lattes, for seven years. They say no one has ever been able to work with him that long… Lattes was a genius, but very unpredictable. He never kept his appointments and would send Orsini and I in his place to meetings at CNPq [Brazilian National Council for Scientific and Technological Development] in Brasília. Lattes was a professor and researcher at UFRJ [Federal University of Rio de Janeiro] and CBPF [Brazilian Center for Physics Research] and came to USP in 1960 on invitation from Mário Schenberg [1914–1990], who was head of the Department of Physics of the School of Sciences and Languages and Literature [now FFLCH] at USP. Lattes hated competitive examinations [concurso público]. I remember he was once planning on taking an exam for a position at the National School of Philosophy in Rio. Orsini and I took his thesis and completed the registration process, but he never showed up for the exam. Then, here at USP, he insisted on organizing a concurso for a professorship at the School of Philosophy, then on Rua Maria Antonia. They launched the concurso, and again Orsini and I signed him up, and he again didn’t show up. Jayme Tiomno [1920–2011] got the post instead, but didn’t last long.

Why?
As a full professor, Tiomno was invited to give inaugural lectures at USP in 1969. The dean at USP at the time was Mário Guimarães Ferri [1918–1985], but he was traveling. In his absence he was replaced by Alfredo Buzaid [1914–1991], who was very active in the military regime. Tiomno had worked at the University of Brasília and thought it was a good opportunity to vent about what he had been through in the capital. And he really held nothing back in describing the problems he had faced in Brasília. Buzaid looked daggers at him from his desk. That same year, Buzaid was appointed Minister of Justice under the Médici administration [1969–1974], and heads soon began to roll, including Tiomno’s. Many physics professors were removed from their positions during that terrible period. When we showed up for work, we had to go through a police checkpoint first at the entrance to the campus.

Personal archive Serra Geral, a mountain range in southern Brazil, spans an area of 2,000 km2 and has dozens of basalt flowsPersonal archive

Were you under any kind of pressure?
I was not very active in politics. But I often visited Schenberg—who had also been relieved of his position and was detained at the DOPS [Department of Political and Social Order]—to discuss issues or get his signature. Ousted professors were prohibited from entering the university and were compulsorily retired. José Goldemberg [president of FAPESP since 2015] and Oscar Sala [1922–2010, president of FAPESP from 1985 to 1995] were also highly respected professors but not politically active. Sala was in fact a foreigner, born in Milan, Italy. And besides the professors, there were also students who were very involved in politics.

Why did you decide to study physics?
By a twist of fate. I was poor as a child and had to work from an early age to help my mother. My father left my family when my sister was 5 years old and I was about to be born. We lived downtown for many years, on Rua Asdrúbal do Nascimento. It was there that I got my first formal job. At 14, I started working as an office boy at Light’s construction department, earning half a minimum wage. I continued there from 1944 to 1946 then went to work at a telegraph company, All America Cables and Radio Inc. Communications were entirely by telegraph at the time. Stock prices on the London or New York stock exchange were transmitted by submarine cables—not in real time, but with a 15-minute lag. Between completing high school and starting college, I spent a gap period working to help my mother. I took an entrance examination for an engineering degree program, but failed mathematics. As there was a second exam open at the School of Philosophy, I applied for the chemistry program, but there were few vacancies left midyear and the exam was canceled. So I applied for physics and was successful, in 1959, scoring 10 in the written and oral math exams. It was a fortuitous choice, but a happy one.

And how did you get into geophysics?
In 1967, Lattes accepted a position as a full professor at the newly created Institute of Physics at UNICAMP [University of Campinas]. So he moved to Campinas and the research group at USP disbanded. I was attracted by geophysics because of the opportunity to work in a field that had to do with Brazil. It bothered me to be doing science that was disconnected from the country, but there was then nothing geophysics-related happening in Brazil. IAG was then still just an ancillary institute at USP. It came under the auspices of USP in 1955 because one of its directors was difficult to deal with and the institute had jumped from one state department to another until at one point it was left orphaned. In 1968, when they did the university reform and created education and research institutes, IAG was left out. There were vacancies for geophysics professors at the Department of Physics, but they were never filled. I began to look at the major questions of geophysics in seismology—an important field because it tells us what Earth is like in its interior—and in gravimetry, or the study of gravity. I was already friends with Umberto Cordani, a professor at the Institute of Geosciences who was instrumental in the early days of geophysics. One day Cordani told me that an English geophysicist, Kenneth Michael Creer, one of the pioneers of paleomagnetism, had created a laboratory in Curitiba but had abandoned the project and left his equipment there. In England, Creer and Patrick Blackett [1897–1974], who was president of the Royal Society, had begun to investigate magnetic field orientation in rocks from different ages in Europe.

Personal archive Pacca during a field trip in 1997 in Vila Bela da Santíssima Trindade, Mato GrossoPersonal archive

What did they find?
They found that the magnetism in these rocks had a polarity opposite to that of Earth’s current geomagnetic field. Because they believed further clues to solving the mystery could be gotten from rocks from more than one continent, Creer collected rock samples in Brazil, Argentina, and Uruguay in the 1960s to study continental drift. There was a fierce battle raging between fixists, who thought continents were immobile, and mobilists, who argued that continents wandered. Continental drift was hardly a novel idea. It was intuitive from the way the coasts of Africa and South America seemed to fit together. In 1913 Alfred Wegener [1880–1930] published the book The Origin of Continents and Oceans, but his arguments were exaggerated and, for this and other reasons, his work was not very well accepted at the time. The battle raged on until the 1970s. We spoke to Creer and he agreed to leave us his equipment, and Cordani and I went to pick it up in Curitiba. It essentially consisted of demagnetizing and Helmholtz coils. These are used to cancel out the earth’s magnetic field which, while not very strong, has to be contained to measure remanent magnetism in rocks. We set up our paleomagnetism laboratory at the Institute of Physics.

What were your first endeavors in this field?
After setting up the laboratory, we hired an Argentine geophysicist—Daniel Valencio, who had worked with Creer in England—and started working. He spent some time here and then I spent a few months in Argentina. Our first study dealt with remanent magnetism in rocks from the Abrolhos archipelago, which had been previously investigated by Cordani and could provide an understanding of the geological history of the islands since they began to form 60 million years ago. Our paper was published in 1972 in Nature Physical Science, which illustrates the growing interest the field had attracted at the time. We continued at the Physics department until 1972, when IAG was officially organized as an education and research unit in its own right at USP. Before then it was uncertain what would become of IAG—which traces its roots to the Empire. There were many possibilities, such as a merger into the Institute of Geosciences, the Institute of Physics, or the Polytechnic School, but none of these were satisfactory to IAG’s then director, Abrahão de Moraes [1917–1970], who taught calculus at the Polytechnic School. I was fortunate to study under him for two years. He wanted IAG to be an institute for training and research in astronomy, geophysics, and meteorology.

How was the fate of IAG decided?
Nothing had yet been decided at the time of Abrahão’s untimely death, in 1970. The dean was then Miguel Reale [1910–2006], who was fond of Abrahão. At the funeral, he made a speech at his tomb-side: “That institute you wanted so much, Abrahão, I’m going to do my best to make it happen.” And the institute was finally created as a training and research unit at USP in 1972. Three departments—astronomy, geophysics, and meteorology—were created under it. Astronomy wasn’t a problem as it was then already an established field of activity at the institute. In addition, three students who Abrahão had sent for doctoral study abroad—Paulo Benevides Soares [1939–2017], José de Freitas Pacheco and Sílvio Ferraz-Mello—had returned and joined the institute. In meteorology, the institute had a weather station at Parque do Estado, in the south of São Paulo City, but no professors. The then director of the institute, Giorgio Giacaglia, tried hiring foreign professors but this didn’t work well. So instead they sent Pedro Leite da Silva Dias [now director of IAG] and Maria Assumpção Dias, who had recently graduated in mathematics, to do a doctorate in the United States and later return to fill professorship and research positions in meteorology. But the geophysics department had to start from scratch. The university required the department to have at least five faculty members in three categories. One was Giacaglia himself, who was a professor at the Polytechnic School and had previously worked with dynamic geodesy. Two geologists were borrowed from the Department of Geosciences, Cordani and Koji Kawashita. I became head of the department because I had done my doctorate in physics and was the only full-time faculty member. We hired an assistant, Francisco Hiodo, and through Creer we secured a doctoral position in Scotland for Marcelo Assumpção, whom we had recently hired and who would later fill a position as professor here. I helped to select the names. In addition to serving as department head, I taught geophysics. Our first master’s degree program was taught by Márcia Ernesto, in 1978. She had earned a scientific initiation and master’s degree in paleomagnetism under me. With the department now actively training geophysicists, we became more confident it would succeed.

I was attracted by geophysics because of the opportunity to work in a field that had to do with Brazil

How were undergraduate programs first implemented at IAG?
We hoped to create undergraduate programs quickly. The meteorology program was easier as it was already an established field at the institute, and was launched in 1975. The geophysics program was more of a challenge. We faced a lot of opposition from geologists’ associations, who wanted no competition, just as mining engineers resisted competition when geology programs were created in Brazil in the late 1960s, at a time when Brazil had discovered a wealth of mineral resources but lacked skilled labor in sufficient quantity and of sufficient quality do develop them. We were able to demonstrate that geologists and geophysicists required a different set of skills, and the program finally began in 1985. The undergraduate program in astronomy came much later because, in the early days of the institute, faculty members felt that an undergraduate program was unnecessary and graduate students would suffice. Today, all three courses and departments are very strong, perhaps the strongest in Brazil, thanks to the quality of our faculty and students. In geophysics, we have made significant advances through international collaborations, dealing with scientific problems that are of interest to foreign scientists.

What scientific work are you most proud of?
I would say it was an extensive magnetism study in Serra Geral, a mountain range stretching across the South of Brazil and parts of Paraguay, Argentina, and Uruguay. It was a collaboration with Italian groups and especially Enzo Piccirillo of the University of Padua and later Trieste, and Piero Comin-Chiaramonti of Verona. Serra Geral is a plateau with an area of 2,000 km2 and dozens of basalt flows. There are places where they are almost 2 km thick. Geologists had previously studied this region, but there were two major unanswered questions. The first was the age of the basalt flows, because in the 1970s when we started our work there we were still using potassium dating methods that carried very high uncertainty. The other was the rate at which basalt flows occurred in the region. Geologists thought they had taken a long time to occur. Both of these questions were answered through paleomagnetism. We found that basalt activity in the region began about 150 million years ago, peaked at 127 million years ago, and then gradually abated. We also found that the process was relatively fast. Six flows in less than 1 million years is a short time considering the volume of lava. There was an incredibly large volume escaping through cracks in the crust of the continent formed by South America and Africa. When continents were larger than they are today, heat would build up until it melted the lithosphere—the outermost layer of Earth—and escaped. Our work in Serra Geral continued for some 20 years, training many masters and PhDs in the process.

Do you still go on field trips?
My most recent trip was in 2008. I went to Cameroon, Africa, and spent a month collecting rock samples inland. Cameroon has volcanic formations running in a more or less straight line from the ocean onto the continent. This is very relevant for anyone studying the paleomagnetism of the earth. The trip was also unforgettable for another reason. We were in backcountry in Cameroon, which has a culture that is completely different from ours, and one of the locals, seeing that one of the researchers in my group seemed to be very hard working, asked me if I would trade her for some goats.

Republish