Deciphering the mechanisms of environmental change from traces of extinction-level celestial impacts

What caused the extinction of the dinosaurs? The first thing that might come to mind is a meteorite crashing into the Earth. Assistant Professor Honami Sato, a geology researcher at the Faculty of Science, searches for traces of celestial impacts in geological strata to decipher the mechanisms of environmental changes on the planet. We spoke with Sato about her methods of stratigraphic surveying and her fascination with geology.

Discovering the Earth’s history through geological formations

- Could you tell us about your current research?

My fields of interest are geology and geochemistry. In particular, I conduct research that searches for evidence of celestial impact events in strata to elucidate the mechanisms of environmental change. A celestial impact event is just that—a phenomenon in which a celestial body strikes the surface of the Earth. Such impact events range in scale from large-scale events that may have caused the extinction of organisms that lived at the time of the impact to small events that have little or no effect on the Earth’s environment. In my research, I have been focusing on the large-scale celestial impacts.

- How do you investigate the traces of these celestial impacts?

I visit sites in Japan and abroad to collect rock samples and conduct analyses in our lab. I mostly investigate geological formations in Europe and the U.S

My main research targets strata as old as three billion years old and as recent as about 66 million years old, which is around the time the dinosaurs went extinct. I often go to Europe because it has deposits from about 200 million years ago that are still in very good condition.

- It sounds like your research takes place in some harsh environments.

In many cases, a survey site is deep in the mountains and the path is steep, so we must be well-equipped. On the way up, my backpack is nearly empty, which is fine, but I’m carrying a backpack full of rock samples on the way down, which makes that part of the hike really tough.

- How do you figure out where the impact sites are?

Large-scale celestial impacts cause craters on the planet’s surface, but craters are easily affected by the surrounding environment and disappear over time, limiting the scope of our research.

We work on the assumption that material scattered by the celestial impact must have accumulated and remained in the strata, so we use written records and other sources to identify and investigate strata around the same age as the crater formations.

- How do you determine what material to look for in your collected samples?

There are two methods. One is to use a microscope and look for particles with a round surface in the strata. These particles tend to have a rounded surface after melting in the heat of impact.

The second method focuses on highly siderophile elements like osmium, iridium, and platinum, which are abundant in astronomical bodies. Using high-precision analysis equipment, we determine if there are any highly siderophile elements in collected samples. If these elements are found in unusual concentrations, it likely indicates evidence of a celestial impact.

With either method, it is difficult to determine if the rock contains the target material at the time of collection. So, that means we have to keep repeating the process of collecting and observing a large volume of samples.

The thrill of holding a piece of the history of the universe in one’s hands

- How did you become interested in the field of geology?

When I was in high school, I wanted to study astronomy at university. Once I got there, I started studying at the School of Science, and one lecture I took was on detecting cosmic materials in strata. That sparked my interest in geology, allowing us to learn about past cosmic events through geological strata.

- It sounds exciting to be able to catch glimpses of the history of the universe.

Yes, it is. For my undergraduate thesis, I explored the possibility of finding evidence of large-scale celestial impacts in strata in Japan. It was exhilarating to discover traces of their existence along the Kiso River in Gifu Prefecture.

I think the great appeal of geology is the ability to discover cosmic materials in strata—materials that once traveled through space to reach Earth—and to feel that you are holding the history of the universe in your hands.

- What are some of your most memorable moments since becoming a researcher?

In 2016, I participated in an international project to excavate a crater formed in the same time period as the dinosaur extinction. I recall feeling emotional when we retrieved sediment from the seabed and saw the thick, unusual strata one would rarely see beneath a crater. It was a profound moment for me, realizing that such celestial events had actually occurred and had left a significant impact on the Earth's environment.

Focusing on strata reveals hidden insights

- What is unique about your laboratory?

I think our great strength is that we can detect cosmic material not from astronomical objects themselves, but from the Earth’s strata. It is difficult to detect such traces in strata because they are attenuated by the rocks that have always existed on the planet, but in my laboratory, we have refined our methodologies to make such detection possible.

- Why do you survey the strata rather than the objects themselves?

Investigating geological strata allows us to obtain high-resolution temporal data. This allows us to observe in detail the environmental changes that occurred before and after a single stratum. Studying strata reveals information not discernible through the examination of craters or celestial bodies alone. That's why I believe it's essential for in-depth investigations into environmental changes.

- Please tell us about the research for which you received the Commendation for Science and Technology by the Minister of Education, Culture, Sports, Science and Technology in 2023.

This research project was something I had been working on with my advisor and other collaborators from my student days. We were trying to determine the influx history of all kinds of cosmic materials in geological strata—everything from tiny dust particles of less than one millimeter in diameter, called cosmic dust, to celestial impact events.

Large-scale celestial impacts have a huge effect on living organisms and creatures at the time of impact by causing environmental changes. I think this research could be useful for predicting future changes in the Earth’s environment based on such cases from the distant past.

Unearthing the fun in geology

- What do you try to keep in mind when teaching students?

I bear in mind that each student has a different area of interest. So, while I prepare a range of research materials and potential survey areas, I consciously avoid making too many suggestions. Instead, I try to allow students to determine the directions and methodologies for their research on their own.

- What kind of people do you want your students to become?

Regardless of whether they go on to work in research, I hope they never forget to bring a spirit of inquiry to their work. I expect them to take pride in their work, always think proactively about what they can contribute, and strive to generate added value in whatever they do.

- What are your goals for the future?

My goal is to establish a laboratory that specializes in celestial impacts. The world is filled with traces from various epochs, with innumerable samples to study. I aim to establish a laboratory robust enough to handle all areas of investigation, a center where every aspect of celestial impacts can be explored.

The celestial impact event is the most widely recognized theory for what caused the extinction of the dinosaurs. This area of research is widely understood by the general public, especially children, and is a good starting point for outreach. My hope is to unearth opportunities for more people to learn about the joys of geology.

- Finally, do you have any advice for high school students who are still deciding whether to pursue higher education?

Geology is a fascinating field that offers glimpses into the history of the cosmos and is filled with romance and discovery, yet it is often overlooked in high school science. I'd be thrilled if you found the field as captivating as I do.

Universities offer an environment conducive to studying any number of fields across the humanities or sciences. I encourage you to discover whatever interests them and pursue it wholeheartedly.

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About Kyushu University 
Founded in 1911, Kyushu University is one of Japan's leading research-oriented institutes of higher education, consistently ranking as one of the top ten Japanese universities in the Times Higher Education World University Rankings and the QS World Rankings. The university is one of the seven national universities in Japan, located in Fukuoka, on the island of Kyushu—the most southwestern of Japan’s four main islands with a population and land size slightly larger than Belgium. Kyushu U’s multiple campuses—home to around 19,000 students and 8000 faculty and staff—are located around Fukuoka City, a coastal metropolis that is frequently ranked among the world's most livable cities and historically known as Japan's gateway to Asia. Through its VISION 2030, Kyushu U will “drive social change with integrative knowledge.” By fusing the spectrum of knowledge, from the humanities and arts to engineering and medical sciences, Kyushu U will strengthen its research in the key areas of decarbonization, medicine and health, and environment and food, to tackle society’s most pressing issues.