Postdoc Spotlight: Geochemist Jonathan Tucker

Jonathan Tucker joined the Department of Terrestrial Magnetism's geochemistry group in November 2016. Photo by Roberto Molar Candanosa, DTM.
Wednesday, April 11, 2018 

There are nearly 3,700 confirmed exoplanets out there, with some 900 of those being terrestrial worlds. But as far as we know, none of them have that pleasant, habitable, and gentle environment that we can call home.

How the Earth evolved to offer the right conditions for human life is a question astronomers devote lifetimes to explore. For Jonathan Tucker, a postdoc in the geochemistry group at Carnegie's Department of Terrestrial Magnetism (DTM), that question might well be answered from a geological standpoint.

Tucker's research focuses on the geological processes that have shaped the Earth since its formation 4.5 billion years ago. In this Postdoc Spotlight, he discusses his research in a career that included stints in astronomy and planetary science before delving into geoscience.

DTM: What exactly does your research involve?

JT: I study volatiles in the Earth, which include the noble gases, carbon, and water. They're volatile—meaning that they don't easily form compounds with other elements. It's not usually thought that these are important elements in rocks inside the Earth. But they actually can tell us a whole lot about Earth's formation and subsequent history, as well as modern-day processes like volcanic eruptions.

What can these volatiles tell us?

Water (hydrogen) and carbon are two of the most important elements that sustain us as living beings and sustain the Earth's surface as clement and habitable. They also affect properties of the Earth's interior such as how and where it melts. The solid Earth outgasses these volatiles in volcanoes and ingasses them at subduction zones. It's kind of like the Earth breathing. By studying volatiles in mantle-derived rocks, I am trying to quantify the fluxes of volatiles between the mantle and ocean-atmosphere both in the modern day and the distant geologic past.

The noble gases are also tracers of the long-term processes that move material between the surface and the mantle. There are a lot of radioactive isotopes with different half-lives that produce noble gases. So by studying the noble gases, we have access to clocks that record processes all the way from Earth's formation and accretion, through long-term history of plate tectonics and mantle evolution, and up until modern day.

How do you get these samples?

Many of the samples I study are from mid-ocean ridges, part of the 60,000 km chain of underwater volcanoes. The volcanoes might be a few kilometers underwater, so they can be hard to get to. Sending a vehicle down to the bottom of the ocean is tricky and expensive, so we go out above the volcanoes on a research vessel, drop a big metal basket over the side, drag it along the bottom, and pull it up. Hopefully, that basket comes up full of volcanic rocks that have erupted from the bottom of the ocean. Samples from volcanoes on land are usually much easier to get. I study both the rocks themselves and little tiny drops of magma trapped inside crystals in the rocks.

What is the goal of your research?

The ultimate question I'm trying to understand is how the Earth operates on a basic, fundamental, and planetary level. The tool I use is the volatile elements.

We don't often observe the solid Earth changing much in our human timescales. But the Earth has been very dynamic over its 4.5-billion-year history. It's that history that has set the stage and allowed for the surface that we live on to become the nice, habitable, and clement environment with which we are familiar. There's no directive of planetary science that says that a planetary surface will turn out this way. So, what are the processes that have been operating on Earth over 4.5 billion years that have led us to this point? That's really the fundamental driving question.

Jonathan TuckerPhotomicrograph of a vesicular mid-ocean ridge basalt glass. Image courtesy of Jonathan Tucker.

How did you become a geochemist?

I always have been interested in Earth science, specifically. But probably like a lot of scientists, it was a circuitous path that has led to this point.

I was always exposed to nature despite having grown up in Los Angeles. When I was very young, my family and I would go hiking in the Berkshires in Massachusetts. We collected interesting-looking rocks and put them out on display. It was very important for my family to take me out traveling, especially to natural places all over the country. In elementary school I even made my parents drive me hours into the California desert to go hunt for rocks. So I've been interested in geoscience since an early age. Also, my mom has a Ph.D. in biochemistry, so I was always around scientific thought and conversation.

I took a long break from Earth science after elementary school, and I was more interested in math, physics, and chemistry. I had a little stint with oceanography, and then I studied astronomy pretty extensively in college. I actually majored in chemistry and astronomy. I took an intro geology class, and it was really interesting, but I didn't know if that was the direction I wanted to go. I was more interested in doing astronomy.

I ended up working on NASA's Mars Curiosity Rover mission while I was in college and for a year afterwards, before grad school. That experience changed my focus from astronomy to planetary science. But this was also before the rover landed on? Mars, so I was mostly working on instrument development, and was itching to do some actual science. Eventually I realized that while studying Mars was really interesting, there's actually a whole lot that we don't know about the Earth as well. So that experience brought me back full circle to my interest in Earth science that I had from a very young age.

How are planetary and Earth science different?

There's probably more that makes them similar than makes them different. Mission science, like what I was involved with on the Mars rover, is a particular part of planetary science. It's very exciting, but it doesn't really fit with my philosophy of science. They go to another planet, take as much data as they can, and then could take decades before they really understand what's going on. That's because the data they have is a tiny snapshot from the planet's surface. On Mars, you have a very limited toolset that you can use to investigate the planet. But on Earth, you can approach a question from every angle. You say, "well, I don't understand this aspect of it, so instead of waiting ten years for another mission, I'm going to do another type of measurement that might solve that question." You can be much more holistic about the science, and I think that's really what sort of meshes with my philosophy towards science. It's just a different approach to science, not that one is better than the other.

What would you be if you weren't a geoscientist?

If I was in another science, it would be astronomy. If I weren't a scientist, maybe I would be an engineer or something like that.

When I was in high school, I did a summer program based on observational astronomy. The goal of the program was to make photographic observations of an asteroid (in my case, 88 Thisbe), and from these observations calculate the asteroid's orbital trajectory around the Sun. We had to learn and perform all the techniques of observational astronomy. We also had to learn all of the calculus, the celestial mechanics, spherical geometry, and computer programming in order to accomplish this. It was a fantastic program and had a lot to do with my confidence in terms of being able to really perform science.

I sort of came of age scientifically and intellectually through that program. That was also the time when people were discovering the first exoplanets—people like Paul Butler, who I actually met at the program! Since then, exoplanet research has revolutionized an entire field of science. I could have gotten in on the ground floor of exoplanet research had I gone in that direction. And that might very well be what I would have been doing.

How did you get to DTM?

My Ph.D. advisor, Sujoy Mukhopadhyay, was actually a postdoc here with Larry Nittler back in the not too distant past. Another lab mate of mine from grad school, Rita Parai, was also a postdoc here and left shortly before I arrived. So, I was very well aware of this place, and I knew that it was a fantastic place to do the type of research I wanted to do. It was also an opportunity to study something a little bit different than what I had been doing in my Ph.D. I was mostly studying the noble gases, and I wanted to switch to studying other volatiles like water and carbon, which I'm doing now with Erik Hauri. So, it was a great opportunity to gain experience in different elemental systems, instrumental techniques, and types of data and analysis, while still falling within the same broad scope of my main interests in volatiles on Earth.

What has it been like since you got to DTM?

This is a wonderful community. There are a lot of people who are specifically interested in the things I'm interested in, so it's great to be able to have very in-depth conversations, as well as getting different perspectives in the types of issues that I'm interested in. But it's also great that there are a lot of people who are studying things peripherally related to my field or not related at all, and I enjoy getting to know what they do. It's sort of a unique community in that sense. If I were sitting in some postdoc office at a university, I would have no idea what the astronomers were doing, no idea what the material scientists were doing.

What have you been up to here?

I mostly have been studying carbon, working on a project to measure how much carbon dioxide is naturally coming out of the Hawaiian volcanoes. This is for many purposes. It's a good number to know how much carbon is being emitted naturally by the Earth and be able to compare that to anthropogenic emissions. I would say the amount of carbon coming naturally out of the Earth is actually surprisingly not well known. It's known in kind of an order of magnitude sense, but there's a lot of disagreement in the scientific literature regarding the specific details.

The amount of carbon coming out of Hawaii is also very important because the material coming out of the Hawaiian volcanoes originates in the very deep mantle, possibly very close to Earth's core. So Hawaii is a great place to look if we want to understand how material like carbon moves globally through the mantle because it is sampling a really deep source.

What kind of research do you wish to pursue in the future?

I definitely want to get back into the noble gas field. The community of noble gas geochemists is not very large, but I think my experience here at DTM also gives me a unique ability and opportunity to integrate noble gas studies with studies of other volatiles, such as carbon and water. In the end it's not really the noble gas systems we want to know about. We're just using them as a tool, and in a lot of ways, we're just using them to understand the other volatiles like carbon, water, and things that do have an influence on the mantle and the surface. Integrating noble gas measurements with measurements of other volatiles is something I've started here and want to continue doing.

Earth, a dynamic planet, as seen on July 6, 2015. Credit: NASA.

What do you think will be different in the next 100 or so years in your field?

My field of science has only been around for about 40 years, so that's very hard to know. One major advance will come in studying Earth's formation and earliest history. This has been an elusive pursuit because there are no geological materials that remain from Earth's formation 4.5 billion years ago. But we're discovering ways of getting around that problem using isotopic tracers. These can be extremely difficult measurements to make, and they often end up giving negative results. But the development of these isotopic tools to study the very early conditions of the Earth, how it formed, and how it went from being a violent chaotic place during its initial formation will be a major advancement that's starting even now.

So would you say that in a way, your work is actually related to astronomy?

Absolutely, yes. One of the big fields in astronomy right now is exoplanets and studying solar systems beyond our own. We used to think of our Solar System as the example: A nice ordered system with a bunch of planets in circular orbits, small and rocky in the inside and big and gaseous on the outside. We're finding out that is not true at all in the rest of the galaxy. Some of the most important questions guiding the exoplanet community are: Can we use our own Solar System as an example for other systems in the galaxy? Are our Solar System and the Earth unique? Are they special, or are they just one of many random outcomes? I am addressing those same questions, just by looking down, rather than up.

Learn more about Jonathan Tucker at

—Roberto Molar Candanosa