18 September 2014

Planetary atmospheres and climate: An interview with Dr. Jonathan Mitchell

Posted by Nanci Bompey

This post is the sixth of a series of profiles on planetary scientists by Mark Hilverda, geoscientist and web manager for AGU’s Planetary Sciences Section. A more complete version of this interview that includes a list of reading resources was published on the Planetary Sciences website.

Dr. Jon Mitchell

Dr. Jon Mitchell

Dr. Jonathan Mitchell is Assistant Professor in the Department of Earth & Space Sciences and the Department of Atmospheric and Oceanic Sciences at UCLA. Dr. Mitchell’s research interests include surface-atmosphere interactions on Titan, superrotating atmospheres, tidal interactions of synchronous satellites, and Earth’s paleoclimate.

How did you first become interested in planetary science?

It was a bit by luck. As an undergrad, I studied Physics and did research in several different areas — superconductors, nuclear physics, and cosmology. I liked cosmology the best, and so I went to the University of Chicago to pursue a graduate degree in it.

After my first two years of classwork and research, I realized cosmology just wasn’t for me. I really wanted to know more about the physics of climate. Here’s where I lucked out. The University of Chicago has a very open academic environment and so I was able to switch research direction to planetary climate with an advisor in Geophysical Sciences without having to start over or reapply.

I was also fortunate that my new advisor, Ray Pierrehumbert, wanted to significantly expand into this area and he gave me a huge amount of freedom to do what I wanted to do. We began by thinking about exoplanets, but there were so few observations back then and Cassini was about to arrive at Saturn, so we decided Titan would be a good compromise. It turned out to be a great decision!


A large portion of your research focuses on the climate and weather of Saturn’s moon Titan. What drew you to investigate these processes on this distant moon?

Picking up with the last question, I knew I wanted to learn more about the physics of climate. As we’ve come to understand now, the weather and climate of Titan is in many ways very similar to Earth’s tropical climate, in that surface temperatures are nearly uniform and weather is concentrated into specific latitudinal zones by the atmospheric circulation. In a sense, Titan is simpler than Earth because it’s all one tropical ‘climate zone’, whereas Earth has at least three climate zones. But it’s Titan’s uniqueness that makes it such an interesting study for climate science.

For example, Titan has a strong antigreenhouse effect (as well as a greenhouse), and this helps us understand extreme climates in Earth’s history like Snowball Earth and the effect of large impacts on the energy budget of Earth. Another example is the strong seasonal cycle of Titan’s weather patterns, which may help us understand the mechanisms behind Earth’s monsoons. Seeing and then modeling these global phenomena on Titan are helping us to understand similar processes on Earth.

Model vs observation: the left column shows storms from Mitchel et al's Titan General Circulation Model (GCM) and the right shows Cassini observations, which seem to compare well. From Mitchell et al., “Locally enhanced precipitation organized by planetary-scale waves on Titan”, Nature Geoscience, 4, 589-592, doi:10.1038/ngeo1219 (2011).

Model vs observation: the left column shows storms from Mitchel et al’s Titan General Circulation Model (GCM) and the right shows Cassini observations, which seem to compare well. From Mitchell et al., “Locally enhanced precipitation organized by planetary-scale waves on Titan”, Nature Geoscience, 4, 589-592, doi:10.1038/ngeo1219 (2011).

You have a background in both astronomy and geoscience which has led you to studying extrasolar planets. Can you tell us about how these two fields assist you in your extrasolar planetary research?

The ‘jargon barrier’ is one of the most significant challenges facing extrasolar planet research, because this new research is bringing together two mature fields with their own ‘languages’. I have it a little easier than most during this transition time because I know a good part of the jargon from both fields. But I think the differences between astronomers and geoscientists goes well beyond jargon and into worldviews.

Very roughly speaking, astronomers are often interested in studying classes of many objects while geoscientists are often interested in studying particular objects. This leads to quite different approaches to the way science is done. For instance, an astronomer interested in ice giant planets might be satisfied with observational constraints or theoretical predictions for a few global parameters, while an atmospheric scientist will be interested in knowing finer detail like the presence of zones/bands of jet streams and clouds. Until now, only the global parameters have been measurable, but the next generation of telescopes will be able to reach finer detail. This is when the interactions between astronomers and geoscientists will get even more interesting.

Some of your more recent work is focused on our home planet in the areas of paleoclimate and regional climate sensitivity. How has your previous research of other worlds assisted in these recent studies?

I partly answered this earlier, but to expand I think of Earth’s paleoclimate as nearly identical in spirit to planetary science. In both cases, the data is limited in coverage and completeness, and that lends itself to more of the ‘big picture’ questions. A classical example is thinking about how our sibling planets, Venus and Mars, reached such drastically different climate states than us, and how Earth may have survived potential climate catastrophes like Snowball Earth, runaway greenhouse or atmospheric escape/loss.

A more challenging question is how specific regions or zones of Earth’s climate have changed in the past. This is an important question for our future, since we all live in regions and would like to know how our regional climate might change. My work on Titan’s climate has assisted me in researching regional climate change by focusing my attention on the ‘big picture’. For instance, a stationary wave living on the Earth’s jet stream will cause alternating patterns of warmer/colder regions along the latitude of the jet because the subpolar front dividing warm subtropical air from colder subpolar air coincides with the jet stream. This is a large-scale phenomenon felt around the globe, i.e., it fits in the ‘big picture’, and it makes testable predictions for the regional pattern of temperature and precipitation during climate states with these large stationary waves.

We discovered that climate models (GCMs) could only fit our data from the last ice age if they have this strong stationary wave response, and only a few of the GCMs we use to predict climate change have them. Now we’re collecting more data from other regions to test this ice age hypothesis, with the hope that GCMs will improve their regional climate modeling capabilities to provide better predictions of our changing climate.


You were the recipient of the Ronald Greeley Early Career Award in Planetary Science at the 2013 AGU Fall Meeting. What does this award mean to you?

I’m really very honored to receive this recognition from my colleagues. Although I didn’t get to meet Ron, I did get to know his wife over lunch at the AGU last year, and she said wonderful things about him. In particular, she said he was very good at mentoring young planetary scientists, and that is a quality I hope to carry into my mid-career and beyond. I was fortunate to have very good examples to follow as a student and postdoc, and that set a strong foundation for my early career.

Jonathan Mitchell (and his trusty companion) at his favorite place on Earth - Grand Teton National Park.

Jonathan Mitchell (and his trusty companion) at his favorite place on Earth – Grand Teton National Park.

What advice can you give for those considering or pursuing planetary science as a career?

Remember to have fun. Science is obviously challenging, but it’s also very rewarding when you discover new things. I’ve also found it challenging at times to fund the research I’m interested in, but I think it’s important to follow your instincts and interests despite any apparent roadblocks.

Lastly, if you could choose any planetary object to obtain new or additional data for understanding its weather and climate, what world would you choose?

This one is difficult for me to answer, so let me give two answers. Within the Solar System, I would like to learn more about Venus, and have a detailed history of it’s evolution. We’re learning this about Mars now, and I hope we can do the same on Venus next.

As for exoplanets, I would like to learn about the ‘super-Earth’ to ‘mini-Neptune’ transitional class of planets, the difference being the presence of a very deep atmosphere. How does a planet become one or the other? Understanding this would be sure to reveal new and interesting physics about planetary formation and evolution.