How to build a habitable planet? An interview with Charles Langmuir

【Author】Xu Yigang

【Translation】Zhao Siyu, Yang Yang

The Earth is the only known habitable planet in the solar system. Exploring how the Earth evolved its unique habitability has always been a frontier area of ​​Earth science and has become one of the current focuses of deep space exploration. What are the key factors that determine the formation of habitable planets? How do solid Earth processes affect the origin of life and the regulation of the surface environment? Is the study of Earth's habitability closely related to the many challenges currently facing humanity? These questions have attracted widespread interest from the scientific community and the public. National Science Review interviewed Professor Charles H. Langmuir, a solid Earth geochemist from Harvard University in the United States. He has conducted systematic research on many aspects of plate tectonic geochemical cycles, covering mid-ocean ridges, convergent plate margins, and intraplate volcanic activity. Professor Langmuir's book How to Build a Habitable Planet was named the best Earth science book of 2012 by the American Publishers Association.

The Importance and History of Earth Habitability Research

NSR : A few years ago, you co-authored How to Build a Habitable Planet with Wally Block, a book that had a profound impact on the earth science community and the public. Could you share the original intention and process of writing this book?

Langmuir : Wally first published the book in 1984. It was a short, seven-chapter book that told a grand narrative from the Big Bang to the human age. Wally later gave me the assignment to teach the course at Columbia University. I loved the book, but I came to realize that the course needed more solid Earth science and that some of the material in the book was outdated.

At first, I thought this work would be just a side project, but when I started working on it, I quickly realized that there were a lot of omissions. In addition to the lack of content on solid Earth science, the book also lacks discussions of geobiology and Earth history—the emerging field of exoplanet research has not yet been included. So this "side project" turned into an intensive work that lasted ten years, during which I had to learn a lot of areas that I was not familiar with before, such as the origin of life and the history of the Earth. But this also became a rare opportunity for me to think about the entire universe from a macro perspective and place the Earth and humans in this broad context. "I realized that this was an emerging theme about planetary evolution, which is a universal process, and I began to look for those universal basic principles that transcend the specific details of what happened on Earth.

NSR : Why is the study of Earth's habitability so important? Is it relevant to the challenges currently faced by humanity, such as global warming?

Langmuir : It is now generally accepted that global warming is the core problem we face and that we must do something about it or it will have a negative impact on our comfort and economic well-being. From this perspective, solving global warming seems to be more of a task for economists and engineers—how much should we invest to mitigate global warming? Which engineering solutions are most cost-effective? So we look for solutions such as renewable energy, electric vehicles, and rebuilding the power grid. At the same time, people also want to solve the problem without making sacrifices, pursuing a win-win strategy that protects the environment without affecting economic growth.

However, in the process of writing this book, I gradually came to a completely different view. If we ask a hypothetical question: "If we could solve the CO2 emissions problem tomorrow, would all our problems be solved?" The answer is of course no. The more difficult problem is the more pervasive and widespread process of planetary destruction. We are over-depleting groundwater, eroding soils, and destroying other life on Earth at the same scale and speed as past mass extinction events. As our population and economy grow, we continue to cut down forests, build new cities, and increase our consumption of meat, inevitably killing large numbers of other plants and animals and destroying the Earth system on which we and all other living things depend. In today's world, even if CO2 emissions were reduced to zero, economic growth would still lead to continued planetary destruction.

So I think we need a Planetary Damage Index, reported as thoroughly and frequently as GDP. Atmospheric change is just one component, along with other components such as biodiversity loss, soil degradation, groundwater depletion, habitat destruction, ocean acidification, and demographic issues. If you think about it, every component of this damage index is increasing, even if we make big cuts in greenhouse gas emissions.

This view may sound rather pessimistic, but there are also positive aspects in the long-term history and evolution of the Earth. The Earth started out as a barren rock, went through a long period of quiescence, and then underwent radical changes as the planet evolved to a new functional state. For example, the Earth was initially lifeless for perhaps hundreds of millions of years. The earliest life was tiny prokaryotic cells in the primordial oceans, when there was no oxygen in the atmosphere, and this lasted for about a billion years. The development of photosynthesis provided oxygen to the Earth, but oxygen was toxic to early organisms, so life took about another billion years to adapt to oxygen and use it to extract more energy from food. This increase in energy led to the emergence of more complex single-celled organisms, such as eukaryotic cells like amoebas and paramecium. Eventually, the Earth's surface became oxygenated, atmospheric oxygen levels rose, and multicellular life began to evolve over hundreds of millions of years, including the invasion of land, the evolution of vertebrates, and trees. Looking at these stages from a systemic perspective, rather than focusing on specific details, each stage involved an increase in energy acquisition pathways and an increase in the scale of relationships.

Viewed in this systemic context, human civilization is the next stage in planetary evolution. We have undergone a revolution in energy capture that has not only given us dominance over all life, but has also supported super-exponential population growth and built a global network that connects us instantaneously through language, print, and various forms of electronic communication. There has been a lot of discussion about whether we have entered the Anthropocene—a very tiny time boundary in geological time. In our book, we argue that we may be at the beginning of the Anthropological Era, an epoch that is more significant than any of the major changes in planetary function in the past in terms of its impact on planetary function. Our ability to look back and forward into the future, and to understand scales from the subatomic to the cosmic, is incredible. Evolution is no longer dependent on random changes in DNA. These are the biggest changes in planetary function in 4.5 billion years, and they are happening at an incredible rate.

NSR : How to view Earth’s habitability from a planetary perspective?

Langmuir : Looking at the evolution of planets over billions of years shows that their evolutionary paths are by no means inevitable. Most planets may not be able to support life at all, and some that do support life may remain in the single-cell stage forever. For example, Venus may have supported early life, but because it is too close to the sun, the greenhouse effect got out of control. Mars may have supported life in the past, and may still support life in its underground cracks, but its small size and distance from the sun make it difficult for life to continue to develop. Other planets may not have experienced the oxygen revolution, or they may have been ended by external factors such as giant meteorite impacts and meteorite passes. Just as a female salmon lays thousands of eggs, but only a few survive to adulthood, each stage of development is full of challenges and dangers. The universe may be like this too, with countless planets, but only a few will undergo the full evolutionary process that the Earth has experienced.

From this perspective, human civilization is also at risk. We are in the process of planetary destruction, and there is no guarantee that existing civilization will survive. All civilizations in history have experienced collapse, although these collapses were limited to certain regions, but in today's globalized world, the collapse of any civilization will have global consequences.

Global warming is just one of many issues in this chain, and even when it comes to global warming, our concerns are often selfish - how will it affect my life? What will happen to economic growth? How will it threaten the environment I live in? We need to live as protectors and maintainers of the planet, not just as users, and this requires fundamental changes in our individual and societal behavior.

Exploring Earth's history and evolution may inspire us to ask these global and personal questions, helping us understand our origins and feel grateful for the Earth and the universe that gave us life and provided for all our needs. When we feel grateful, we may develop a sense of responsibility, and if we can truly do that, then our relationship with the planetary system may change fundamentally. So why is it important to study Earth's history and evolution? Because it provides us with a grand context and perspective that is closely related to the meaning and purpose of human existence.

Key factors affecting Earth's habitability

NSR : When we look at the habitability problem from a macro perspective and take Earth as our research object, we find that many specific factors contribute to Earth's habitability. What conditions are needed to build a habitable planet, and what is the probability that these conditions are prevalent in the universe?

Langmuir : A fundamental requirement for habitability is climate stability, which depends on the planet's concentration of volatiles, especially carbon dioxide and oxygen. Three of these elements (C, H, O) also make up more than 90% of the mass of living organisms. With the exception of helium, which does not participate in the reaction, these three elements (C, H, O) are the most common elements in the Milky Way. Therefore, the universe is full of molecules required to maintain a stable climate and support life. However, on rocky planets such as the Earth, volatiles are not accumulated efficiently, making their total amount on the Earth small relative to other elements. Fortunately, they can be dissolved in deep magma and released at the surface, resulting in the enrichment of these volatiles in the surface layer of the planet. But there are still many factors that may cause the huge differences in the volatile content on terrestrial planets.

How important is it to have the right concentration of volatiles in the atmosphere? The Earth's surface has just enough water to fill up the ocean basins, and the coexistence of oceans and continents provides the basis for long-term climate stability. Is this a lucky accident, or the result of some feedback mechanism that we don't yet understand? Would a planet with more water become a "water world"? Would such a world still be able to maintain a stable climate and life? The exact concentration of carbon dioxide in the atmosphere is crucial to the temperature of a planet. We already know that changes in atmospheric carbon dioxide concentration have an impact on climate, but what if a planet had ten times less or ten times more carbon dioxide?

The abundance of oxygen is central to Earth's ecosystems today, and it has remained roughly constant within a narrow range for hundreds of millions of years. What feedback mechanisms control atmospheric oxygen concentrations? How do these mechanisms operate over planetary history? How do they vary on different planets? An often overlooked key is the role played by the solid Earth. Plate tectonics and volcanism are key in determining the abundance of surface volatiles. The evolution of atmospheric oxygen depends not only on "sources" of oxygen formed by photosynthesis and burial of organic carbon, but also on "sinks" of oxygen, most of which are achieved through redox reactions of iron and sulfur in the solid Earth.

Exploring the universality of planetary habitability raises new questions. For example, is there some kind of feedback mechanism that ensures that a planet has the right amount of water on its surface? Does the emergence of life often accompany a transition from a reduced to an oxidized state on the planet's surface? The operation of plate tectonics and its efficiency in linking different layers of a planet seem to depend largely on the mass of the planet. In order to maintain the right concentration of volatiles, does a planet need to be both located in the habitable zone of its star and have a moderate size? Research on these questions is just beginning, and we currently rely on idealized models that may not provide us with definite answers.

There may be some general principles that apply to planetary environments that transcend specific circumstances. First, habitability breeds habitability. Life that destroys the environment it depends on will eventually fail, while those that create more habitable environments will thrive. Second, there is an evolutionary advantage to life in terms of being able to access more energy. This means that life will evolve to exploit more and more energy, as we observe on Earth. Third, there are advantages to symbiosis, feedback, and relationships. Stable ecosystems can survive for long periods of time, which allows for success in the evolution of organisms. Although these principles may vary greatly from planet to planet in terms of their specific biological expression and planetary evolution, aren’t they universal? Finally, in the case of Earth, over time, planets will develop abundant resources—soil, groundwater, mineral deposits, fossil fuels, and so on. These resources await the emergence of life that can use them. If these principles are universal, then barring catastrophic events, isn’t intelligent civilization the ultimate outcome of planetary development?

NSR : Which discoveries are most critical to solving these problems?

Langmuir : I am far from the only one who thinks it is extremely important to ask how common life is in the Milky Way. Is life the “norm on planets” or is it extremely rare? If we find life elsewhere, we will be able to answer this question with certainty. The argument that life is rare and that Earth is unique implies that, statistically speaking, it is extremely unlikely that life exists. How can highly ordered life arise from a chaotic collection of molecules? There are 100 to 200 billion stars in the Milky Way, and astronomers tell us that most of them have planets around them, which gives us about a trillion opportunities for life to exist. Even if the probability of life arising is one in a million, there are still a million planets in the Milky Way that have life. However, the chance that we stumble upon one of them is also one in a million. Astronomers have only discovered a few thousand other planets so far, so we still have countless generations of exploration to go. So if we find evidence of life elsewhere—whether on Mars, Europa, or a planet in another solar system—it will show that life is extremely abundant in the universe, not by chance but as a property of the universe.

Another landmark discovery would be the ability to generate life in the laboratory. Remarkable progress has already been made in this area. Given the short time that humans have existed, and the limited imagination we have compared to the diversity of environments that may have existed on early Earth, the origin of life becomes relatively simple if we can create the conditions in the laboratory that could support life. Once we can create a primitive life form in the laboratory, biological evolution over time could give rise to an amazing diversity of life. Making a simple protocell, if we can do that on a tiny human timescale, then life is likely to be ubiquitous in the universe.

I would love to witness these discoveries in person, because they would be a turning point in human history—we would be firmly aware that we are not alone; we are part of this vibrant universe.

A message to the younger generation of researchers

NSR : What advice can you give to students and young scientists working in the field of volatile cycling and Earth habitability?

Langmuir : Young scientists often ask me how to get published in Nature or Science, because these publications are often crucial to their career advancement. The key to getting published in these top journals is to ask good questions, be passionate about your research, explore the problem deeply, and be open to discovery. If you do discover something, with luck and the right editor and reviewers, your research will be published in Nature. This cannot be artificially created. In my career, many important papers were not published in Nature and Science. What matters is not where you publish, but the quality of the paper.

The common trait of great scientists is that they keep an open mind to the world and question what they observe, so they sometimes discover new things that others have overlooked. If you can find the right question and follow through, it often makes people say, "This is so obvious! Why didn't I think of this before?" My great uncle won the Nobel Prize in Chemistry, but one of his most important discoveries was in physical oceanography. He loved the outdoors, even in bad weather conditions. One day, he was on a ship crossing the Atlantic when he encountered a violent storm. He went on deck and felt the wind and waves. He looked at the water and saw that the seaweed was arranged in equidistant lines parallel to the wind. When the wind changed direction, this arrangement changed within half an hour. This may have been obvious to sailors for thousands of years, but he saw it and asked himself, how could this happen? Later, he went to his lake house in the summer, and when a storm came, he rowed a boat and dipped his umbrella in the water and found that the umbrella rotated with the waves, showing convection cells parallel to the wind direction. He not only defined this flow, but also measured its speed. The Langmuir Circulation is now considered to be the main form of water mixing in shallow ocean areas and is at the core of research in the field of physical oceanography.

Another time, while boating on a calm day, he noticed that a drop of oil from the engine spread out into a large sheet on the water, and he asked himself, “How thick is that layer?” Measurements were made, and single-molecule films were discovered, and there is now a journal on surface chemistry named after him.

He made these discoveries not because he wanted to publish in Science or Nature. He achieved what he did because he was curious about the world and followed his curiosity through clever experiments and publications. In his later years, he enjoyed his entire career.

As a child, my father continued this tradition by doing fun science experiments with me—putting candles in milk bottles to create a vacuum so that hard-boiled eggs would be sucked into the bottle, making bubbles from carbon dioxide float, inhaling helium to make sounds high-pitched, making simple instruments. I learned that science was practical and fun. In my career, I have been very lucky to find joy in doing things I love. I never thought about winning awards or getting rich through scientific research. Of course, there was some luck—I was lucky to go into volcanic geochemistry at a time when there was very little data and very limited exploration of the oceans. Of course, under the current system, I had to write research proposals with hypotheses, as if scientific innovations could be designed in advance. But when the ship went out to sea to explore something new, or when new data came in, I was alert to possible discoveries. In fact, such discoveries did occasionally appear.
Looking back on my career, I think there are several aspects that are very important. The first is to obtain high-quality data. Be reasonable in your skepticism about the quality of the data and work hard to ensure its accuracy. High-quality data has lasting value. Second, make sure to simulate the data quantitatively and consider all kinds of data, not just your analysis of a certain isotope ratio. Otherwise, the ideas and theories you come up with may not work and will not stand the test of time. Finally, although the scientific field can be competitive, interpersonal relationships are more important. If you have many important papers but no friends at the end of your career, you will feel unsatisfied. Therefore, be generous in sharing your samples with students, postdocs, and colleagues. Science is a collective activity - progress is achieved through interpersonal networks, and we need to build and cultivate these connections.

【Xu Yigang, researcher at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, editorial board member of National Science Review, guest editor of the special topic "Origin and Cycle of Volatiles in Habitable Planets". English version of this article: How to build a habitable planet: an interview with Charles H. Langmuir, DOI: 10.1093‍/nsr/nwae060】