by Sally Blumenthal
Hades, according to Greek mythology, is the God of the Underworld and last regurgitated son of Cronos. He is also the namesake for the earliest geologic period in Earth’s history. The Hadean era earns its name from the fiery, molten state of the Earth 4.5 billion years ago. How did we go from hellscape to a lush planet with oceans? Planetary scientists from all over this planet are trying to answer this question by building a framework to understand terrestrial planet histories. To date, there are 5,307 confirmed exoplanets. An exoplanet, shortened from extra-solar planet, is a planet orbiting a star outside of our solar system. With so many planets, we are able to look at snapshots of planets throughout their lifetime. These population studies may give us the necessary insights to better understand the formation history of our own planet.
In 2004, we discovered one of the first Super-Earths orbiting a sun-like star, 55 Cancri e. This extremely hot world doesn’t look like Earth, yet. Covered in magma, what exists there already is water, and possibly lots of it. In our universe, water forms easily or is thermodynamically favorable. Once a planet is formed, it has an initial water inventory. Sometimes this is altered by events such as collisions with other bodies, like comets, or even other planets, but for now we will keep it simple. The water exists as a vapour in the magma. As the planet cools the water vapour will undergo a phase transition and become liquid. Now that we’ve formed an ocean, we’ve cracked it, right? Unfortunately not as we cannot ignore the role of an atmosphere when considering a planet. Just as on Earth, water vapour can reside in the atmosphere. If given the opportunity it can cool enough to turn into clouds and rain, or it will persist as a gas. This process of where in the planet the water resides is called partitioning.
We do not have to look outside our own system to start answering this question. Venus is considered “Earth’s Twin” as it is roughly the same size as Earth. A marked difference between Earth and Venus is its lack of an ocean, making them more like fraternal twins. So where did Venus’s ocean go? Venus lies closer to the sun than Earth and thus receives more UV radiation. This UV radiation is the key to how Venus lost its ocean. The water vapour that was in Venus’s atmosphere underwent UV photolysis, where water, H2O, was broken into parts, H and OH. As a hydrogen weighs less than an oxygen and hydrogen together called a hydroxyl– it takes less energy for it to escape from Venus. This newly liberated hydrogen escapes to space before it can re-form water, leaving behind oxygen to form O2.
Combining the concepts of water partitioning and UV-driven escape, we can now begin to separate planets into two categories to understand their formation histories. Planets like Venus, closer to their host star comprise the first category, while planets farther from their host star comprise the second category. In their hellscape period, planets in category one have more water partitioned into their atmosphere than planets in category two, meaning that planets in category one stay molten for longer. This is counter-intuitive because we think of water as cooling, but as a vapour, water acts as a greenhouse gas, trapping heat, just like on Earth. Thus, as the planetary atmosphere traps heat it prolongs the course to the solidification of a crust. Planets in category two, experience less UV exposure and have less water partitioned into the atmosphere than the magma. As these planets cool and thus solidify, the water retained has the potential to form oceans.
The question of where in the planet that water is retained is an interesting one. Frank Herbert examines this question on the planet Arrakis, creating one of the most influential science fiction novels to date in Dune. Does the water form a sub-surface ocean like on Europa? Does it hide in mantle rocks? Insights into the details of a planet’s formation history, especially the timescale, can inform us about the necessary conditions for a habitable world– What are the conditions necessary for the emergence of plate tectonics, and for fostering the emergence of life? It can also inform us about more familiar and pressing issues like greenhouse gas feedbacks. How is it that we live on a planet with both life-sustaining oceans and an atmosphere that we can breathe? By developing a framework for understanding the vast population of terrestrial planets we can converge on how rare or not rare is a habitable world.
Indeed by researching the question of the origins of oceans on terrestrial planets we produce more questions than answers. With new exoplanet space missions like the James Webb Space Telescope (JWST), the Planetary Climate Dynamics group in the Atmospheric, Oceanic, and Planetary Physics Department is working on models to help answer some of these questions. Our group leverages a vast Earth Science background to put terrestrial formation into context in the past, present, and future to help solve issues from climate change. As a student at St Edmund Hall, my contribution to the group is creating a model to marry together planetary dynamics (escape and winds) and chemistry (UV-catalysed chemistry and chemical kinetics) to model planet formation scenarios. Maybe Frank Herbert helped us start the conversation about water retention in terrestrial formation, but now we are on the path to provide answers in that conversation. Like cell phones in Star Trek or unmanned cars in Total Recall, science fiction often inspires innovation. Here we want to collect details to transform myth into an informed reality.