Why did the ocean on Mars disappear?
Scientists believe that Mars, formed some 4.6 billion years ago, once had a massive amount of water on its surface and the Martian climate was warm and wet. There are many findings, such as a basin, riverbed, valley and clay minerals, that point to the presence of liquid water on the planet at one time.
Fast forward to today, however, and Mars is an extremely cold and dry planet covered by sand and rocks containing iron oxide, giving the Red Planet its rusty appearance. A small amount of ice in polar caps and water vapor have been discovered, but no water in liquid form has been spotted on the planet. Where did all the water go? Shohei Aoki, a lecturer at the Graduate School of Frontier Sciences, is on a mission to find the answer to that question.
“I want to find out the reason why Mars lost its water, how it came to be the desolate wasteland it is today. I want to unravel and trace the path of the planet’s evolutionary process,” said Aoki, who in 2020 won the Baron Nicolet Prize in the science classes of the Royal Academy of Belgium, awarded to researchers under the age of 40 for distinguished research on planetary atmospheres. The young planetologist studies the evolution of the planetary environment and atmosphere by examining data obtained by instruments installed on spacecraft and large telescopes that sit atop mountains. After research positions at universities and research institutions in Japan, Italy and Belgium, Aoki was selected as a University of Tokyo Excellent Young Researcher in 2021 and joined the university’s faculty the following year.
“The atmosphere is a key determinant of supporting life and shaping the environment of a planet,” he said. “I believe understanding the Martian atmosphere could shed light on understanding the planetary conditions for nurturing life like that on Earth.”
Getting to know the Red Planet at home and abroad
Day in and day out, Aoki pores over data obtained in space. But he said he has never been an astronomy enthusiast, which may come as a surprise to many. “People often point to a star in the sky and ask me its name, but I have no idea,” he laughed. “I’m still not that into astronomy.”
He said it was a TV program on Mars, which he happened to watch by chance when he was in high school, that drew him into the world of planets. Back then, he was devoting much of his time to baseball with the hope of playing in the national high school baseball championship tournament. The TV program exposed him to the great mysteries of the universe, such as its origin and whether life exists on Mars. And Aoki was captivated. “Seeing the interviews with researchers, inserted into different parts of the program, also opened my eyes to the possibility of studying Mars at a university, which sparked my interest,” Aoki recalled.
After searching for a university that offered a course in astronomy, he entered Tohoku University and enrolled in the Department of Astronomy and Geophysics at the Faculty of Science. There, he began his research on planetary atmospheres, with a focus on Mars, and continued his studies at the university’s Graduate School of Science.
According to Aoki, there are various approaches to investigating planetary atmospheres, such as through numerical simulations or developing instruments mounted onto satellites. His approach is to use data obtained by space probes and ground-based large telescopes to study the atmospheric composition, temperature and circulation of a planet.
“Today, Mars has a thin atmosphere composed mostly of carbon dioxide. It also includes gases present in small amounts, called trace gases, such as water vapor and carbon monoxide,” he said. “By observing these components, I’m trying to trace the evolution of the planet’s environment and find out where its water escaped to.”
Back in his graduate school days, Japan did not have planetary probes that had successfully entered orbit around Mars. So, Aoki went to Europe, hoping to delve deeper into research, using the European Space Agency’s Mars Express orbiter, which was the state-of-the-art Mars probe at the time. There, Aoki conducted research at the Institute for Space Astrophysics and Planetology in Italy, the Royal Belgian Institute for Space Aeronomy and the University of Liège in Belgium. During his stays in Europe, he also participated in the European Space Agency’s ExoMars mission as a member of a team analyzing the Martian atmosphere, using the mission’s Trace Gas Orbiter.
Aoki also uses ground-based large telescopes installed with large-size, high-performance instruments that cannot be mounted on spacecraft. To observe Mars, Aoki went to the Subaru Telescope near the summit of Mauna Kea in the U.S. state of Hawaii, operated by the National Astronomical Observatory of Japan, and the ALMA Observatory in the Atacama Desert in Chile, which are located at an altitude of 4,000-5,000 meters.
“I can obtain all my data remotely and carry on my work that way, but that does not give me a sense of the circumstances surrounding the observatories at any given moment. Being physically there on-site enables me to see what’s going on in the sky. For example, if we suddenly lose a planet’s signal, I can pop outside and check that it was due to cloudy skies,” Aoki said. “Also, I can give instructions directly to the person operating the telescopes, which allows me to observe and collect data according to the method I want to try.”
By analyzing the spectral data of the various colors of light collected by telescopes and Mars probes, we can deduce the chemical composition and other information of the planet’s atmosphere, he said.
Lost ocean of Mars
The research Aoki has been engaged in for some time is studying the ratio of “normal” water (H2O) and heavier water (HDO) in Mars’ atmosphere to estimate the amount of water that once existed on the planet. While water is composed of two hydrogen atoms and a single oxygen atom, one of the hydrogen components of HDO is a heavier form of hydrogen, deuterium (D), which has a neutron in its nucleus that gives it more atomic mass than ordinary hydrogen (H). Experts say most of the water that existed on Mars escaped into space for various reasons. Given that it is easier for light water to shake off Mars’ gravity and escape into space, and more likely for heavier water to linger in the atmosphere, scientists estimated that the amount of water on Mars was six to seven times greater than that on Earth, based on these properties of water in nature. They established the current ratio of HDO to H2O on Mars, according to data collected and measured through observation, and compared it to the ratio of the two types of water on Earth to calculate the amount of water lost on Mars. They estimate there was about a 100-meters-deep ocean in the northern hemisphere of Mars at one time.
A study of the vertical distribution of water vapor from the surface to an altitude of 100 kilometers revealed that water on Mars escaped into space during the warmer months. “Mars has four seasons. During the cooler fall and winter seasons in the southern hemisphere, water vapor turns into ice and does not rise to the upper atmosphere. But during the warmer summer period, water vapor circulates in the atmosphere together with dust storms and is carried to the upper atmosphere and disappears into space,” he said.
Aoki said the best part of his research is when he sees data gathered by a new planetary probe for the first time. “Using a new probe, we can obtain data we’ve never seen before, such as discovering a new component in the atmosphere or not finding any trace of a substance that we thought existed,” he explained. “The same can be said for telescopes: We can discover new things by employing new techniques, such as using a different wavelength for observation. When I see those new data, my heart beats faster with excitement.”
Aoki is also participating in the Martian Moons eXploration (MMX) mission led by the Japan Aerospace Exploration Agency, or JAXA, as a member of its spectral analysis team. Scheduled to launch in 2026, MMX is the world’s first mission planning to touch down and bring back materials from the surface of one of Mars’ two moons. It is expected to land on the Martian moon Phobos and collect samples considered to contain materials that flew over from Mars. They are also planning to observe the Martian atmosphere’s climate and isotope ratio. Isotopes are different forms of the same element differentiated by the number of neutrons in their nucleus, like normal hydrogen (H) and its heavier form deuterium (D).
As for the future, Aoki is hoping to continue his research to study Venus, which, like Mars, may have had liquid water a long, long time ago. Europe and the United States are scheduled to launch Venus probes around 2030, and Aoki is scheduled to participate as a member of a spectral analysis team of the European mission.
“I want to acquire a systematic understanding of the atmospheres of terrestrial planets such as Mars and Venus,” he said. In recent years, a hunt for an Earth-like planet outside our solar system is drawing increasing attention. But information that can be obtained from observing exoplanets is limited. “If we can deepen our understanding of the evolution of the environment and atmosphere of terrestrial planets in our solar system, like Mars and Venus, it can be useful when we discover an Earth-like exoplanet in the future. I want to contribute to that,” he said.
