Roughly 4 billion years ago, Mars looked a lot different than it does today. For starters, its atmosphere was thicker and warmer, and liquid water flowed across its surface. This included rivers, standing lakes, and even a deep ocean that covered much of the northern hemisphere. Evidence of this warm, watery past has been preserved all over the planet in the form of lakebeds, river valleys, and river deltas.

For some time, scientists have been trying to answer a simple question: where did all that water go? Did it escape into space after Mars lost its atmosphere, or retreat somewhere? According to new research from Caltech and the NASA Jet Propulsion Laboratory (JPL), between 30% and 90% of Mars’ water went underground. These findings contradict the widely-accepted theory that Mars lost its water to space over the course of eons.

The research was led by Eva Scheller, a Ph.D. candidate at the California Institute of Technology (Caltech). She was joined by Caltech Prof. Bethany Ehlmann, who is also the associate director for the Keck Institute for Space Studies; Caltech Prof. Yuk Yung, a senior research scientist with NASA JPL; Caltech graduate student Danica Adams; and JPL research scientist Renyu Hu.

In the past two decades, NASA and other space agencies have dispatched over a dozen robotic explorers to the Red Planet to characterize its geology, climate, surface, atmosphere, and evolution. In the process, they learned that Mars once had enough water on its surface to cover the entire planet in an ocean between 100 and 1,500 meters (330 to 4920 ft) in depth – a volume equal to half of the Atlantic Ocean.

By 3 billion years ago, Mars’ surface water had disappeared and the landscape became as it is today (freezing cold and desiccated). Given how much water once flowed there, scientists wondered how it could have disappeared so thoroughly. Until recently, scientists theorized that atmospheric escape was the key, where water is chemically disassociated and then lost to space.

This process is known as photodissociation, where exposure to solar radiation breaks down water molecules into hydrogen and oxygen. At this point, the theory goes, Mars’ low gravity allowed for it to be stripped from the atmosphere by solar wind. While this mechanism is sure to have played a role, scientists have concluded that it cannot account for the majority of Mars’ lost water.

For the sake of their study, the team analyzed data from Martian meteorites, rover, and orbiter missions to determine how the ratio of deuterium to hydrogen (D/H) changed over time. They also analyzed the composition of Mars’ atmosphere and crust today, which allowed them to place constraints on how much water existed on Mars over time.

Deuterium (aka. “heavy water”) is a stable isotope of hydrogen that has both a proton and neutron in its nucleus, whereas normal hydrogen (protium) is made up of a single proton orbited by one electron. This heavier isotope accounts for a tiny fraction of hydrogen in the known Universe (about 0.02%) and has a harder time breaking free of a planet’s gravity and escaping into space.

Because of this, the loss of a planet’s water to space would leave a telltale signature in the atmosphere in the form of a larger-than-normal level of deuterium. However, this is inconsistent with the observed ratio of deuterium to protium in Mars’ atmosphere, hence why Scheller and her colleagues propose that much of the water was absorbed by minerals in the planet’s crust. As Ehlmann explained in a recent Caltech news release:

“Atmospheric escape clearly had a role in water loss, but findings from the last decade of Mars missions have pointed to the fact that there was this huge reservoir of ancient hydrated minerals whose formation certainly decreased water availability over time.”

Jezero Crater on Mars is the landing site for NASA’s Mars 2020 rover. Image Credit: NASA/JPL-Caltech/ASU

On Earth, flowing water weathers rocks to form clays and hydrous minerals, which contain water as part of their mineral structure. Since Earth is tectonically active, hydrated minerals are endlessly cycled between the mantle and the atmosphere (through volcanism). Clays and hydrated minerals have also been found on Mars, an indication that water once flowed there.

But since Mars is tectonically inactive (for the most part), its surface water was sequestered early on and never cycled back out. Thus, the features that indicate the past presence of water were preserved by the permanent drying of the surface. Meanwhile, a significant portion of that water was preserved by becoming absorbed beneath the surface.

This study not only addresses the question of how Mars’ water disappeared billions of years ago. It could also be good news for future crewed missions to Mars, which will depend on locally-harvested ice and water. Previously, co-authors Ehlmann, Huh, and Yung collaborated on research that traced the history of carbon on Mars – since carbon dioxide is the principle constituent of the Martian atmosphere.

The study that describes their findings recently appeared in the journal Science, titled “Long-term Drying of Mars Caused by Sequestration of Ocean-scale Volumes of Water in the Crust,” and was presented on March 16th during the Lunar and Planetary Science Conference (LPSC). Due to COVID restrictions, this year’s conference was virtual and took place from March 15th to 19th.

The research was made possible with support provided by NASA Habitable Worlds award, a NASA Earth and Space Science Fellowship (NESSF) award, and a NASA Future Investigator in NASA Earth and Space Science and Technology (FINESST) award.