Maybe Mars hasn’t lost its water after all. He’s still trapped on the planet

About 4 billion years ago, Mars was very different from what it is today. For starters, its atmosphere was thicker and warmer, and liquid water flowed over its surface. This included rivers, stagnant 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 lake beds, river valleys and river deltas.

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

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

Over the past two decades, NASA and other space agencies have dispatched more than 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 4,920 feet) deep – a volume equal to half of that. Atlantic Ocean.

3 billion years ago, the surface water of Mars had disappeared and the landscape became what it is today (freezing cold and parched). Considering how much water once flowed into it, scientists wondered how it could have disappeared so completely. Until recently, scientists theorized that atmospheric leakage was the key, where water is chemically dissociated and then lost into 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 is, Mars’ low gravity allowed it to be pulled out of the atmosphere by the solar wind. While this mechanism certainly played a role, scientists concluded that it couldn’t explain the majority of the water lost on Mars.

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

Deuterium (aka “heavy water”) is a stable isotope of hydrogen that has both a proton and a neutron in its nucleus, while normal hydrogen (protium) consists of a single proton orbiting it. of an electron. This heavier isotope is a tiny fraction of hydrogen in the known Universe (about 0.02%) and has a harder time breaking free from a planet’s gravity and escaping into space.

For this reason, the loss of water from a planet in space would leave a telltale signature in the atmosphere in the form of a higher than normal level of deuterium. However, this is incompatible with the observed relationship between deuterium and protium in the atmosphere of Mars, hence the reason why Scheller and his colleagues propose that much of the water was taken up by the minerals in the crust of the planet. As Ehlmann explains in a recent Caltech press release:

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

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

But since Mars is tectonically inactive (for the most part), its surface waters were sequestered early on and were never recycled. Thus, the characteristics which indicate the past presence of water have been preserved by the permanent drying of the surface. During this time, a significant portion of this water was preserved by becoming absorbed below the surface.

This study is not just about how the water on Mars disappeared billions of years ago. This 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 traces the history of carbon on Mars – since carbon dioxide is the main constituent of the Martian atmosphere.

Going forward, the team plans to continue analyzing isotopic and mineral composition data to determine what happened to nitrogen and sulfur minerals on Mars. In addition, Scheller plans to expand his research into what happened to the water of Mars by conducting laboratory experiments that simulate Martian weathering processes and observing the ancient crust in the Jezero crater (where Perseverance is currently exploring).

Scheller and Ehlmann are also scheduled to assist in the operations of the Perseverance rover when it comes time to collect rock samples and drill. These will be returned to Earth by a subsequent NASA-ESA mission, where researchers can examine them. For Scheller, Ehlmann and their colleagues, this will allow them to test their theories about climate change on Mars and what drives it.

The study that describes their results was recently published in the journal Science, titled “Long-term drying of Mars caused by sequestration of ocean-scale water volumes in the crust,” and was presented on March 16^e during the Lunar and Planetary Sciences Conference (LPSC). Due to COVID restrictions, this year’s conference was virtual and took place from March 15^e to 19^e.

The research was made possible through support provided by the 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.

Further reading: Caltech, Caltech Authors