Why Mars lost its water: Study suggests the rotational axis may have caused the loss of its lakes and oceans

Mars is an extremely arid planet, whose pressure and temperature conditions prevent liquid water from existing on its surface . However, geological and mineralogical evidence suggests that, in the distant past, the red planet hosted large volumes of water in the form of rivers, lakes, and even oceans .

Despite decades of research, one of the great mysteries of Martian history remains unresolved: what happened to all that water?

A new study, led by the Institute of Astrophysics of Andalusia (IAA-CSIC), has analyzed the role of obliquity —the tilt of the planet's rotational axis—in the loss of hydrogen, and therefore water, from the Martian atmosphere over time. The work has been published in the journal Nature Astronomy .

“To understand the study, it's important to keep in mind that the obliquity of Mars has changed significantly throughout its history,” says Gabriella Gilli, an IAA-CSIC researcher who co-led the work.

"The three-dimensional climate model we used suggests that, during periods of high obliquity, the escape rate could have been nearly twenty times higher than today," he explains.

The study's co-lead author, Francisco González-Galindo, notes: "If we were to combine all the water present on Mars between 3 and 4 billion years ago, we would have a global ocean more than 100 meters deep."

Some of that water may still be present below the surface today, trapped in the form of ice or embedded in hydrated minerals. However, another portion has been lost to space through a process known as "atmospheric escape," in which atoms and molecules acquire enough energy to overcome the planet's gravitational pull and escape into the interplanetary medium.

The current rate of hydrogen escape alone is insufficient to explain the loss of the enormous amount of water that existed in the past. Mars' orbit experiences periodic variations that significantly influence its climate.

One of the most significant is the change in the tilt of its rotational axis, known as obliquity. "Although this value is currently similar to Earth's—around 25 degrees—on Mars it has fluctuated widely over the past hundreds of millions of years, with an average of around 35 degrees," Gilli says.

Although these variations are known to have a significant influence on the planet's water cycle, how they affect water loss through atmospheric escape has not been investigated until now.

The study explored the relationship between Mars' obliquity and water loss over time , revealing that during periods when the axial tilt reached high values, insolation at the poles increased.

This intensified the water cycle and generated a warmer, more humid atmosphere. Under these conditions, water vapor reached higher layers of the atmosphere, where it was more vulnerable to solar radiation, which broke it down into hydrogen and oxygen atoms.

Being very light, hydrogen atoms could escape more easily into space, thus contributing to the loss of water from the planet .

The research team estimates that hydrogen loss during periods of high obliquity could explain the disappearance of an amount of water equivalent to a global ocean about 80 meters deep. This value coincides with the lower limit of estimates of the water that Mars once held. "Although it may seem modest compared to Earth, on Mars it represents a significant fraction of its former water, so its impact is significant," comments Gabriella Gilli.

The key tool used in this study, the Mars Planetary Climate Model (Mars-PCM ), was initially developed by the Laboratoire de Météorologie Dynamique in Paris, in collaboration with other international institutions.

For this study, the IAA-CSIC has incorporated fundamental improvements to the global climate model of Mars, including new compounds and chemical reactions that have made it possible, for the first time, to accurately reproduce observations of hydrogen escape made by, among others, the Maven (NASA) and Mars Express (ESA) missions.

The team has also carried out simulations that show how changes in the tilt of the Martian axis have influenced the loss of water to space .

"Our results indicate that hydrogen escape played a more important role in the drying process of Mars than previously thought, which is key to reconstructing how much water the planet has lost to space throughout its history," says Francisco González-Galindo.

In this context, the study has astrobiological implications, since understanding how changes in the tilt of the planet's axis have intensified the water cycle and favored its loss to space allows us to refine the search for possible periods in which Mars could have been habitable .

“Knowing when and how the right conditions arose—and when they ceased to exist—is critical to assessing whether the red planet could have supported life at some point in its history,” Gilli emphasizes.

Furthermore, the work also highlights the extent to which orbital parameters can transform a planet's climate.

"While on Earth the variations are mild thanks to the stabilization exerted by the Moon, on Mars they have caused drastic changes that affected water, the atmosphere, and, ultimately, its potential to sustain life," says González-Galindo.

“This long-term view of planetary climate change also offers valuable insight into the fragility of the balances that make habitability possible , and underscores the importance of protecting our planet,” concludes Gabriela Gilli.