Scientists debunk long-held theory about how Mars lost its water

A team of scientists is challenging a long-held theory of how Mars lost its water billions of years ago, suggesting that the planet’s ancient water may have been trapped beneath the surface rather than escaping through space.

Their findings were published Tuesday in the journal Science, and have major implications for how we understand Mars’ history and its habitability.

Mounting evidence suggests Mars was once a wet, warm, and possibly habitable planet.

Over time, the planet lost its water and became the desolate world it is today. Scientists weren’t quite sure how Mars’ water dried up, but a popular theory suggested that water molecules escaped from the planet to outer space through atmospheric escape. But new evidence suggests it went underground.

WHAT'S NEW — Eva Scheller, a graduate student in geology at the California Institute of Technology and lead author of the new study, was interested in recreating the history of water on Mars in order to better understand when the planet may have been habitable.

“Mars has always been very interesting from a habitability standpoint,” Scheller tells Inverse. “Water plays a huge role in that.”

The models suggest that 30-99 percent of Mars’ water may have been buried beneath the surface of Mars, incorporated into minerals in the planet’s crust, becoming a part of the molecular structure of the minerals rather than in its liquid form.

The new study also answers the question of when Mars lost its water, which affects the history of the planet’s habitability.

Scientists know that the planet’s crust began forming around 4 billion years ago, therefore the model suggests that Mars lost its water between 4.1 to 3.7 billion years ago.

“That's really important because that means Mars already becomes pretty arid 3 billion years ago,” Scheller says. “That kind of constraints when we think that Mars would have had this water, which ultimately has implications for habitability.”

That means that if Mars was once able to host life, it would have been around 3 billion years ago. Around that time is when life began to appear on Earth.

HERE’S THE BACKGROUND — Anywhere there’s water on Earth, some form of life has managed to survive. Scientists believe that Mars once had flowing rivers, lakes and maybe even an ocean.

Tanya Harrison, a planetary scientist and director of science strategy for Planet Labs who was not involved in the study, says that knowing that Mars had water in the past is important for understanding if life ever arose on Mars.

“The fact that we can tell that there used to be a lot of water on Mars has really big implications for the potential for Mars to have had life in the past,” Harrison tells Inverse.

How they did it — In order to reconstruct the process by which Mars lost its atmosphere — which led to the loss of its water — the team behind the new study used data gathered by NASA’s Curiosity rover of the Gale Crater on Mars, which scientists believe had water over its early history, as well as measurements of the NWA 7034 meteorite, which has the highest water content ever found in a Martian meteorite.

Curiosity discovered clay minerals in mudstones throughout its journey at Gale Crater, which often have high water content.

The team measured the deuterium-to-hydrogen isotopic ratio, which is connected to which reservoirs water is moving to and from. They then modeled the total mass of water on Mars, with the input including release of water from volcanoes on Mars.

The scientists worked under two hypotheses: that the water evaporated and was lost to space as Mars lost its atmosphere, or it became locked underneath the surface.

Harrison says that the team was able to get these results today because of all the data accumulated on Mars through ongoing international robotic missions.

“I think that seems consistent with what we were expecting,” Harrison says. “I'm not sure if we've had any solid measurements before to tell us how much water was lost to space versus was locked up in the ground, because it's not something we've been able to study all that well.”

The new results present another way through which to consider water loss on Mars.

“We're saying that by taking into account water loss to the crust, that it is actually equally as important as water loss to space, or actually much more important,” Scheller says.

The same process by which the study suggests Mars lost the majority of its water takes place on Earth through an interaction between water and rocks which forms hydrated minerals like clay, which contain water in their crystal structures.

However, on Earth, the water is released again through volcanic processes that did not take place on Mars, according to the researchers.

WHAT'S NEXT — On February 18, NASA’s Perseverance rover landed on Mars to begin its mission of searching for clues of ancient life.

Perseverance will also be the first rover to study parts of the Martian crust where these hydrated minerals are located, which will help scientists nail down what the exact process was that took place where the water interacted with the rock to form hydrated minerals.

“So in a sense, our model is a sort of framework for future work that can be done with the Perseverance rover,” Scheller says.

ABSTRACT: Geological evidence shows that ancient Mars had large volumes of liquid water. Models of past hydrogen escape to space, calibrated with observations of the current escape rate, cannot explain the present-day D/H isotope ratio. We simulate volcanic degassing, atmospheric escape, and crustal hydration on Mars, incorporating observational constraints from spacecraft, rovers and meteorites. We find ancient water volumes equivalent to a 100- to 1500-meter global layer are simultaneously compatible with the geological evidence, loss rate estimates, and D/H measurements. In our model, the volume of water participating in the hydrological cycle decreased by 40 to 95% over the Noachian period (~3.7 to 4.1 billion years ago), reaching present-day values by ~3.0 billion years ago. Between 30 and 99% of Martian water was sequestered by crustal hydration, demonstrating that irreversible chemical weathering can increase the aridity of terrestrial planets.