Mars’s current atmosphere is frankly dim, turning less than 1% into the pressure of the earth, but there is good evidence that it was substantially thicker in the past. The researchers have now directly observed the atoms that escape in a way not observed so far.
This process, known as atmospheric spraying, may have facilitated the transition from Mars of an aqueous planet to the arid world that is today, the team reported in Scientific advances.
“I’ve been looking for this since it was a postdoc.”
Since the early 2010s, planetary scientist Shannon Curry At the University of Colorado, Boulder has studied Mars’s data, looking for signs that the atmosphere of the red planet is eroding. It has been a long trip, he said. “I’ve been looking for this since it was a postdoc.” The colleagues even went to Curry so that their search could be crazy. “Every year, I executed my code and looked for it,” he said. “We started joking that it was like a unicorn.”
But Curry, NASA’s main researcher Mars’s atmosphere mission and volatile evolution (Maven)Now she has reasons to celebrate: she and her colleagues believe that they have finally captured the first direct observations to spray on Mars.
Escaping for kicks
Planetary atmospheres constantly change; Everything, from solar eclipses to volcanic eruptions and burning fossil fuels, can alter its composition, density and structure. Atmospheres can also erode through several processes. One is photodisociation, in which photons separate the molecules, creating lighter components that can escape. Sputtering is another. This process implies high energy ions, accelerated by the electric field of the sun, crossing the upper atmosphere of a planet and colliding with neutral atoms. These energy kicks give enough energy to the neutral particles that escape the gravitational field of the planet.
The spray plays only a minor role in the escape of the atmosphere of Mars today: the spray rate is currently several lower magnitude orders than that of photodisociation. “But we believe that billions of years ago, it was the main escape driver,” Curry said.
Thanks to almost a decade of observations of Maven, Curry and its collaborators had access to detailed records of the electric field and neutral particles in the atmosphere of Mars. They focused on the neutral argon, a heavy noble gas. In general, it is difficult to eliminate the argon from the Martian atmosphere in other ways, he said Manuel ScherfAstrophysicist of the Space Research Institute of the Academy of Sciences of Austria in Graz, Austria, who did not participate in the investigation. “The only really efficient escape mechanism at this time is spray.”
Keep the darkness
“We have to get out of sunlight to detect spray.”
The team used Mars atmosphere simulations where they could find a spray signal. Looking over an altitude of approximately 360 kilometers seemed to be key, the modeling revealed. In addition, the team knew that it was essential to look at Mars’s side pointing away from the sun. That is because photodisociation dominates during the day. “We have to get out of sunlight to detect spray,” he said Janet LuhmannA space scientist at the University of California, Berkeley, and member of the research team.
The researchers compared Argon’s abundance in the Martian atmosphere in two altitude containers: 250–300 and 350–400 kilometers. They also compared periods during which the electric field pointed to or outside Mars. The spray should occur preferably in the highest altitude container when the electric field points to Mars, it is then when the ions accelerate towards the atmosphere of the planet. In fact, Curry and his colleagues found statistically higher densities of Argon in that data group.
The team calculated that Argon was being polvited at a rate of approximately 1023 atoms per second. That may seem a large number, but in reality it is approximately 100 times lower than the current rate of photodisociation, Luhmann said. But billions of years ago, the electric field was probably much stronger than it is today, and spray rates could have been much higher, possibly being the dominant taxpayer to the erosion of the atmosphere of Mars.
Such change could help explain what happened with Mars’s water.
There is an abundant evidence that there was once liquid water on the surface of Mars: valleys rivers, dry lake beds and other characteristics carved in water persist until today. This means that Mars’s atmosphere must have been thick enough to withstand liquid water. “You need that atmospheric pressure that push down on the water to be a liquid,” Curry said. But the red planet today is an arid world devoid of visible water. The spray could explain, at least partially, how pressure loss occurred.
And because liquid water is closely linked to our conception of life, these results have an important meaning, said Scherf. “You can’t know if life can exist somewhere if you don’t understand the atmosphere and how it behaves.”
Curry and his colleagues hope to use Maven data in the coming years, but the team recently learned that they may not have that opportunity: the mission is scheduled to be canceled in the proposed 2026 federal budget. That has been a great emotional blow, Curry said, but the team does not yet give up. “United States at this time is number one on Mars Exploration,” Curry said. “We will lose that if we cancel these assets.”
—Katherine Kornei (@Katherinekornei), Scientific writer
Citation: Kornei, K. (2025), scientists spotting in Mars, EOS, 106, https://doi.org/10.1029/2025EO250231. Posted on June 24, 2025.
Text © 2025. The authors. CC BY-NC -nd 3.0
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