Mars, once a planet full of water, today presents an arid, desert landscape. The characteristics of its surface bear witness to a past where water flowed abundantly:
- River valleys
- Dry deltas
- Hydrated minerals
Over the last 3 billion years, some of this water has seeped into the Martian subsoil. On the other hand, the fate of the rest of this precious resource remained an enigma for years. John Clarkea researcher at the Center for Space Physics at Boston University, leads a team determined to solve this planetary puzzle.
The future of Martian water: two main hypotheses
According to Clarke, water on Mars had only two possible destinations:
- Solidify in the ground
- Breaking down into atoms escaping into space
“To understand how much water was once present and how it evolved, we need to study how atoms escape into space,” says Clarke. This atmospheric leak is at the heart of current research on Mars.
Atmospheric exhaust: key to the Martian enigma
Analysis of data collected by Hubble and MAVEN allowed Clarke's team to track the current rate of escape of hydrogen atoms from the Martian atmosphere. By extrapolating this information, they were able to trace the planet's water loss over time, providing a clearer picture of Mars during its wetter and warmer past.
On Mars, sunlight breaks down water molecules in the atmosphere into hydrogen and oxygen atoms. This process, called photodissociation, releases two types of hydrogen: regular hydrogen and a heavier isotope, deuterium. Because of its greater mass, deuterium escapes from the Martian atmosphere more slowly than normal hydrogen.
| Isotope | Masse | Exhaust speed |
|---|---|---|
| Hydrogen | 1 | Fast |
| Deuterium | 2 | Spring |
Over time, more hydrogen escapes than deuterium, causing the deuterium-to-hydrogen ratio in the atmosphere to increase. Measuring this ratio allows scientists to estimate the quantity of water once present on the Red Planet.
A dynamic and complex Martian atmosphere
Recent research has revealed that Mars' atmosphere is much more dynamic than previously thought a decade ago. It heats up and cools down quickly, sometimes in just a few hours. This turbulence is influenced by Mars' varying distance from the Sun, which fluctuates by up to 40% over the course of a Martian year.
These findings led to two major conclusions:
- When Mars is closer to the Sun, water molecules rise more quickly into the atmosphere, releasing hydrogen and deuterium at high altitudes.
- Rapid fluctuations in the escape rates of hydrogen and deuterium suggest that these atoms require additional energy to escape Martian gravity.
These seasonal and energetic variations explain why hydrogen and deuterium escape rates vary significantly throughout the Martian year. This in-depth understanding of the Martian atmosphere could have important implications for our understanding of the universe and the search for extraterrestrial life.
Studying the history of water on Mars is crucial not only for understanding the Red Planet, but also for studying other planets similar in size to Earth in distant star systems. Mars, along with Earth and Venus, is in or near the “habitable zone” of our solar system, where liquid water could exist. Each of these planets evolved under very different conditions, providing scientists with natural laboratories to study the evolution of planets in habitable zones over time.
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