Billions of years ago, Mars had vast oceans, rivers, and lakes. It is believed that this water was eventually lost to space over time. Solar wind and UV light slowly but surely eroded the tenuous Martian atmosphere, but a new study suggests that the amount of water lost in that way can't account for a planet as dry as modern-day Mars.
The researcher simulated a range of scenarios for where the water might have gone. Obviously, there's the long-running idea that it mostly evaporated and escaped into space. Volcanic degassing could have helped to push water into the air. Another possibility is chemical weathering. Water altered rocks and became trapped in minerals now buried within the Red Planet’s crust.
The new study, published in Science, showed that this crustal hydration scenario might be a major player in the disappearance of the ocean. At least 30 percent of water could be trapped in rocks under the surface of Mars. But the model shows that uncertainties in the model might mean that up to 99 percent of the ancient oceans might still be on the Red Planet.
The simulated scenario is based on observational constraints from orbiting spacecraft, rovers, and meteorites from Mars found on Earth. This allowed the researchers to not only understand the role crustal hydration played in the evolution of Mars but now the team has a better idea of when the changes to the planet took place.
Mars formed roughly 4.5 billion years ago and its ocean came to be a few billion years afterward. According to their model, Mars began with enough water to cover the entire planet around 100 meters (330 feet) deep. The Martian water cycle had already massively decreased by 3.7 billion years ago. The team thinks that between 40 and 95 percent of water had disappeared by that point. Mars became as arid as it is now around 3 billion years ago. The team posits Mars's crust sucked up the water.
The phenomenon of crustal hydration happens also on Earth, but plate tectonics allows for material in the crust to be melted down and the water content to be once again released into the atmosphere. Based on the observations collected so far, such a cycle was not present on Mars. Older minerals have a higher water content, compared to those formed closer to our time.
Studying rocks from different time periods could be key to confirming this new model. A particular chemical signature would be different in rocks from the “wetter” period compared to the rocks from the dryer period. Current and upcoming missions might soon be able to provide the data to test this idea.