Mars’s long-lost atmosphere may have been hiding in plain sight all along.
A recent study suggests as much as 80% of Mars’s missing air could be trapped in the planet’s red clay soil, captured inside a mineral known as smectite.
![Frosty white water ice clouds and swirling orange dust storms above a vivid rusty landscape reveal Mars as a dynamic planet in this sharpest view ever obtained by an Earth-based telescope. The Earth-orbiting Hubble telescope snapped this picture on June 26, when Mars was approximately 43 million miles (68 million km) from Earth - its closest approach to our planet since 1988. Hubble can see details as small as 10 miles (16 km) across. Especially striking is the large amount of seasonal dust storm activity seen in this image. One large storm system is churning high above the northern polar cap [top of image], and a smaller dust storm cloud can be seen nearby. Another large duststorm is spilling out of the giant Hellas impact basin in the Southern Hemisphere [lower right]. Acknowledgements: J. Bell (Cornell U.), P. James (U. Toledo), M. Wolff (Space Science Institute), A. Lubenow (STScI), J. Neubert (MIT/Cornell)](https://c02.purpledshub.com/uploads/sites/48/2021/05/Mars-d1e4afb.jpg?webp=1&w=1200)
Mars was once a much wetter world, its surface water kept in place by a thick carbon dioxide atmosphere.
Then, around 3.5 billion years ago, both the water and the atmosphere disappeared.
Planetary geologists have studied Mars for years trying to discover where its atmosphere went, but the answer may finally have come from those looking at our own planet, and a group of researchers at MIT who were investigating smectites.
Each grain of the minerals contains multiple folds that can trap carbon-bearing molecules for billions of years.
By chance, the research team looked at a map of Mars and realised the Red Planet is covered in smectites.
"These smectite clays have so much capacity to store carbon,” says Joshua Murray from MIT, who led the study.
"So then we used existing knowledge of how these minerals are stored in clays on Earth, and extrapolated to say, if the Martian surface has this much clay in it, how much [carbon] can you store in
those clays?"
Mars’s smectite clays started life as rocks packed with an iron-rich mineral called olivine.
Over time, the oxygen in the water reacted with the olivine’s iron, oxidising it and turning it the rust-red colour that Mars is famous for.
Meanwhile, the hydrogen atoms in the water molecules reacted with any available carbon dioxide, forming methane.
"At this time in Mars’s history, we think carbon dioxide is everywhere, in every nook and cranny, and the water percolating through the rocks is full of carbon dioxide too," says Murray.
Over millions of years, the clays then continued reacting with water to form smectites, which then trapped the methane within them, the MIT researchers say.
Their study estimates that as much as 80% of Mars’s early thick atmosphere could have been sequestered in this way.
"This methane could still be present and maybe even used as an energy source on Mars in the future," says geology professor Oliver Jagoutz, also from MIT, who co-authored the research.
