There is ample evidence that Mars once had liquid water in the form of lakes and rivers, but it was unclear whether these bodies of water came from one-off events, such as volcanic eruptions melting ice or meteor impacts, or whether they were tied to a more global weather cycle.
Now, William Rapin at the University of Toulouse, France, and his colleagues have examined imaged from Curiosity and found a distinctive pattern of hexagonal ridges in mud from the Gale crater, a former lake, which they say can only be formed from repeated wet and dry environments, of around a year or less.
“It’s the first time we can show that the climate sustained hydrological change seasonally, or wet and dry seasons” says Rapin. “We knew the Earth had them, but we didn’t know of any other planets that did. Now we know Mars had seasons.”
The researchers think the ridges were originally cracks formed in mud, which tend to intersect at specific angles, that had dried out.
These cracks would have been filled in by flooding and minerals, acting as a mould for a more resilient mix of mud and rock that remained as ridges when the rest of the material was washed away. “Only a seasonal climate – something with high frequency, geologically speaking – can produce those cracks in the mud that got fossilised,” says Rapin.
The hexagons are all about 4 centimetres wide, which Rapin and his colleagues used to estimate that the water depth was about 2 centimetres. This suggests that these cycles were fairly regular, lasting around a Martian year at the time, and may have persisted for millions of years.
Some environments on Earth display similar patterns, such as in California’s Racetrack Playa, which is a dry lake for most of the year but fills with a shallow layer of water in the rainy season.
These rock formations appear to be about 3.6 billion years old, which is important because it is around the same time we know that life first emerged on Earth, which means there should be enough time for life to have emerged on Mars, too. “If you have life on Earth, then why not life on Mars, if conditions on both planets were about the same,” says Mark Sephton at Imperial College London.
The seasonal weather could also have helped form molecules essential for life, like RNA and proteins, from small building blocks of organic matter, such as amino acids and nucleotides. Lab experiments have shown that the chemical reactions required, like polymerisation and condensation reactions, often require periods of dehydration.
“If you’ve got a primordial soup, and you dry things out, there’s a chance that things will stick together, as long as they don’t get degraded by radiation or oxidation,” says Sephton.
Earth lacks a geological record for when the building blocks of life may first have appeared, but Mars does have a rock record from that period. “This is a giant experiment for polymerising organic matter and self-organising it, and it’s all preserved,” says Rapin.