Ancient Mars had seasonal weather similar to Earth’s, with alternating wet and dry seasons, according to mud patterns discovered by NASA’s Curiosity rover. These seasonal cycles may have helped form some of the more complex building blocks for life, such as RNA and basic proteins.
There is ample evidence that Mars once had liquid water in the form of lakes and rivers, but it was unclear whether these came from one-off events, such as meteor impacts or volcanic eruptions melting ice, 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 images 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, each lasting around a Martian 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 in mud that have dried out. The cracks, which tend to intersect at specific angles, would have been filled in by flooding and minerals. Some of this material would have been washed away, but a more resilient mix of mud and rock would have remained, forming the ridges. “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. This is around the time we know that life first emerged on Earth, which means there should have been 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 need 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 first 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.