Mars may not have always been a dry and barren desert place. New evidence suggests that climatic conditions on the planet varied, perhaps seasonally, and created favorable conditions for the emergence of life.
One important finding is the discovery of hexagonal patterns in Gale Crater, which indicate a history of recurring cycles of wet and dry conditions. During dry periods, minerals may have dried out between wet periods, resulting in specific formations preserved as fossils.
“We found dried sulfate-enriched polymer ridges clustered in Y-shaped joints that reflect cracks formed in fresh mud due to repeated cycles of equal intensity,” writes the team led by geochemist William Rapen of the University of Paul Sabatier in France.
“Instead of random hydrologic activity caused by a meteorite strike or volcanic activity, our results point to a stable, cyclic, possibly seasonal climate on early Mars.”
These findings have implications not only for understanding the more Earth-like climate on Mars. They also add weight to existing evidence that conditions on early Mars were favorable for the emergence of biochemistry – the molecular basis of life.
Because of the lack of tectonic activity, the surface of Mars preserves an extensive geologic history of the planet that goes back as far as 4.3 billion years. In this history, we find abundant evidence of a past wet Mars, with lakes, rivers, and oceans that shone on the planet’s surface an era ago.
However, the hydroclimate of early Mars is harder to reconstruct. But Rapen and his team have shown that it is possible if you know how to look. At the bottom of Gale Crater, where the Curiosity rover is exploring, they found a series of well-preserved hexagonal patterns in sedimentary rock rich in calcium and magnesium salts that are about 3.6 to 3.8 billion years old, overlapping two time periods known as Noachian and Hesperian.
We know, based on what we see on Earth, what processes can create such patterns in dry pools. The researchers, after exploring the options, concluded that the most likely explanation is the drying of wet mud.
And not just one-time drying. Wet mud that dries out forms T-shaped cracks. Repeated drying cycles result in Y-shaped cracks and, as a result, a hexagonal pattern.
The concentration of salts in the sedimentary rock is much higher than in the basalt rock, indicating that the salts were transported there – probably salt water permeated the mud, then evaporated, leaving the salt behind.
Finally, the thickness of the patterned rock indicates that regular wet and dry conditions persisted on Mars for a long time – thousands to perhaps millions of years.
We don’t yet have evidence of microbial life on Mars, but these cyclic conditions would have been favorable for organizing organic molecules into complex compounds. We know that there are organic molecules on Mars and that they are probably widespread. Weather cycles are another piece of the habitability puzzle.