Even with all we’ve learned about Mars in recent years, it doesn’t stack up against all we still don’t know and all we hope to find out. We know that Mars was once warm and wet, a conclusion that was less certain a couple of decades ago. Now, scientists are working on uncovering the details of Mars’s ancient water.
New research shows that the Gale Crater, the landing spot for NASA’s MSL Curiosity, held water for a longer time than scientists thought.
Life needs water, and it needs stability. So, if Gale Crater held water for a long time, it strengthens the idea that Mars could’ve supported life. We know that Gale Crater is an ancient paleolake, and this research suggests that the region could’ve been exposed to water for a longer duration than thought. But was it liquid water?
The research is titled “Ice? Salt? Pressure? Sediment deformation structures as evidence of late-stage shallow groundwater in Gale crater, Mars.” It’s published in the journal Geology, and the lead author is Steven Banham. Banham is from the Imperial College of London’s Department of Earth, Science, and Engineering.
The research centers on desert sandstone that Curiosity found.
We know that water played a role in shaping the Martian surface. Multiple rovers and orbiters have given us ample evidence of that. Orbital images show clear examples of ancient deltas. We also have many images of sedimentary rock, with its tell-tale layered structure, laid down in the presence of water. But beyond the initial creation of Martian sandstone, the details of the rock can tell scientists about what happened long after it formed.
This research focuses on Gale Crater and the landforms within it. Mount Sharp (aka Aeolis Mons) is the dominant feature in the crater and rises 5.5 km or about 18,000 feet. It’s made up of sedimentary layers that have been eroded over time. But it has substructures that show its detailed history.
One structure overlays Mount Sharp and post-dates Mount Sharp’s erosion. It’s characterized by the accumulation of aeolian strata under arid conditions. That means windborne deposits instead of waterborne deposits. So scientists can tell that there was a wet period during which fluviolacustrine sediments built Mt. Sharp. They can also tell that a dry period followed, during which wind-borne sediment created the overlying structure. That’s what you’d expect to find if the story ended here: Mars was wet, then it wasn’t.
“Surprisingly, we found that these wind-deposited layers were contorted into strange shapes, which suggests the sand had been deformed shortly after being laid down. These structures point to the presence of water just below the surface.”
Amelie Roberts, study co-author, Imperial College London’s Department of Earth Science and Engineering.
But scientists found something odd in the overlying windborne sandstone: deformed layers that could only have been formed in the presence of water. “The sandstone revealed that water was probably abundant more recently, and for longer, than previously thought – but by which process did the water leave these clues?” Banham said in a press release.
That’s more difficult to determine.
“This water might have been pressurized liquid, forced into and deforming the sediment; frozen, with the repeat freezing and thawing process causing the deformation; or briny, and subject to large temperature swings,” Banham said.
“What’s clear is that behind each of these potential ways to deform this sandstone, water is the common link.”
There’s a generally accepted understanding of Martian water among scientists. By the middle of Mars’ Hesperian Period, the planet lost its water. The Hesperian’s boundaries in time are uncertain, but it’s generally thought of as the transition from the heavy bombardment period to the dry Mars we know today. The Hesperian could’ve ended between 3.2 and 2.0 billion years ago. The Noachian preceded it, and the Amazonian followed it.
This research presents a new wrinkle. It suggests that Mars had abundant subsurface water toward the end of the Hesperian. The evidence is in MSL Curiosity’s images of different sedimentary rocks on Gale Crater’s Mt. Sharp.
“When sediments are moved by flowing water in rivers, or by the wind blowing, they leave characteristic structures which can act like fingerprints of the ancient processes that formed them,” said Banham.
MSL Curiosity slowly worked its way up Mt. Sharp, studying the rocks at different elevations as it ascended. As expected, it found younger rocks the higher it went. Eventually, it reached the Stimson formation. The Stimson formation is the remnant of an ancient windborne desert dune field.
An analysis of Curiosity’s images shows that Stimson formed after Mt. Sharp when Mars was thought to be dry. But Stimson isn’t entirely uniform. One of its features is named the Feòrachas structure, and it contains features that were clearly influenced by the presence of water.
“Usually, the wind deposits sediment in a very regular, predictable way,” said study co-author Amelie Roberts, a PhD candidate from Imperial College London’s Department of Earth Science and Engineering. “Surprisingly, we found that these wind-deposited layers were contorted into strange shapes, which suggests the sand had been deformed shortly after being laid down. These structures point to the presence of water just below the surface.”
In the Brackenberry outcrop feature, the sedimentary rocks show evidence of deformation by water. There are laminations in various states of deformity, becoming more pronounced in the feature geologists call the cusp core. In the cusp core, wind-ripple laminations bend toward the vertical and become incoherent.
The authors explain that there are three mechanisms that can explain the deformed features, and they all involve water. They’re also not mutually exclusive.
High-pressure water could’ve overcome the strength of the rock and deformed it. Large ice deposits on top of the structure could’ve caused deformation, as could freeze/thaw cycles of water inside the rock. The third explanation involves sediment rock weakly bound together by evaporites. Thermal expansion and contraction of the evaporites can deform the rock.
“The layers of sediment in the crater reveal a shift from a wet environment to a drier one over time – reflecting Mars’ transition from humid and habitable environment to inhospitable desert world,” said co-author Roberts. “But these water-formed structures in the desert sandstone show that water persisted on Mars much later than previously thought.”
Mars is no exoplanet, but it’s inadvertently teaching us a lot about our quest to understand exoplanets and habitability.
“Determining whether Mars and other planets were once able to support life has been a major driving force for planetary research for more than half a century,” said Dr. Banham. “Our findings reveal new avenues for exploration – shedding light on Mars’ potential to support life and highlighting where we should continue hunting for new clues.”
“Our finding extends the timeline of water persisting in the region surrounding Gale crater, and so the whole region could have been habitable for longer than previously thought,” said Amelie.
Maybe one day in the far distant future, one of our rovers on a distant exoplanet will flip over a rock and watch something scuttle away. It’s easy to imagine.
But Mars is an instructive example. If it remained habitable for longer than we thought, it was likely only marginally inhabitable. We can’t say for sure, but complex life seems to be out of the question. This should prepare humanity for what we can expect to find in our quest for habitable exoplanets.
There are a bewildering number of variables that go into making Earth the living oasis that it is. We’re much more likely to stumble on other planets like Mars, which were once habitable and maybe even harboured simple life. If Earth’s long-lived habitability is the outlier, and Mars’ marginal, interrupted habitability is more likely, we can expect to find many planets like it that were once alive but are now long dead.