When it comes to Mars and our ongoing efforts to understand that planet's ancient habitability, evidence is so far elusive. But scientists have an idea where the evidence is likely hiding: in sediments.
New research shows that an ancient mud lake is a good place to explore.
Where should we search for evidence of life on Mars? It's not obvious, though regions with plenty of sediments are a good starting point.
That's one of the reasons NASA's Perseverance rover is in Jezero Crater, the site of an ancient paleolake, where sediment deposits could be one km. deep. But there are many sediment-rich regions.
Where else should scientists search to widen their scope?
"The quest for past Martian life hinges on locating surface formations linked to ancient habitability," the researchers write in their paper.
One region that attracts attention is where massive outflow channels carried material from Mars' Southern Highlands into its Northern Lowlands, to the east of Valles Marineris.
Massive amounts of sediments accumulated in these northern lowlands, and that might suggest that it's a good place to explore. But the authors of a new paper say that might be a mistake.
The paper is "Exploring the evidence of Middle Amazonian aquifer sedimentary outburst residues in a Martian chaotic terrain." It's published in Nature Scientific Reports, and the lead author is Alexis Rodriguez, Senior Researcher at the Planetary Science Institute.
Instead of exploring the region where the sediments flowed into, scientists should focus on the source of the sediments.
"Venturing into the northern plains for sampling could prove precarious, as distinguishing between materials sourced from the aquifers and those eroded and transported during channel formation could become an intricate task," said Rodriguez.
Instead, researchers should focus on the Hydraotes Chaos region, a sub-region of Oxia Palus. Hydraotes Chaos might contain an ancient mud lake in the form of plains, and the sediments there could be hiding evidence of life.
"The plains, situated within Hydraotes Chaos, offer a unique glimpse into ancient aquifer materials. These plains, which we think formed from mud extruding into a basin directly above their source aquifer, provide a more targeted exploration opportunity," Rodriguez said.
The vast flood channels stretching from the highlands into the lowlands are a region that's so vast it's almost impossible to explore. There's also the complicated task of differentiating between sediments from different sources in the enormous region. The flat basin in Hydraotes Chaos could simplify exploration.
"Unlike vast flood channels with their complex erosion patterns, this finding simplifies the examination of Martian aquifers, reducing the risk of overland sedimentary acquisition, and opens a new window into Mars' geological past," said Rodriguez.
This basin is more directly connected to Mars' subsurface, and if the planet was ever habitable, its subsurface would've likely remained habitable longer than the surface.
When Mars' magnetic shield failed around four billion years ago, the surface became an inhospitable place. But simple life could've persisted underground, in the right conditions.
"Our research focuses on a sedimentary unit within Hydraotes Chaos, which we interpret to be the remnants of a mud lake formed by discharges from gas-charged mudstone stratigraphy dating back to nearly 4 billion years ago, a time when the surface of Mars was likely habitable. These sediments might harbor evidence of life from that or subsequent periods," said Rodriguez.
"It is important to remember that the subsurface of Mars might have included habitability lasting the duration of life's history on Earth," added Rodriguez.
Mars' surface and its geology is a puzzle that's not easy to solve, even though scientists have made progress. Sometimes it involves some deep sleuthing, as in this research. The ancient mud lake isn't where life persisted, but it's intricately linked to where it could have existed, if it did at all.
Here's what the researchers think happened in this region in Mars' deep past.
The ancient mud lake had a source aquifer of mudstone directly under the lake. Within the mudstone, igneous activity triggered phase segregation, forming vast chambers of mostly liquid water that were several kilometers wide and hundreds of meters in depth. The chaotic nature of Hydraotes Chaos' terrain suggest there were many of them, and that they were interlinked. Earth has similar features, but nowhere near as large.
But all that water and sediment didn't stay in the aquifer.
"Initially, biomolecules could have been dispersed throughout the volume of large groundwater filled cavities." Rodriguez said. "As the water was released to the surface and ponded, the water went away leaving behind lags of sediments and potentially high concentrations of biomolecules."
Potentially high concentrations is a good place to start, and NASA has taken note.
"NASA Ames is considering the plains as a possible landing site for a mission to search for evidence of biomarkers, specifically lipids. These biomolecules are extremely resistant and could have endured billions of years on Mars," co-author Mary Beth Wilhelm of NASA Ames Research Center said.
The region holds other interesting features as well, and they're also pieces of the Martian puzzle. There are widespread mud volcanoes and also diapirs, igneous intrusions of deformable material forced through brittle overlying rock. That gives researchers a glimpse of underground processes and structures without having to dig for them.
In addition, the study region includes widespread mud volcanoes and possible diapirs, providing additional windows into subsurface, potentially habitable rocks," said co-author Jeffrey Kargel, also from NASA AMES.
"A small rover could within short distances sample the mud lake sediments and these materials, dramatically increasing the odds of biosignature detection."
The region's age also aligns with Mars exploration goals, and the planet's history. The plains may be only one billion years old, meaning any biomolecules wouldn't have been on the surface for three or more billion years like some other regions. That boosts the chances of finding any intact biomolecules, since the surface exposure would degrade them.
"Our crater counts indicate that the plains are relatively recent, returning an age of 1 billion years. This age is good news for our search for life. This age is way younger than the ages of most aquifer releases on Mars, dating back to approximately 3.4 billion years ago. So, the materials spent a huge amount of time in the subsurface." co-author Berman said.
A small rover could visit many of these features without travelling great distances, and that's always an attractive idea. The farther a rover needs to travel, the larger, more complicated, and more expensive it usually needs to be. There's also more risk of failure, breakdown, or other problems.
The ESA's upcoming Rosalind Franklin rover will land in Oxia Planum sometime after 2028, but it's landing spot is far from this region.
There are currently no plans for a mission to Hydraotes Chaos and the ancient mud lake. But the team has targeted a particular landing site for a potential mission to the region.
Who knows how long or how many missions it'll take before we find the evidence we're looking for. Maybe we never will. But as long as we keep looking, there's a chance.
The better we understand the planet as a whole, the more we know where to look. This work points out what could be a prime location for exploration, but we'll have to wait and see.