There is no evidence of anything having lived on Mars (yet), but what we do know now is that hypothetical fossilized microbes could have been obliterated.
NASA’s Curiosity rover got really curious when it observed some areas of Gale Crater. It saw areas of the rock record that had been mysteriously destroyed, and if there was fossilized life there, it could have gone with it. Curiosity’s CheMin instrument found the culprit. Billions of years ago, brines seeped through the bottom of one of the crater’s lakes as it was drying out. They ended up changing the chemistry of the subsurface.
What are now layers of mudstone have had their record completely rewritten — which could mean evidence of life that spawned later. Even more fascinating, as CheMin principal investigator Tom Bristow observed in a study recently published in Science, is that there are unaltered areas of the same layer of rock just over a thousand feet away.
“With CheMin, we found large differences in the content of clay minerals and iron oxide minerals in the rocks in these two places,” Bristow tells SYFY WIRE. “These differences cannot be explained by lake processes, so we started to think about other sources and types of fluids that could cause the difference in minerals.”
CheMin revealed how vastly different the chemistry of the two sites are. Lakes that have long since dried out first formed in the crater around 3.5 billion years ago. Previous observations beamed back by spacecraft had already revealed that layers of rock with high levels of magnesium sulfates eventually covered up the original deposits in those lakes. The presence of these highly soluble sulfates, gave away that most of the lake water in that part of Gale Crater had evaporated as the Martian climate, thought to have once been habitable, dried up.
When a shift in climate vaporizes water, brines tend to form (a phenomenon that has already been seen in the remains of ancient lakes on Earth). They are the product of buried salt deposits dissolving into whatever water that is left. These brines will make their way into the sediment beneath, and lakes can shrink or expand depending on the conditions surrounding them. As mudstone slowly morphed into sedimentary rock through diagenesis, a process that involves both physical and chemical changes, its past was overwritten.
“The physics that determines what types of minerals are stable under particular physical and chemical conditions is the same on Earth and Mars,” Bristow says. “Because of this, minerals are essential recorders of what conditions were like on Mars billions of years in the past.”
It isn’t impossible for things to live in salty lakes. Even if there was freshwater life that perished when the lakes of Mars evaporated, other things may have taken over. Diagenesis can create an ideal habitat for microbes and other forms of life. Brine shrimp — which you might know better as sea monkeys — thrive in salty environments. These creatures can dry out in harsh conditions and reanimate when things improve. What scientists really need to investigate is whether the region and time period in which the minerals formed make them potential indicators of life.
Minerals from that strange former lake in Gale Crater have some things to say about that. While they may not be screaming “aliens,” it became obvious the brines were oxidizing. This explains how Mars got to be the Red Planet. Lava from long-dead Martian volcanoes hardened into igneous rocks that turned red as they were oxidized during diagenesis. There are microbes right here on Earth that are actually involved in this process. Curiosity is now climbing Mount Sharp, also formed by layers of sediment, to investigate the younger layers.
Where Mount Sharp ends, so do deposits of sediment. The higher up Curiosity crawls, the newer the sediment, meaning a prospect of finding more evidence for what changes to the sulfates in this relatively young sediment could mean. Bristow is eager to continue investigating the reactions that could have occurred on Mars and whether they indicate life.
“Studying the mineralogy and physical characteristics of these rocks will give us further insights into the brines along with their formation, reactions, and possible sources of energy for life,” he says. “It will be fascinating to see if the rocks provide clues about broader climate change on Mars.”