If life exists on Mars, it still hasn’t showed itself—but recent evidence from the Red Planet increasingly supports the possibility. Life could have developed there. Most of the conditions are right, and nothing found so far rules out the possibility, either in the distant past or today.

If something is or was alive on the Red Planet, it’s probably tiny. Because microbes make up the vast majority of life on Earth and live in its most inhospitable environments, they are the most likely thing to find somewhere else. It’s not so easy figuring out what “alive” means on another planet—let alone discovering a living microbe. Scientists are still puzzled by life on Earth— struggling to understand how life started, what it requires to survive, what it looked like 4 four billion years ago, and how to recognize traces of ancient life today.

Solving these mysteries demands the latest and best laboratory tools and lots of samples from the field—two things that are hard to come by when you’re talking about a planet tens of millions of miles away from Earth. So it doesn’t mean much that we have not yet seen life on Mars.

The Case for Varnish—and a Mineral Called Manganese

The upcoming Mars 2020 mission and its rover-based mobile laboratory will take another giant step toward answering the question of life on the planet. First, though, science needs to answer some questions about life here on Earth. It turns out that the dark, hard varnish coating cliff faces and rocks throughout the American Southwest and elsewhere—often as a canvas for ancient carved petroglyphs—has a lot to say about the subject, for a few reasons.

Several minerals make up varnish: manganese oxides, iron oxides, and (mostly) clays. Manganese oxides require oxygen to form. On Earth, oxygen only appeared in the atmosphere after cyanobacteria—life—began photosynthesis. It’s not clear what other mechanisms could produce such high abundances of atmospheric oxygen, although some have been proposed. Living microbes play a not- yet- fully understood role in forming varnish, perhaps by producing manganese oxides from the manganese that falls on rock as dust.

The microbes in varnish could be well suited to Mars. They can repair radiation damage to their DNA. They tolerate extreme hot and cold and thrive in the driest deserts on Earth. They need only a trace of dew or water vapor. They can even survive in a vacuum. Because Mars is cold, dry, dusty, and bombarded by solar radiation, these traits make rock varnish a tantalizing place to look for the signature of life on Mars.

As on Earth, So on Mars

The case for manganese got a lot stronger when the ChemCam instrument, developed at Los Alamos National Laboratory and currently operating on the Curiosity rover on Mars, identified veins of manganese oxides and what looks like manganese-rich coatings on Martian rocks. The presence of manganese oxides suggests Mars once had an oxygen-rich atmosphere.

Because life and the oxygen in the atmosphere on Earth are interrelated on Earth, do these discoveries mean that life could have helped to create rock varnish on Mars?

Maybe. Los Alamos’s next-generation instrument SuperCam, set to fly on the upcoming Mars 2020 mission, will help us further explore the chemical compounds on Mars that might be signatures of life. And because chemistry here is the same as the chemistry on Mars, similar processes can be expected to work in both places.

To make sense of those future findings, the Mars research team first needs to understand everything about manganese and varnish on Earth. Now that it has larger implications,varnish deserves a closer look than as a canvas for early humans to detail their world through petroglyphs. To that end, on in an internally funded project at Los Alamos, we are studying both the geology and biology of rock varnish on Earth—still our best laboratory—using the kinds of instruments that will go to Mars in 2020.

Establishing how microbial varnish forms on Earth will tell us what to look for on Mars. Manganese has a complicated chemistry. One type of manganese oxide might indicate a life-friendly Mars, and another, might not. Likewise, one type might have been formed by microbes, and another, might not have.

Our research into varnish relies on SuperCam-type instruments we use to study the mineralogy of different manganese oxides. Doing so is key to spotting microbe-formed varnish. We will also focus on identifying and understanding the various microbe species of microbe that form varnish, identifying the unique fingerprints distinguishing microbially formed varnish from the chemically formed kind, and understanding the related chemistry. We are working with rock varnish samples gathered from Arizona, New Mexico, Utah, and, Wyoming, including the spectacular Mesa Prieta site in northern New Mexico, with its vast gallery of ancient images pecked into varnished basalt.

Since the Mars 2020 rover is looking for the signatures of life, knowing everything about manganese will help the research team pick targets for SuperCam and for study with other instruments onboard the rover. SuperCam will also help the team pick samples to be cached—a major goal of Mars 2020—so a future mission could one day return them to Earth for study in a fully equipped lab.

Did life sign its name in manganese oxides on Mars? For a big claim like that, you need big proof. Scientists often assemble proof from jigsaw-puzzle pieces revealed by a series of discoveries, not just one eureka! moment. Careful work leading to a better understanding of common rock varnish might just provide a big piece in solving the puzzle of life beyond Earth.