How NASA can avoid a false positive discovery of Martian microfossils

Buried within an ancient mudstone slab in what was once an inlet leading to a lake bounded by Mars’ Crater Lake, sample number 25 of NASA’s Perseverance rover is potentially a Martian astrobiological game changer.

The rock core was drilled just last month and offers telltale signs that microbial life may once have had a foothold here.

The “leopard spot” inclusions in rocks are similar to features that on Earth are often associated with fossilized microbial life, the University of California at Berkeley noted last week.

But such leopard spots can also be made abiotically.

That’s the challenge, so you need to know the context of the collection point, Persistence science team member Sandra Siljestrom, an astrobiologist and geochemist at the Research Institute of Sweden (RISE), told me in her office in Stockholm. I’m a Persistence sample bar, my job is just to make sure the best sample is selected, she told me. There are rocks everywhere; you have to choose, says Siljestrom. We don’t know exactly where life might have been if there was life on Mars, so we sample for diversity, she says.

Siljestrom served as the ‘sample shepherd’ on Persistence samples 22 through 25, meaning she led the team’s discussions about where the rover should drill.

First you have to prove that it’s an environment that could have existed where life could have lived and could have preserved the data, says Siljestrom.

What makes Champion 25 special?

Sample 25 is really a mudstone sample, and this is a material where you can store organics for a long time, Pascale Ehrenfreund, an astrobiologist at George Washington University in Washington, DC, told me at a recent astrobiology conference in Copenhagen. It could have been that microbial communities changed the rock and made these particular shapes and these leopard spots, she says.

But the elephant in the room remains how NASA can prevent another false positive like the one after the initial findings from the Allan Hills Martian meteorite.

The Allan Hills meteorite was found quite by accident nearly twenty years ago in the Far Western Icefield region of Antarctica. And when it was returned to the lab in warmer climates, sample ALH84001 was examined under a scanning electron microscope and was initially thought to have embedded microfossil-like morphologies.

But after a world-first press conference in 1996 announcing the findings, as the hours, days and weeks passed, it soon became clear that the Allan Hills meteor had never contained life. It contains some quite wild-looking completely natural carbonate morphologies, but no microfossils.

Part of the problem with the ALH84001 study was simply that too little was known about where it originated on Mars. In contrast, NASA knows exactly where its Martian samples originated, and such context is a powerful harbinger of what ancient Martian biology might be.

Ironically, at the young age of 16, Siljestrom was in large part inspired to pursue astrobiology and geochemistry after following news of the Allan Hills meteorite. Although it turned out to be a negative result, Siljestrom was attached to the idea of ​​searching for life on Mars and understanding the origin of life here on earth.

Scanning electron microscopes in use today are much better than they were in 1996, says Siljestrom. So we now have a more critical eye and more insight into how different chemical compounds can be produced abiotically, she says.

A morphological reality check

Another way NASA can avoid false positives is to compare Martian samples with the chemical composition and morphologies of microfossils found here on Earth, Siljestrom says. It’s a good reality check, she says.

But in situ analysis of Perseverance rock core samples remains limited.

Persistence has an instrument that can detect organics, but it can’t actually look at the specifics of organics, Siljestrom says.

This is why these Martian samples deserve to be returned to earth as soon as possible. Last April, NASA selected several aerospace companies to evaluate and improve the current sample return architecture. Lockheed Martin, SpaceX and Northrop Grumman are among the aerospace companies chosen to examine the most affordable and fastest methods of returning samples from the Martian surface to Earth, NASA says. The European Space Agency (ESA) is also expected to be part of the final sample return mission when it launches.

If we could analyze the samples with high-resolution instruments in the lab, we would know a lot more, Ehrenfreund says. But these samples will be preserved and waiting for us, even if it takes ten years to get there, she says.