In order to understand where we should look for life on other worlds, we need to understand where life exists on Earth. Or maybe a better way to put that is that we need to understand where life doesn’t exist. The problem is, no one knows where life can’t exist. We keep looking, and we keep finding life everywhere. Life. Is. Everywhere.

A couple of weeks ago, in a back-of-the-envelope calculation, Pamela assumed life on Earth is within ten kilometers of the surface of the planet. While this felt like a huge expansion on what is realistic, this is apparently less and less so as researchers look deeper and deeper beneath the surface of the Earth and at the bottom of the seas. As we mentioned a few weeks ago, scientists working in Antarctica found life as advanced as sponges and related sea creatures beneath 3000 feet of ice.

Now, researchers looking 5000 feet beneath the Earth in an old gold mine have discovered biofilms and other life living without light or oxygen. This former gold mine is part of the Sanford Underground Research Facility (SURF), located in Lead, South Dakota, and utilized the Deep Mine Microbial Observatory. In these environments where I never imagined life would be found, it turns out that different kinds of minerals are capable of providing the nutrients microbes need. 

According to the project lead, Caitlin Casar: Our results demonstrate the strong spatial dependence of biofilm colonization on minerals in rock surfaces. We think that this spatial dependence is due to microbes getting their energy from the minerals they colonize. 

These results show that biofilms can meaningfully contribute to the global nutrient cycles. Casar goes on to add that these biofilms, and I quote, “have astrobiological implications as these findings provide insight into biomass distributions in a Mars analog system.”

While Casar and her team explore beneath the Earth, researchers like Tanja Bosak are building on these results to search on the surface of Mars. The Perseverance rover carries 43 sample tubes, and it is hoped that sometime around 2031 a second mission will be able to pick up these samples and return them to Earth for analysis we can’t do with the instruments on Percy. The trick is trying to find the 43 rocks most likely to tell the story of Mars’ past, possible life and today’s still-maybe possible life. 

As a field geologist, Bosak is using Percy to do the work she does with her hands and tools on Earth. According to Bosak: We work to understand the chemical conditions under which microbes make certain minerals or fossilize in certain ways because that helps us understand and reconstruct what the environment was like. 

She goes on to explain, regarding the Jezero crater, that: We have a habitable environment — we have a lake. We have all the sediments that are known to preserve the signs of life on Earth. 

Now she just needs to find the rocks with those preserved signs, and we have to send a follow mission, and we have to wait more than a decade, but science is a long game and we can’t wait to learn what, in many years from now, we learn about Mars.

More Information

Phys.org article

“Rock-Hosted Subsurface Biofilms: Mineral Selectivity Drives Hotspots for Intraterrestrial Life,” Caitlin P. Casar, Brittany R. Kruger, and Magdalena R. Osburn, 2021 April 9, Frontiers in Microbiology