Rock varnish is a dark stain primarily found on rocks in desert environments, a smooth coating that provides a perfect canvas for rock carvings, or petroglyphs. How these stains got there, however, is a long-standing scientific mystery written about in the 1800s by both Charles Darwin and Alexander von Humboldt, and still debated to this day.

Since rock varnish is primarily found on surfaces with high sun exposure that also tend to have trickling water or dew accumulation, light and water are known to be important for the formation process. However, while most of the material in varnish comes from airborne dust and weathered rocks, this thin coating also has an extremely high concentration of manganese compared to the surrounding environment, and scientists haven’t been able to explain where that extra metal comes from.

Until now. Using the Stanford Synchrotron Radiation Lightsource, scientists were recently able to map the different compounds in rock varnish, finding that the high concentration of manganese in continuous layers must be related to ongoing biological processes as opposed to a long-term chemical reactions on the rock surface due to sun exposure. Genomic sequencing of the stain material also revealed an abundance of Chroococcidiopsis bacteria, which produce extra manganese to protect against harsh solar radiation, effectively creating a natural sunscreen for themselves. That extra metal is left behind when the cells die and then oxidized to form the dark, smooth surface.

This research concludes that rock varnish is a byproduct of magnesium-rich bacteria over millennia, linking the thin, dark stain to the existence of life. And if they’re right, that life might not be limited to our own planet — the Curiosity rover has found rock varnish on Mars. We haven’t seen any petroglyphs on the red planet just yet, but scientists are only beginning to scratch the (metaphorical) surface of what the existence of rock varnish on Mars means in the search for extraterrestrial life.