To most people, the NASA image below looks like a photo of a rock. But to geobiologist Nora Noffke, that rock could contain an ancient shadow of life on Mars, fossilized on the surface of a 3 billion-year-old lake bed.

Noffke, a scientist at Old Dominion University in Virginia, was sorting through the images that Curiosity routinely sends to Earth from its distant tour of the red planet. In one of those photographs, she spotted a pattern on a rock in a dried-up lake bed that is now the Gale Crater.

Noffke took a closer look, studying what she suspected could be fossils in the rock from microbe structures that lived in the lake billions of years ago, carpeting the edges. In a detailed study published in the journal Astrobiology, Noffke compared Curiosity’s images from Mars to how microbes behave on Earth, and the mark they leave behind billions of years later.

To confirm that what Noffke saw is actually a fossil would require an analysis of the rock sample. Curiosity won’t be returning to Earth to deliver that kind of evidence, so scientists can only speculate on what could be proof that life existed outside the Earth and on our own neighboring planet.

And then again, it could just be a rock.