PASADENA, July 5 -- Two ancient mudstones at the edge of what was once a Martian river delta have forced a recalibration of what scientists know about that planet's chemistry. The rocks, drilled by NASA's Perseverance rover from a light-colored outcrop in Jezero Crater called Bright Angel, contain macromolecular organic carbon in concentrations and distributions that no prior instrument on Mars has matched. The question of whether that carbon was ever part of something living is one the rover cannot answer. What it can say, plainly, is that Mars had the chemistry.

The findings appear in a study published Wednesday in Science Advances, co-led by Kyle Uckert, a research scientist at NASA's Jet Propulsion Laboratory, and Ashley Murphy, a po...