NASA’s Perseverance rover is currently investigating rock outcrops alongside the rim of Mars’ Belva Crater. Some 2,300 miles (3,700 kilometers) away, NASA’s Curiosity rover recently drilled a sample at a location called “Ubajara.” The crater bears an official name; the drill location is identified by a nickname, hence the quotation marks.

Both names are among thousands applied by NASA missions not just to craters and hills, but also to every boulder, pebble, and rock surface they study.

“The No. 1 reason we pick all these names is to help the team keep track of what they’re finding each day,” said Ashwin Vasavada, the Curiosity mission’s project scientist at NASA’s Jet Propulsion Laboratory in Southern California. “Later on, we can refer to the many hills and rocks by name as we discuss them and eventually document our discoveries.”

Just how scientists come up with the identifiers has evolved since the early days, 25 years ago, when they used cartoon character names. Here’s a closer look.

The difference between an official name on Mars and an unofficial one is seemingly simple: Official monikers have been approved by a body of scientists known as the International Astronomical Union (IAU). The IAU sets standards for naming planetary features and logs the names in the Gazetteer of Planetary Nomenclature.

For example, craters larger than 37 miles (60 kilometers) are named for famous scientists or science-fiction authors; smaller craters are named after towns with populations of less than 100,000 people. Jezero Crater, which Perseverance has been exploring, shares the name from a Bosnian town; Belva, an impact crater within Jezero, is named after a West Virginia town that is, in turn, named after Belva Lockwood, the suffragist who ran for president in 1884 and 1888.

More than 2,000 locations on Mars bear official names, but even more unofficial nicknames dot the Martian map.

Early Mars missions sometimes took a whimsical route with nicknames, even using cartoon character names. “Yogi Rock,” “Casper,” and “Scooby-Doo” were among the unofficial names applied by the team behind NASA’s first rover, Sojourner, in the late 1990s.

The philosophy changed with the Spirit and Opportunity rovers, whose teams started using more intentional names. For instance, the Opportunity team nicknamed a crater “Endurance” to honor the ship that carried explorer Ernest Shackleton’s ill-fated expedition to Antarctica. The names for the spots where Curiosity and Perseverance landed honor science-fiction writers Ray Bradbury and Octavia E. Butler, respectively. The InSight team named a rock that had been jostled by the lander’s retrorockets during touchdown “Rolling Stones Rock,” after the band. And the Curiosity team named a Martian hill after their colleague Rafael Navarro-González, who died from COVID-19 complications.