After NASA's Opportunity Rover landed on Mars in 2004, the probe revealed a puzzling phenomenon — tiny spheres, or spherules, just one-eighth of an inch (0.3 centimeters) in diameter, embedded in Martian rock. They appeared dark blue due to a high concentration of the mineral hematite, an iron ore. According to NASA, the tiny objects presented a minor problem for Opportunity, because they obstructed the grinding tool that the probe used to gather samples of the Martian surface.
The spherules were dubbed blueberries, after one scientist observed that the objects resembled blueberries in a muffin, as a 2004 article from Astrobiology Magazine noted.
But the Martian blueberries were more puzzling than appetizing, as scientists wondered how they originated and what significance they might have in the red planet's natural history. As Astrobiology reported, some scientists initially hypothesized that the blueberries were volcanic ash propelled into the Martian atmosphere, where the ash formed tiny globes before falling back to the surface. Others believed that they formed when water flowed through rock and carried away minerals that then precipitated and formed spheres.
More recently, in 2014, scientists suggested the blueberries actually were the remains of small meteorites that crashed into the Martian surface, as this 2014 National Geographic article describes.
Now, a paper by researchers from Japanese, Mongolian and British institutions, published in December 2018 in the journal Scientific Advances, supports the idea that the blueberries were formed by water. The scientists studied analogous iron-ore formations on Earth. They found that similarities to the Martian blueberries, combined with evidence of the influence of acid sulfate water on Mars, suggest that the blueberries also formed from the dissolving of carbonate spherules that were possibly formed in the dense atmosphere of early Mars.
But as this Space.com article on the research details, scientists can't fully solve the mystery until they have a probe with more sophisticated instruments yielding higher resolution on the part of the Martian surface where the blueberries were discovered. The Mars 2020 Rover will have such capabilities, but unfortunately it will be exploring a different area on Mars.