The thirteenth circle / MarcyKate Connolly and Kathryn Holmes.

Enhanced descriptions from Syndetics:

The X-Files meets Scooby-Doo in THE THIRTEENTH CIRCLE, a middle-grade mystery from MarcyKate Connolly and Kathryn Holmes, featuring two unexpected friends, crop circles, science fairs, and Men in Black, perfect for both the highly scientific and cryptid enthusiasts alike.

Cat knows aliens are real, and she's determined to prove it. By studying the Weston Farm Circles, her town's legendary crop circle phenomenon, she'll not only demonstrate the existence of extraterrestrial life, but also win the grand prize in the McMurray Youth Science Competition--a feat she's sure will impress her distant NASA scientist father.

Dani most certainly does not believe in aliens. How can she, when they go against every scientific principle she's been taught? So when Dani is paired with Cat to enter the McMurray Youth Science Competition--which she has to win to avoid going to her parents' artsy summer camp--she knows she's at a disadvantage. Her solution? Disprove Cat's theory, of course . . . without telling her partner her true intentions.

But as the girls bond over science, it becomes clear that there is something strange about the Weston Farm Circles. And when Dani and Cat's project is threatened by suspicious forces, they'll have to work together to expose the truth, once and for all.