It turns out that they are almost definitely used to calibrate China's spy satellites.
So says Jonathon Hill, a research technician and mission planner at the Mars Space Flight Facility at Arizona State University, which operates many of the cameras used during NASA's Mars missions.
...
Satellite cameras focus on the grids, which measure approximately 0.65 miles wide by 1.15 miles long, and use them to orient themselves in space.
...
The calibration targets are larger than might have been expected, he said, suggesting that the satellite cameras they are being used to calibrate have surprisingly poor ground resolution.
Another strange image taken not far away shows a Stonehenge-like arrangement of objects radiating outward, with fighter jets parked at its center. "This is almost certainly a calibration/test target for orbital radar instruments," Hill said. "Since a significant amount of radar return is due to differences in surface roughness, they're probably testing ways of making the areas around planes 'bumpy' enough that the planes are partially masked."