In concept simulation 10.2 you can explore the concepts that are important in this problem. astronauts on a distant planet set up a simple pendulum of length 1.2 m. the pendulum executes simple harmonic motion and makes 100 complete oscillations in 360 s. what is the magnitude of the acceleration due to gravity on this planet

Period of a simple pendulum = 2π √(L/G)

(360s/100) = 2π √(1.2m/G)

1.8s / π = √1.2m / √G

√G · (1.8s/π) = √1.2m

√G = (π · √1.2m) / 1.8s

Square each side:

G = π² · 1.2m / 3.24 s²

G = (1.2 · π² / 3.24)  m/s²

G = 3.66 m/s²

So I just went and looked up Mars gravity. Floogle says it's 3.711 m/s² there.
That seems awfully close ... only 1.4% greater than our astronauts measured. 
You don't suppose . . . . .