Mars has a mass of 6.42 × 10²³ kg and a radius of 3.39 × 10⁶ m…

Step 1. Phobos. $T = 2\pi\sqrt{\dfrac{(9.38 \times 10^{6}\ \text{m})^3}{4.28 \times 10^{13}\ \text{m}^3/\text{s}^2}} = 27\,580\ \text{s} = 7.66\ \text{h}$.

Step 2. Skimming. $T = 2\pi\sqrt{\dfrac{(3.39 \times 10^{6}\ \text{m})^3}{4.28 \times 10^{13}\ \text{m}^3/\text{s}^2}} = 5993\ \text{s} = 99.9\ \text{min}$.