Phobos orbits Mars in 27,553 s at a distance of 9.378 × 106 m. What is the mass of Mars?

For a moon in a circular orbit, the orbital period and radius give the planet’s mass: M = 4π²r³/(GT²). Substituting r = 9.378 × 10⁶ m, T = 27,553 s, and G = 6.674 × 10⁻¹¹ N·m²/kg² gives M ≈ 6.43 × 10²³ kg. Therefore, the mass of Mars is choice C, 6.43 × 10²³ kg.

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