The Moons of Mars for Lowell Was Right!

Phobos

Phobos is Mars’s moon, about 11 km in radius. Long ago, it was reached by Martians and turned into a space station. The interior was mined and hollowed out, and the ends capped with transparent hemispheres of unknown composition. Phobos was then put under rotation, enough to give .5 Martian Gs - equivalent to the gravity of the Galilean moons, or Luna. Inside live a species of Martian unknown on the planet - very tall and delicate, with paper thin ornamental shells. They service ships arriving from elsewhere, and sell the produce of their little hollow world.

Deimos

Deimos was originally thought to be the smaller of Mars’ two moons, about 15km long and 10 km wide. It is actually a gigantic derelict spaceship - according to legend, the one that destroyed the fifth planet. Several exploring parties have penetrated far into its interior, with one - the infamous Henderson party - never returning.