6. The moons of Mars

The moons of Mars—Phobos and Deimos—were first discovered in 1877 by the American Asaph Hall. ‘The moons of Mars: captured asteroids’ explains that Phobos and Deimos are small rocky bodies in synchronous rotation. They are far too small for their own gravity to pull their shapes into hydrostatic equilibrium. Their densities, just less than twice that of water, are too low for them to be solid rock. It is more likely that below the surface regolith, their interiors consist of chunks of loosely packed rubble. Spectroscopically, both Phobos and Deimos resemble asteroids that are believed to equate to a class of meteorite known as carbonaceous chondrites.