Deimos: A Small Martian Moon . ... Credit: Viking Project , JPL , NASA Explanation: Mars has two tiny moons, Phobos and Deimos . Pictured above is Deimos , the smaller moon of Mars. ... The diminutive Martian moons were discovered in 1877 by Asaph Hall , an American astronomer working at the US Naval Observatory in Washington D.C. The existence of two Martian moons was predicted around 1610 by Johannes Kepler , the astronomer who derived the laws of planetary motion . ...