Moons of Uranus: Science Behind Tilted Planet’s Satellites

Uranus and its six largest moons compared at their proper relative sizes and relative positions. From left to right: Puck, Miranda, Ariel, Umbriel, Titania, and Oberon

Uranus, the seventh known planet of the Solar System, has 27 discovered moons, most of which are named after characters that appear in, or are mentioned in, the works of Alexander Pope and William Shakespeare.

Uranus’s moons are divided into three groups: 

  1. Five major moons,
  2. Thirteen inner moons,
  3. And nine irregular moons. 

The major and inner moons all have prograde orbits, while the orbits of the irregulars are mostly reversed. The inner moons are tiny dark bodies that share common origins and properties with Uranus’s rings. 

The five major moons are relatively ellipsoidal, meaning that they reached hydrostatic equilibrium at some point in their past (and may still be in peace). Four of them show signs of internally driven processes such as volcanism and canyon formation on their surfaces. The largest of these five, Titania, is the eighth-largest moon in the Solar System and 1,578 km in diameter, about one-twentieth the mass of the Earth’s Moon. The orbits of the normal moons are nearly coplanar with Uranus’s equator, which is tilted 97.77° to its known orbit. Uranus’s irregular moons have strongly inclined (mostly retrograde) and elliptical orbits at large distances from Uranus.

Characteristics and Science

The Uranian natural satellite system is the least bulky among those of the giant planets. Indeed, the five significant satellites’ combined mass is less than half that of Triton (the seventh-largest known moon in the Solar System) alone. The largest of the satellites, Titania, has a radius of 788.9 km, or less than half that of the moon, but slightly more than that of Rhea, the second-largest Moon of Saturn, making Titania the eighth-largest moon in the Solar System. Uranus is around 10,000 times more massive than its natural moons.

Uranus’s known irregular moons range in size from 20 km (Trinculo) to 120–200 km (Sycorax). Unlike Jupiter’s irregulars, Uranus exhibits no correlation of axis with angle. Instead, the reversed moons can be split into two groups based on orbital eccentricity/axis. The average inclinations 60° < i < 140° are void of known moons due to the Kozai mobility. In this instability region, solar perturbations at apoapse make the moons acquire large eccentricities that lead to ejection or crashes with inner satellites. The existence of moons in the uncertainty region is from 10 million to a billion years. Margaret is the only recognized irregular prograde moon of Uranus, and it currently has the most unusual orbit of any moon in the Solar System.

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