Uranus’ Moons - 5 larger moons 10 smaller moons The smaller moons were discovered by Voyager 2.

All the moons orbit in the rotational plane, so they experience the same extreme seasons as the planet.

The five larger moons are, in order from nearest to farthest from the planet: Miranda, Ariel, Umbriel, Titania, and Oberon. All have circular, synchronous orbits.

All the smaller moons lie inside Miranda’s orbit. Many of the smaller moons help to produce the ring system. Many of the smaller moons are named after characters in Shakespearean plays.

Puck, a small Uranian moon

There is a minor mystery involving the larger moons orbiting in the rotational plane. They are all so similar in composition, it is inconceivable that they were captured individually.

They must have been original moons. But, if that is true, what could have tilted the planet without expelling the moons?

The five larger moons are all probably made of ice and rock. They are similar to the six mid-sized moons of Saturn.

The outermost is Oberon, and the next is Titania.They are both heavily cratered and show no signs of geologic activity.

Oberon and Titania have albedos of 0.3 to 0.4. (The icy surfaces may be dirty.)

It is believed that these moon are dark because of radiation darkening.

Radiation Darkening - High energy particles striking the surface can break up molecules on the surface, leading to chemical reactions that build up a layer of dark, organic material. The surface is darker because it contains more carbon-bearing ice.

Umbriel is the next moon. It has the darkest albedo of Uranus’ moons at 0.2. The next moon is Ariel.

The innermost of the five larger moons is Miranda. Miranda has a wide range of surface terrains.

Miranda appears as though it has been catastrophically disrupted several times with the pieces falling back together in a jumbled, chaotic way.

Neptune’s Moons - Neptune has no regular moon system. It is the only Jovian planet that does not.

Two moons can be seen from Earth: Triton and Nereid. Voyager 2 found six more, all within Nereid’s orbit.

Triton is the largest moon of Neptune. It is also the farthest from the planet.

Triton has a circular, retrograde orbit. It is the only large moon in the solar system that orbits retrograde.

Triton has numerous frozen “lakes” of H 2 O ice. Voyager 2 observed nitrogen gas geysers erupting. This may cause the thin N 2 atmosphere that has been observed.

Triton has a tidal bulge that causes it to spiral toward the planet rather than away from it due to its retrograde orbit.

Eventually (after about 100 million years) Triton will move inside Neptune’s Roche limit and be torn apart to form a ring.

Nereid has an extremely elongated orbit with a perigee of 1.4 million km and an apogee of 9.7 million km.

Neptune’s second largest moon is Proteus. But Proteus was not discovered until 1983 by Voyager 2, 33 years after the discovery of Nereid. Proteus is very dark and its orbit is closer to Neptune, both of which made it hard to detect.

Ring Systems - Uranus’ rings were discovered in 1977 when Uranus passed in front of a bright star and each ring dimmed the light (stellar occultation).

Uranus actually had the first rings observed after Saturn. Jupiter’s were seen in 1979 by Voyager 1 & 2 and Neptune’s in 1989 by Voyager 2.

Uranus’ Rings

Neptune’s rings have arcs where the ring is brighter than other areas.

The rings of Uranus and Neptune require shepherding satellites like the F-ring of Saturn.

Neptune’s three brightest rings are named Galle, Leverrier, and Adams.

A view back by Voyager 2 as it leaves Neptune.