General orientation to the Solar System VI

The Saturn system

Saturn from NASA.


Items of interest:

The major satellites

Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus, Earth, and the Moon to scale.

Saturn has 62 known named satellites. Unlike Jupiter, with its four large moons, Saturn has one large moon, Titan, six small icy ones, and a vast collection of minor bodies whose orbits extend to 0.17 AU. All of the major moons are icy in composition. They mostly resemble small versions of Callisto with crater-saturated surfaces covered by impact craters and cut by rift valleys where the crust seems to have pulled apart. (E.G. Dione). They are:

Three, however, deserve special mention:

The ice giants

Uranus from University of Hawaii - Astronomy 110.


Orbital Specs:

The closest major planet that is not visible to the naked eye and, hence, unknown in antiquity. Discovered by William Herschel in 1781. Visited by only one spacecraft, Voyager 2 in 1986, Uranus is a nondescript greenish sphere with an interesting history.

Items of interest

Miranda, Ariel, Umbriel, Titania, Oberon, Earth, and the Moon to scale.
The major satellites

Uranus has 27 known satellites of which five are proper icy worlds, similar to the major moons of Saturn (minus Titan, which is unique in the Solar System). Titania, the largest, is typical. Like Saturn's moons, they show subtle contrasts of ancient terrain and more active regions of parallel faults and lines. They are far enough from the Sun, however, that methane, like water and CO2 would be exist as a solid. Although icy, their surfaces are darkened by space weathering (for later discussion.) They are:

Miranda from NOAA.


Actually smaller than Enceladus with radius of 236 km, shows bold stripes reminiscent of Enceladus and an enigmatic distinction between ancient and striped terrain. Its topography features the tallest cliffs in the Solar System (See also an artist's imagining). Might it also have active geysers? The answer awaits a future generation of high-end planetary missions.

Earth and Neptune to scale from Wikipedia.


Orbital Specs:

When Galileo discovered the moons of Jupiter in 1613, Jupiter and Neptune were close together in the sky. Neptune appears in his notebooks in a different color ink from the background stars. Galileo seems to have recognized that it was something different but failed to grasp that it was a planet. Its proper discovery came in 1846 after a search prompted by observations of perturbations of Uranus' orbit by the gravity of some unseen body. Visited by only one spacecraft, Voyager 2 in 1989. Even though its cloud-top temperature is a mere 54 K, it is surprisingly lively.

Items of interest

Proteus, Triton, Earth, and the Moon to scale.
The major satellites

We know of thirteen moons of Neptune. Unlike the moons of Uranus, those of Neptune have wide range of orbital inclinations (they don't all orbit in a single plane.) Four have retrograde orbits. Some orbit very far from Neptune. Only one, Triton, is a world of any size. Proteus, the next largest, is just barely unable to pull itself into a spherical shape.

Triton from Astronomy Picture of the Day.


Physical specs:

Orbital specs:


Pluto as seen by New Horizons in 2015 from NASA.

Trans-Neptunian Objects:

The region beyond the orbit of Neptune (30 AU) is inhabited by untold numbers of icy minor bodies. Most reside in the Kuiper Belt (trans-Neptunian objects with low orbital inclination and eccentricity.) Others have more irregular orbits that carry them beyond the Kuiper Belt. These are sometimes deflected into the inner Solar System where we see them as comets. We currently know over 1000, but speculate that there are many times this number.

Items of interest:

Major Trans-Neptunian objects from Wikipedia.
New discoveries:

Asteroid 2015 BZ509 from Wikipedia.
Interlopers: Interstellar Objects

Finally, 2018 has seen the identification of two interlopers from beyond our Solar System:

Key concepts and vocabulary.
Additional reading: