Moons of the Solar System

Young Astronomers Blog Volume 29, Number 16.

On many nights when you look up into the night sky you can see a bright object known simply as the Moon. It is Earth’s only natural satellite and is unusual in that it is one of the seven largest moons of the solar system. This is not something we would expect for a small rocky planet like the Earth. For the story of our moon, see The Moon. For some information about observing the Moon, see Observing the Moon and Planets.

Moons are rare in the inner part of the solar system and tend to be found farther out orbiting the gas and ice giant planets. Mars  is the only other of inner terrestrial planets with moons, however, they are small and irregular; probably captured asteroids.

• Phobos is the largest, although not by much. It moves around Mars faster than Mars’ 24 ½ day rotation. So, it appears to rise in the west and set the east. Someday the orbit of Phobos will decay causing it to crash into the surface of Mars.
• Deimos is the smaller of the two. It has a higher and slower orbit and will likely escape from Mars’ gravitational attraction in the distant future.

The large outer planets, Jupiter (79), Saturn (82), Uranus (27), and Neptune (14) have over 200 moons among them. Seventeen are large and spherical, similar to the Earth’s moon. With one exception, Neptune’s Triton, they are thought to have formed during or shortly after the formation of each planet. Clouds of dust, gas, and ice swirling around the newly formed planets were pulled together by gravity and condensed into the large moons we see today. These large moons are all tidally locked with their planet, which means they always keep the same side facing the planet. There is also strong evidence of liquid water beneath the surface of several of these moons (see Underground Oceans).

Jupiter has just under eighty moons, however all but four are extremely small. The exceptions are the four Galilean moons, which were discovered by Galileo Galilee back in 1610. These four are so large that they are really worlds in themselves.

• Io is the closest to Jupiter and speeds around the planet in just over a day. The gravitational pull from Jupiter and from the other Galilean moons results in severe tides that disturb the surface by as much as 300 feet. This constant pulling on Io heats up the interior resulting in active sulfur volcanos that cover its surface.
• Europa is coated in a layer of water ice with groves and ridges. The gravitational pull from Jupiter and the other moons creates friction that heats up the interior of the moon resulting in a possible underground ocean of liquid water. In 2014 and 2016, the Hubble telescope spotted plumes rising from the surface of Europa. Additional evidence for water plumes was found in 2019 using the Keck telescope in Hawaii. Just recently, in 2021, Hubble found evidence of water vapor in Europa’s atmosphere.
• Ganymede is the largest moon in the solar system. Its surface is divided into areas of older rocky terrain intermingled with areas of newer icy terrain. Ganymede might also have an underground ocean of liquid water.
• Callisto is almost the same size as the planet Mercury. Its surface is a mixture of rock and ice with extensive cratering.

Saturn has probably the most interesting system of moons. It, as of the fall 2021, has the most moons of any planet in our solar system with 82. Like Jupiter, many are small, but there are seven large enough to be spherical and one other that is almost spherical.

Four of these, Mimas, Tethys, Dione, and Rhea, have similar structures of rock and water ice.

Enceladus has geysers of icy material rising from four long canyons nicknamed tiger stripes near its southern pole. The geysers are thought to originate from a moon wide underground ocean of liquid water. These plumes are the source for Saturn’s E ring.

Titan, Saturn’s largest moon, is just a bit smaller than Jupiter’s Ganymede, but still larger than the planet Mercury. Titan is unique in having an atmosphere thicker than that of the Earth. The atmosphere is composed of nitrogen, like the Earth, but with methane rather than oxygen. It is so cold on Titan’s surface that its landscape is composed of water ice (rather than silicate rock), and it has liquid methane in the form of rivers, lakes, and seas (rather than liquid water). Highlands such as Xanadu and lowlands filled with flowing dunes such as Shangri-La are found along its equator. The largest seas are Kraken Mare, Ligeia Mare, and Punga Mare at the north pole. There is also a large lake, Ontario Lacus, near its southern pole.

Saturn’s moon Hyperion is found just outside the orbit of Titan. At around 150 to 200 miles wide, it is the largest non-spherical object in the solar system.

Iapetus, the farthest of the large moons from Saturn, has two distinct hemispheres. One is a dark reddish color, the other a lighter color. The latest thinking is that Iapetus’ dark hemisphere is caused by dust from a ring of material found near the irregular moon Phoebe. Because Iapetus is moving in the opposite direction from the ring, material collects on the forward-facing side and not the other side.

Uranus has five semi-large moons, similar in size to those of Saturn (excluding Titan). While most solar system moons are named for mythological figures, the moons of Uranus are named after characters in British literature (primarily Shakespeare). The two largest moons are Titania and Oberon, named for the king and queen of the fairies in A Mid Summers Night Dream. Ariel and Umbriel, named for characters from a poem by Alexander Pope, are slightly smaller and found closer to the planet. The smallest of the five, Miranda, named after the heroine in The Tempest, has a jumbled surface and possibly underwent some type of collision or upheaval in the distant past.

Neptune has only one large moon, Triton. It orbits Neptune in the opposite direction of Neptune’s rotation and is thought to have been a dwarf planet captured by Neptune’s gravity in the distance past. Triton also appears to have active geysers sending nitrogen up onto Neptune’s surface.

Pluto’s large moon, Charon, is half the diameter of Pluto itself. Charon and Pluto are tidally locked with each other. So, Pluto always keeps one side facing Charon, and Charon always keeps one side facing Pluto. This gives Pluto and Charon a common 6.2-day orbit and rotational period. Pluto has four other extremely small moons, Styx, Nix, Kerberos, and Hydra.

Three of the other dwarf planets also have moons, although all are extremely small.

• Hi’iaka and Namaka are the two moons of Haumea.
• Dysnomia is Eris’s only moon.
• MK2 is the recently discovered moon of Makemake.