Imagine looking up in the sky one night and seeing a half-dozen or more moons. A few are big and appear as a disk, but others are smaller and, while bright, look more like the planet Venus. One of them zips through the skies, rising in the west and setting in the east! One pair of moons are in resonance—they meet up at the same time every few days. Another moon rises at the same time every day. These weird sightings are not so far out; things like this do happen on other planets. One wonders what astrology would be like if Earth had a multitude of moons.
Our single Moon fascinates us all. It's a shape-shifter—changing form over the course of the month as its angle with the Sun changes. Yet we never see the back of the Moon. This is because the Moon and Earth are tidally locked. We know about ocean tides on Earth that are pulled by the Moon (there are also much smaller land tides), but imagine how much tidal force the much bigger Earth exerts on the Moon. Over very long periods of time, the Earth's gravitational pull on the Moon has distorted it, causing one side to bulge and lock in direction to the Earth, and there it has stayed. Tidal locking is common in our solar system, as we will see in our tour of the moons.
The process of moons orbiting planets came about in different ways. One is similar to the way the entire solar system formed. A spinning dust cloud shaped like a disk, called a protoplanetary disk, slowly condenses to form a planet. Condensing dust farther out from the planet forms moons. Moons formed this way more or less orbit in a flat plane and are called regular moons. Other moons may be bodies captured by the planet's gravity. These moons can have a variety of orbits and are called irregular moons. Earth's moon doesn't exactly fall into these categories and appears to be the result of a collision. The leading hypothesis today argues that long, long ago, a body half the size of Earth, called Theia, smashed into Earth, creating a debris cloud made up of Theia and a large part of the outer layers of the Earth. This debris coalesced into the Moon we see today.
Mercury and Venus have no moons, but Mars has two small ones—Phobos ("fear") and Deimos ("terror"). Asaph Hall discovered them by telescope in 1877, but in a way they were "inadvertently predicted" one hundred and fifty years earlier by none other than Jonathan Swift, in his book Gulliver's Travels. These moons were named after two characters in Greek mythology who fought wars with their father Ares (the Roman name is Mars). Phobos is the larger moon but is not spherical; it is oblong, measuring about 17 × 13 × 3 miles across. It is very close to Mars and moves through the sky faster than Mars rotates, so Phobos actually rises in the west and sets in the east. Phobos eclipses the Sun periodically, but it is too small to create a total eclipse. Phobos is further distinguished by a massive crater on one side that Hall named after his wife Angeline (who was a suffragist and abolitionist). Phobos's density is very low, prompting speculation that it is hollow or possibly artificial. It's most likely made up of more ice than rock.
Mars's other moon is smaller, under four miles in diameter, and more distant—appearing more like a very bright star. Its orbital period is not much longer than the Martian day of about twenty-five hours, so it will rise in the east and take almost three days to cross the sky and set in the west. Both Phobos and Deimos are tidally locked to Mars. Based on their composition, both moons are most likely captured asteroids, but this is not settled. An alternate hypothesis is that a large impact threw big chunks of Mars into orbit and all but two fell back to the planet. Regardless, Phobos will crash into Mars in about fifty million years as its orbit slowly decays.
Jupiter has an abundance of moons—sixty-seven as of this writing. Most of them have been named after characters associated with Zeus (the Roman equivalent of Jupiter). With Jupiter, we have eight regular moons that have more or less circular orbits in the plane of Jupiter's equator, along with many irregular moons.
The irregular moons are very small and have extreme orbits, suggesting that they are captured objects, probably asteroids. The eight regular moons most likely formed when Jupiter did. These include four small ones: Metis, Adrastea, Amalthea, and Thebe. They orbit Jupiter closely, circling the planet in under one Jupiter day, which is really fast—about ten hours. The other four, farther from Jupiter, are the well-known Galilean moons: Io, Europa, Ganymede, and Callisto. They were simultaneously discovered in 1610 by Simon Marius in Germany and Galileo Galilei in Italy.
Galileo published first and received credit for the discovery, but later astronomers decided to use the names that Marius proposed. These are big moons—all nearly two thousand miles or more in diameter.
We have learned a great deal about Jupiter's moons from the Galileo spacecraft that was launched by NASA in 1989 and which arrived in Jupiter's neighborhood in 1995. Io was named for the mythological priestess of Hera who was one of Zeus's lovers. It's known for its incredible volcanic activity—it has at least four hundred active volcanoes. Io also has rugged mountains, some taller than Mt. Everest. These features on Io have been named for characters and places from the Io myths and also from Dante's Inferno. The intense geological activity that produces these features stems from tidal friction—the friction produced as Io experiences extreme tides from massive Jupiter. Io is Jupiter's closest Galilean moon.
Farther out from Io comes Europa, a bit smaller than Earth's Moon, but still the sixth-largest moon in the solar system. It was named after the queen of Crete, who was also a lover of Zeus. Europa, orbiting between Io and Ganymede, is locked into orbital resonance with them. For every two orbits of Europa, Io makes four orbits and Ganymede makes one. But the most astounding feature of Europa is that its surface is smooth, though with many cracks. It appears that its icy crust is riding on a subsurface salty ocean that may be over sixty miles deep. That's a lot of water—about twice the volume of all of Earth's oceans. Speculations abound as to whether or not this ocean may harbor life. We will learn more when NASA's Europa Clipper and the European Space Agency's Icy Moon Explorer reach Europa in the mid-2020s.
Ganymede, the next moon out, is big—about 3,300 miles in diameter and 8 percent larger than Mercury, though not as dense, as it is made up of light silicate rock and ice. It was named for the youth who was abducted by Zeus and made cupbearer of the gods. Like Europa, it may have layers of liquid water and ice, though its surface has both smooth and cratered sections. Ganymede has polar caps and its own magnetic field.
Callisto is just behind Ganymede in size at around 3,000 miles in diameter. It was named for a nymph who was associated with the hunter goddess Artemis, daughter of Zeus. Callisto is tidally locked to Jupiter, rotating once every revolution and showing the planet only one face. As the farthest of the Galilean moons from Jupiter, it is not tidally heated and receives less radiation, so it may be the best choice for a future human base. It is not dense, but its surface is heavily cratered.
Moving farther out from the Sun, we arrive at Saturn with its sixty-two moons. Of these moons, twenty-four are regular and the rest are orbiting in all directions. But let's not forget Saturn's rings, which are composed of millions of tiny satellites, sometimes called moonlets, that range from the size of dust to the size of a football field.
Saturn's most famous moon is Titan, discovered by Christiaan Huygens in 1655. By 1684, Giovanni Domenico Cassini discovered four additional moons orbiting Saturn, and in 1789, William Herschel discovered two more. Herschel proposed naming the Saturn's regular moons after the Titans, the brothers and sisters of Cronus (the Roman equivalent of Saturn). Later, when NASA's Voyager 1 and Voyager 2 passed Saturn, even more moons were discovered, and the names of Inuit, Gallic, and Norse giants were used for these irregular moons. In 1997, NASA launched the Cassini Mission to explore Saturn and its moons. It reached Saturn in 2004 and began orbiting the ringed planet and sending back data. Its mission has recently been extended, and Cassini will continue to explore until at least 2017. In early 2005, Cassini deployed the Huygens probe, which parachuted to the surface of Titan and sent back data during its descent and pictures of its landing.
Titan is unique among our solar system's moons—its orange color is due to its thick atmosphere, which is mostly nitrogen and some methane. It's a big moon, larger than the planet Mercury and second only to Ganymede as a moon. Titan is tidally locked with Saturn and its orbit is highly inclined, which suggests that there were serious collisions early in its history. We have learned from Cassini and Huygens that Titan is made of ice and rock, and the moon even has seasons, though it's cold—about minus 180 degrees F. Titan has a methane cycle in which methane, a gas at Earth's temperatures, changes from liquid to gas and back to liquid. Liquid methane collects in lakes on Titan's mostly smooth surface; it flows like rivers, evaporates, and then rains back down. A large, dark area of Titan, called the Xanadu region, is mountainous and may be geologically active. The Huygens probe landed on a plain and took pictures of eroded ice rocks (ice that is as hard as rock). Some scientists see Titan as an analog for a very early Earth, though much colder.
While Titan is Saturn's only large moon, there are many other interesting bodies in the Saturn system. The moon Rhea is under one thousand miles in diameter and is mostly ice with some rock. Its surface, heavily cratered, is an ice crust that breaks at the edges and slides downhill into slumps. Iapetus, roughly the same size as Rhea, is also heavily cratered and has some huge impact basins. It has an odd equatorial ridge, like a wall, that is about twelve miles high and eight hundred miles long. Iapetus is tidally locked with Saturn and has two distinct hemispheres—one light and one dark. Tethys, a bit smaller, is mostly ice and highly reflective. It has a massive crater named Odysseus that is almost three hundred miles across. Tethys also has a massive valley, called the Ithaca Chasma, which is two miles deep and wraps around three-fourths of the moon. Mimas is a small moon, only about 250 miles in diameter, but it has a giant impact crater, named Herschel, making Mimas resemble the Death Star in Star Wars.
Enceladus is a small moon of Saturn, only about three hundred miles in diameter, but it has been getting a lot of attention. Its northern hemisphere is cratered, but its southern hemisphere has long cracks, called "tiger stripes," from which tidally-heated geysers and cryovolcanoes erupt and spew liquids and gases into space. Cryovolcanoes exist on very cold planets and release not hot magma, but water, methane, and ammonia. The Cassini probe has made several close passes to Enceladus and has recorded the contents of its eruptions, mostly water vapor with some organic compounds, leading to speculations about the possibility of life on this little moon. Two other moons of Saturn are Dione, which is locked in a resonance with Enceladus, and Phoebe, which is an irregular moon.
Uranus, the next planet out from the Sun, has twenty-seven moons that are either named for characters in Shakespeare's plays, or from the poetry of Alexander Pope. Herschel, who discovered Uranus, also discovered two moons, Titania and Oberon, in 1787. More moons were discovered in 1851 by William Lassell and are named Ariel and Umbriel. Miranda, discovered in 1948 by Gerard Kuiper, is named after Miranda from Shakespeare's play The Tempest. Miranda has a close orbit to Uranus and is tidally locked with it. This moon also has some extreme topography with canyons and escarpments.
Uranus has only been visited by Voyager 2, which added to the list of moons orbiting Uranus. But what's really different from other moon systems is that Uranus has an extreme tilt to its axis, causing the regular moons to orbit at right angles to the plane of the solar system, which then causes extreme seasonal cycles.
Titania, named after the queen of the fairies in Shakespeare's A Midsummer Night's Dream, is Uranus's largest moon at a diameter of about one thousand miles. It's about half rock and half ice, and it may have a subsurface ocean since it is tidally locked with Uranus. As a result, the surfaces change over relatively short periods of time and resurface ancient craters. Oberon, a bit smaller, is the outer-most regular moon of Uranus and is very heavily cratered. It was named after the king of the fairies in A Midsummer Night's Dream. Umbriel is named after a character from Pope's poem "The Rape of the Lock," as is Ariel after both Pope's poem and Shakespeare's The Tempest. Both are similar in size, about seven hundred miles in diameter. Ariel is cratered and has some rugged terrain.
Neptune has fourteen known moons that are, appropriately, named after water deities. As with the other planets, there are regular and irregular moons. Neptune's rotation is about thirteen hours and all but the outer two moons in this list orbit synchronously with its rotation. The irregular moons have both prograde and retrograde orbits and were probably captured by Neptune's gravity. Of the irregular moons, Psamathe and Neso have very large orbits that take them twenty-five years to orbit Neptune.
Triton is Neptune's largest moon, with Proteus being Neptune's second-largest moon. Triton is much bigger than the rest of Neptune's moons combined, having 99.5 percent of the combined mass of these moons. It was discovered in 1846 by William Lassell, but it wasn't seen up close until Voyager 2 passed through the Neptune system in 1989. Triton has a retrograde orbit, which means it orbits in the opposite direction to Neptune’s rotation. This strongly suggests that it is a captured body, probably coming from the Kuiper Belt. Triton is slightly larger than Pluto and from what Voyager 2 found, it is similar in composition and surface features to Pluto—flat plains of frozen nitrogen and ice with few impact craters.
With Pluto, we come to the end of the solar system as we have known it. While Pluto has been downgraded from official planet status into a new category of dwarf planets, it is still one of the most compelling bodies to orbit the Sun. Pluto is small, not even as big as Earth's Moon, and a bit smaller than Triton, yet it amazingly has its own collection of moons. These moons are, in order of distance from Pluto, Charon, Styx, Nix, Kerberos, and Hydra. Charon, the ferryman of Hades in Greek mythology, is the largest of these, about 750 miles in diameter and about half the size of Pluto itself. Charon's gravity is such that Charon and Pluto are more like a double planet, each orbiting a point in space that lies between them. They are tidally locked and together form the gravitational center that the other moons orbit. Four small and elongated, non-spherical moons orbit farther out in the same plane as Charon. The shifting gravitational fields from Pluto and Charon cause these moons to rotate erratically. Nix, named after the mother of Charon, and Hydra, the nine-headed serpent that fought Hercules, are small moons with diameters of about twenty-nine and thirty-seven miles, respectively. In between them, Kerberos—named after Cerberus, the dog that guards the underworld—is in orbit. Kerberos is roughly ten–fifteen miles in diameter. Styx, Nix, and Hyrdra are in a complex orbital resonance, which suggests the moons were formed as a result of a collision. Pluto's moon system is really quite weird.
Our tour of the moons of our solar system ends here, but what a strange trip it has been. There is much to explore and no doubt some extraordinary discoveries will be made as humans make their way into the outer solar system. Today, many scientists believe that if there is life in the solar system outside of Earth, it will not be on a planet—it will be on a moon.
Excerpted from Llewellyn's 2017 Moon Sign Book. Click here for current-year calendars, almanacs, and datebooks.