A moon is an object in space that orbits around a planet. The most famous example is the Earth’s moon, of course, but there are moons going around all the planets in the Solar System except for Mercury and Venus. Even Pluto and many asteroids have moons.
The largest moon in the Solar System is Jupiter’s Ganymede, measuring 5,268 km across. This is actually bigger than Mercury. The second largest moon is Saturn’s Titan at 5,152 km across. The largest moon orbiting a dwarf planet is Pluto’s Charon, measuring 1,186 km across.
Astronomers believe that moons were formed out of the same protoplanetary disk that formed the planet. Excess material didn’t join the planet, but spun out into a moon orbiting the planet.
Most moons orbit in the same direction as the rotation of their planet. This is called a regular or prograde orbit. Most moons have a regular orbit, but there are some exceptions, like Neptune’s Triton, which orbits in a retrograde direction.
Some moons are tidally locked to their planet. This is a situation where the moon’s rotation time matches its orbital period. Our Moon is a perfect example of this. It takes the same amount of time to complete one orbit around the Earth as it takes to complete one rotation on its axis. Because of this, the Moon always shows the same face to the Earth.
If you’d like more information on moons, here’s some of them around the solar system:
Jupiter‘s four largest moons – aka. the Galilean Moons, consisting of Io, Europa, Ganymede and Callisto – are nothing if not fascinating. Ever since their discovery over four centuries ago, these moons have been a source of many great discoveries. These include the possibility of internal oceans, the presence of atmospheres, volcanic activity, one has a magnetosphere (Ganymede), and possibly having more water than even Earth.
But arguably, the most fascinating of the Galilean Moons is Europa: the sixth closest moon to Jupiter, the smallest of the four, and the sixth largest moon in the Solar System. In addition to having an icy surface and a possible warm-water interior, this moon is considered to be one of the most-likely candidates for possessing life outside of Earth.
In ancient Greek lore, the Titans were giant deities of incredible strength who ruled during the legendary Golden Age and gave birth to the Olympian gods we all know and love. Saturn‘s largest moon, known as Titan, is therefore appropriately named. In addition to being Saturn’s largest moon – and the second-largest moon in the Solar System (after Jupiter’s moon Ganymede) – it is larger by volume than even the smallest planet, Mercury.
Beyond its size, Titan is also fascinating because it is the only natural satellite known to have a dense atmosphere, a fact which has made it very difficult to study until recently. On top of all that, it is the only object other than Earth where clear evidence of stable bodies of surface liquid has been found. All of this makes Titan the focal point of a great deal of curiosity, and a prime location for future scientific missions.
With 67 confirmed satellites, Jupiter has the largest system of moons in the Solar System. The greatest of these are the four major moons of Io, Europa, Ganymede and Callisto – otherwise known as the Galilean Moons. Named in honor of their founder, these moons are not only comparable in size to some planets (such as Mercury), they are also some of the few places outside of Earth where liquid water exists, and perhaps even life.
But it is Callisto, the fourth and farthest moon of Jupiter, that may be the most rewarding when it comes to scientific research. In addition to the possibility of a subsurface ocean, this moon is the only Galilean far enough outside of Jupiter’s powerful magnetosphere that it does not experience harmful levels of radiation. This, and the prospect of finding life, make Callisto a prime candidate for future exploration.
Over the course of the past decade, many amazing discoveries have been made at the edge of the Solar System. Thanks to the work of astronomers working out of Earth-based observatories, with the Hubble Space Telescope, and those behind the recent New Horizons mission, not only have new objects been discovered, but additional discoveries have been made about the ones we already knew about.
For example, in 2005, two additional satellites were discovered in orbit of Pluto – Hydra and Nix. The discovery of these moons (which has since been followed by the discovery of two more) has taught astronomers much about the far-flung system of Pluto, and helped to advance our understanding of the Kuiper Belt.
In the ongoing drive to unlock the secrets of Saturn and its system of moons, some truly fascinating and awe-inspiring things have been discovered. In addition to things like methane lakes and propane-rich atmospheres (Titan) to moon’s that resemble the Death Star (Mimas), it is also becoming abundantly clear that planet’s beyond Earth may harbor interior oceans and even the extra-terrestrial organisms.
Nowhere is this more apparent than on Enceladus, Saturn’s sixth largest moon, which also possesses some of the most interesting characteristics in the outer Solar System. These include long veins of blue ice that resemble stripes, not to mention amazing plumes of water ice that have been spotted periodically blasting out of the moon’s southern pole. These, in turn, raise the possibility of liquid water beneath the surface, and possibly even life!
The planets of the outer Solar System are known for being strange, as are their many moons. This is especially true of Triton, Neptune’s largest moon. In addition to being the seventh-largest moon in the Solar System, it is also the only major moon that has a retrograde orbit – i.e. it revolves in the direction opposite to the planet’s rotation. This suggests that Triton did not form in orbit around Neptune, but is a cosmic visitor that passed by one day and decided to stay.
And like most moons in the outer Solar System, Triton is believed to be composed of an icy surface and a rocky core. But unlike most Solar moons, Triton is one of the few that is known to be geologically active. This results in cryovolcanism, where geysers periodically break through the crust and turn the surface Triton into what is sure to be a psychedelic experience!
Exploring the Solar System is like peeling an onion. With every layer removed, one finds fresh mysteries to ponder over, each one more confounding than the last. And this is certainly the case when it comes to Jupiter’s system of moons, particularly its four largest – Io, Europa, Ganymede and Callisto. Known as the Galilean Moons, in honor of their founder, these moons possess enough natural wonders to keep scientists busy for centuries.
Consider Io. As Jupiter’s innermost moon, it is also the fourth-largest moon in the Solar System, has the highest density of any known moon, and is the driest known object in the Solar System. It is also one of only four known bodies that experiences active volcanism and – with over 400 active volcanoes – it is the most geologically active body in the Solar System.
Like all of the Solar Systems’ gas giants, Uranus has an extensive system of moons. In fact, astronomers can now account for 27 moons in orbit around Uranus. Of these, none are greater in size, mass, or surface area than Titania. One of the first moon’s to be discovered around Uranus, this heavily cratered and scarred moon was appropriately named after the fictional Queen of the Fairies.
Beginning in 1978, astronomers began to discover that Pluto – the most distant known object from the Sun (at the time) – had its own moons. What had once been thought to be a solitary body occupying the outer edge of our Solar System suddenly appeared to have a system. And as time went on, a total of four moons would be discovered.
Of these, Charon is the largest and most easily observed, hence why it was discovered first. In addition to being the biggest of its peers, its also quite large in comparison to Pluto. As such, Charon has always had something of a unique relationship with its parent body, and stands apart as far as objects in the outer Solar System are concerned.
In 1930, Pluto was observed for the first time. For many decades, astronomers thought that the “ninth planet of the Solar System” was a solitary object. But by 1978, astronomers discovered that it also had a moon roughly half its size. This moon would come to be known as Charon, and it would be the first of many discoveries made within the Pluto’s system.
In fact, within the last decade, four additional satellites have been discovered in orbit of Pluto. Of these, the outermost to be observed is the moon now known as Hydra.
Fabio Evagelista is a Brazilian writer.
Crossed Paths is the first book of the Myra-Hati trilogy, an epic adventure in a post-apocalyptic world, for the lovers of sci-fi / fantasy genre. This is the author’s first work published in America.