There are currently 174 named moons in our Solar System, orbiting the six outermost planets, with more being discovered and debated every day. Some are more fascinating than others, whether it’s their landscapes, features, orbits or environments – so we ranked our top 13 most interesting moons, because, well, why not
Dactyl – The Alternative Moon
Orbits: Asteroid 243 Ida
Discovered in 1995 by the Galileo probe, this moon is less than a mile in diameter and is the natural satellite to the asteroid Ida, a Koronis asteroid located in the belt between Mars and Jupiter. Before this, scientists had no evidence that asteroids could have moons, but since its discovery a further 24 have been found to orbit them.
The origin of the unorthodox moon is debated, and could either be from the asteroid itself, or a captured object.
Enceladus – The Exciting Possibility
Enceladus is one of the brightest objects in our Solar System, as its water ice surface reflects almost 100 per cent of the Sun’s light – but this physical characteristic isn’t what makes it one of the most thrilling places in our Solar System.
The icy moon is often described as the most scientifically compelling place in our Solar System, and rightly so, as it has the most promising conditions for harbouring life outside of our planet.
In 2005, the Cassini probe discovered an incredibly exciting feature of the moon – ice geysers. The plumes from the geysers include ice particles, as well as gaseous water vapour, carbon dioxide, methane, ammonia and nitrogen. These eruptions replenish the surface with new ice, as well as provide the icy material to Saturn’s E-ring. Water vapour is heavily located above a geothermal feature on its surface known as the ‘Tiger Stripes’, deep crevasses in the otherwise smooth moon. The source of heat is likely provided through tidal forces, warming the core of the planet and causing the geological activity.
The moon is also hiding an important secret – a global-sized ocean of water, as evidenced by the wobble in its orbit, which can only be caused by a liquid interior. The factors of internal heat, chemistry and oceanic presence lead Enceladus to its great potential of the existence of life.
Charon – Pluto’s Other Half
A perhaps controversial decision to select a moon of an ex-planet, but an interesting moon all the same!
At half the size of Pluto, the pair are often referred to as a double dwarf planet system, particularly as they orbit around a central point in space as opposed to Pluto being the centre of orbit.
The moon was discovered when the Hubble Space Telescope captured images of Pluto looking more elongated than its usual spherical shape.
Its name comes from the mythological ferryman who used to carry souls across to the realm of the dead, requiring a coin to allow the dead to proceed to the underworld. The New Horizons spacecraft symbolically carried a Florida state quarter to pay the ferryman, as it passed by Charon and Pluto in 2015.
Atlas – The UFO
Similar to Pan, Saturn’s other innermost moon, Atlas has an equatorial ridge giving the moon its distinctive flying saucer shape. With an average radius of 15 km, the small moon Atlas was discovered in 1980 using images from the Voyager 1 probe during its flyby of Saturn.
Its close proximity to Saturn means it completes one orbit of its mother planet in just 14.4 hours.
Hyperion – The Space Sponge
Hyperion is an irregular shaped moon, in that it is not spherical, and is likely a fragment of a much larger, ancient moon destroyed from an impact in the early Solar System.
The moon has a very low density, nearly half that of water, and alongside its deeply cratered surface, gives Hyperion its porous, spongy appearance. The craters remain as it is one of Saturn’s outermost moons, experiencing almost no tidal forces, which would slowly fill these deeply excavated impact craters.
Mimas – That’s No Moon…
The resemblance of Saturn’s moon Mimas to an iconic moon-shaped fictional space station is largely the fault of a giant impact crater that covers a third of its diameter.
The crater measures 130 km across with a 5 km surrounding walls is known as the Herschel Crater, after William Herschel who spotted the moon in 1789. The body that struck the moon almost ripped it apart, as evidenced by the fractures on the opposite face of Mimas. The moon is littered with craters, showing a lack of surface replenishment despite its close proximity to Saturn and elliptical orbit, which should provide enough heat through gravitational tidal activity.
Mimas is also tidally locked, with the same face towards Saturn throughout its 22.5 hour orbit of the planet. Mimas also disrupts the orbits of much smaller moons, and speeds up when passing the large moons Enceladus and Dione.
Oh, and if you haven’t worked it out by now, the moon looks like the Death Star from Star Wars.
Phobos – On Course for Trouble
Phobos is the larger of Mars’ two moons, and is irregular, non-spherical in shape measuring 27x22x18 km. The moon orbits so close to its parent planet that it completes three orbits in one day!
Its most noticeable feature is the Stickney crater, 9.7 km across, left behind from an impact that nearly shattered the moon. Constant bombardment of meteors has left the surface covered in a fine dust.
At first glance, Phobos seems like an uninteresting object in the sky over Mars, but has a catastrophic future ahead, and it’s this eventual demise that gets Phobos onto our top 20 list of interesting moons.
The moon is gradually getting closer to the Red Planet at a rate of 1.8 m every hundred years, and the two will ultimately collide in 50 million years. Alternatively, the moon will be broken down by impacts, forming a fine dusty ring around its parent planet.
Ganymede – The King of the Moons
The Jovian moon is the largest in the Solar System, and even larger than the first planet in our system, Mercury – it would definitely be given planetary status if it orbited the Sun directly. The icy-rock surface has both craters and grooves, and a molten core, which allows it to possess its own magnetosphere within that of Jupiter.
In 1996, the Hubble Telescope detected a thin atmosphere surrounding the moon, containing oxygen. However, its far too thin to support life – at least that we know of.
Pan – The Ring Sculptor
This small moon was first discovered by the Voyager 2 probe in 1990 after capturing an image of the innermost rings, featuring the tiny speck of Pan (14 km in diameter) in the 325 km wide Encke Gap.
The saucer shape moon influences the particles in the ring system, by producing kinks known as wakes. When the fast moving particles pass Pan, the moon gives them a gravitational ‘kick’, and they bunch up to produce the waves that can extend many hundreds of miles into the rings.
Miranda – Frankenstein’s Moon
At 500 km in diameter, the small moon was unlikely to exhibit any tectonic activity, yet Miranda’s surface is littered with canyons, 12 times as deep as Earth’s Grand Canyon. Miranda’s surface can be divided into separate regions with differing levels of features, from smooth to cratered, and was first imaged during the Voyager 2 flyby of Uranus in 1986 – the closest object in the probe’s trajectory.
The possibility for the moon’s mismatched surface is that the moon had been previously destroyed in an impact event in the early Uranian system, and then reaggregated due to the gravitational attraction of the large fragments.
Epimetheus and Janus – The Twins
This may appear as cheating to choose two moons, but these irregular rocky bodies were originally combined as one moon in an earlier Saturn system, and when discovered, were thought to be the same object.
The interesting feature of this pair is their co-orbital condition, as they follow the same path around Saturn, but one is 50 km further from the planet. This means the inner moon travels slightly faster around the planet, and catches up to the outer moon every four years. At this point, the gravitational influence on one another causes the moons to essentially swap places, so the outer becomes the inner and vice versa.
They are located in a faint dusty ring of Saturn, which was likely produced by the material ejected after meteor impacts on both Janus and Epimetheus.
Triton – A Volcanic Space Melon
Triton is likely a captured Kuiper Belt object, held in orbit by the strong gravitational pull of Neptune. The moon possesses a thin atmosphere, with abundances of nitrogen and methane, produced through volcanic activity at its surface.
When Voyager 2 passed by Triton in 1989, erupting geysers were an unexpected feature on the cold, scaly ‘cantaloupe’ moon. The surprising trait was revealed through images taken of the southern pink cap, showing dark streaks left behind on the surface from the carbonaceous plumes. It is one of the few volcanically active bodies in our Solar System, despite its 4.5 billion km distance from the Sun.
It still remains one of the coldest places, with most of the nitrogen existing as a frost, which gives the moon its highly reflective surface.
Titan – A Terrain fit for Tolkien
At only two per cent smaller than Ganymede, Titan is the second largest moon in our Solar System. It is the only moon known to exhibit a thick atmosphere, dense enough that the solid core of the planet is completely obscured by the nitrogen and methane clouds. UV radiation from the Sun also initiates reactions to occur between these gases, producing many different organic molecules, existing as trace abundances in Titan’s atmosphere.
The Cassini probe revealed lakes and rivers of ethane and methane, which are filled by the rains from the orange clouds, as well as dark sand-like dunes comprised of hydrocarbon grains. Any volcanic activity is likely to produce water instead of the molten rock as we know here on Earth. Mountains on the surface of Titan are named after those found in JRR Tolkien’s Middle Earth.