Moons — a variety of objects

Moons are a variety of objects in the Solar System. So, what are their mysteries, sizes and other properties?

Why Mercury and Venus don’t have any moons

When you look at the list of moons, you’d see Mercury and Venus have no moons. Then why? For Mercury, things are simple. Being the smallest and the closest planet from Sun, it is also the hardest to keep a satellite. If it manages to catch a moon, it would quickly be in an unstable orbit and fly away. For Venus, its slow retrograde rotation makes any moon in orbit unstable, making it also moonless.

Earth’s Moon

Earth has a large moon. It has about 1.234% the mass of Earth and 27% the diameter. It also has a sixth of Earth’s gravitational acceleration. Moreover, it is thought to be originated from a large impact, due to some similarities of Moon rocks and Earth rocks. Also, the far side and the near side are very different and is explained in Moon — Our Only Moon.

Moon is the only place other than Earth humans have set foot on because of its proximity. In fact, Moon is the closest natural celestial object to Earth, unless some asteroids invade the space between them. Also, it has no atmosphere but an exosphere, which is a nebulous molecular cloud. So, temperatures can get as hot as 100 Celsius in the daytime and -170 Celsius at night. Temperatures can drop to -230 Celsius in shaded areas!

It is also a factor for life on Earth. It provides ocean tides, which may trigger life to migrate from ocean to land. Also, it can help marine inhabitants navigate through the sea. Moreover, this force stabilizes the axial tilt of Earth, making the climate relatively stable.

Mars’s Moons

Mars has 2 small moons, named Phobos and Deimos. They are all thought to be asteroids in the Asteroid Belt in the first place because they are all C-type asteroids. Then, they got close to Jupiter and is being transferred to Mars by Jupiter’s massive gravity.

Phobos and Deimos are both quite tiny because they are 22 and 12 kilometers across respectively. They are also very close to Mars. Deimos orbits once every 30 hours and three times a day for Phobos. Phobos’s orbit is spiraling towards Mars and will one day either crash into it or form a beautiful ring around the red planet.

Asteroids

Although asteroids’ gravitational pulls seem not to be powerful, they can capture moons too. The first moon found belonging to an asteroid is Dactyl, which orbits 243 Ida. Soon after, more and more moons of asteroids are found and now we get more than 150 asteroids with a moon, and some have two. Also, there are binary asteroids, with 2 asteroids about the same mass orbit each other.

In the 5 dwarf planets, 4 have moons. Haumea has 2, Makemake and Eris have 1 respectively, and Pluto has 5 moons! Its ability to catch moons is even better than Earth’s because it is very far from Sun.

Jupiter’s Moons

Jupiter has 79 moons, the second-largest number of moons in the Solar System! Their sizes also vary from 1 kilometer across to a giant larger than Mercury. In fact, 4 moons are round due to self-gravitation, while others are asteroids in the system. The 4 large ones are Io, Europa, Ganymede and Callisto.

Io is the most volcanically active object in the Solar System. It has hundreds of volcanoes despite having the same size as Moon. Its volcanism is because of tidal heating, making large amounts of lava reshape its surface constantly. Also, it has its own magnetic field and it forms a radiation belt that made Jupiter’s magnetic field 2 times larger than expected. It is also the densest moon in the Solar System, being 3.5 times as dense as water.

Europa is the smallest of the 4 large moons, but yet it has the most to explore. Its surface is covered with water ice ,and deep beneath the surface, there is an ocean having more water than all Earth’s oceans. Like Io, it is tidally locked to Jupiter and it orbits once every 3.5 days.

Ganymede is a moon larger than Mercury. It is 5,268 kilometres across but it only has half the mass. It is also in an orbital resonance with Io and Europa. When it circles the planet once every 7 days, Europa circles twice and Io orbits 4 times. Callisto is about as large as Mercury, and is one of the most heavily cratered objects in the Solar System.

Saturn’s Moons

Saturn has 82 moons, the most in the Solar System. It has 7 major moons which are round and 75 minor ones. Many of them are in the rings as Shepherd Moons, which controls the flow of the ring. These inner moons include Pallene, Pan, Aegaeon, Daphnis, Atlas, Pandora, etc. But there are 4 exceptional moons of Saturn that we’ll mention below.

Enceladus has plumes spewing out organic molecules, bringing the evidence for life. It is relatively small for a major moon, only 500 kilometres wide. Furthermore, the temperature there is similar to the coldest corners on Moon, reaching about -200 Celsius. That’s because it is 9.5 times farther than Earth to Sun, reflecting a lot of light. When it orbits Saturn twice, Dione, a larger moon, orbits Saturn once. Also, it is the primary origin of Saturn’s E ring.

Titan is the only moon in the Solar System with an atmosphere. In reality, it is even thicker than Earth’s despite its weak gravity! It is mostly nitrogen, a bit of methane and other trace gases. There are also lakes on Titan’s surface. But unfortunately, it is not water. It is made of liquid methane. By the way, the temperatures there is -179 Celsius, cold enough for many gases to liquefy. It is also larger than the planet Mercury.

Rhea is a 1,600-kilometre-wide moon with a possible ring system on its own, which has never seen on another moon in the Solar System. Meanwhile, Iapetus is a strange moon with an extremely dark side and a very bright side. This is because of its neighbor, Phoebe, who brought the dust there by micrometoroids.

Moons of Uranus and Neptune

Uranus has 27 moons. 5 of them are major with 4 large and 1 small. Ariel, Umbriel, Titania and Oberon are all more than 1,000 kilometres across. Miranda is only half the size, about 470 kilometres in diameter. There are also 22 irregular moons. 2 of them, Ophelia and Cordelia, keep one of Uranus’s 13 rings in place.

Neptune has 14 moons but only 1 is spherical. It is the former dwarf planet Triton. It is the 7th largest moon, a bit larger than Pluto. Still, it orbits the planet backwards, also known as a retrograde orbit. It then disturbed other captured objects once it is in Neptune’s orbit, leaving so little moons existing there. However, maybe the additional ones aren’t found yet as Voyager 2 is the only spacecraft that flew by Uranus or Neptune.