You Gotta Know: Planetary Moons By: Grace Meng

How many moons does each planet have? Mercury – 0 Venus – 0 Earth – 1 Mars – 0 Jupiter – 63 (the most) Saturn – 62 Uranus – 27 Neptune – 13

Earth’s moon (AKA Luna) Orbital period: 27.33 days The moon is about 27% of the size of the Earth The moon’s gravity causes the Earth’s tides The leading explanation for the origin of the moon is that there was a large impact from an object about the size of Mars, to the early molten Earth, which knocked away the pieces that would later coalesce into the moon. The ancient Greeks believed the dark spots on the moon were seas and the lighter regions were land. This influenced the modern names for these places, “maria” and “terrae”, Latin for sea and land respectively. For example, the Mare Tranquillitatis or Sea of Tranquility, where the first manned landing on the moon in 1969 occurred.

Phobos and Deimos Their names respectively mean Fear and Panic, named after the horses who pulled the chariot of the Greek god Ares (Roman name Mars, which explains a lot). The moons have surface materials similar to asteroids in the outer asteroid belt, which leads most scientists to believe that they are captured asteroids.

Jupiter There are 63 moons of Jupiter, the most of any solar system planet, and it would be highly illogical to list them all here. The four best known moons of Jupiter were discovered by Galileo (and are sensibly called the Galilean Moons). They were discovered when Galileo used a telescope to examine the sky. They are Callisto, Europa, Ganymede, and Io. These are all names of figures in Greek mythology. Although as to why you would name a moon after someone who got turned into a cow, I’m not sure. Ganymede is the largest moon in the Solar System, and is even bigger than Mercury. Not bad for a cupbearer. Io is the innermost moon of Jupiter, and is the most geologically active object in the solar system.

An illustration of Jupiter and the Galilean moons from NASA’s website

The inner three Galilean moons revolve in a 1:2:4 resonance

Saturn Saturn has 62 moons, almost as many as Jupiter. The most noteworthy is Titan, which has a nitrogen-rich Earth like atmosphere and a landscape that includes hydrocarbon lakes and dry river networks as well as large ice caps. It comprises more than 96% of the mass in orbit around the planet. Enceladus, another moon of Saturn, emits jets of gas and dust and may harbor liquid water under its south pole region. The largest of Saturn’s irregular moons is Phoebe.

Artist’s conception of Saturn and its major icy moons

Uranus Uranus has 27 known moons, all of which are named after characters from William Shakespeare and Alexander Pope. The largest are Puck, Miranda, Ariel, Umbriel, Titania, and Oberon, all of which were discovered by 1948. The remaining moons were discovered after 1985, during the Voyager 2 flyby or with the aid of Earth-based telescopes.

Uranus and its six largest moons compared at their proper relative sizes and relative positions. From left to right: Puck, Miranda, Ariel, Umbriel, Titania, and Oberon

Neptune Neptune has 14 moons and they are all named after minor water deities in Greek mythology. The largest is Triton, which was discovered just 17 days after the discovery of Neptune itself, in 1846. The second natural satellite of Neptune wasn’t discovered for over a century after that, in 1949. The moon orbiting farthest from its planet is Neso and has an orbital period of about 26 years.

Triton and Neptune, three days after the Voyager 2 flyby in 1989