Inner satellite facts for kids

An inner moon is a small natural satellite that orbits very close to its planet. It moves in the same direction as the planet spins (this is called a prograde orbit). These moons also orbit almost flat with the planet's middle.

Scientists think most inner moons formed right where they are, at the same time as their planets. Neptune's inner moons are different. They probably formed from pieces of older moons that broke apart. This happened after Neptune captured its giant moon, Triton.

What makes inner moons special? They are very close to their planet. They orbit super fast, often in less than a day. They are also small, light, and often have unusual shapes.

Discovering Inner Moons

Thirty inner moons are known so far. They orbit all four of the giant planets: Jupiter, Saturn, Uranus, and Neptune. Because they are so small and close to their bright planets, they are hard to see from Earth. Some, like Pan and Daphnis at Saturn, have only been seen by spacecraft.

The first inner moon found was Amalthea in 1892. It was discovered by E. E. Barnard. Next were Saturn's moons Epimetheus and Janus in 1966. These two moons share the same orbit. It was confusing until the Voyager 1 spacecraft flew by in 1980.

Most other inner moons were found by the Voyager 1 and Voyager 2 spacecraft. They flew past Jupiter (1979), Saturn (1980), Uranus (1986), and Neptune (1989). More recently, the Cassini spacecraft found more inner moons at Saturn (2005-2009). The Hubble Space Telescope found new ones at Uranus (2003) and Neptune (2013).

How Inner Moons Orbit

All inner moons travel in nearly perfect circles. They also orbit in the same direction as their planet spins. Their orbits are very flat, almost perfectly lined up with their planet's equator. Only one, Neptune's closest moon Naiad, is tilted more than one degree.

The moons closest to their planets orbit inside the planet's rings. They are well within the Roche limit. This is a distance where a planet's gravity could tear a moon apart. But these moons stay together because their material is strong and has friction. This means if a tiny pebble were on the side of the moon furthest from the planet, the planet's pull might be stronger than the moon's own gravity. This is why these moons look clean of loose dust and rocks.

Saturn's moon Pan is an extreme example. It orbits inside the rings at only 70% of its fluid Roche limit. Neptune's moon Naiad is also very close to its limit.

Moons that orbit faster than their planet spins slowly spiral inward. This is called tidal deceleration. In the far future, these moons will either crash into their planet or break apart. This affects moons like Metis and Adrastea at Jupiter. It also affects most inner moons of Uranus and Neptune. Saturn's moons are safe from this because Saturn spins very fast.

What Inner Moons Look Like

Size and Shape

Inner moons are small compared to their planet's main moons. They are too small to be perfectly round like Earth's Moon. Many are stretched out, like Amalthea, which is twice as long as it is wide.

The largest inner moon is Proteus. It is about 440 km across at its longest part. It is almost round, but not quite. Most known inner moons are 50 to 200 km across. The smallest confirmed one is Daphnis, which is only 6 to 8 km in size.

How They Spin

All inner moons are tidally locked. This means they spin at the same rate they orbit. So, they always show the same side to their planet, just like Earth's Moon always shows us the same face. Their longest side usually points towards their planet.

Their Surfaces

The inner moons of Jupiter, Uranus, and Neptune have very dark surfaces. They reflect only a small amount of light. Saturn's inner moons, however, are very bright. This is likely because fresh ice particles from Saturn's rings cover their surfaces. The other planets' inner moons may have been darkened by space radiation. None of the known inner moons have an atmosphere.

Craters on Moons

The inner moons that we have seen up close have many craters. Craters are made when objects crash into the moon's surface. Moons orbiting close to giant planets get hit more often. This is because the planet's strong gravity pulls in space rocks. Scientists think that very small inner moons would be destroyed by impacts over time. This means there's a minimum size for inner moons that can last a long time.

Gathering Ring Material

At least two of Saturn's inner moons, Atlas and Prometheus, have ridges around their middle. The ridge on Atlas is very clear. Also, Pandora is covered in some fine dust. Scientists think these features are from ring material sticking to the moons. This also explains why these moons are not very dense and are very bright. Prometheus has even been seen pulling in dusty material from Saturn's F ring.

Inner Moons of Our Solar System

Jupiter's inner moons and rings

Jupiter's Inner Moons

Jupiter has the smallest group of inner moons, with only four known ones:

• Metis
• Adrastea
• Amalthea
• Thebe

The Galileo spacecraft might have seen even smaller moonlets near Amalthea's orbit.

Saturn's Inner Moons

Saturn has nine known inner moons. They are closely linked to its ring system. Many of them orbit inside the rings, creating gaps or "shepherding" ringlets.

• S/2009 S 1 – a tiny moonlet in the B-ring
• Pan – a shepherd satellite in the Encke division of Saturn's A Ring
• Daphnis – a shepherd satellite in the Keeler Gap of Saturn's A Ring
• Atlas – a shepherd satellite for the outer edge of Saturn's A Ring
• Prometheus – an inner shepherd satellite for Saturn's narrow F Ring
• Pandora
• Epimetheus and Janus – these two moons share an orbit and swap places over time.
• Aegaeon – a moonlet in the G-ring

S/2009 S 1 and Aegaeon are not big enough to clear their own paths in the rings. The same is true for the small moons Methone, Anthe, and Pallene. These orbit between the larger Saturnian moons Mimas and Enceladus.

Scientists have also seen other objects near the F ring. These include S/2004 S 3, S/2004 S 4, and S/2004 S 6. They sometimes have a dusty cloud around them. It's not clear if they are temporary clumps of dust or small solid moons.

Schematic of the Uranian satellite and ring system

Uranus's Inner Moons

Uranus has the most inner moons, with thirteen known ones:

• Cordelia and Ophelia – these are inner and outer shepherd satellites for Uranus' brightest ring, the narrow ε ring.
• Bianca
• Cressida
• Desdemona
• Juliet
• Portia
• Rosalind
• Cupid
• Belinda
• Perdita
• Puck
• Mab – connected to the μ ring.

Neptune's Inner Moons

Diagram of Neptune's inner satellites.

Neptune has seven known inner moons:

• Naiad
• Thalassa
• Despina
• Galatea – an inner shepherd satellite for the very narrow Adams ring. This ring is broken into arcs.
• Larissa
• Hippocamp
• Proteus

Scientists believe these moons are "rubble piles." This means they formed again from pieces of Neptune's original moons. These original moons were disturbed when Neptune captured its giant moon Triton. The disturbance caused collisions, and the pieces that didn't get lost came back together to form the inner moons we see today.

Exploring Inner Moons

Most inner moons have been photographed by the spacecraft Voyager 1 and Voyager 2. Many of them only appear as single dots or a few pixels in these images. However, some moons show more detailed surface features.