Clone moons of Saturn..

 

What you're seeing are three different moons of Saturn 🪐, named Tethys, Dione and Rhea.

It's very hard to distinguish between them. But that isn't the only reason we can consider them to be clones.

Tethys and Dione are very similar in size. A difference of just 30 km in their radii. Rhea is quite larger, though.

They're all mostly made of water ice 🧊, and heavily cratered, which means they’ve not changed since billions of years— old scars of impacts are still there.

Here are enhanced colour images of their surfaces:

It's obvious that all of them have one side much darker than the other.

There's an interesting reason behind it.

The three moons orbit in Saturn’s E ring, a faint ring of ice, created by another tiny moon Enceladus, whose geysers blast off water into space.

Being tidally locked to Saturn (keeping the same face towards their planet), their one side continuously faces the ring while other is shielded.

This creates a thin layer of ice on half of the moon, making it brighter than the other.

Here's a stunning image taken by Cassini spacecraft. Rhea passes behind Dione. They blend seamlessly into a snowman. ⛄

Are there any "gas giant" moons?

 

We have not yet discovered any exomoons. Many inevitably exist, as all planets in our system except Mercury and Venus have moons. Our instruments for detecting them elsewhere in the universe are too crude yet. Gas giant exomoons will likely be found in the future and will not be much smaller than the exoplanet they orbit.

As the above image shows, gas giant planets don't get much more significant in diameter with the increase in mass due to compression from high gravity. Brown dwarf stars start from 13 times the mass of Jupiter, while lowest-mass red dwarf stars are at least 80 times as massive as Jupiter, yet they aren't that much bigger in diameter. We found some outliers in this regard. Some gas giant exoplanets orbit their stars so close that they get puffed up from being fried by high temperatures.

A very massive gas-giant exoplanet that is nearly 13 times the mass of Jupiter can potentially have moons that are Neptune, Saturn, or even Jupiter-like, and they would be gas-giant moons.

In the past, we had doubts about whether such moons could form, but in October 2023, the James Webb Space Telescope discovered many Jupiter-like double rogue exoplanets floating in space without stars in the Orion Nebula. The current leading theory is that they formed in typical star systems and were ejected. Some planets can end up jettisoned from systems during planetary formation, collide, or fall into the star and be destroyed before the orbits settle on stable configurations.

If such double Jupiter-like planets indeed formed in star systems and not directly in space like suns form, then we will find double gas giant planets and massive gas giant planets with gas giant Jupiter- or Saturn-size exomoons in the future.

In our system, Titan, the moon of Saturn, is larger than the planet Mercury but less massive and has a thicker atmosphere than Earth. It is the closest we get to a gas giant moon in our backyard.