In our solar system, every planet except Mercury and Venus has one or more natural moons. Earth's moon, of course, as we know it, is a beautiful, bleak, lifeless world shaped by ancient volcanoes and countless impact craters. Although the moon is our most familiar natural satellite, it is not the most interesting. Each giant planet in the outer solar system has a host of moons, many of which formed at the same time as the main planet and, like the main planet, are made of ice-rich material. Despite their distance from the sun and lack of sunlight and heat, these moons are still as colorful as their host planets.
Here, we set out to discover some of the strangest and most exciting magical worlds. Some moons, like Callisto (also known as Callisto) and Mimas (mimas), have been frozen solid for billions of years, but impacts from space have left extraordinary scars on their surfaces. Other moons, such as Saturn's shepherd moons Epimetheus (" Pan ") and Enceladus (" Atlantis ") and Neptune's solitary moon Tradus (" Nellard "), are affected by their mutual attraction to the surrounding bodies during their lifetimes. Most exciting of all, some of these exotic worlds are heated by the host planet's powerful tidal forces, triggering periods of intense activity like the one that shaped Uranus's Frankenstein moon Carnesto (" Miranda "). In some cases, these powerful tidal forces are still at work today, creating fascinating bodies like the tortured moon IO (" IO ") and the icy moon Enceladus (" Enceladus "). Enceladus's placid exterior may even be hiding the solar system's biggest secret: alien life.
enceladus
Since NASA's Cassini probe arrived at Saturn in 2004, the tiny inner moon of the beautiful ringed planet, Enceladus, has become one of the most studied and discussed planets in the entire solar system. Enceladus was most recently known for its discovery of huge plumes of water ice spewing into space from cracks in the moon's southern hemisphere, suggesting liquid water lurks beneath its thin icy crust.
Prior to Cassini's arrival, early images showed the moon's surface to be unusually bright, with craters appearing to be covered in ice. So it's hard not to wonder if there's some strange activity going on on Enceladus. Still, cassini's image of the plumes as it flew past water ice plumes is certainly spectacular evidence that Enceladus is indeed an active world.
Enceladus has a diameter of 504 kilometers and is made of rock/ice. Like many of Saturn's moons, Enceladus should have been a frozen planet billions of years ago. But tidal forces from the gravitational tug of war between Saturn and its larger moon Dione keep its interior warm and active, making enceladus a prime target in the search for alien life within the solar system.
As most of the water ice falls back and covers the surface of Enceladus, some of it escapes due to weak gravity and enters Saturn's orbit. Here, the water ice unwinds, forming a doughnut-shaped E ring, the outermost and thinnest of Saturn's rings.
callisto
Callisto is the most distant of Jupiter's four Galilean moons and the third largest in the solar system, just a little smaller than Mercury. Callisto is famous for being the hardest hit object in the solar system; Its dark surface is studded with indistinguishable impact craters, the deepest of which are mostly bare of subsurface ice and strewn with bright "jet" debris.
Callisto's position in the Jovian system results in craters all over its surface. The giant planet's gravity exerts a powerful influence, disrupting the orbits of comets passing Jupiter and often pulling comets toward their doom. In 1994, comet Shoemaker-Levy 9 collided with Jupiter in one of the most famous celestial impacts.
While some of Jupiter's larger moons are the most vulnerable to impacts, Callisto's other inner neighbors (which are more affected by tidal forces) have all gone through geological processes that obliterated ancient craters. But Callisto's surface has remained largely unchanged for more than 4.5 billion years, creating a spectacular array of craters that span hundreds of millions of years.
dactyl
Ai Star (Asteroid 243) is an asteroid. It also has a satellite called Ivey (" Dactyl "). Along its longest axis, Aiwei is only 1.6 kilometers in diameter. Alena's gravity is weak, making it unlikely that she could be captured in her orbit. It is more likely that Ai and Ai Wei formed at the same time, but there are many questions about this possibility.
Sagna is a leading member of the Corvida group, a family of more than 300 asteroids. The Ravenstar asteroids all have similar orbits and are thought to have formed from an asteroid impact about 1 billion to 2 billion years ago. It is possible that Aiwei was a piece of smaller debris from the collision that ended up in Alveira's orbit. But there's a problem -- computer models suggest another asteroid impact would be enough to destroy Aiwei.
So how did Ivey survive for over a billion years?
One theory is that the Ravenadas may be younger than they look, and that the severe impact craters on Alva also came from the impact storms triggered by the initial decomposition. Another theory is that Ivey suffered a devastating impact but, as NASA discovered, pulled itself back into its original orbit, which may also explain its unusually spherical shape.
iapetus
Iapetus (" Iapetus ") ranks among the strangest moons for two reasons. The first reason is obvious. When iapetus was discovered in 1671, it was noted that one side of the moon was much dimmer than the other. Iapetus's front hemisphere, the side that always faces Saturn as it orbits it, is dark brown, while the back hemisphere is light gray. One early theory to explain the color difference between the front and back hemispheres suggested that iapetus's front hemisphere was covered in dust from tiny meteorites hitting the outer moon, making it appear dimmer.
But cassini's images tell a more complicated story. Most of the dark material appears to come from iapetus's interior, the coarse debris left over from sublimation of dusty ice on the moon's surface. At first, dust from the outer moon slowly accumulated in iapetus's front hemisphere, causing it to deepen in color and begin to absorb heat, creating a sublimation effect.
Another mystery of Iapetus is the equatorial ridge that surrounds the moon. Iapetus's equatorial ridge is about 13 kilometers high and 20 kilometers wide. The equatorial ridge gives Iapetus a unique look, a bit like a walnut. The formation of the equatorial ridge remains a mystery. Some theories suggest that iapetus long ago was spinning much faster than it is today, creating a bulge near the equator that left the equatorial ridge we see today, while others believe that iapetus was once surrounded by a ring system that collapsed to form the equatorial ridge.
nereid
The second moon found orbiting Neptune, Tradus is on the list because of its extreme orbit. Tradus is between 1.4 million and 9.7 million kilometers away from Neptune. Such orbits tend to be typical of captured satellites. The intense gravity of the giant exoplanet would pull passing asteroids and comets into extremely eccentric orbits. But Tradus's unusual trajectory tells a more interesting story.
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