October 21st, 2018
Saturn's moon Iapetus has a very dark side.
There are several theories as to why but one outweighs the rest with computer modeling that aligns with observational evidence.
On Twitter I posted the below comment about Saturn's moon Iapetus. 

Perspective
Saturn's moon Iapetus' equatorial ridge is 13 km high.
Commercial airlines fly 9-12 km 30-40 k feet
A flight from Iapetus' north to south pole would end in a collision with its equatorial ridge
How many mountains on Earth does a commercial flight impact at 11 km?

Followed by a second tweet
Iapetus is also dirty on one side but that's a whole different story

A fellow twitterer @Xurde69 gave me the impression (by retweeting the second comment twice) that he wanted me to explain the dark side of Iapetus. Tweets are too short to do the subject justice hence the motivation behind the creation of this page.

Thanks for the motivation @Xurde69

Five of Saturn's seven largest (rounded) moons in their orbital position and roughly to scale.
Mimas, Enceladus, Tethys, Dion, Rhea. >>>>>>>>>>>>> Titan >>>>>>>>>>>>>> Iapetus
Albedo is a measure of the brightness or reflectiveness of a cosmic body.

An albedo of 1 is the equivalent of 100% of the light that hits the object reflecting off that object. This definition comes with an exception.

In space, typically the albedo of one object is compared to the brightness of another object, this is called geometric albedo and it can be greater than 100%.

Most definitions of bond vs geometric albedo are not easy to understand so I wrote my own.
                                              >>>>>>>>>>>

In the above table you can see Enceladus and Tethys have geo albedos greater than 100%
Its not unusual for Saturn moons to have brightness variations on their leading and trailing hemispheres.

Of the seven largest rounded Saturn moons, only Titan and Enceladus don't exhibit this trait most likely because both have active geology.
There is a significant difference, however, between the dark material on Mimas, Tethys, Dione and Rhea from that of Iapetus.
Iapetus' dark side (named Cassini Regio) is much much darker than the other moons dark side.
Only Iapetus has the darker side on its leading hemisphere while all the others exhibit slightly dark material on the trailing hemisphere.

What then can explain this difference?
Iapetus is outside the orbit of Titan while the other rounded moons are inside. Iapetus takes 79 days to orbit Saturn and is tidally locked meaning that like our Moon, it always shows one face to the parent planet and turns once per orbit.

Iapetus is dark and red/brown on its leading hemisphere and primarily bright white H2O ice on its trailing.

The difference in brightness (albedo) is about 0.05 on its dark red/brown side and about ten times brighter (one order of magnitude) or 0.55 on the opposite side.

Iapetus aims its dark leading hemisphere towards the Sun roughly every 38 days (4 weeks) per orbital revolution.

Its orbit is inclined 15.5 degrees relative to Saturn’s equator which is much steeper than inward orbiting Saturn moons. 

The most plausible explanation for Iapetus’ darkened color variance has to do with Saturn’s moon Phoebe which itself orbits 4 times further out than Iapetus.
Phoebe travels around Saturn in a retrograde (counter rotating) orbit and is considered a captured moon.
 
Saturn has bright ice rings that extend outward as far as 10 Saturn radii (Sr), one Sr = 60,330 km. Saturn also has a dark dust ring.

This ring is too faint to be seen by telescopes in the light spectrum. It is only visible in the infrared. This dust ring extends from at least 128 to 207 Sr. Phoebe orbits the planet at an average distance of 107 Sr.

The dust ring is 20 Sr wide and is 40 Sr from the lower to upper tilted edge which is in line with Phoebe's orbit. The ring is thus thought to be sublimated or crater gardened material ejected off Phoebe. The ring is assumed to be orbiting retrograde relative to Saturn and Iapetus, same as Phoebe. 

That means Iapetus is orbiting around Saturn crashing headlong into all these dust particles. Since it is tidally locked to its parent planet one side of Iapetus is always facing toward this dust cloud.
These images give an idea of the size, shape, angle and distance of the dust ring relative to the inner icy rings. 

Moons inside the orbit of Titan pick up bright ice crystals mixed with small amounts of dust or gravel while moons outside Titan's orbit will tend to pick up dark Phoebe dust dominant particles.


It was logically reasoned that Iapetus is picking up inward falling dust from the Phoebe formed dust ring.

As the dust accumulated on Iapetus’ leading hemisphere, its darkness absorbed more sunlight energy during the month it was facing the Sun increasing temperatures enough to sublimate (evaporate) its water ice.

As temperatures rise to 125 K, H2O ice sublimates and is ejected to atmospheric elevations of 570 km which allowed transfer of those ices to the trailing hemisphere because of Iapetus' size (gravity).

A model was successfully used to recreate this leading hemisphere darkening as well as the trailing hemisphere brightening ice deposition process over the course of 2.4 byr.

This model is based on direct observational evidence, data gathered by telescopes and data from the Cassini probe.
As is often the case, when I study one phenomenon I stumble onto others.
Take for example Phoebe.
At first glance it would appear Phoebe has been battered a lot by impactors, in turn, forming all these craters.
But
I learned something important about craters when I wrote my Pit Chains page 102.

Impact craters have shallow broad based bowl shapes with steep sidewalls and raised ridges along their rims.

Pit Chains are another beast entirely.
Two dominant features define a pit chain.
  1. They have conically shaped side walls similar to an ice cream cone (they are not bowl shaped, unless really old and eroded). The more conical the cone and smooth the walls, the younger the pit.
  2. They don't have raised ridges around their outer edges.
In addition to the above two traits, they also tend to form in lines as they are the result of subsurface slip fault fractures.

Subsurface cracks develop as the crust slumps inward (the result of gravitational compaction or thermal cooling) these cracks provide access to deeper cavities.

Surface sediment aka regolith (sandy material) then drains through the cracks into the below void forming a cone shape on the surface without a raised rim ridge.

The cones form just like you see in the sand of an hour glass.
When we look closely at Phoebe, there are some obvious signs of pit chains rather than the commonly interpreted impact crater.


In this section of Phoebe, you can clearly see that several of these craters don't have raised rims and their shape is conical not bowl shaped.

The inside wall of the central pit shows light material sliding down the side walls while near the upper edge there is clear evidence of a darker subsurface layer near the rim.

This darker material is likely the same ejected dust that forms the dust ring. With an escape velocity of 100 m/s, it wouldn't be difficult for material to eject into space.

These conically shaped no ridge rimmed pits imply there is a subsurface crack into which the surface regolith is draining.

These conically shaped no ridge craters form a bending line up into the main collapsed crater.

A subsurface fracture may follow this general line.

There are broad shaped pits within the largest collapsed crater (upper) area that follow in a diagonal line down the outside.
Phoebe has a density of 1.638 g/cm^3 and is considered differentiated containing about 50% rock as opposed to the inner satellites of Saturn which are thought to contain 35% rock. Iapetus has a density of 1.088 meaning it is mostly a water ice ball.

Basically Phoebe has a core of rock surrounded by ice infused with gravel.
As the gravel is able to do so, it collapses inward through the rock hard ice and the ice sublimates as orbital temperatures increase kicking dust into a ring of debris.

Do you see evidence of another pit chain (subsurface slip fault) in this next image?
As of December 3rd, 2018 an interesting article was written about the deuterium ratio in Phoebe. This ratio of heavy hydrogen further supports the idea Phoebe formed further out in the colder zones of the solar system and was captured by Saturn.
Quote
The discovery of an unusual deuterium to hydrogen isotopic ratio (D/H) for Saturn’s moon Phoebe means it was formed in and comes from a far part of the Solar System, Clark said. “Phoebe’s D/H ratio is the highest value yet measured in the Solar System, implying an origin in the cold outer Solar System far beyond Saturn.” 


Hyperion is the next moon inward from Iapetus and it shows the same surface features as Phoebe.

Take a close look at Hyperion and you will see sink holes aka pits as well as slumped land slides with dark gravel resting in the center of many pits
Just like Phoebe, Hyperion has a massive crater with a bulge in the middle. But there is a significant difference between these two bodies. The density of Hyperion is only 0.544 g/cm^3 while Phoebe is 1.638. Hyperion is mostly extremely porous water ice. Hyperion is darker than the inner moons of Saturn and is reddish in color similar to the dark material on Iapetus but with a much lighter coating. Hyperion's axial rotation is chaotic, meaning the deposited dust does not accumulate on a leading or trailing hemisphere as there is none. 

Granules of accumulated dust/gravel are visible inside most of Hyperion's pits.

Phoebe with its high density appears to be mostly rock from some other place in the solar system. Phoebe appears to be ejecting material off its surface via sublimation as its orbit is significantly closer to the Sun's energy and it's also collapsing inward through thrust faults.

Hyperion with its density appears to be mostly loosely held together ice that is getting showered lightly with dust from Phoebe. As the dust accumulates on the surface it compresses inward creating many of the pits. Sun light would also act to heat the darker dust more than the ice in turn focusing the dust grains into groups as they coalesce at the bottom of the pits. This process is called sublimation degradation.

Download a high res image of Hyperion here and look closely at the center of the pits. The dust/gravel build up and subsequent ice pit formation is obvious. This makes it much more difficult to delineate between which of these craters are dust pits, slip fault pits or impact craters, however, most of these pits are conical without raised ridges on their rims and have a dust/gravel accumulation at the center.

Four for the price of one.

You know why Iapetus is dark on one side and light on the other.

You understand why geometric albedo can be greater than 100%.

You know why Hyperion is darkened with dust grains.

You know how to identify the difference between slip fault pit chains and impact craters.