How to Live on Other Planets: Saturn

So far away, yet arguably the most beautiful of celestial bodies. The Sun is a pale star and the Earth practically invisible from where it orbits... 

...can we live in the Saturnian system?
Description
Saturn is the second-largest planet in the Solar System, just a bit wider than nine times Earth and weighing nearly a hundred times more. It is distinguished by its vivid set of rings and large moons.

Like a typical gas giant, it is mostly hydrogen (96%) and helium (3%), with these gasses compressed into a metallic state by the pressure at the core. 

The giant's atmosphere is less well explored than that of Jupiter, as only three missions have sent probes to Saturn compared to eight for the former. Hopefully we will learn much more as the Cassini probe, which has orbited Saturn since 2004, spirals to its end in the upper atmosphere.

0km is set at the altitude where atmospheric pressure is 1atm 

Saturn's atmosphere is much cooler than Jupiter's. The mass difference between the gas giants begins to show as the gasses are much less compressed. At the altitude where atmospheric pressure is equal to Earth's sea level (1atm), we find a frigid haze (-173 degrees Celsius) that obscures the lower levels. 

Going deeper reveals three colourful cloud layers. Ammonia, being the lightest, floats on top. Underneath it is a layer of ammonium hydrosulfide (ammonia + water + sulphur) that extends for another hundred kilometers. Beneath both are water ices, starting at 10 atm pressure. Atmospheric temperatures only become comfortable (>20 degrees Celsius) at 300km below the 1 atm altitude, where pressures approach a hundred times sea level. 

As stated above, one of Saturn's most impressive features is its ring system. These are composed of 99% water ice and vary between 1m and 1km thickness.

The near-invisible outer rings extend to 300000km.

Particles composing the rings are generally very small, most a few centimeters in diameter with few ranging up to a few meters in size. The latter mass around a handful of tons each.  

Between these rings are moons.

The most notable are Titan, Iapetus, Rhea, Dione and Tethys. All are larger than 1000km across, with Titan standing as second-largest moon in the Solar System with 5150km diameter. 

There are 62 known moons, of which sixteen are between 18 and 500km in diameter. Saturn's smaller moons are interesting targets for colonization to provide support for efforts on larger bodies.

Habitability

If we actually decided to live on Saturn itself, we'd be facing a difficult task... but with fewer hardships than attempting to do the same on Jupiter.

Artist's impression of the view from Cassini's closest approach.

There is no solid ground, so a colonization attempt will have to stay in the atmosphere. At lower altitudes, temperatures and pressures become crushing, so only the top layer of Saturn's atmosphere is realistically accessible.

Two options are available: -100km altitude or -300km altitude.

At -100km, the atmospheric pressure is equal to Earth's sea level pressure. However, external temperature would be a chilling 134K, equivalent to -139 degrees Celsius. At -300km, temperatures rise to a much more comfortable 20 degrees Celsius, but the pressure is 10 atmospheres. This is the same as the pressure found 90 meters under the sea.

Staying warm through insulation and heating requires much less mass than reinforcing the habitats to withstand high pressures. So, it is very likely that colonization attempts will focus on staying at higher altitudes than at lower altitudes.

The main challenge is staying in the air.

Buoyancy is an option. A balloon containing gasses less dense than the surrounding atmosphere generates a lifting force equal to the difference in density between the internal and external gasses.

On Earth, a helium balloon is very effective because the internal gasses have a density of 0.178kg/m^3. The external gass, air, has a density of 1.292kg/m^3 at sea level. The difference, 1.114kg/m^3, means that each cubic meter of volume in a helium balloon can lift up to 1.114kg off the ground. 

On Saturn, buoyancy is much harder to achieve. At -100km altitude, it is believed that Saturn's atmosphere has a density of 0.19kg/m^3. A helium balloon would barely float in the atmosphere.

Better balloons would need a combination of lower density gas (hydrogen), higher temperatures and higher pressures to lower internal gas densities. A perfect vacuum balloon would have a density close to 0kg/m^3... but even so it would only be able to lift a measly 190 grams per cubic meter, from which must be deduced the mass of the balloon's pressure vessel. With such a low lifting ability, a single ton of equipment on Saturn would require a lifting balloon at a minimum 20 meters in diameter.

In short, trying to stay afloat in Saturn's atmosphere is possible, but would require absolutely massive balloons for even the smallest payloads. Very advanced materials technology would have to be developed for temperature-resistant, shear-resistant and ultra-lightweight balloon envelopes to make this a feasible proposition.

The other option is lifting power from a wing.
A colonization effort of Saturn might fly through the upper atmosphere and maintain altitude through lifting surfaces. Saturn's surface gravity is 1.065g, which is very close to the surface gravity on Earth. What this means is that to keep one ton of equipment in flight, one ton of lifting force must be generated by the wings. 

The main difficulty with this approach is the very low density of the atmosphere. A wing in Saturn's atmosphere would generate 14.7% of the lift that it would on Earth. To compensate for this, the wing must be made 6.8 times larger or fly through the air 2.6 times faster. A larger wing masses proportionately more, so attempting to fly at jet airliner speeds would require planes that look more like the Solar Impulse plane than the Airbus A340: long thin wings that make up most of the plane's size.

Flying instead 2.6 times faster would mean that for an airplane of familiar size and proportion, supersonic speeds are needed just to take off!

Thankfully, lift and drag are linked. If lift is hard to generate, then there is not much drag either. Generating enough lift would create similar levels of drag as here on Earth. This would mean that the most efficient way to fly on Saturn would be by using some sort of supersonic flying wing. 

NASA's N3-X, powered by electric turbojets.

Efficiency in this case would be critical. The most efficient engines are turbojets, but their propellers are subject to the same constraints as wings on Saturn: producing the necessary thrust would require propellers that spin nearly seven times faster than on Earth, which is not realistic due to material constraints and turbulent flow losses. The same applies to turbofans and high bypass turbojets.

The nuclear-ramjet-powered SLAM.

The best thruster would work like a ramjet: the incoming hydrogen airflow is compressed without any turbines. It is them heated and accelerated by burning it in on-board liquid oxygen or with the exposed core of a nuclear reactor. 

Of course, staying afloat is only half the challenge. 
Resources such as energy and materials must be acquired to ensure that a floating colony survives longer than for a short visit. 

All that would be needed is scarce in Saturn's atmosphere.

Hydrogen and helium compose 99.5% of the gasses the flying colony would have access to. Traces of methane and ammonia fill the remaining 0.5%, with vital oxygen compounds or water measured in parts per million. It is the gaseous equivalent of a dry desert. If the gasses are collected, liquefied and fractioned to extract the methane and ammonia, less than 3.2 grams of methane and 0.1 grams of ammonia per cubic meter would be obtained!

Aramid fibres.

Probably the only consolation is that ammonia and methane provide the carbon, nitrogen and hydrogen needed to form aramid fibres. They are a high tensile strength material which provide a robust yet lightweight construction material to make more habitats and pressurized balloons from. Solid containers might use carbon fibres instead. 

However, powering even critical systems such as heating and life support, measured in kilowatts per person, is a challenge. Less than 140W/m^2 of sunlight reaches Saturn. At -100km altitude, less than half of that amount is expected to reach a flying colony. Even highly efficient solar cells would need over 35m^2 for a single kilowatt of electric output... a small colony would need several acres of solar panels just to stay alive! This is not a practical design if it drag, mass and structural requirements are considered. 

Chemical, aerothermal, wind and nuclear energy are the other options.

Since reactive elements to burn in hydrogen are extremely rare and usually already bound in stable molecules, chemical energy sources are a no-go.
Aerothermal energy exploits the differences in temperature between different altitudes in an atmosphere, like a geothermal power station does with different depths. Saturn's core is expected to radiate away its energy at 12000K, while the troposphere is a frigid 80K. A lot of energy can be extracted from this temperature gradient... if the two layers were not separated by incredible distances and pressure differences. 
Due to the gas giant's massive size and low density, it takes altitude differences of hundreds of kilometers to obtain temperature differences of a few dozen kelvin. An aerothermal power station on Saturn would need to circulate a coolant up and down these distances, making it a huge investment. Obtaining the resources to build it would be very hard, making it float and survive the buffeting winds would be even more difficult. 

NASA photo of a Saturn storm.

Wind energy is an excellent option. Storms push winds to over 400m/s at the equator, six times faster than even the strongest hurricanes on Earth. Wind power scales with cubed increases in velocity, so even in the lower density atmosphere of Saturn, megawatts of electricity could be generated by even relatively small wind turbines. Of course, there are challenges and huge investments in time, resources and effort to make the most of this power source: they will be discussed in a dedicated section below. 

Convair B-36s were used as test-beds for flying nuclear reactors.

Of all the possible sources of energy, nuclear power is the only realistic option for the foreseeable future. Powerful, compact and long-lasting, it would likely arrive pre-built from Earth and keep running for decades. Having access to cold gasses outside the colony would also make waste heat management of a reactor quite easy. The downside is that Saturn, its moons and most of the Solar System is devoid of the fissile fuels needed to keep the reactors running forever. 

At a first glance, Saturn should have a clement environment to live in. The atmosphere above a floating colony would provide sufficient protection against cosmic rays and other sources of radiation. 1g gravity and sea-level pressure should mean living conditions are no worse than on Arctic research stations.  

In fact, the colony would constantly be buffeted by extreme winds. Looking out of a window would show only a glimmer of light illuminating endless, turbulent clouds stretching from horizon to horizon. There is a constant risk of the habitats being punctured or wings failing due to wear (a flying colony cannot just 'stop' for repairs). In many ways, it is worse than the vacuum of space, a situation worsened by the simple unpredictability of the winds and temperatures from day to day. 

Like this, but much darker.

If, despite all these hardships, a permanent presence is established, it would likely be utterly dependent on supplies coming in from bases on other moons. Orbital mechanics therefore add another layer to the challenges the colony would face: reaching low orbit around Saturn requires an orbital velocity of 25km/s. Thankfully, Saturn rotates at an incredible 9.87km/s at the equator. Launching in the direction of the planet's rotation reduces the deltaV budget to low orbit down to about 15km/s.

The deltaV required to send down supplies from Saturn's moons down to Saturn's equator, if full use of aerobraking is available, ranges from 5.7km/s from Mimas to 9.5km/s from Titan. This is a significant cost, comparable to sending payloads into orbit from Earth's surface.

Sending anything the other way, from Saturn's upper atmosphere to a moon, such as scientific experiments or sea-sick colonists, would require 21 to 25km/s! 

For near future propulsion technology, such as hydrogen-oxygen or nuclear thermal rockets with a performance of 400 to 1000 seconds Isp, Low Saturn Orbit O) is a one-way street. They would require incredibly high mass ratios (12 to 587) to climb back out of low orbit. Because of these deltaV budgets, a station in Low Saturn Orbit is actually harder to supply than a station in the atmosphere where aerobraking can be used.

Titan

The largest moon, the most interesting moon and the best target for a colonization attempt.

It is 50% larger than Earth's moon and nearly twice as massive, weighing in as second-largest moon of the Solar System. Its surface gravity of 0.14g helps it retain a unique feature: a thick and dense atmosphere composed nearly entirely of nitrogen (98.4%). 

It has liquid lakes and rivers on the surface, but these are made of methane and ethane. Titan receives 1% of the sunlight that Earth does, and after passing through the thick clouds, less than 0.1% remains to heat up the surface to a frigid 94 K (−179.2 degrees Celsius).

The 'underground ocean' model is displayed here.

Titan's surface is a thick layer of ice run through by lakes of liquid methane and ethane. A thick petrochemical haze floats over these lakes. Between the lakes run rivers that have carved endless valleys, crevasses and channels over the millennia. 

A view of Titan's second-largest lake, Ligeia Mare, composed from images taken by Cassini.

Ammonia is lighter than water in either liquid or solid forms. It is therefore concentrated in the upper layers of Titan's icy crust, permeating the porous ice at the surface and forming water/ammonia/methane clathrates underneath. 

As you go deeper, water becomes the prevalent component, potentially forming a vast sub-surface ocean upon which the icy surface floats. It is thought that the ice layers are on average 200km thick, which an ocean another 300km deep underneath that. 

Because the moon underwent a period of relative warmth during its formation, it enjoyed nearly fully liquid crust for over three billion years. As seen below, the outer layer would have allowed heavier elements such as rocks, metals and minerals to sink to the bottom of the ocean. This means that today, after most of the ocean froze over, the surface should contain very little of anything heavier or denser than water ice. 

Evolution of Titan's crust over the past 4.5 billion years. 

The residual heat now powers a cryo-volcanic cycle in which methane is expelled into the atmosphere to replenish losses due to photochemical reactions driven by ultraviolet radiations from the Sun. The result of those reactions in a range of petrochemical and organic molecules called tholins. 

'Da' stands for Daltons, a unit equal to 12 atomic mass units.

The weather cycle on Titan.

Tholins are large molecules (over 12 atoms) composed of hydrogen, carbon and nitrogen. They fall as large, slow raindrops and accumulate at the bottom of methane lakes as a sludge layer that NASA simply calls 'gunk'. Tholins are what gives Titan's atmosphere its reddish haze. 

Titan will be difficult to live on.

The main issue is the cryogenic temperatures on the surface. Unlike the airless void where the only way to lose heat is through the slow process of radiation, the thick atmosphere of Titan actively sucks heat away through the much faster process of conduction. From a 293K interior to a 94K exterior, a space-grade 1mm thick aluminium wall would lose more than 40MW/m^2! A human wandering onto the surface would freeze solid in seconds. 

Directly dealing with this problem costs energy and mass. Thick insulators will be needed to slow down conduction. 10cm of polyurethane foam can reduce heat losses to less than 4W/m^2. Regular human body heat is sufficient to warm a well insulated environment. A 10m long, 5m wide half-cylindrical habitat would need a constant 314W of heating if it utilized the thick polyurethane insulation described above - humans emit 100W of body heat on average.

The definitive solution for this will be apply the same colonization methods as on the moons of Jupiter: dig under the surface and create underground volumes for habitation. Having to cover every exposed surface with thick insulation is a burden on an early colony's industrial capacity. Ice, on the other hand, is a natural insulator. Humans living inside an ice-walled habitat can actually heat it up quickly enough to melt the ice.

However, Titan's surface ices are not suitable for construction. Solid water ice is heavier than the methane-riddled, porous ammonia/water ice on the surface, so it will sink. You cannot build out of the surface ices either, as they are porous and filled with methane. Exposing it to the heat of a habitat would cause the methane to bubble out and leave channels and empty pockets big enough to cause any structure to collapse. 

With the atmosphere protecting against radiation, you can build directly on the surface.

Finding an appropriate is best done at the poles, where there is less methane rainfall, or at the edges of a lake, so that liquid methane can be used to cool the habitats' exteriors and keep the ices from melting or out-gassing. There, you can build directly on the surface with no digging required. 

Most of a colony's needs can be met: plentiful water for oxygen and hydrogen, ammonia for fertilizer and organic compounds, methane for plastics. Construction materials would revolve around carbon composites, aramid fibres and polyethylene plastics. No metals or elements heavier than carbon would be accessible - essentials such as sulphur, potassium and phosphorus would have to imported from elsewhere. 

The other major problem is energy.

Even if heating demands are greatly reduced, power will be needed to drive life support's recyclers, circulate air, light the plantations, power the mining equipment, heat the chemical reactors and so on. Every expansion or intensification of activities is a proportional increase in demand for energy.

Producing electricity on Titan would be quite difficult. Solar power is too faint to be practically exploited at 14W/m^2 at the surface. Titan's thick icy crust precludes the use of geothermal power, and the wind speed at surface level is so low that wind power would produce 1500 times less power than on Earth.

The low gravity and the generally flat topography makes hydropower an incredibly poor investment of resources. For reasons stated above, there is an extremely low chance of finding fissile ores to power nuclear reactors.

So what options are there for producing energy on Titan?

Unlike on Earth, Titan is mostly devoid of reactive chemicals. Nitrogen is very stable and there is no free oxygen to burn the liquid methane in. Extracting oxygen from water costs more energy than burning the products in methane produces. Free hydrogen is present in the upper atmosphere, and it can be forced to react with nitrogen to produce ammonia in an exothermic process called the Haber Process. Hydrogen composes 0.1% of Titan's air and the reaction nets only 30.8kJ per mole of hydrogen.  

Another option is the thermal decomposition of tholins. They are large molecules kept stable only by the extremely low temperatures. Heated slightly, they are likely to decompose into simpler elements such as ammonia, nitrogen and methane while releasing energy. However, it is unknown if they can be collected in significant quantities, but scraping lake-beds would be a good start.

Finally, there is wind energy.

Due to having a very thick and cold atmosphere, wind speeds are very low on Titan. A gentle 0.5m/s can be felt on the ground, rising to about 2m/s at 3000m altitude. Compare this to Earth, where wind speeds at the same altitude are at 5m/s. At an even higher altitude of 40km, Titan wind speeds reach 20m/s, up to 30m/s at 60km. The strongest winds are present over the largest lakes, like oceanic storms back home.
The formula for calculating wind power is the following:

• Wind Power=0.5 × Turbine Efficiency × Area × Density × Wind Speed^3 

Wind power is in watts and turbine efficiency is a dimensionless factor. Modern turbines achieve an efficiency of about 40%, with 59.3% being the maximum theoretical limit. Area is the disk covered by the turbine blades, in m^2. Density is that of the atmosphere at the turbine's altitude. On Titan, ground level air density is about 5.77kg/m^2. At altitude of 40km, it is about 0.73kg/m^3; density is proportionate to pressure (0.1 bar at 40km to 1.5 bar at 0km) and inversely proportional to temperature (75K to 94K). Wind speed is in m/s.

If we take the most powerful wind turbine currently produced, the Vestas V-164 with 21000m^2 swept area, and place it at a 40km altitude on Titan, it would produce 24.5MW of power. This is 35% more power than it would have produced in our atmosphere at similar altitudes. 

Wind turbines have several advantages on Titan. The lower gravity would reduce structural requirements. The dense atmosphere allows more mass to be floated to higher altitudes - the greater the turbine high, the faster the winds it catches and power it generates. There are many current designs for high altitude turbines, as are presented below, and on Titan they will be smaller and lighter. 

Another advantage a Titan wind turbine has is that the atmospheric temperature is so low, certain superconductors can operate without needing any cooling or protection.

A confusor uses the Coanda effect to increase wind speeds entering the turbine by up to 30-40%, more than doubling output.

Wind power is a large-scale solution to energy problems a Titan colony would have. It is also limited in power density: each turbine would need a large clearing for anchoring the balloons. The industrial capacity to produce the large power generating equipment will not be available in the early stages of colonization, and most of the heavy elements that go into producing superconductors and electromagnets will have to sourced from off-planet. 

The Saturn Energy Network

Unlike Jupiter, Saturn does not have an overwhelming magnetosphere that can easily be tapped for power. 

What it does have however is a relatively clement environment and very strong winds. Novel turbine designs can attempt to harness these winds with a potential output of terawatts. The downside is that much more time and resources must be invested into the Saturn system to obtain the power output needed to drive a full colonization attempt compared to Jupiter.

The key problem with harnessing wind energy on Saturn is that you cannot extract energy from a wind if you are also being dragged along by it at the same velocity. Like a heat engine, you need a gradient to extract energy. On a planet with a solid surface, this is easy: you can anchor yourself to the ground. On Saturn, you must build turbines in a slower wind zone that can extend into a faster wind zone. 

Wind speeds on Saturn collected by various missions. 0km starts at 100 mbar.

Three wind speed gradients can be found on Saturn: vertical, horizontal and local.

The vertical gradient is between the upper atmospheric layers and the lower layers. On Earth, winds regularly reach gale forces in the stratosphere, culminating in the Jet Stream's 60-75m/s at 24km altitude. Saturn makes the Beaufort scale seem inadequate with 300m/s at 100km altitude and increasing up to 500m/s down at -150km altitude. We don't know if winds reach even greater velocities in the high-pressure depths. 

The horizontal gradient is between different latitudes. Equatorial winds are the fastest while polar winds are the slowest, with peaks and reversals near the poles.

A wind turbine could extend horizontally and be spun by the wind speed differences. The flatter the curve on the chart above, the greater the wind speed difference between latitudes. For example, at the -40 degree latitude, a wind speed difference of nearly 140m/s can exploited over a 5 degree difference... equivalent to about 95000km. 

Local wind gradients are caused by storms and convection cells. Storms are frequent and powerful on the gas giant. They have sharply defined edges where wind speed gradients are extreme, to be exploited by relatively small turbines. 

A sound frequency based explanation for why Saturn displays hexagonal storm patterns. 

Convection cells move large masses of air vertically. Wind speeds are lower, but they are much safer and more predictable than storms. 

Wind turbines designed to exploit any of these gradients will be positively massive: tens of kilometers long for the vertical wind gradients, thousands of kilometers long for those attempting to draw on the difference between latitudes. On the upside, gigawatts of 'free' and continuous energy is available per turbine.

We will now explore two of these designs.

Kite-drawn Wind Turbine:

Not to scale.

A kite in a lower altitude, higher velocity wind layer drags a floating wind turbine through higher altitude, lower velocity winds. The difference in wind speed makes the turbine spin and generate electricity.

The longer the tether, the greater the difference in wind speeds. It operates best in the deeper atmospheric layers as the increased density makes keeping the turbine aloft easier. In low density air, it might have to deploy wings to increase lift at the cost of more drag.

A wind turbine floating at -100km altitude at the equator, dipping its kite into the -150km wind layer, might see a wind speed difference of over 150m/s. A well-designed wind funnel at the turbine's entrance can further increase this to 200m/s. This would allow for more than 380kW/m^2 with a 50% efficient design, so a 100m diameter design would produce 2.98GW. If the turbine operates at lower altitudes, it can use a smaller balloon to float and more energy can be extracted from the denser airflow. A smaller balloon allows for less drag, which in turn reduces the size of the kite necessary.  

Latitude-line Wind Wheel:

To scale.

A much more massive project would be the latitude-line wind wheel. It exploits the differences in wind speeds between different latitudes on Saturn. Instead of using the wind to spin turbine blades, the entire structure rotates with edges catching winds moving in opposite directions. 

The edge ring is equipped with sails to catch the wind. Using sailing techniques developed for water speed records, these sails are individually angled into the wind to travel faster downwind than the actual wind speed. Sail speed can be nearly doubled.  

The rotation of the entire structure can be used to drive an airfoil around a hub. Like a helicopter, it would provide lift and preclude the use of voluminous, drag-inducing lifting balloons. At the hub, a vertically spinning generator applies torque against the giant wheel's tilt. The wheel compensates for this using its angular momentum and variable lift. Terawatts of electricity are expected from each wheel. 

Generating electricity in Saturn's atmosphere is not the final step: it must be delivered to where it is likely to be needed, which is on industrial sites on Saturn's moons or a colony on Titan.

Laser power beaming is already being explored as an option for increasing UAV endurance.

There are four steps to delivering the wind energy. All involve the use of beamed transmission to tackle the vast distances involved. The first is carrying the electricity out of the thick lower altitudes up to the upper atmosphere. From there, it must reach re-transmission stations on low orbit. Then, it is delivered to the moons. The last step is to collect the energy and convert it into a useful form.

As mentioned above, wind turbines work best in Saturn's lower altitudes due to the combination of more violent wind speeds and higher density air. It would be converted into a laser beam using equipment on the turbine itself. On Earth, the selection of which wavelengths to use is dominated by the presence of atmospheric water vapour. On Saturn, hydrogen's absorption spectrum determines which wavelengths are favoured. The presence of other trace gasses produces the second graph below.

The large coloured gaps between black lines on the absorption spectrum indicate wavelengths that are 'safe' to use. Saturn's reflectance spectrum indicates which wavelengths are not absorbed (high reflectivity) and which are effectively absorbed (low reflectivity). We want shorter wavelengths as they allow for better focused beams by smaller and lighter focusing arrays, so taking both data-sets into account, we select a 680nm wavelength. 

We do  not want to burden the wind turbines will massive laser generators, so Free Electron Lasers are out of the question unless they drastically improve in power density compared to current technology. A good alternative choice would be massed Fibre lasers with an efficiency of about 50%.

The beam will have to travel from the lower altitudes to a relay station in orbit. These stations are required as an equatorial wind turbine would be spinning around the planet at velocities of up to 10km/s and would not be able to keep a lock on a target on a moon many millions of kilometers away. 

The atmospheric transmission distance should be about 2000 to 2500km. Focusing a laser over such a distance is not too hard for the massive latitude-line wind wheel, which can afford to mount a large focusing array on its stationary hub, but will be quite difficult for a rapidly moving kite-drawn wind turbine. The solution is to have an independently aligned high-altitude balloon carrying a re-focusing mirror that the wind turbine's laser can focus from a much smaller distance. Several balloons are increasing altitude can be used before the beam is bounced to its destination on an orbital relay station.

The orbital relay station can use the same laser beam and simply re-focus it across the 0.185 to 3.5 million km distance to Saturn's moons. Another option is to use frequency doubling to reduce the laser's wavelength at the cost of efficiency, but allows for much easier focusing over long distances.

Once the beam arrives at the target moon, it must be converted back into electricity. Photovoltaic cells tuned to the laser's wavelength can perform this task at 30 to 40% efficiency. A laser-thermal steam turbine is much more massive but can achieve efficiencies of 50%, or a laser-thermal magnetohydrodynamic system that can surpass 60%. 

Titan is a special case. It is extremely difficult to devise a laser beam that can both cross the large distance between Saturn and the moon AND penetrate the atmosphere. The solution to this problem is an alternative energy transmission system.

A purely laser-based power transmission system is likely to have a low overall efficiency. A four-step process as described above might suffer energy losses on the order of 80%. For every 1kW produced from Saturn's winds, 200W arrives where it is needed in the form of electricity, and would still need to overcome the hurdle that is Titan's atmosphere. 

The alternative is physical transport of hydrogen. Instead of generating electricity to power laser beams, the wind turbines are used to collect, liquefy and store hydrogen. The liquid hydrogen is fed to a highly stress-resistant tank covered in ablative heat shielding and equipped with parachutes. 

The tank is shot by coilgun straight up and out of Saturn. It is intercepted by Titan at the peak of its trajectory, where it performs an aerobraking maneuver at the moon's orbital velocity of 5.57km/s before parachuting to the ground.

Hydrogen can be burned in Titan's nitrogen atmosphere to produce ammonia and 15MJ of energy per kg. This heat energy can be converted into electricity at efficiencies of 40 to 60% by gas turbines or MHD generators, so we can expect 7.5MJ/kg.

The gravitational potential energy of 1kg of mass at Titan's altitude above Saturn (5.683e26 kg) is about 31MJ. An 80% efficient coilgun would need 38.7MJ to deliver 1kg of hydrogen to Titan.

The alternative hydrogen-based power transmission system delivers 7.5MJ of energy per kilogram while consuming 38.7MJ, meaning an overall efficiency of 19.4%. This is lowered by the mass of the aeroshell, heat-shield and parachutes required, but it would remain competitive compared to a purely laser-based transmission system. 

Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus

Most of these moons fit on Saturn's massive E-ring.

These six moons range from 396 to 1527km in diameter, making them of special interest in the Saturn system. Travel between these moons is much cheaper in terms of deltaV and duration than travel to other planets. It is therefore much more interesting for a colony to develop the capacity to exploit these moons and extract the materials required for its growth in-situ that to try and ship them in from elsewhere. 

Mimas:

It is the smallest of the six moons and the smallest object in the Solar System to maintain a round shape through self-gravitation. Its density of 1150kg/m^3 suggests that it is made up nearly entirely of water ice, with a very small collection of rock dust at the center. 

Mimas receives nearly no tidal, radioactive or solar heating, making it a cold and dead iceball only notable for its proximity to Saturn. This places it well within Saturn's magnetosphere, which protects it from solar radiation. 

Here is a deltaV table for travel from Mimas:

Low orbit is barely 500 meters above the surface, with an orbital velocity of about 96m/s. 

We can see that going from Mimas to Low Saturn Orbit (LSO) costs a hefty 9.7km/s. This is about the deltaV required to reach Low Orbit on Earth. Mimas is therefore not very useful as a 'staging area' for delivering supplies to a colony on Saturn itself, saving only about 1km/s of deltaV on a trip down from the much more interesting neighbour Enceladus.

The moon's becomes even more dull when we consider that most of Mimas's features are shared with Janus and Epimethieus, two moonlets even closer to Saturn, or the larger asteroids in the A and B rings. The only resource Mimas can be exploited for, water, is already available in vast quantities throughout the Saturn system. 

Enceladus:

Enceladus is a shining white disk in the E ring measuring 500km in diameter. It orbits at a 238000km distance from Saturn and only manages a 0.0113g surface gravity.

Despite its diminutive size, it is the most fascinating moon in the Saturn system after Titan. 

It orbits in resonance with the moon Dione, forcing a push and pull that results in tidal heating of the core. This heat is the source of visible cryovolcanic activity, a constantly renewed crust of ice and a unique subsurface ocean. 

The large rocky core contributes to most of the moon's mass and contains enough metals and minerals to raise the average density to 1609kg/m^3. It expels its heat in the form of geysers that end up launching a mixture of ice and frozen methane into space.

The heat has also eroded the usual icy shell found on Saturn's moons down to a thin crust only 5km thick at the south pole. 65km of salty water separate it from the warm bedrock below. 

Enceladus's ocean is unique in being covered by such a thin layer of ice and shallow enough to prevent the formation of high-pressure ice at its bottom, granting direct access to the rocky core. Pressures at the bottom should range between 50 and 100 bar, equivalent to 500 to 1000m under the sea. At the ice/water interface nearer the surface, pressures could be as low as 5 bars.

A colonization attempt of the Saturn system would start at Titan but would focus on Enceladus second. 5km of ice softened by geothermal activity and held together by only 1% of Earth's gravity would be easy to dig through. The only difficulty would be penetrating the hardened outer layer at -189 degrees Celsius, but that same surface provides a firm foothold for digging machines. Digging can be made even easier if it is started at the bottom of canyons up to 1km deep or a surface vent that channels down all the way to the ocean. 

Building habitats that can survive these pressures unlocks an immense reward: a warm, protected environment to live in and direct access to the core of the moon. 

The water itself is rich in minerals and contains dissolved methane. Sodium can be directly filtered from the water, and chlorine in addition to carbon monoxide is an essential component in the carbochlorination process. This process allows the extraction of pure metals such as aluminium or platinum from raw ores. CO- can be obtained by reacting methane with water.  

Practically unlimited amounts of elements such as potassium, calcium, phosphorus and sodium could be extracted from Enceladus's core. These are necessary for extended human presence in space. It is also very likely that deposits of lighter metals, such as aluminium, magnesium or even iron can be found near the core's surface. Traces of zinc, nickel and even uranium might be accessible if we dig into the core. A comparison to the thermal activity of Dione suggests that Enceladus's heat output is supplemented by fission decay.

A source of heavy elements so far from the inner planets is worth more than its weight in gold! It would massively boost any colonization attempt in Saturn either as a trading commodity or as a source of steel and aluminium. Other colonies in the Solar System would have to use amide fibres or carbon-based materials. 

A stream of protons is stripped away from Enceladus and impacts Saturn's north pole, creating impressive aurorae. 

Industrial activity might initially be powered would have to be powered externally. The best method is beaming energy directly from Saturn's wind farms. Solar panels tuned to the laser's wavelength can convert this to electricity. The electricity can be conducted down to where it is needed using wires, or used to create hydrogen peroxide out of the surrounding ices. The H2O2 is easy to store, not much denser than water and can be used like gasoline in mobile vehicles.  

Here is a deltaV table for travel from Enceladus:

The low gravity and low deltaV to orbit means that very efficient electric engines can be used directly from the surface. Landing or liftoff on other larger moons and planetary bodies usually requires a separate high-thrust set of engines. 

Shipping payloads from Enceladus to Titan costs a hefty 5.45km/s. 

However, an aerobrake maneuver in Titan's atmosphere can reduce this figure down to 4km/s. For an electric rocket like the Arcjet, a mass ratio of only 1.22 is required to make this trip. Hydrogen propellant is plentiful in a world made up mostly of water and powering the rocket can be done with the same solar panel and laser combo used to power Enceladus. Chemical rockets will use either liquid hydrogen/liquid oxygen (450s Isp) or liquid methane/liquid oxygen (380s Isp), with liquid hydrogen obtained through electrolysis and liquid methane from the gasses dissolved in the subsurface ocean. They require much higher mass ratios (3 and 2.92) but are much more likely to endure aerobraking than flimsy solar panels.

Tethys:

Tethys, showing the Odysseus impact basin

Tethys is third closest to Saturn, orbiting at 295 thousand km. Due to its proximity to the gas giant, it is in the unenviable position of being just within  the tail end of Saturn's magnetosphere and is therefore constantly bombarded by energetic particles. 

Tethys, like Mimas, is a large ball of hardened ice with no measurable tidal heating effects. It is quite unlikely that the moon hides a subsurface ocean, and if it did, it would be a thin layer of high pressure fluids under many dozens of kilometers of rigid crust. 

Numerous craters mar the ancient surface of the moon. They are a good place to hide from the radiation by providing a head start to digging into the ice. Some of the craters are over 40km wide and over a kilometer deep. 

There is a chance that one of the impacts was caused by a metallic asteroid. Without any tectonic or thermal activity to disturb the ice, the asteroid's remains would be left concentrated at the bottom of the crater.  It would form an easily accessible deposit of metals and minerals. It would be an extremely rare occurrence however, and the only hope of incentivising a colonization of the otherwise bland moon.

Colourful blue streaks on Tethys's surface.

Tethys shares its orbit with Telesto and Calypso, large moonlets of more than 20km in diameter. They are thought to be composed entirely of loosely packed water ice and other frozen volatiles. 

Here is a deltaV table for travelling from Tethys:

In terms of deltaV cost, Tethys is far away from both Saturn and Titan. 

Dione:

Dione as seen by Cassini during a close fly-by. Saturn's rings in the background.

Dione is a 1123km diameter moon that orbits at 377 thousand km from Saturn. It orbits in a 1:2 resonance with Enceladus, causing tidal heating effects that warms both moons' cores and raises internal temperatures enough to allow for a sub-surface liquid ocean. Tectonic activity has slowly but surely cracked and remade the surface of Dione over time - trying to dig habitats into the crust is quite risky when tunnels can collapse at any time. 

This moon is most similar to Enceladus. However, its size makes it a less attractive target for colonization. 

For one, the moon's ice crust is over 65km deep compared to Enceladus's 5km depth at the South poles. Its gravity is 0.024g, meaning that a digging operation to reach the subsurface ocean would be more than 26 times more difficult than on Enceladus. 

An ocean under such depth of ice would be pressurized. It would start at approximately 21.5MPa (211 atmospheres) at the ice/water interface. Diving all the way to the bottom of the sub-surface ocean, we would encounter pressures of about 36.5MPa (360 atmospheres). The equivalent ocean depths on Earth is -2110 meters and -3600 meters. 

Dione's density of 1478kg/m^2 is about 9% lower than that of Enceladus. It suggests that there are fewer heavy elements available in the core... its size would also make them harder to access as lighter elements would have floated to the top and heavier elements sunk to the center during its formation. 

Consider the following deltaV table for travel from Dione:

Dione is situated in between Enceladus and Titan in terms of deltaV cost. Payloads from Dione would require only 3.7km/s of deltaV to reach Titan, which can further be reduced to about 2.1km/s with aerobraking. Compared to the minimum 4km/s deltaV from Enceladus to Titan, it is three times cheaper to ship payloads from Dione.  

In the short term and during early colonization efforts, Enceladus is the clear winner when it comes to supplying heavier elements to Titan's colony. However, in later stages of colonization, the deltaV savings might finally make Dione an interesting alternative to Enceladus. 

Rhea:

Rhea is a large yet lightweight moon composed of a homogenous mixture of ice and dust. It has no core, only a center of compressed ice.

It receives too little energy to support a liquid ocean. The deeply scarred surface implies a very stable crust with little to disturb it. 

Some craters are over 40km wide.

In that regard, it is similar to Jupiter's moon Callisto. It is an excellent spot for a colony with a heavy investment in digging. Unlike on Enceladus or Dione, where several kilometers of ice needs to be excavated to reach the 'good stuff', we might find rocky minerals and even metals mixed into the ice near the surface. 

The deeper a Rhea colony digs, the more non-volatiles it will have access to. It would provide a more immediate reward compared to the ocean moons, and no specialized sea-going technology for high-pressure environments would be needed. In fact, it might be the first moon colonists would consider when looking for sources of the lithophile elements.

Looking at the deltaV table for travel from Rhea, we can solidify this supposition with the fact that an early industrialization of this moon instead of Enceladus would save nearly 3km/s of deltaV per payload delivery:

Iapetus:

Nicknamed the 'Death Star' moon for its large peaked crater, or the 'ying-yang moon' for its dark and white faces, Iapetus is visually striking. 

It orbits far from all the other major moons at a distance of over 3.56 million km from Saturn.

In red, Iapetus's orbit. It is highly inclined compared to the other moons.

Iapetus is nearly identical in size and density to Rhea - a good guess is that their internal structure is very similar. This means that the same techniques can be used to exploit either moon: dig into the hard ice, melt it to a slush and shake it until dust particles drop to the bottom. 

The major difference is that Iapetus is far away from Saturn, so transmitting power through laser beam will incur heavy efficiency losses. 

The deltaV table shows that sending a payload from Iapetus to Titan only costs a bit less (2.1km/s) than the same payload from Rhea (2.7km/s). What it doesn't show is that the inclination difference between Iapetus and Titan greatly reduces the number of minimum deltaV Hohmann transfer windows available. 

Travelling outside of these windows will cost more deltaV and nullifies Iapetus's advantage. On the other hand, sending payloads out of Saturn and to other planets in the Solar System is best done from this moon. An 'extracronian' delivery would cost about 3.3km/s more if sent from Rhea. The tempering factor however is that it is unlikely that any colony on other planets would have need of dust filtered from ice around Saturn. The dust is best employed as a source of building materials for the development of valuable industries on Enceladus.

The rings

The reason for Saturn's beauty, the rings are vast and complex structures that extends for hundreds of thousands of kilometers into space. 

Despite their apparent size, they are actually very thin disks composed of small particles measuring less than a millimeter to over a meter in size. 

These particles are very widely distributed, so collecting significant masses involves scooping up huge areas in the rings. 99.9% of the mass in the rings is composed of water ice. This makes the rings a poor source of anything needed for a colonization attempt, and water is already plentiful on Saturn's moons in addition to being present in a more easily exploited form.

Other than looking pretty and presenting a hazard to anyone trying to cross the rings at high relative velocity, the only potential use the rings have is supplying water to spaceships who cannot or do not want to land on a moon. Normally moonlets such as Janus would provide the easy access to water for this function (and could serve as a propellant depot near LSO), but the potential for 'wilderness refueling' without any restrictions on where it is performed has political and military implications. Warships attacking Saturn could refill their propellant tanks by scooping up ring material after a costly insertion burn instead of slowly spiralling in on high-efficiency propulsion to save of propellant. This reduces the 'home advantage' of Saturn defenders. 

The colony

A colonization attempt of the Saturn system would concentrate people on two locations: Enceladus and Titan. The colonist's preferences and the opportunities on each moon will determine their relative populations

On Enceladus, nuclear power will be essential until wind farms on Saturn are set up. They will provide the energy needed to dig through the ice on the moon's South poles and open up the sub-surface ocean below for habitation.

For a long time, colonists will have to live in artificially lit, enclosed environments with low gravity and devoid of luxuries. Diggers sink to the bottom of the ocean to scrape off minerals and metals from the moon's rocky core, then float back to processing centers near the top. These resources are used to expand habitation spaces and industrial centers, and will eventually start being sent to Saturn itself to build the wind farms. 

An early enceladus colony reliant on nuclear power. 

If the colonists wish for more freedom, they can take a swim near the ocean's surface. A pressure of 5 bar is equivalent to 40 meters under the sea, a depth scuba divers regularly reach. In fact, sea dwellers might prefer to equalize the pressure between the sea and the interior environments of the habitats: this would allow quick exit and entry for anyone who wants to swim or operate underwater vehicles. People who feel uneasy or claustrophobic when surrounded by water might prefer the ice tunnels further above. 

Titan would be easier to set up from pre-built components. Habitats would be placed at ground level and their interiors would be set to pressures very slightly higher than the exterior. Energy capacity can be immediately increased by lifting up wind turbines on large balloons and tethering them to the ground. Being able to walk outside and fly away with nothing more than a pair of wings, an oxygen tanks and a thick suit of insulating material would be quite liberating. 

On the downside, industrial capacity and long-term viability would depend on shipments from bases on other moons. There is always the risk of any breach leading to near-instantaneous death by freezing of all the occupants of the areas affected. Rockets can aerobrake and parachute to the surface easily, but climbing back up requires altitude-compensating nozzles and drag-reducing trajectories. 

Evolution

The development of colonies on Saturn revolves around the energy sources available and their management. This energy-starved state will continue until the vast storms on Saturn are harnessed and converted into laser beams that distribute energy throughout the Saturn system.

With that energy, rocket propellant can be produced from ice. The propellant allows regular shipments of building materials from Enceladus to Titan and elsewhere, allowing for the rapid development of comfortable habitats and prosperous industry. In turn, these resources are converted into wind turbines to be installed on Saturn, unlocking even more power.

Eventually, there will be enough leftover resources and power that Saturn colonies can start trading with the rest of the Solar System.

Trading products commonly found throughout the Solar System, such as nitrogen or methane, is a losing proposition due to the deltaV cost and distances involved in travel between Saturn and the other planets. Instead, they would be sent to even harder to reach places from the other planets, such as Neptune and Uranus. Alternatively, Enceladus and Dione could be converted into gold mines that would ship heavy elements to more desolate places, such as Jupiter's moons.  

Once the Saturn energy network is up and running, some power can be diverted to coilguns that shoot projectiles at 9.7km/s. This is also Saturn's orbital velocity... if the railguns are angled against Saturn's direction of travel, the projectiles would end up with zero orbital velocity around the sun.

The projectiles would be inert masses handled by a solar-electric drone that guides them a trajectory that skims across planets in lower orbits. 

If they fall sunwards long enough, the projectiles would gain massive amounts of kinetic energy. Spaceships can time an intercept with these projectiles to convert the kinetic energy into their own motion, either by deflecting them with magnetic fields or vaporizing them inside a propellant cloud. 

Here is a recreating of the trajectory of a projectile shot retrograde 9.7km/s from Saturn's orbit, falling all the way to Earth's orbit:

The bright blue line is the projectile's trajectory.

The projectile reaches 39km/s at 1A.U. from the Sun. If a spaceship were to intercept this projectile and use it for propulsion, we'd have to add the orbital velocity of the Earth to this figure, for an impact at 69km/s. A spaceship orbiting in LEO would face a relative velocity of nearly 77km/s. 

What this all means is that Saturn can export its energy production as kinetic impactors that can be used for propulsion. There is a 63x multiplier with Earth LEO as destination, so each kJ provided at Saturn becomes 63kJ at destination. 

This is not much better than Jupiter's performance for the same destination. What makes Saturn unique is that it can sell this kinetic energy to Jupiter and Jupiter's Trojan asteroids as well. The relative velocity in that case is 24km/s, which is a 6x multiplier, rising to 64km/s relative velocity with ships sitting in Low Jupiter Orbit. 

Jupiter cannot sell kinetic impactors to itself, so this gives Saturn a unique market opportunity. It is especially useful considering that Jupiters intra-system travel costs so much deltaV. Kinetic impactors would provide high Isp and high thrust for arbitrarily high propulsive power, solving this problem.