Continuing with our "Definitive Guide to Terraforming",

we present our guide to terraforming Solar System's Moons.

 

Much like terraforming the inner Solar System,

it might be feasible someday.

 

But should we?

 

 

 

 

 

 

 

 

 

 




How Do We Terraform...

Jupiter's Moons?
by Matt Williams

22 April, 2016

from UniverseToday Website

 

 

 

 

Surface Features of the Four members

at different levels of zoom in each row
 

 

Fans of Arthur C. Clarke may recall how in his novel, 2010 - Odyssey Two (or the movie adaptation called 2010: The Year We Make Contact), an alien species turned Jupiter into a new star.

 

In so doing, Jupiter's moon Europa was permanently terraformed, as its icy surface melted, an atmosphere formed, and all the life living in the moon's oceans began to emerge and thrive on the surface.

As we explained in a previous video ("Could Jupiter Become a Star") turning Jupiter into a star is not exactly doable (not yet, anyway). However, there are several proposals on how we could go about transforming some of Jupiter's moons in order to make them habitable by human beings.

 

In short, it is possible that humans could terraform one of more of the Jovians to make it suitable for full-scale human settlement someday.

 

 

 


The Jovian Moons

Within the Jupiter system, there are 67 confirmed moons of varying size, shape and composition.

 

In honor of Jupiter's namesake, they are sometimes collectively referred to as the Jovians. Of these, the four largest,

...are known as the Galileans (in honor of their founder, Galileo Galilei).

 

These four moons are among the largest in the Solar System, with Ganymede being the largest of them all, and even larger than the planet Mercury.

In addition, three of these moons - Europa, Ganymede and Callisto - are all believed or known to have interior oceans at or near their core-mantle boundary. The presence of warm water oceans is not only considered an indication of potential life on these moons, but is also cited as a reason for possible human habitation.
 

 

 

 

 


Of the Galilean Moons, Io, Europa and Ganymede are all in orbital resonance with each other. Io has a 2:1 mean-motion orbital resonance with Europa and a 4:1 resonance with Ganymede, which means that it completes two orbits of Jupiter for every one orbit of Europa, and four orbits for every orbit Ganymede.

 

This resonance helps maintain these moons' orbital eccentricities, which in turn triggers tidal flexing their interiors.

Naturally, each moon presents its own share of advantages and disadvantages when it comes to exploration, settlement, and terraforming.

 

Ultimately, these come down to the particular moon's structure and composition, its proximity to Jupiter, the availability of water, and whether or not the moon in question is dominated by Jupiter's powerful magnetic field.

 

 

 


Possible Methods

The process of converting Jupiter's Galilean moons is really quite simple.

 

Basically, its all about leveraging the indigenous resources and the moons' own interactions with Jupiter's magnetic field to create a breathable atmosphere.

 

The process would begin by heating the surface in order to sublimate the ice, a process which could involve orbital mirrors to focus sunlight onto the surface, nuclear detonators, or crashing comets/meteors into the surface.
 

 


"Terraforming" by Wasteland-3D.

Credit: deviantart.com/Wasteland-3D
 


Once the surface ice begins to melt, it would form dense clouds of water vapor and gaseous volatiles (such as carbon dioxide, methane and ammonia).

 

These would in turn create a greenhouse effect, warming the surface even more, and triggering a process known as radiolysis (the dissociation of molecules through exposure to nuclear radiation).

Basically, the exposure of water vapor to Jupiter's radiation would result in the creation of hydrogen and oxygen gas, the former of which would escape into space while the latter remained closer to the surface.

 

This process already takes place around Europa, Ganymede and Callisto, and is responsible for their tenuous atmospheres (which contain oxygen gas).

And since ammonia is predominantly composed of nitrogen, it could be converted into nitrogen gas (N²) through the introduction of certain strains of bacteria. These would include members of the Nitrosomonas, Pseudomonas and Clostridium species, which would convert ammonia gas into nitrites (NO²-), and then nitrites into nitrogen gas.

 

With nitrogen acting as a buffer gas, a nitrogen-oxygen atmosphere with sufficient air pressure to sustain humans could be created.
 

 

An engineer suggests building a roof over a small planet

so that Earthlike conditions could be maintained.

Credit: by Karl Tate, Infographics Artist

See more at:

http://www.space.com/23082-shell-worlds-planet-terraforming-technology-infographic.html#sthash.LB9CyN2g.dpuf
 


Another option falls under the heading of "paraterraforming" - a process where a world is enclosed (in whole or in part) in an artificial shell in order to transform its environment.

 

In the case of the Jovians, this would involve building large "Shell Worlds" to encase them, keeping the atmospheres inside long enough to effect long-term changes.

Within this shell, Europa, Ganymede and Callisto could have their temperatures slowly raised, the water-vapor atmospheres could be exposed to ultra-violet radiation from internal UV lights, bacteria could then be introduced, and other elements added as needed.

 

Such a shell would ensure that the process of creating of an atmosphere could be carefully controlled and none would be lost before the process was complete.

 

 

 


Io

With a mean radius of 1821.6 ± 0.5 km, and an average distance (semi-major axis) of 421,700 km from Jupiter, Io is the innermost of the Galileans.

 

Because of this, Io is completely enveloped by Jupiter's powerful magnetic field, which also why the surface is exposed to significant amounts of harmful radiation.

 

In fact, Io receives an estimated 3,600 rem (36 Sv) of ionizing radiation per day, whereas living organisms here on Earth experience an average of 24 rem per year!
 

 

Enhanced-color image of Jupiter's moon Io,

showing sulfur dioxide frost (white and grey)

and other types of sulfur deposits as yellow and brown.

Recent volcanic activity is marked

by red and black blotches.

Credit: NASA

 


The moon has the shortest orbital period of any of the Galileans, taking roughly 42.5 hours to complete a single orbit around the gas giant.

 

The moon's 2:1 and 4:1 orbital resonance with Europa and Ganymede (see below) also contributes to its orbital eccentricity of 0.0041, which is the primary reason for Io's geologic activity.

With a mean density of 3.528 ± 0.006 g/cm3, Io has the highest density of any moon in the Solar System, and is significantly denser than the other Galilean Moons. Composed primarily of silicate rock and iron, it is closer in bulk composition to the terrestrial planets than to other satellites in the outer Solar System, which are mostly composed of a mix of water ice and silicates.

Unlike its Jovian cousins, Io has no warm-water ocean beneath its surface.

 

In fact, based on magnetic measurements and heat-flow observations, a magma ocean is believed to exist some 50 km below the surface, which itself is about 50 km thick and makes up 10% of the mantle. It is estimated that the temperature in the magma ocean reaches 1473 K (1200 °C/2192 °F).

The main source of internal heat that allows for this comes from tidal flexing, which is the result of Io's orbital resonance with Europa and Ganymede.

 

The friction or dissipation produced in Io's interior due to this varying tidal pull creates significant tidal heating within Io's interior, melting a significant amount of Io's mantle and core.
 

 

 

 

 


This heat is also responsible for Io's volcanic activity and its observed heat flow, and periodically causes lava to erupt up to 500 km (300 mi) into space.

 

Consistently, the surface of is covered in smooth plains dotted with tall mountains, pits of various shapes and sizes, and volcanic lava flows.

 

It's colorful appearance (a combination of orange, yellow, green, white/grey, etc.) is also indicative of volcanic activity which has covered the surface in sulfuric and silicate compounds and leads to surface renewal.

Io contains little to no water, though small pockets of water ice or hydrated minerals have been tentatively identified, most notably on the northwest flank of the mountain Gish Bar Mons. In fact, Io has the least amount of water of any known body in the Solar System, which is likely due to Jupiter being hot enough early in the evolution of the Solar System to drive volatile materials like water off its surface.

Taken together, all of this adds up to Io being a total non-starter when it comes to terraforming or settlement.

 

The planet is far too hostile, far too dry, and far too volcanically active to ever be turned into something habitable!
 

 

 

 

Europa

Europa, by contrast, has a lot of appeal for proponents of terraforming.

 

If Io could be characterized as hellish, lava-spewing place (and it certainly can!), then Europa would be calm, icy and watery by comparison.

 

With a mean radius of about 1560 km and a mass of 4.7998 ×1022 kg, Europa is also slightly smaller than Earth's Moon, which makes it the sixth-largest moon and fifteenth largest object in the Solar System.
 

 

 

 

 


It's orbit is nearly circular, with a eccentricity of 0.09, and lies at an average distance of 670 900 km from Jupiter. The moon takes 3.55 Earth days to complete a single orbit around Jupiter, and is tidally locked with the planet (though some theories say that this may not be absolute).

 

At this distance from Jupiter, Europa still experiences quite a bit of radiation, averaging about 540 rem per day.

Europa is significantly more dense than the other Galilean Moons (except for Io), which indicates that its interior is differentiated between a rock interior composed of silicate rock and a possible iron core. Above this rocky interior is layer of water ice that is estimated to be around 100 km (62 mi) thick, likely differentiated between a frozen upper crust and a liquid water ocean beneath.

If present, this ocean is likely a warm-water, salty ocean that contains organic molecules, is oxygenated, and heated by Europa's geologically-active core.

 

Given the combination of these factors, it is considered a strong possibility that organic life also exists in this ocean, possibly in microbial or even multi-celled form, most likely in environments similar to Earth's deep-ocean hydrothermal vents.

Because of its abundant water, which comes in both liquid and solid form, Europa is a popular candidate for proponents of colonization and terraforming.

 

Using nuclear devices, cometary impacts, or some other means to increase the surface temperature, Europa's surface ice could be sublimated and form a massive atmosphere of water vapor.

 

 

 

 

This vapor would then undergo radiolysis due to exposure to Jupiter's magnetic field, converting it into oxygen gas (which would stay close to the planet) and hydrogen that would escape into space.

 

The resulting planet would be an ocean world, where floating settlements could be built that floated across the surface (due to oceans depths of ~100 km, they could not be anchored).

 

Because Europa is tidally-locked, these colonies could move from the day-side to the night-side in order to create the illusion of a diurnal cycle.

 

 

 


Ganymede

Ganymede's is the third most distant moon from Jupiter, and orbits at an average distance (semi-major axis) of 1,070,400 km - varying from 1,069,200 km at periapsis to at 1,071,600 km apoapsis.

 

At this distance, it takes seven days and three hours to completes a single revolution. Like most known moons, Ganymede is tidally locked, with one side always facing toward the planet.

With a mean radius of 2634.1 ± 0.3 kilometers (the equivalent of 0.413 Earths), Ganymede is the largest moon in the Solar System, even larger than the planet Mercury.

 

However, with a mass of 1.4819 x 10²³ kg (the equivalent of 0.025 Earths), it is only half as massive, which is due to its composition, which consists of water ice and silicate rock.

Ganymede is considered another possible candidate for human settlement - and even terraforming - for several reasons. For one, as Jupiter's largest moon, Ganymede has a gravitational force of 1.428 m/s2 (the equivalent of 0.146 g) which is comparable to Earth's Moon.

 

Sufficient enough to limit the effects of muscle and bone degeneration, this lower gravity also means that the moon has a lower escape velocity - which means it would take considerably less fuel for rockets to take off from the surface.
 

 


Artist's cut-away representation

of the internal structure of Ganymede.

Credit: Wikipedia Commons/kelvinsong
 


What's more, the presence of a magnetosphere means that colonists would be better shielded from cosmic radiation than on other bodies, and more shielded from Jupiter's radiation than Europa or Io.

 

All told, Ganymede receives about 8 rem of radiation per day - a significant reduction from Europa and Io, but still well above human tolerances.

The prevalence of water ice means that colonists could also produce breathable oxygen, their own drinking water, and would be able to synthesize rocket fuel. Like Europa, this could be done by heating up the surface through various means, sublimating the water ice, and allowing radiolysis to convert it into oxygen.

 

Again, the result would be an ocean world, but one with significantly deeper oceans (~800 km).

And then there is the distinct possibility that Ganymede, like Europa, has an interior ocean due to the heat created by tidal flexing in its mantle. This heat could be transferred into the water via hydrothermal vents, which could provide the necessary heat and energy to sustain life.

 

Combined with oxygenated water, life forms could exist at the core-mantle boundary in the form of extremophiles, much like on Europa.

 

 

 


Callisto

Callisto is the outermost of the Galileans, orbiting Jupiter at an average distance (semi-major axis) of 1,882,700 km.

 

With a mean radius of 2410.3 ± 1.5 km (0.378 Earths) and a mass of 1.0759 × 1023 kg (0.018 Earths), Callisto is the second largest of Jupiter's moons (after Ganymede) and the third largest satellite in the solar system.

 

It is similarly comparable in size to Mercury - being 99% as large - but due to its mixed composition, it has less than one-third of Mercury mass.
 

 


Model of Callisto's internal structure

showing a surface ice layer,

a possible liquid water layer, and an ice–rock interior.

Credit: NASA/JPL
 


Compared to the other Galileans, Callisto presents numerous advantages as far as colonization is concerned.

 

Much like the others, the moon has an abundant supply of water in the form of surface ice (but also possibly liquid water beneath the surface). But unlike the others, Callisto's distance from Jupiter means that colonists would have far less to worry about in terms of radiation.

 

In fact, with a surface exposure of about 0.01 rem a day, Callisto is well within human tolerances.

Much like Europa and Ganymede, and Saturn's moons of Enceladus, Mimas, Dione, Titan, the possible existence of a subsurface ocean on Callisto has led many scientists to speculate about the possibility of life.

 

This is particularly likely if the interior ocean is made up of salt-water, since halophiles (which thrive in high salt concentrations) could live there.

However, the environmental conditions necessary for life to appear (which include the presence of sufficient heat due to tidal flexing) are more likely on Europa and Ganymede. The main difference is the lack of contact between the rocky material and the interior ocean, as well as the lower heat flux in Callisto's interior.

 

In essence, while Callisto possesses the necessary pre-biotic chemistry to host life, it lacks the necessary energy.

Like Europa and Ganymede, the process of terraforming Callisto would involve heating up the surface in order to sublimate the surface ice and create an atmosphere, one which produces oxygen through radiolysis.

 

The resulting world would be an ocean planet, but with oceans that reached to depths of between 130 and 350 km.

 

 


 

 

 

Potential Challenges

Okay, we've covered the potential methods and targets, which means its time for the bad news.

 

To break it down, converting one or more of the Galileans into something habitable to humans presents many difficulties, some of which may prove to be insurmountable.

 

These include, but are are not limited to:

  • Distance

  • Resources/Infrastructure

  • Natural Hazards

  • Sustainability

  • Ethical Considerations

Basically, the Jovian system is pretty far from Earth.

 

On average, the distance between Jupiter and Earth is 628,411,977 million km (4.2 AU), roughly four times the distance between the Earth and the Sun. To put that into perspective, it took the Voyager probes between 18 months and two years to reach Jupiter from Earth.

 

Ships designed to haul human passengers (with enough supplies and equipment to sustain them) would be much larger and heavier, which would make the travel time even longer.

In addition, depending on the method used, transforming the surfaces of Europa, Ganymede, and/or Callisto could require harvesting asteroids from the Main Belt or from Jupiter's Trojans and Greeks. And since missions to this region of space would need to haul back several tons of icy cargo, they too would need powerful propulsion systems to make the journey in a reasonable amount of time.

Ergo, any vessels transporting human crews to the Jovian system would likely have to rely on cryogenics or hibernation-related technology in order to be smaller, faster and more cost-effective.

 

While this sort of technology is being investigated for crewed missions to Mars, it is still very much in the research and development phase.
 

 


The Crew Transfer Vehicle (CTV)

using its nuclear-thermal rocket engines

to slow down and establish orbit around Mars.

Credit: NASA
 


As for transport missions to and from the Asteroid Belt, these could be equipped with systems like Nuclear-Thermal Propulsion (NTP), Fusion-drive systems, or some other advanced concept.

 

But thus far, no such drive systems exist, with some being decades or more away from feasibility.

Also, all this talk of transport and space hauling brings up the second aspect of this challenge, which is the problem of infrastructure. In order to mount multiple crewed missions to the Jovian system, as well as asteroid retrieval missions, a considerable amount of infrastructure would be needed that either does not exist or is severely lacking.

This includes having lots of spaceships, which would also need advanced propulsion systems.

 

Just as important is the need for refueling and supply stations between Earth and the Jovian System - like an outpost on the Moon, a permanent base on Mars, and bases on Ceres and in the Asteroid Belt.

Where "Shell Worlds" are concerned, the challenge remains the same.

 

Building an enveloping structure big enough for an entire moon - which range from 3121.6 km to 5262.4 km in diameter - would require massive amounts of material.

 

While these could be harvested from the nearby Asteroid Belt, it would require thousands of ships and robot workers to mine, haul, and assemble the minerals into large enough shells.
 

 


The magnetic field of Jupiter

and co-rotation enforcing currents.

Credit: Wikipedia Commons/Ruslik0
 


Third, radiation would be a significant issue for humans living on Europa or Ganymede.

 

As noted already, Earth organisms are exposed to an average of 24 rem per year, which works out to 0.0657 rem per day. An exposure of approximately 75 rems over a period of a few days is enough to cause radiation poisoning, while about 500 rems over a few days would be fatal.

 

Of all the Galileans, only Callisto falls beneath this terminal limit.

As a result, any settlements established on Europa or Ganymede would require radiation shielding, even after the creation of viable atmospheres. This in turn would require large shields to be built in orbit of the moons (requiring another massive investment in resources), or would dictate that all settlements built on the surfaces include heavy radiation shielding.

On top of that, as the surfaces of Europa, Ganymede and Callisto (especially Callisto!) will attest, the Jovian system is frequented by space rocks. In fact, most of Jupiter's satellites are asteroids it picked up as they sailed through the system.

 

These satellites are lost on a regular basis, and new ones are added all the time.

 

So colonists would naturally have to worry about space rocks slamming into their ocean world, causing massive waves and blotting out the sky with thick clouds of water vapor.

Fourth, the issue of sustainability, has to do with the fact that all of the Jovian moons either do not have a magnetosphere or, in the case of Ganymede, are not powerful enough to block the effects of Jupiter's magnetic field. Because of this, any atmosphere created would be slowly stripped away, much as Mars' atmosphere was slowly stripped away after it lost its magnetosphere about 4.3 billion years ago.

 

In order to maintain the effects of terraforming, colonists would need to replenish the atmosphere over time.
 

 


Illustration of Jupiter

and the Galilean satellites.

Credit: NASA
 


Another aspect of sustainability, one which is often overlooked, has to do with the kinds of planets that would result from terraforming.

 

While estimates vary, transforming Europa, Ganymede and Callisto would result in oceans that varied in depth - from 100 km (in the case of Europa) to extreme depths of up to 800 km (in the case of Ganymede).

 

In contrast, the greatest depth ever measured here on Earth was only about 10 km (6 miles) deep, in the Pacific's Mariana Trench.

With oceans this deep, all settlements would have to take the form of floating cities that could not be anchored to solid ground. And in the case of Ganymede, the oceans would account for a considerable portion of the planet. What the physicals effects of this would be are hard to imagine.

 

But it is a safe bet that they would result in tremendously high tides (at best) to water being lost to space.

And finally, there is the issue of the ethics of terraforming.

 

If, as scientists currently suspect, there is in fact indigenous life on one or more of the Jovian moons, then the effects of terraforming could have severe consequences or them. For instance, if bacterial life forms exist on the underside of Europa's icy surface, then melting it would mean death for these organisms, since it would remove their only source of protection from radiation.

Life forms that exist close to the core-mantle boundary, most likely around hydrothermal vents, would be less effected by the presence of humans on the surface. However, any changes to the ecological balance could lead to a chain reaction that would destroy the natural life cycle.

 

And the presence of organisms introduced by humans (i.e. germs), could have a similarly devastating effect.
 

 


Artist's impression of a hypothetical ocean cryobot

(a robot capable of penetrating water ice)

in Europa.

Credit: NASA
 


So basically, if we choose to alter the natural environment of one or more of the Jovian moons, we will effectively be risking the annihilation of any indigenous life forms.

 

Such an act would be tantamount to genocide (or xenocide, as the case may be), and exposure to alien organisms would surely pose health risks for human colonists as well.

 

 

 


Conclusions

All in all, it appears that terraforming the outer Solar System might be a bit of a non-starter. While the prospect of doing it is certainly exciting, and presents many interesting opportunities, the challenges involved do seem to add up.

 

For starters, it doesn't seem likely or practical for us to contemplate doing this until we've established a presence on,

  • the Moon

  • Mars

  • the Asteroid Belt

Second, terraforming any of Jupiter's moons would involve a considerable amount of time, energy and resources.

 

And given that a lot of these moon's resources could be harvested for terraforming other worlds (such as Mars and Venus), would it not make sense to terraform these worlds first and circle back to the outer Solar System later?

Third, a terraformed Europa, Ganymede and Callisto would all be water worlds with extremely deep oceans.

  • Would it even be possible to build floating cities on such a world?

  • Or would they be swallowed up by massive tidal waves; or worse, swept off into space by waves so high, they slipped the bonds of the planet's gravity?

  • And how often would the atmosphere need to be replenished in order to ensure it didn't get stripped away?

 

 

 

And last, but not least, any act of terraforming these moons would invariably threaten any life that already exists there.

 

And the threat caused by exposure wouldn't exactly be one-way. Under all of these circumstances, would it not be better to simply establish outposts on the surface, or perhaps within or directly underneath the ice?

All valid questions, and ones which we will no doubt begin to explore once we start mounting research missions to Europa and the other Jovian moons in the future. And depending on what we find there, we might just choose to put down some roots.

 

And in time, we might even begin thinking about renovating the places so more of our kin can drop by.

 

Before we do any of that, we had better make sure we know what we're doing, and be sure we aren't doing any harm in the process!










 

 


 

How Do We Terraform...

Saturn's Moons?
by Matt Williams
27 April, 2016

from UniverseToday Website

 

 

 

 

 


Around the distant gas giant Saturn lies a system of rings and moons that is unrivaled in terms of beauty.

 

Within this system, there is also enough resources that if humanity were to harness them - i.e. if the issues of transport and infrastructure could be addressed - we would be living in an age a post-scarcity.

 

But on top of that, many of these moons might even be suited to terraforming, where they would be transformed to accommodate human settlers.

As with the case for terraforming Jupiter's moons (far above article), or the terrestrial planets of Mars and Venus, doing so presents many advantages and challenges. At the same time, it presents many moral and ethical dilemmas.

 

And between all of that, terraforming Saturn's moons would require a massive commitment in time, energy and resources, not to mention reliance on some advanced technologies (some of which haven't been invented yet).
 

 

 


The Cronian Moons

All told, Saturn system is second only to Jupiter in terms of its number of satellites, with 62 confirmed moons.

 

Of these, the largest moons are divided into two groups: the inner large moons (those that orbit close to Saturn within its tenuous E-Ring) and the outer large moons (those beyond the E-Ring).

 

They are, in order of distance from Saturn,

 


Saturn's moons.

Credit: NASA/JPL
 


These moons are all composed primarily of water ice and rock, and are believed to be differentiated between a rocky core and an icy mantle and crust.

 

Among them, Titan is appropriately named, being the largest and most massive of all the inner or outer moons (to the point that it is larger and more massive than all the others combined).

In terms of their suitability for human habitation, each one present its own share of pros and cons. These include their respective sizes and compositions, the presence (or absence) of an atmosphere, gravity, and the availability of water (in ice form and subsurface oceans).

 

And in the end, it is the presence of these moons around Saturn that makes the system an attractive option for exploration and colonization.

As aerospace engineer and author Robert Zubrin stated in his book Entering Space: Creating a Spacefaring Civilization, Saturn, Uranus and Neptune could one day become "the Persian Gulf of the Solar System", due to their abundance of hydrogen and other resources.

 

Of these systems, Saturn would be the most important, thanks to its relative proximity to Earth, low radiation, and excellent system of moons.

 

 


 

 

 

Possible Methods

Terraforming one or more of Jupiter's moons would be a relatively straightforward process.

 

In all cases, this would involve heating the surfaces through various means - like thermonuclear devices, impacting the surface with asteroids or comets, or focusing sunlight with orbital mirrors - to the point that surface ice would sublimate, releasing water vapor and volatiles (such as ammonia and methane) to form an atmosphere.

However, due to the comparatively low amounts of radiation coming from Saturn (compared to Jupiter), these atmospheres would have to be converted to a nitrogen-oxygen rich environment through means other than radiolysis.

 

This could be done by using the same orbital mirrors to focus sunlight onto the surfaces, triggering the creation of oxygen and hydrogen gas from water ice through photolysis.

 

While the oxygen would remain closer to the surface, the hydrogen would escape into space.

The presence of ammonia in many of the moon's ices would also mean that a ready supply of nitrogen could be created to act as a buffer gas. By introducing specific strains of bacteria into the newly created atmospheres - such as the Nitrosomonas, Pseudomonas and Clostridium species - the sublimated ammonia could be converted into nitrites (NO²-) and then nitrogen gas.

Another option would be to employ a process known as "paraterraforming" - where a world is enclosed (in whole or in part) in an artificial shell in order to transform its environment.

 

In the case of the Cronian moons, this would involve building large "Shell Worlds" to encase them, keeping the newly-created atmospheres inside long enough to effect long-term changes.
 

 


An engineer suggests building a roof over a small planet

 so that Earthlike conditions could be maintained.

Credit: Karl Tate/Infographics Artist
 


Within this shell, a Cronian moon could have its temperatures slowly raised, the water-vapor atmospheres could be exposed to ultra-violet radiation from internal UV lights, bacteria could then be introduced, and other elements added as needed.

 

Such a shell would ensure that the process of creating of an atmosphere could be carefully controlled and none would be lost before the process was complete.

 

 

 


Mimas

With a diameter of 396 km and a mass of 0.4×1020 kg, Mimas is the smallest and least massive of these moons.

 

It is ovoid in shape and orbits Saturn at a distance of 185,539 km with an orbital period of 0.9 days. The low density of Mimas, which is estimated to be 1.15 g/cm³ (just slightly higher than that of water), indicates that it is composed mostly of water ice with only a small amount of rock.

As a result of this, Mimas is not a good candidate for terraforming.

 

Any atmosphere that could be created by melting its ice would likely be lost to space. In addition, its low density would mean that the vast majority of the planet would be ocean, with only a small core of rock.

 

This, in turn, makes any plans to settle on the surface impractical.

 

 

 


Enceladus

Enceladus, meanwhile, has a diameter of 504 km, a mass of 1.1×1020 km and is spherical in shape.

 

It orbits Saturn at a distance of 237,948 km and takes 1.4 days to complete a single orbit. Though it is one of the smaller spherical moons, it is the only Cronian moon that is geologically active - and one of the smallest known bodies in the Solar System where this is the case.

 

This results in features like the famous "tiger stripes" - a series of continuous, ridged, slightly curved and roughly parallel faults within the moon's southern polar latitudes.
 

 


The "tiger stripes" of Enceladus

as pictured by the Cassini space probe.

Credit: NASA/JPL/ESA
 


Large geysers have also been observed in the southern polar region that periodically release plumes of water ice, gas and dust which replenish Saturn's E-ring.

 

These jets are one of several indications that Enceladus has liquid water beneath it's icy crust, where geothermal processes release enough heat to maintain a warm water ocean closer to its core.

The presence of a warm-water liquid ocean makes Enceladus an appealing candidate for terraforming.

 

The composition of the plumes also indicate that the subsurface ocean is salty, and contains organic molecules and volatiles. These include ammonia and simple hydrocarbons like methane, propane, acetylene, and formaldehyde.

Ergo, once the icy surface was sublimated, these compounds would be released, triggering a natural greenhouse effect.

 

Combined with photolysis, radiolysis, and bacteria, the water vapor and ammonia could also be converted to a nitrogen-oxygen atmosphere. The higher density of Enceladus (~1.61 g/cm3) indicates that it has a larger than average silicate and iron core (for a Cronian moon).

 

This could provide materials for any operations on the surface, and also means that if the surface ice were to be sublimated, Enceladus would not consist mainly of incredibly deep oceans.

However, the presence of this liquid salt-water ocean, organic molecules and volatiles also indicates that the interior of Enceladus experiences hydrothermal activity.

 

This energy source, combined with organic molecules, nutrients, and the prebiotic conditions for life, means that is possible that Enceladus is home to extraterrestrial life.

 

 


Gravity measurements by

NASA's Cassini spacecraft and Deep Space Network

suggest that Saturn's moon Enceladus,

which has jets of water vapor and ice gushing from its south pole,

also harbors a large interior ocean beneath an ice shell,

as this illustration depicts.
Artist's depiction of Enceladus' interior ocean

and the jets of water vapor that periodically erupt

from its southern polar region.

Credit: NASA/JPL-Caltech
 


Much like Europa and Ganymede, these would probably take the form of extremophiles living in environments similar to Earth's deep-ocean hydrothermal vents.

 

As a result, terraforming Enceladus could result in the destruction of the natural life cycle on the moon, or release life forms that could prove harmful to any future colonists.

 

 

 


Tethys

At 1066 km in diameter, Tethys is the second-largest of Saturn's inner moons and the 16th-largest moon in the Solar System.

 

The majority of its surface is made up of heavily cratered and hilly terrain and a smaller and smoother plains region. Its most prominent features are the large impact crater of Odysseus, which measures 400 km in diameter, and a vast canyon system named Ithaca Chasma - which is concentric with Odysseus and measures 100 km wide, 3 to 5 km deep and 2,000 km long.

With a mean density of 0.984 ± 0.003 grams per cubic centimeter, Tethys is believed to be comprised almost entirely of water ice.

 

It is not currently known whether Tethys is differentiated into a rocky core and ice mantle. However, given the fact that rock accounts for less 6% of its mass, a differentiated Tethys would have a core that did not exceed 145 km in radius.

 

On the other hand, Tethys' shape - which resembles that of a triaxial ellipsoid - is consistent with it having a homogeneous interior (i.e. a mix of ice and rock).
 

 

 

 

Because of this, Tethys is also off the terraforming list. If in fact it has a tiny rocky interior, treating the surface to heating would mean that the vast majority of the moon would melt and be lost to space.

 

Alternately, if the interior is a homogeneous mix of rock and ice, then all that would remain after melting occurred would be a cloud of debris.

 

 

 


Dione

With a diameter and mass of 1,123 km and 11×1020 kg, Dione is the fourth largest moon of Saturn.

 

The majority of Dione's surface is heavily cratered old terrain, with craters that measure up to 250 km in diameter. With an orbital distance of 377,396 km from Saturn, the moon takes 2.7 days to complete a single rotation.

Dione's mean density of about 1.478 g/cm³ indicates that it is composed mainly of water ice, with a small remainder likely consisting of a silicate rock core. Dione also has a very thin atmosphere of oxygen ions (O+²), which was first detected by the Cassini space probe in 2010.

 

While the source of this atmosphere is currently unknown, it is believed that it is the product of radiolysis, where charged particles from Saturn's radiation belt interact with water ice on the surface to create hydrogen and oxygen (similar to what happens on Europa).

Because of this tenuous atmosphere, it is already known that sublimating Dione's ice could produce an oxygen atmosphere.

 

However, it is not currently known if Dione possesses the right combination of volatilizes to ensure that nitrogen gas can be created, or that a greenhouse effect will be triggered.

 

Combined with Dione's low density, this makes it an unattractive target for terraforming.
 

 


Saturn's moon Dione,

with Saturn's rings visible in the background.

Credit: NASA/JPL

 

 

 


Rhea

Measuring 1,527 km in diameter and 23×1020 kg in mass, Rhea is the second largest of Saturn's moons and the ninth largest moon of the Solar System.

 

With an orbital radius of 527,108 km, it is the fifth-most distant of the larger moons, and takes 4.5 days to complete an orbit. Like other Cronian satellites, Rhea has a rather heavily cratered surface, and a few large fractures on its trailing hemisphere.

With a mean density of about 1.236 g/cm³, Rhea is estimated to be composed of 75% water ice (with a density of roughly 0.93 g/cm³) and 25% of silicate rock (with a density of around 3.25 g/cm³).

 

This low density means that although Rhea is the ninth-largest moon in the Solar System, it is also the tenth-most massive.

In terms of its interior, Rhea was originally suspected of being differentiated between a rocky core and an icy mantle. However, more recent measurements would seem to indicate that Rhea is either only partly differentiated, or has a homogeneous interior - likely consisting of both silicate rock and ice together (similar to Jupiter's moon Callisto).

Models of Rhea's interior also suggest that it may have an internal liquid-water ocean, similar to Enceladus and Titan.

 

This liquid-water ocean, should it exist, would likely be located at the core-mantle boundary, and would be sustained by the heating caused from the decay of radioactive elements in its core.

 

Interior ocean or not, the fact that the vast majority of the moon is composed of ice water makes it an unattractive option for terraforming.
 

 


Views of Saturn's moon Rhea.

Credit: NASA/JPL/Space Science Institute

 

 

 


Titan

As already noted, Titan is the largest of the Cronian moons.

 

In fact, at 5,150 km in diameter, and 1,350×1020 kg in mass, Titan is Saturn's largest moon and comprises more than 96% of the mass in orbit around the planet.

 

Based on its bulk density of 1.88 g/cm3, Titan's composition is half water ice and half rocky material - most likely differentiated into several layers with a 3,400 km rocky center surrounded by several layers of icy material.

It is also the only large moon to have its own atmosphere, which is cold, dense, and is the only nitrogen-rich dense atmosphere in the Solar System aside from Earth's (with small amounts of methane). Scientists have also noted the presence of polycyclic aromatic hydrocarbons in the upper atmosphere, as well as methane ice crystals.

 

Another thing Titan has in common with Earth, unlike every other moon and planet in the Solar System, is atmospheric pressure. On the surface of Titan, the air pressure is estimated to be around 1.469 bars (1.45 times that of Earth).

The surface of Titan, which is difficult to observe due to persistent atmospheric haze, shows only a few impact craters, evidence of cryovolcanos, and longitudinal dune fields that were apparently shaped by tidal winds.

 

Titan is also the only body in the Solar System beside Earth with bodies of liquid on its surface, in the form of methane-ethane lakes in Titan's north and south polar regions.

With an orbital distance of 1,221,870 km, it is the second-farthest large moon from Saturn, and completes a single orbit every 16 days. Like Europa and Ganymede, it is believed that Titan has a subsurface ocean made of water mixed with ammonia, which can erupt to the surface of the moon and lead to cryovolcanism.

 

The presence of this ocean, plus the prebiotic environment on Titan, has led some to suggest that life may exist there as well.
 

 


Titan's dense, hydrocarbon rich atmosphere

remains a focal point of scientific research.

Credit: NASA
 


Such life could take the form of microbes and extremophiles in the interior ocean (similar to what is thought to exist on Enceladus and Europa), or could take the even more extreme form of methanogenic life forms.

 

As has been suggested, life could exist in Titan's lakes of liquid methane just as organisms on Earth live in water. Such organisms would inhale dihydrogen (H²) in place of oxygen gas (O²), metabolize it with acetylene instead of glucose, and then exhale methane instead of carbon dioxide.

However, NASA has gone on record as stating that these theories remain entirely hypothetical. So while the prebiotic conditions that are associated with organic chemistry exist on Titan, life itself may not.

 

However, the existence of these conditions remains a subject of fascination among scientists.

 

And since its atmosphere is thought to be analogous to Earth's in the distant past, proponents of terraforming emphasize that Titan's atmosphere could be converted in much the same way.

Beyond that, there are several reasons why Titan is a good candidate. For starters, it possess an abundance of all the elements necessary to support life (atmospheric nitrogen and methane), liquid methane, and liquid water and ammonia.

 

Additionally, Titan has an atmospheric pressure one and a half times that of Earth, which means that the interior air pressure of landing craft and habitats could be set equal or close to the exterior pressure.

This would significantly reduce the difficulty and complexity of structural engineering for landing craft and habitats compared with low or zero pressure environments such as on the Moon, Mars, or the Asteroid Belt.

 

The thick atmosphere also makes radiation a non-issue, unlike with other planets or Jupiter's moons.
 

 

 

 

And while Titan's atmosphere does contain flammable compounds, these only present a danger if they are mixed with sufficient enough oxygen - otherwise, combustion cannot be achieved or sustained.

 

Finally, the very high ratio of atmospheric density to surface gravity also greatly reduces the wingspan needed for aircraft to maintain lift.

With all these things going for it, turning Titan into a livable world would be feasible given the right conditions.

 

For starters, orbital mirrors could be used to direct more sunlight onto the surface. Combined with the moon's already dense and greenhouse gas-rich atmosphere, this would lead to a considerable greenhouse effect that would melt the ice and release water vapor into the air.

Once again, this could be converted into a nitrogen/oxygen-rich mix, and more easily than with other Cronian moons since the atmosphere is already very rich in nitrogen. The presence of nitrogen, methane and ammonia could also be used to produce chemical fertilizers to grow food.

 

However, the orbital mirrors would need to remain in place in order to ensure the environment did not become extremely cold again and revert to an icy state.

 

 

 


Iapetus

At 1,470 km in diameter and 18×1020 kg in mass, Iapetus is the third-largest of Saturn's large moons.

 

And at a distance of 3,560,820 km from Saturn, it is the most distant of the large moons, and takes 79 days to complete a single orbit. Due to its unusual color and composition - its leading hemisphere is dark and black whereas its trailing hemisphere is much brighter - it is often called the "yin and yang" of Saturn's moons.
 

 

 

 

 


With an average distance (semi major axis) of 3,560,820 km, Iapetus takes 79.32 days to complete an single orbit of Saturn. Despite being Saturn's third-largest moon, Iapetus orbits much farther from Saturn than its next closest major satellite (Titan).

 

Like many of Saturn's moons - particularly Tethys, Mimas and Rhea - Iapetus has a low density (1.088 ± 0.013 g/cm³) which indicates that it is composed primary of water ice and only about 20% rock.

But unlike most of Saturn's larger moons, its overall shape is neither spherical or ellipsoid, instead consisting of flattened poles and a bulging waistline. Its large and unusually high equatorial ridge also contributes to its disproportionate shape. Because of this, Iapetus is the largest known moon to not have achieved hydrostatic equilibrium.

 

Though rounded in appearance, its bulging appearance disqualifies it from being classified as spherical. Because of this, Iapetus is not a likely contender for terraforming.

 

If in fact its surface were melted, it too would be an ocean world with unrealistically deep seas, and this water would likely be lost to space.

 

 

 


Potential Challenges

To break it down, only Enceladus and Titan appear to be viable candidates for terraforming.

 

However, in both cases, the process of turning them into habitable worlds where human beings could exist without the need for pressurized structures or protective suits would be a long and costly one.

 

And much like terraforming the Jovian moons, the challenges can be broken down categorically:

  • Distance

  • Resources and Infrastructure

  • Hazards

  • Sustainability

  • Ethical Considerations

In short, while Saturn may be abundant in resources and closer to Earth than either Uranus or Neptune, its really very far.

 

On average, Saturn is approximately 1,429,240,400,000 kms away from Earth (or ~8.5 AU the equivalent of eight and a half times the average distance between the Earth and the Sun). To put that in perspective, it took the Voyager 1 probe roughly thirty-eight months to reach the Saturn system from Earth.

 

For crewed spacecraft, carrying colonists and all the equipment needed to terraform the surface, it would take considerably longer to get there.
 

 


This portrait looking down on Saturn and its rings

was created from images obtained by NASA's

Cassini spacecraft on Oct. 10, 2013.

Credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic
 


These ships, in order to avoid being overly large and expensive, would need to rely on cryogenics or hibernation-related technology in order to be smaller, faster and more cost-effective.

 

While this sort of technology is being investigated for crewed missions to Mars, it is still very much in the research and development phase.

 

What's more, a large fleet of robotic spaceships and support craft would also be needed to build the orbital mirrors, capture asteroids or debris to use as impactors, and provide logistical support to crewed spaceships.

Unlike the crewed vessels, which could keep crews in stasis until their arrival, these ships would need to have advanced propulsion systems to ensure that they were able to make the trips to and from the Cronian moons in a realistic amount of time.

 

All of this, in turn, raises the crucial issue of infrastructure. Basically, any fleet operating between Earth and Saturn would require a network of bases between here and there to keep them supplied and fueled.

So really, any plans to terraform Saturn's moons would have to wait upon the creation of permanent bases on the Moon, Mars, the Asteroid Belt, and the Jovian moons.

 

In addition, building orbital mirrors would require considerable amounts of minerals and other resources, many of which could be harvested from the Asteroid Belt or from Jupiter's Trojans.

This process would be punitively expensive by current standards and (again) would require a fleet of ships with advanced drive systems.

 

And paraterraforming using Shell Worlds would be no different, requiring multiple trips to and from the Asteroid Belt, hundreds (if not thousands) of construction and support craft, and all the necessary bases in between.
 

 


Saturn and its moon, Titan,

appear together in this view from the Cassini spacecraft.

Credit: NASA/JPL-Caltech/SSI
 


And while radiation is not a major threat in the Cronian system (unlike around Jupiter), the moons have been subject to a great deal of impacts over the course of their history.

 

As a result, any settlements built on the surface would likely need additional protection in orbit, like a string of defensive satellites that could redirect comets and asteroids before they reached orbit.

Fourth, terraforming Saturn's moons presents the same challenges as Jupiter's. Namely, every moon that was terraformed would be an ocean planet And whereas most of Saturn's moons are untenable due to their high concentrations of water ice, Titan and Enceladus are not that much better off.

 

In fact, if all of Titan's ice were melted, including the layer that is believed to sit beneath its interior ocean, its sea level would be up to 1700 km in depth!

Not only that, but this sea would surround a hydrous core, which would likely make the planet unstable.

 

Enceladus would not fair any better, as gravity measurements by Cassini have shown that the density of the core is low, indicating that the core contains water in addition to silicates. So in addition to a deep ocean on its surface, its core might also be unstable.

And last, there are the ethical considerations. If both Enceladus and Titan are home to extra-terrestrial life, than any efforts to alter their environments could result in their destruction.

 

Barring that, melting the surface ice could cause any indigenous life forms to proliferate and mutate, and exposure to them could prove to be a health hazard for human settlers.
 

 


Saturn's satellites arrange according to scale (top)

and their positions relative to Saturn's ring structures.

Credit: ESA

 

 

 


Conclusions

Once again, when faced with all of these considerations, one is forced to ask,

  • Why bother?

  • Why bother altering the natural environment of the Cronian moons when we could settle on them as is, and use their natural resources to usher in an age of post-scarcity?

Quite literally, there is enough water ice, volatiles, hydrocarbons, organic molecules, and minerals in the Saturn system to keep humanity supplied indefinitely.

What's more, without the effects of terraforming, settlements on Titan and Enceladus would probably be a lot more tenable. We could also fathom building settlements on the moons of Tethys, Dione, Rhea, and Iapetus as well, which would prove much more beneficial in terms of being able to harness the system's resources.

And, as with Jupiter's moons of Europa, Ganymede, and Callisto, foregoing the act of terraforming would mean there would be an abundant supply of resources that could be used to terraform other places - namely, Venus and Mars.

 

As has been argued many times over, the abundance of methane, ammonia, and water ices in the Cronian system would be very useful in helping to turn "Earths twins" into "Earth-like" planets.

Once again, it would seem that the answer to the question "can/should we?" is a disappointing no.