Rhea Simulation

Rhea moon — A terraformed Rhea made by Triniwiki in Universe Sandbox 2.

This is a simulation of what one would expect to find on a terraformed Rhea, using formulas from Math And Terraforming. Please note that not even the supercomputers at NASA can provide us with a perfect simulation. The information showed here is only an approximation.

Basic data[]

Distance from Sun: 1453.53 million km
Distance from Saturn: 0.527 million km
Diameter: 1528 km
Solar Constant: 0.0209
Mass: 0.00039 Earths
Mean density: 1.236 kg/l
Saturn's period: 29.457 Earth years
Day length: 4.518 Earth days
Rotation axial tilt: 17 degrees

Atmosphere[]

See Atmosphere Parameters

Given the very low gravity, it will be difficult for Rhea to hold an atmosphere.

During this simulation, we will use an atmosphere with the same pressure at sea level as Earth's and a similar composition.

Atmosphere stability for oxygen molecules:
- Earth's gravity (15 degrees C): 4.116
- Rhea's gravity (15 degrees C): 64.14
- Rhea's gravity (-150 degrees C): 27.41
Atmosphere stability for water molecules:
- Earth's gravity (15 degrees C): 7.320
- Rhea's gravity (15 degrees C): 128.3
- Rhea's gravity (-150 degrees C): 54.83
Atmosphere stability for hydrogen molecules:
- Earth's gravity (15 degrees C): 65.88
- Rhea's gravity (15 degrees C): 1154
- Rhea's gravity (-150 degrees C): 1034

notes: A value below 10 means stability for over a million years, a value between 10 and 100 means stability between 0.1 and 10 millions of years, while a value higher then 100 means stability for less then 10 thousand years.

This calculation does not include solar wind erosion.

Conclusion: The atmosphere of Rhea will be divided in two distinct layers, separated by a greenhouse gas buffer. In the upper layer, where temperature will be low, oxygen and nitrogen can be held for thousands of years. However, water vapors, if they make their way that far, will be lost into space. In the lower layer, even oxygen is nearly unstable. Water vapors will fast rise to the upper layer. Hydrogen, resulting from interaction between water molecules and ionizing radiation, will escape into space very fast.

The good thing is that Rhea lies within Saturn's magnetosphere, so it will be protected from solar wind erosion.

The atmosphere will look like this:

Ground average temperature: 15 degrees C

Surface pressure at sea level: 1
Atmosphere total mass (Earth = 1): 0.90
Atmosphere breathable height: 352 km
Atmosphere total height: 1050 km

Ground average temperature: -150 degrees C

Surface pressure at sea level: 1
Atmosphere total mass (Earth = 1): 0.56
Atmosphere breathable height: 241 km
Atmosphere total height: 717 km

Combined values

Atmosphere total mass (Earth = 1): 0.73
Atmosphere breathable height: 297 km
Atmosphere total height: 880 km.

As one can see, Rhea will have an atmosphere that extends a bit above its radius above the surface. At that height, gravity is reduced to half, forcing in fact the atmosphere to be even higher. If Rhea will one day be terraformed, the atmosphere will lack stability and will slowly be lost in space. Still, for at least a thousand years, it will remain as it is.

Temperature[]

Main article: Temperature.

The first problem with Rhea is that we need to gain the correct surface temperature. The Solar Constant is small (0.0209), compared to Earth (1.98). We will need Greenhouse Gases. The Greenhouse Calculator shows us that Titan will need 0.656 kg/sqm of sulfur hexafluoride.

Climate Simulation[]

Main article: Climate.

On Earth, the average temperature is +15 degrees C. Technicians will try, with the help of greenhouse gases, to keep this temperature.

Rhea has a smaller diameter then Earth (0.120), so air currents can mix temperatures faster. The atmosphere will be high enough to pass over all Geographic barriers.

Average temperatures for each latitude:

At equinox:

poles: 15.0 C
75 deg: 15.2 C
60 deg: 15.3 C
45 deg: 15.3 C
30 deg: 15.3 C
15 deg: 15.4 C
equator: 15.4 C

At winter solstice:

poles: 14.7 C
75 deg: 14.8 C
60 deg: 15.0 C
45 deg: 15.1 C
30 deg: 15.2 C
15 deg: 15.2 C
equator: 15.2 C

At summer solstice:

poles: 15.2 C
75 deg: 15.2 C
60 deg: 15.2 C
45 deg: 15.2 C
30 deg: 15.2 C
15 deg: 15.2 C
equator: 15.2 C

Day - night cycle variation:

Rhea has a relatively day (4.518 Earth days), but is well protected by its greenhouse layer. So, temperature variations between day and night will not be significant.

Daily temperature variation: 0.3 degrees C
Equator day-night variations:
- Equinox: 15.3 to 15.5 degrees C
- Solstice: 15.1 to 15.3 degrees C
Day - night variations for 45 deg latitude:
- Equinox: 15.2 to 15.4 degrees C
- Winter solstice: 15.0 to 15.2 degrees C
- Summer solstice: 15.1 to 15.3 degrees C

Seasons:

Rhea has its axis tilted with respect to the Sun. However, because of the very strong greenhouse effect, this will result in temperature variations of less then one degree C.

Altitude variations:

Rhea has a Geography dominated by craters. There are some altitude differences, but the atmosphere is very fluffy. Atmospheric pressure does not vary too much with altitude. A mountain that is 10 km high will experience a decrease in temperature like a hill on Earth that is 250 m high. So, temperature variations caused by altitude should be ignored.

Conclusion.

Rhea will experience very little temperature variations, so little that they can be ignored. The temperature will vary between 14.8 and 15.4 on the entire surface. With a temperature difference of only 0.6 degrees C, wind currents will be very slow. As a direct result, air masses will be stagnant. The atmosphere will get filled with moisture up to 100%. Without a decrease in temperature, it will not rain significantly, so the air will be very wet.

In these conditions, we expect the atmosphere to be filled with haze and clouds. Because of the little gravity, rain droplets will fall very slow, further keeping water in the air. It will be such a hazy environment, that you might never see an object that is 100 m in front of you.

Settlers might be tempted to use chemicals to clear the sky or to force rain. This technology is already known. If this will be used, then the atmosphere might be periodically cleaned of excess moisture. An artificial climate cycle can be created.

Geography[]

It appears that Rhea is made of 75% water ice and 25% rock. The moon is not differentiated, it has a homogenous structure.

Terraformers have 4 major ways to transform an icy Outer Planet:

Increase the heat, melt the ice and transform it into an Oceanic Planet, then leave it as it is.
If possible, build Artificial Continents after melting all the ice.
Use Ground Insulation, to save the icy crust, then cover it with solid rock.
Heat the moon, until solid particles from the molten ice will form a natural insulation above the ice crust (see Direct Icy Body Terraforming).

If we go for the first way of terraforming, the moon will need huge amounts of heat, to melt all its ice. In the end, we will have a very deep ocean and a rocky core. In order to do so, we need to wait a long period of time, because only little heat reaches the moon from the Sun. Then, it is possible to do the second step, adding artificial continents.

Because Rhea is made 75% of water ice, the third option could be possible (even if expensive). We could use whatever materials are on the surface to form an insulation layer. Because of the high amounts of water ice that need to be melted to extract materials needed for this, the moon will have large oceans. Construction of the insulation layer can be done more easy with genetically modified organisms (assuming such a technology will be invented in the future).

The last option, which is possible for Iapetus, might not be possible for Rhea. Iapetus has large amounts of dark materials on the surface (dirty ice), which can naturally form an insulation layer.

The third option seems the most feasible. If we use it, then most of the surface will be covered by a planetary ocean, while the rest will be islands, high mountains that reach the surface.

Ocean:

Rhea will have a global ocean, that will reach a few km deep. Because of the lack of temperature variations, the ocean will lack currents. Even on the surface, there will be only very small waves. Some tides can be produced by the passing of other moons.

Because of the lack of currents, deep water will not have enough oxygen. It will be a deep, dead sea, with only the surface containing algae and fish.

Ground:

Those places that will rise above water surface will be the high mountains and crater rims. Terrain will be rough and settlers will use these islands to build their towns.

Conclusion:

Rhea will be almost an oceanic world, protected by a layer of ground insulation. Heat transfer from the ocean to the icy crust will occur at a slow rate, so that the oceans will get deeper. At the same time, bacteria will increase the ground insulation layer, slowing the process.

Rare islands will be places where settlers will prefer to stay. They might appear like the mountain islands of the Pacific, only that they will be always covered by a dense fog.

The Sky[]

As any Outer Planet, Rhea will have a lot of moisture in its atmosphere. Assuming that temperature variations will never exceed one degrees Celsius, fogs and clouds will never vanish. There will never be a piece of blue sky unless people will use chemicals to clean the atmosphere.

Still, from orbit, some celestial bodies will be visible.

The Sun will appear 0.97 units wide (like an object 0.97 mm wide will appear if you look from a distance of 1 m, see Angular Size for details).

Saturn and other satellites visible as disks will be:

Saturn - 221 units
Mimas - 0.56 to 1.16 units
Enceladus - 0.66 to 1.74 units
Tethys - 1.29 to 4.58 units
Dione - 1.24 to 7.49 units
Titan - 2.94 to 7.41 units
Iapetus - 0.36 to 0.48 units

Some planets will also be visible, with a Magnitude as follows:

Mercury: 4.6 to 4.8
Venus: 1.7 to 2.0
Earth: 2.9 to 3.4
Mars: 5.9 to >6
Jupiter: -0.2 to 2.4

However, people from the surface will not see this, unless they fly into outer space.

Human Colonies[]

Population limit: 0.79 million
Land population feeding capacity: 3.3 people fed from one square km
Largest city supported by environment: 3 100 people

Assuming it will have similar types of terrain Earth will have, Rhea can support a Population Limit of 0.79 million people.

An outer planet is in a critical balance. An increase of heat form the surface can result in a hole in the greenhouse layer. This is why population limit is calculated to be so small.

So, Rhea will be sparsely populated. Only towns not rarely exceeding 3000 inhabitants will be possible on the moon. The population can be divided into 316 towns with 2500 inhabitants each. We can imagine these towns spread across the islands, separated by the ocean.

Industry[]

Rhea has the advantage that it is undifferentiated. Because of this, heavy elements are not all compressed into the nucleus. Many of them can be found close to the surface, locked in ice. So, mining will be an option here.

Having minerals available, industrial corporations can develop on the surface. They will be separated by large distances, reducing the effects of Pollution.

Rhea is a small and fragile world, so it will not be a place for large metallurgic corporations. Instead, it will be a good place for manufacturing companies.

Agriculture[]

The islands will have little surface. However, the moon will have large oceans, where fish and algae can develop. This might be the main source of food.

Transportation[]

With little gravity, air transport will be efficient and used on large scale. This will be the main and most efficient way to move people around, while cargo will be shipped mostly by water.

Roads and railways will only be used on a small scale, on the islands.

Since Rhea is tidal locked, there is no place for geosynchronous satellites. Still, there are many safe orbits around the moon for satellites that will provide telecommunications. The large atmosphere will pose major challenges for radio waves.

There will be constructed at least one base on the surface, for space travel. Large interplanetary ships, carrying passengers and cargo to and from Saturn, will dock at Helene. Then, smaller ships will ferry between the little moon Helene and Rhea.

Tourism[]

It is questionable what kind of tourism can exist on Rhea. Its extremely wet climate will be very unpleasant.

Wild Life[]

The islands will be home for birds and mammals, that will find their food in water.

The ocean will have two different layers of water. Above, water will have enough oxygen. Plants and fish will grow, using the little light that reaches them. Below, there will be almost no oxygen and only some bacteria will survive.