Fumarole

(This moon is in the same universe as Cauldron, Cryolith, Phobis Terra, and Behemoth, but is not something I'd plan to make a main part of the actual mod, if it ever gets made.  This description is more for my own enjoyment, and hopefully yours.)

General Description

Fumarole is the innermost large moon of Behemoth.  It is about the size of ganymede, and made of about 1/6 water, 3/4 rock, and the rest metal, with these materials separated into layers.  The moon is heavily tidally heated, and highly active.  Gases escaping from the moon create a surprisingly thick atmosphere, creating some unusual weather patterns, and the atmosphere feeds most of the material found in the magnetic field of Behemoth.

Geology

The tidal heating from behemoth effects both the water layer, and magma chambers within the rock layer.  Activity within the rock layer creates hydrothermal vents and underwater volcanoes, as well as releasing gases and rock dust into the water layer.

Within the water layer, only the top few miles of material is solid, with the rest being a fast convecting liquid ocean.  This ocean also flattens the moon's surface significantly; any bulges from the upper ice layer are quickly eroded by the currents, and mountains cannot support themselves for long. 

At the surface, the most obvious features are numerous geysers, throwing out large amounts of water and gases, often miles above the surface.  These geysers appear to be generated by plumes of hot water from hydrothermal vents at the rock layer.  As these heated plumes reach the upper ice layer, they melt the ice, allowing passage to the surface.  These plumes also carry gas released by the vents below, and lower pressures near the surface release this gas, powering the geyser's eruption.  Depending o the conditions of the geyser, the water may remain liquid for some distance after the eruption.

The immediate area surrounding the geyser is usually darker than normal, due to rock dust, while farther out, terrain is slightly lighter from falling and frosting ice.  Geysers build hills around themselves as well, with slopes getting shallower farther away from the main hole.  Eruptions of geysers create clouds in the atmosphere for several days, and light snowfall is common, lighetning areas downwind from the geyser.

Water volcanoes are also common on the surface of this moon.  These are usually formed where some weakness in the ice allows watery material to push through closer to the surface.  As the pressure decreases, gases within the water escape, and create further cracks in the ice while pulling the water upward, where it eventually breaks through the the surface.  Though the formation process is broadly similar to geysers, these volcanoes lack the energy and extra gas from a hydrothermal plume, and as a result are much less violent.

Most water volcanoes have a number of different outlets, resulting in a large, flat hill, formation, usually with some ridges or bumps where the water escapes.  Some of the larger formations maintain water lakes in the middle, while others only release water occasionally.  All formation release gas, with water lakes usually bubbling, and other water volcanoes releasing vapor streams.

Convection currents also create a plate tectonic system on the surface ice.  This system moves much faster than Earth's system, with speed of several feet a year seem in some places.  Splitting occurs at "ice faults", forming large crack and valley systems within the ice.  Some of these cracks, as with ice volcanoes, fill with liquid water.

At collisions between ice plates, so called "rough terrain" forms.  Large forces within the ice cause it to bulge and buckle, sometimes with one section oversliding another.  Ice that juts down into the water below is quickly eroded, and the resulting terrain forms a jumbled collection of peaks, landslides, and cracks.  Water, and gases absorbed into the ice, are recycled into the ocean layer at these areas.

Despite the activity of this moon, quakes are are rare, and not particularly powerful.  The ice of tis moon is soft compared to rock, and breaks and smooths easily, and water lubricates a lot of the motion of ice plates as well.  Smaller events and landslides do occur, however, and can be dangerous.

Atmosphere

Most of the atmosphere of this moon is nitrogen and carbon dioxide, with small amounts of sulfur gases, water vapor, organic compounds, oxygen, and other such gases.  Most gases are released directly from ice volcanoes, while nitrogen forms from ammonia.  Gases are lost by being absorbed by ice or frozen at the poles and transported into the water layer, and by being stripped by material in Behemoth's magnetic field.  The atmosphere overall has a density at the surface of about 1/15 Earth's.

Winds in the atmosphere mainly travel towards the poles, where carbon dioxide snows and freezes out.  These winds create streaks on the plains, transport ice particles, and erode the surface.

Ice clouds can form throughout the atmosphere, though are more common near ice volcanoes and geysers.  These clouds get much more common after geyser eruptions feed water into the atmosphere.  Ice particles and snow fall from these clouds, and create snow streaks towards the poles from volcanoes and geysers.

Towards the poles, carbon dioxide ice clouds form as the air gets cool enough.  These clouds also create snow, and this material helps recycle carbon dioxide into the liguid water layer of the moon.

The upper atmosphere is the main source of material for Behemoth's magnetic field.  This field also creates auroras near the poles of the moon, which can be seem during unusually dark periods.

Explanation

(Disclaimer:  I am not a geologist, planetary scientist, or any other similar sort of scientist, just someone who finds planetary science interesting.  As a result, a lot of these ideas may be impossible, or at least unrealistic.  This probably applies more to these moons than it does to the previous four planets I've written about.  Any mental anguish, property damage, or other negative consequences from reading this blog post are the responsibility of the reader.)

This moon is meant to combine interesting features of a number of outer planet moons  (Europa, Io, Enceladus, and Triton mainly.)  I don't know if any other the numbers I've written actually make sense (Since i don't have access to, or knowledge of, ways to make geological models), so am even less sure of this world than of the planets I've created in previous posts.  Most of these ideas, though, are based on actual models or things that have been seen.

The geysers are meant to resemble a combination of Io and Enceladus's plumes.  Their power source (hydrothermal vents creating water plumes) comes from a section of a paper i read about for a model of Europa, suggesting an explanation for water pockets within the ice shell of that moon.  Ice faults and collision areas are based off a combination of Europa terrain, and Earth plate tectonics.  Ice volcanoes aren't based off anything in particular, apart from the general idea of what a "cryovolcano" might be like.

The atmosphere is mostly added to make the moon cooler, although without any weather models, or good estimation methods, it's the part of the moon I'm least sure of.  (I know very little about how quickly certain reactions might occur, how much and what material would actually be released by volcanoes, how much material would get stripped, and what temperature the atmosphere would be, determining which parts might freeze out.  All of this would significantly effect what this moon might have if it somehow existed.)  The atmosphere would, however, create enough pressure to allow liquid in the surface.  (Assuming a gravity about the same as Io or Ganymede, and assuming I looked up and calculated everything right.)

The Europa paper I mentioned earlier suggested that most of its ocean would be at about freezing, although I'm not sure of large amounts of extra heat might change this.  As a result, I'm not sure whether liquid water could be kept warm on a moon like this, though for coolness purposes I have it written in.