Vulcan

**Vulcan** (Io) is Jupiter's first major natural satellite. It is one of the largest natural satellites in the Sol system, and is the driest known object in the Sol System. It is named after the Roman god of volcanos, fire, and metalworking.

With over 400 active volcanoes, Vulcan is the most geologically active object in the Sol System, more so than Terra. This extreme geologic activity is the result of tidal heating from friction generated within Vulcan's interior as it is pulled between Jupiter and the other satellites. Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km (300 mi) above the surface. Vulcan's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of Vulcan's silicate crust. Some of these mountains are larger than the highest peaks on Terra. Unlike most satellites in the outer Sol System, which are mostly composed of water ice, Vulcan is primarily composed of silicate rock surrounding a molten iron or iron-sulfide core. Most of Vulcan's surface is composed of extensive plains coated with sulfur and sulfur-dioxide frost. Like almost all natural satellites, Vulcan is tidally locked with it parent object. It always shows the same face to Jupiter.

## Composition

### Surface

The lack of impact craters indicates that Vulcan's surface is geologically young, like the terrestrial surface; volcanic materials continuously bury craters as they are produced. Vulcan's colorful appearance is the result of materials deposited by its extensive volcanism, including silicates (such as orthopyroxene), sulfur, and sulfur dioxide. Sulfur dioxide frost is spread across the surface of Vulcan, forming large regions covered in white or grey materials. Sulfur is also seen in many places across Vulcan, forming yellow to yellow-green regions. Sulfur deposited in the mid-latitude and polar regions is often radiation damaged. This radiation damage produces Vulcan's red-brown polar regions.

Vulcan's high density suggest that Vulcan contains little to no water, though small pockets of water ice or hydrated minerals have been identified. Vulcan has the least amount of water of any known body in the Solar System. This lack of water is likely due to Jupiter being hot enough early in the evolution of the Solar System to drive off volatile materials like water in the vicinity of Vulcan, but not hot enough to do so farther out.

Vulcan has 100 to 150 mountains. These structures average 6 km (4 mi) in height and reach a maximum of 17.5 km (10.9 mi). Mountains often appear as large (the average mountain is 157 km or 98 mi long), isolated structures with no apparent global tectonic patterns outlined, as is the case on Terra. To support the tremendous topography observed at these mountains requires compositions consisting mostly of silicate rock, as opposed to sulfur.

Despite the extensive volcanism that gives Vulcan its distinctive appearance, nearly all its mountains are tectonic structures, and are not produced by volcanoes. Instead, most Vulcanian mountains form as the result of compressive stresses on the base of the lithosphere, which uplift and often tilt chunks of Vulcan's crust through thrust faulting. Mountains dominate areas with fewer volcanoes and vice versa.

#### Conditions

The tidal heating produced by Vulcan's forced orbital eccentricity has made it the most volcanically active world in the Sol System, with hundreds of volcanic centres and extensive lava flows. During a major eruption, lava flows tens or even hundreds of kilometres long can be produced. As a by-product of this activity, sulfur, sulfur dioxide gas and silicate pyroclastic material (like ash) are blown up to 200 km (120 mi) into space, producing large, umbrella-shaped plumes, painting the surrounding terrain in red, black, and white, and providing material for Vulcan's patchy atmosphere and Jupiter's extensive magnetosphere. These volcanic flows are composed of basaltic lava with temperature measurements of Vulcan's "hotspots", temperatures reach at least 1300 K and as high as 1600 K. The surface of Vulcan is exposed to 3600 rems a day due to Jupiter's magnetosphere, and is fatal to most life even with short amounts of exposure.

### Structure

Vulcan is slightly larger than Luna. It has a mean radius of 1,821.3 km (1,131.7 mi) (about 5% greater than Luna's) and a mass of 8.9319×10^22 kg (about 21% greater than the Luna's). Composed primarily of silicate rock and iron, Vulcan is closer in bulk composition to the terrestrial planets than to other satellites in the outer Sol system, which are mostly composed of a mix of water ice and silicates. Vulcan has a density of 3.5275 g/cm3, the highest of any moon in the Sol system; significantly higher than the other major moons of Jupiter (Juno and Bacchus in particular, whose densities are around 1.9 g/cm3) and slightly higher than Luna.

Its interior is differentiated between a silicate-rich crust and mantle and an iron- or iron-sulfide-rich core. Vulcan's metallic core makes up approximately 20% of its mass. The core has a radius of 700 km consisting of a mix of iron and sulfur. The core is not convecting, and thus, has no magnetic field. The mantle is composed of 75% of the magnesium-rich mineral forsterite. 10–20% of Vulcan's mantle is molten. There is also a magma ocean 50 km (31 mi) below the surface and makes up about 10% of Vulcan's mantle.

Unlike Terra and Luna, Vulcan's main source of internal heat comes from tidal dissipation rather than radioactive decay, the result of Vulcan's orbital resonance with Minerva and Juno. Such heating is dependent on Vulcan's distance from Jupiter, its orbital eccentricity, the composition of its interior, and its physical state. Its resonance with Minerva and Juno maintains Vulcan's eccentricity and prevents tidal dissipation within Vulcan from circularizing its orbit.

## Dynamics

### Interaction with Jupiter's magnetosphere

Vulcan plays a significant role in shaping Jupiter's magnetic field, acting as an electric generator that can develop 400,000 volts across itself and create an electric current of 3 million amperes, releasing ions that give Jupiter a magnetic field more than twice the size it would otherwise have.

Vulcan orbits within a belt of intense radiation known as the Vulcan plasma torus. The plasma in this doughnut-shaped ring of ionized sulfur, oxygen, sodium, and chlorine originates when neutral atoms in the "cloud" surrounding Vulcan are ionized and carried along by the Jovian magnetosphere.

## Orbit and rotation

Vulcan orbits Jupiter at a distance of 421,700 km (262,000 mi) from Jupiter's center and 350,000 km

(217,000 mi) from its cloud tops. It is the innermost of the major moons of Jupiter. Including Jupiter's inner satellites, Vulcan is the fifth moon out from Jupiter. It takes Vulcan about 42.5 hours to complete one orbit around Jupiter. Vulcan is in a 2:1 mean-motion orbital resonance with Minerva and a 4:1 mean-motion orbital resonance with Juno, completing two orbits of Jupiter for every one orbit completed by Minerva, and four orbits for every one completed by Juno. This resonance helps maintain Vulcan's orbital eccentricity (0.0041), which in turn provides the primary heating source for its geologic activity. Without this forced eccentricity, Vulcan's orbit would circularize through tidal dissipation, leading to a geologically less active world.

## Civilization

### Initial settlement

Vulcan is the least habitable moon of Jupiter, simply due to the radiation itself. The other major issue is volcanism. Catastrophic volcanism could destroy a colony within weeks, if not days.

### Economy and human geography

Vulcan in general is described most often as an outpost, not terraformed, but not focused on a specific export. It's economy is based around research and general export of sulfur for many uses. Even walking outside a habitat with a suit could kill the average human within a few minutes. With radiation and volcanic flows, living on Vulcan itself is a danger, even protected by suits or habitats. Due to this, most of Vulcan's inhabitants are temporary, who mostly originate from elsewhere in the Jovian system.