Mercury

The smallest planet in orbit around Sol, **Mercury** is a tiny, scorched world hovering just beyond the point at which its silicate surface would melt. The planet, first surveyed by the *Mariner 10* probe in 16 AFO, is pockmarked with craters, likely from minor objects attracted from the fringes of the Vulcanoid belt, and is riven with compressional features caused by the planet's cooling. Mercury is currently a disputed zone between the United Nations of Humanity and the Loriset Intendancy.

## Physical Characteristics

Mercury is a rocky world orbiting close to Sol with a proportionally enormous iron core. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 kilometres. It is also smaller than the natural satellites of Ganymede, Titan, and Eurybia.

### Internal Structure

Mercury appears to have a solid silicate crust and mantle overlying a solid, iron sulfide outer core layer, a deeper liquid core layer, and a solid inner core. The radius of Mercury's core is estimated to be 2,020 ± 30 km (1,255 ± 19 mi), based on advanced models. Mercury's crust is estimated to be 35 km (22 mi) thick, thicker than any planet inwards of it. One distinctive feature of Mercury's surface is the abundance of a number of narrow ridges, extending to several hundred kilometers long. It is thought that these were formed as Mercury's core and mantle cooled and contracted at a time when the crust had already solidified.

There are a number of hypotheses regarding Mercury's enormous iron core. The most popular and widely accepted is that Mercury suffered a powerful impact by a planetesimal late in its formation, stripping 60% of its mass and the vast majority of its mantle away from the planet, the same process was narrowly avoided by Earth and Theia as the two fell into orbit around each other rather than impacting. Alternatively, Mercury may have formed early, before Sol's temperature was stabilized. Temperatures in the Mercurian region may have risen to as high as 10,000 K in this hypothesis, torching off most of Mercury's mass. The existence of Hestia and the Handmaiden Belt cast doubt on this hypothesis, however. A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material and not gathered by Mercury.

### Surface Geology

Mercury's surface is similar to that of Neith or Simud, showing heavy cratering and extensive lava flows from its early history, indicating that functionally no geological activity has taken place in the intervening four billion years. Mercury is substantially more regionally heterogenous than either of the above objects, which each showcase large expanses of similar geology such as Simud's enormous maria.

#### Impact Basins & Craters

Craters on Mercury range in diameter from small bowl-shaped cavities in the surface to monumental multi-ringed impact basins extending hundreds of kilometers in radius. They appear in all states of degradation, from relatively fresh rayed craters to highly degraded crater remnants from the beginning of the Solar System. Mercurian craters differ subtly from Neithian craters in that the area blanketed by their ejecta is much smaller, a consequence of Mercury's stronger surface gravity.

The largest known crater is Caloris Planitia, or Caloris Basin, with a diameter of 1,550 km. The impact that created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric mountainous ring ~2 km tall surrounding the impact crater. The floor of the Caloris Basin is filled by a geologically distinct flat plain, broken up by ridges and fractures in a roughly polygonal pattern. It is not clear whether they are volcanic lava flows induced by the impact or a large sheet of impact melt.

The impact that created Caloris created shockwaves which converged on its antipode, creating high stresses in the crust at that location and forming unusual hilly terrain known simply as "weird terrain".

#### Plains

Gently rolling hilly plains in the zones between large craters are Mercury's oldest visible surface regions, predating the terrain heavily cratered in the Late Heavy Bombardment. The smooth plains are widespread flat areas that fill depressions of various sizes and bear a strong resemblance to the Simudian maria. Unlike the maria, the smooth plains of Mercury have the same albedo as the older inter-crater plains. Despite an absence of certain evidence for volcanism, the localisation and rounded and lobed shape of these plains strongly suggest volcanic origins. All smooth plain regions on Mercury formed significantly after the Caloris basin.

#### Compressional Features

Mercury possesses numerous compression faults across its surface, known as rupes, criscrossing its plains. These also exist on Neith or Simud, but are much more prominent here. As Mercury's interior cooled, it contracted and its surface began to deform, creating wrinkle ridges and lobate scarps associated with thrust faults. The scarps can reach lengths of 1000 km and heights of 3 km. Such features can be seen overlapping others, indicating that they formed over time after the formation of the craters they run through. Mapping of the features has confirmed that Mercury's radius has shrank by up to seven kilometers. Most activity along the major thrust systems probably ended about 3.6–3.7 billion years ago, but small-scale formations have been found, tens of meters in height and with lengths in the range of a few km, that appear to be younger than 50 million years old, indicating that compression of the interior and the surface geological activity caused by this continue to the present.

### Surface Conditions and Exosphere

The temperature of Mercury's surface ranges from 1-7 hundred Kelvin at the most extreme locations, never rising above 180 K at the poles. The subsolar point reaches about 700 K during periapsis (0°W or 180°W), but only 550 K at apoapsis (90° or 270°W). After sunset, the planet's temperature drops to a fairly uniform 110 K.

The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below 102 K, far lower than the global average. This creates a cold trap where ice can accumulate. These enormous ice deposits formed the nuclei of the first and most populous Mercurian colonies. The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet's interior or deposition by impacts of comets.

Mercury is too small and high-temperature for its gravity to retain anything over long periods of time. However, it does have a tenuous surface-bounded exosphere containing hydrogen, helium, oxygen, sodium, calcium, potassium and others at a surface pressure of less than approximately 0.5 nPa (0.005 picobars). This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources. Hydrogen atoms and helium atoms probably come from the solar wind, diffusing into Mercury's magnetosphere before later escaping back into space. Radioactive decay of elements within Mercury's crust also creates helium, as well as sodium and potassium.

### Magnetic Field and Magnetosphere

Despite its tiny size and extremely slow rotation, Mercury possesses a global magnetic field, with a strength at about 1.1% of Earth's. Unlike Earth's, Mercury's magnetic poles are nearly aligned with the planet's spin axis.

## Orbit and Rotation

Mercury is tidally or gravitationally locked with Sol and orbits in a way unseen further out in the system. As seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around Sol, making the sun appear to rise and set just once every two Mercurian years. Mercury's orbit is among the most eccentric of any planetary body in the Solar System, and is highly inclined compared to the other planets.

## History

Known to the ancients of Earth and Theia, Mercury was a fairly unremarkable planet, always visible immediately before sunrise and after sunset. The invention of the telescope revealed absolutely nothing about it other than that it exhibited phases similar to Earth's moons, and the first true knowledge gained about it would have to wait until the launch of Mariner 10 in 16 AFO.

Mariner 10 was launched from Cape Kana and used a close pass over Theia to reach interplanetary space. The spacecraft then flew past Venus twice, using its advanced technology to make detailed images of its rings and moon Neith, before conducting several flybys of Mercury itself. The probe only managed to image one hemisphere due to the flyby geometry, but the later launch Ambassador 2 managed to enter orbit, surveying the entire surface of the planet as well as its tiny asteroid moon, Messenger. Messenger was observed to have an unstable orbit, impacting the planet 40 years after this survey.

The *Rising Moon* manned mission set down the beginnings of the first colony on Mercury before the Serran Wars, and its crew elected to remain on the planet for several years rather than return home to be imprisoned by the rising Serran Empire as political dissidents. Doing so, the *Rising Moon* was set down on Mercury's surface and disassemble in one of the shadowed polar craters, forming the simply named Rising Moon Colony. The Serran Empire's expansion across the system did quickly reach the colony, but Rising Moon's preparation time allowed it to assemble various defenses, fortifying it against siege for almost four years until the Serran forces tunneled beneath the base and invaded, killing much of its small population and imprisoning the rest.

Mercury's population expanded after the conquering of Theia, at which point it served as a launching point for the colonization of Vulcan. Loriset slave labor caused its population to balloon as well. The slave revolt, accompanied by the White Rose Rebellion, freed Mercury's colonies from Serran control well ahead of the Theian War of Independence, and the onset of that war prevented the planet from being retaken by the rapidly collapsing Serran Empire.

The colonies remained mostly independent, but its political leanings led it to ally itself with the United Nations of Humanity rather than the Loriset Intendany. Loriset colonies in the equatorial regions independant of the human-built ones to the north and south began to export large quantities of refined materials to compete economically with the polar states. These hostilities began to escalate, the Human colonies requested military aid from Earth, the Loriset colonies did the same, and the first battle of the Homeworld Wars took place in orbit.

The battle raged for days, with ballistic projectiles winging across Mercury's sphere of influence visible from the surface. The black sky was pierced by the reactor detonations of spacecraft from both sides. Space junk would collect in orbit and make the planet extremely difficult to access until the present day, and orbital bombardment destroyed much of the planetary colonies.

Today, Mercury itself has cooled down from war, but the Homeworld Wars that the first battle kickstarted rage on to this day. The colonies have ceased hostilities in their isolation, developing trade networks to keep each other alive.

## Lunar System

Mercury has a single current moon, but this moon was not always in orbit. There have likely been a number of large asteroids captured into semistable orbits around Mercury, eventually crashing down to the surface.

### Messenger

On the first proper telescope exploration of Mercury, it had a single moon in a highly elliptical, near-polar orbit, tentatively named "Messenger" after its confirmation. When Mariner 10 flew past, however, it had a single moon in a different, nonpolar orbit and a large lava pool on its night side. Since the first Messenger was never surveyed and almost nothing was ever known about it, the new moon inherited its name. Ambassador 1 dropped a tiny probe onto its surface as it was theorized to be a captured member of the Handmaiden Belt, no objects from which had been landed on.

Messenger later impacted Mercury's equatorial region in full view of orbital satellites. The earthquakes were felt from the newly founded Rising Moon Colony, but damage was negligible.

### 31 Crateris

During the Theian War for Independance, a small asteroid on an elliptical orbit from the Handmaiden Belt was observed making several flybys, pulling its orbit closer and closer to Mercury's. A small probe was launched from Tholia to survey the object as a potential mining prospect, finding that the previously unremarkable asteroid was in fact made of almost pure metallic materials and was going to enter orbit after a few more passes. After the close of the war, 31 Crateris, retaining its asteroid number, had its orbit stabilized into a high, circular orbit above the equatorial region. Mining operations were placed by the various colonies on its surface, and the multispecies staff was on the verge of rioting when the Battle of Mercury took place and the planet was cut off from the rest of the system by the shrapnel. The workers were quickly evacuated as a large cloud of debris that was formerly the Loriset Flagship ILS *Drovana* approached the moon, but the later impact destroyed the mining site. The moon remains uninhabited.