Appendix A

          The planet Jopitar is a classic gas giant – a vertiginous ocean of hydrogenous fluid, with no well-defined surface as on a rocky terrestrial world. Nearly 300 thousand kilurets wide at the equator (more than ten times broader than Earth), the planet is somewhat flattened in shape due to rapid rotation (once every eight rohs, or about ten hours in human units). Viewed at a distance from an equatorial perspective, it appears to have a banded structure, dominated by a light zone along the equator, followed by an alternating sequence of parallel darker belts and additional zones toward either pole. These correspond to the visible cloud tops in Jopitar's atmosphere – high, frigid white clouds of ammonia ice crystals in the zones, and deeper, warmer clouds of ammonium hydrosulfide and water in the belts. Trace compounds of phosphorus, sulfur, and various hydrocarbons give the clouds a variety of subtle colors. The alternating cloud band pattern extends to nearly 70 degrees north and south latitude, where it gives way to a pair of more chaotic, mottled, circular belt-like regions centered on either pole. Each of these covers some ten times the total surface area of Earth.
          Because Jopitar is an upside-down world from the perspective of a terrestrial planet dweller, it is more useful to speak of depth rather than altitude when describing vertical location within the atmosphere. Depth is conveniently measured from the tropopause – the top of the active weather layer, just above the tops of typical upper-level ammonia clouds. The tropopause marks the boundary between the clear stratosphere above, and the clouded, turbulent troposphere below. It is the coldest level in the atmosphere, typically a numbing 100 nevlu (161 degrees below zero on the human Celsius scale); temperature increases in both upward and downward directions. Only the most powerful storms penetrate this elevation, because cloud formation requires convection, and convection requires that temperature decrease in the upward direction even more quickly than rising air cools naturally.
          The Jopian atmosphere consists primarily of hydrogen and helium, although the precise composition changes with depth. Just beneath the main deck of water clouds at 223 kilurets depth, the average composition is approximately 73% hydrogen and 23% helium by mass. The remaining 4% consists of heavier elements – in particular, 0.9% carbon, 0.3% nitrogen, 2.6% oxygen, and 0.1% sulfur – which combine with the plentiful hydrogen to form a variety of molecules. Without these molecules, there would be no visible clouds, and no possibility of life. Jopitar would be quite bland, both outwardly and inwardly. In terms of the number of molecules, the atmosphere just below the water clouds averages about 85.9% molecular hydrogen (H₂), 13.5% atomic helium (He), 0.19% methane (CH₄), 0.06% ammonia (NH₃), 0.39% water (H₂O), and 0.01% hydrogen sulfide (H₂S). Because heavier elements tend to sink through the light, buoyant hydrogen, and to condense out of the atmosphere at higher temperatures, the proportion of heavier elements increases with depth. Silicon, which together with oxygen and carbon forms the backbone of Jopian life, doesn't attain significant general atmospheric proportions until nearly 990 kilurets depth. If not for a relative depletion of magnesium in the lower troposphere, most silicon would condense out of the atmosphere much deeper down, and Jopian life as we know it would not be possible.
          Beneath the visible clouds, the gaseous atmosphere becomes progressively denser and warmer with increasing depth. The molecular hydrogen gas becomes a supercritical fluid, in which the distinction between gas and liquid becomes meaningless, at 286 kilurets depth. There is no clear liquid boundary or surface. Between 18 to 22% of the distance to the planet's center, atmospheric pressure becomes so great that electrons are squeezed out of the hydrogen atoms, and the hydrogen undergoes a transition into a liquid metal. Although this metallic hydrogen soup can accommodate a significant concentration of helium and other heavier elements, drops of helium slowly condense and rain down through deeper levels, gradually depleting the helium higher up. At the center of the planet lies a dense, hellish core of rock, with a mass of several Earths, sheathed by a thick layer of dirty hot molten water, ammonia and methane "ice."
          The atmosphere of Jopitar is in a constant state of motion. The currents are mainly powered both by radiant energy received from Suol, the local sun, and from gravitational energy released by the slow contraction of the giant planet (about 15 centurets per jope). Because Jopitar is both so large and so far from Suol, the internal energy source is comparable to the suolar contribution (~67%, versus only ~0.04% for Earth). The suolar energy is absorbed in the upper atmosphere, and heats equatorial latitudes far more than the poles. This drives a convection pattern in the upper troposphere, extending from the tropopause down through the eight-rab level (roughly sea-level pressure on Earth) to about 80 kilurets depth, in which heated gases rise at the equator, move poleward, cool, and sink, returning at a lower level to the equator. In the process, excess heat is transported toward the poles. As in Earth's atmosphere, this overall system gets broken into a series of rolling convection cells (analogous to Earth's Hadley, Ferrell, and Polar cells), tied in Jopitar's case to the zone-belt system. Gases ascend in each zone, cool, and form clouds of ammonia ice crystals. The dried gases then diverge toward the adjacent belts on either size, where they converge with gases arriving from the adjoining zones. These gases sink into the belts, where they again diverge, and travel back to the adjacent zones. The cell pattern ends with gases descending at either pole.
          In contrast, because Jopitar spins so rapidly and has a flattened shape, the internal heat is channeled from the hot core more strongly toward the poles than the equator. This drives a distinct three-layered tier of convection cells at deeper levels, which transports excess heat not only from the core to the surface, but also from the poles to the equator. The most important component for our purposes is the mid-tropospheric convection system. Air in the middle troposphere tends to rise in the belts and sink in the zones, just the opposite of the suolar-driven pattern in the upper troposphere. The two systems are linked, in that gases sinking from higher levels in the belts meet gases rising from deeper levels, forcing each to diverge into the adjoining zones, completing the convection loops.
          Air ascending in the belts expands and cools, leading to the formation of the ammonium hydrosulfide and water clouds visible from space. At progressively greater depths, a variety of thinner ammonium halide, alkali sulfide, alkali halide, and silicate clouds are also spawned. The mid-tropospheric circulation ends at about 1,780 kilurets depth. From here down to 1,970 kilurets, the atmosphere is so transparent that conditions are not well suited to convection, but rather to the upward transfer of energy through infrared (heat) radiation. Massive hot convective plumes nonetheless do frequently break through this barrier from below. Because of the circulation pattern in the lower troposphere, this generally occurs in the zones, but the plumes are then quickly swept by latitudinal currents into the adjoining belts. There they stream up through the middle troposphere, stirring convection at higher levels.
          The upper reaches of the mid-tropospheric belts are the site of intermittent but intense lightning storms, which channel upward a significant portion of the heat percolating outward from the planet's core. These are commonly triggered and fed by the underlying hot plumes. The material inside a rising plume is considerably hotter and more buoyant than the surrounding atmosphere. As a plume rises through the middle troposphere, it chaotically fragments into distinct streams – the rey rivers of fire. Each river can give rise to a cluster of storms at higher levels. These are dramatically energized by the heat released when water condenses in the lower storm clouds, and often spawn towering thunderheads capped by icy ammonia anvils at the top of the troposphere. Mid-size storm clusters are typically a few hundred kilurets in size, and last five to ten yads, but large clusters can grow up to four to ten thousand kilurets across within several yads. These mammoth formations encompass more than one hundred individual storm cells, and may persist for kews or even thoms. They can span a height up to 225 kilurets, from the base of the underlying liquid water clouds to the tops of the uppermost ammonia ice plumes. The most powerful storm towers can breach the tropopause, into the lower stratosphere. Vertical updraft velocities of 60 to 175 rets per noc (some 180 to 540 kilometers per hour) are common.
          The net ascent in belts is not large-scale, but carried mainly by the vigorous upward motions in localized storms. There is even a gradual subsidence of gases between storms, as well as penetration of some dry air from higher altitudes to levels below the water clouds. A general, widespread upwelling in the belts would actually produce thick zone-like water stratus clouds instead of individual storms, and create conditions inimical to thunderstorm formation.
          High-speed jet currents blow along the zone-belt boundaries, alternating eastward (in the same direction as the planet's rotation) on the equator side of each belt, and westward on the pole side. The belts are thus cyclonic (they rotate counterclockwise in the northern hemisphere, and clockwise in the southern), while the zones are anticyclonic. Normally, regions of relative low pressure generate cyclonic flow, while high pressure regions engender anticyclonic flow. These rotation patterns are caused by the Coriolis force, as air moves naturally from higher to lower pressure. In the absence of storms, pressure tends to be higher in the zones than in the belts at both the top of the upper- and the bottom of the mid-tropospheric convection systems, supporting the prevailing jets. At the transition between systems, however, relative pressures in the zones and belts are reversed, generating contrary winds. These are overcome by the strong belt lightning storms, which penetrate through the transition region. As on Earth, such storms are cyclonic from lower through mid levels, and don't become anticyclonic until approaching the cloud tops. The strong outer wind fields feed directly into the existing jets, overwhelming the weak opposing currents. The net result is that jet wind speeds are nearly constant from great depths up through the 35-rab level (at roughly 175 kilurets depth). Above this, they do not reverse direction, but only diminish in intensity through the tropopause.
          Imbedded in the zones are major anticyclonic ovals, typically bright white or (less often) ruddy in color, which normally extend into the neighboring belt on the equator side. These are the tops of giant, upwelling hurricane-like vortices, crowned by ammonia clouds that rise higher than typical zonal clouds. The larger systems may persist for octujopes. While inhabitants of rocky terrestrial planets usually think of storms as being cyclonic, this is true only from a perspective beneath the clouds, where air is rushing inward and upward through a region of low pressure. Above a cyclonic storm, a relative high-pressure area develops, and the air can only spread outward, producing anticyclonic flow.
          The anticyclonic ovals are powered and maintained by the powerful lightning storms in the belts. While these storms are cyclonic at low to mid levels, they are anticyclonic at the top. The storms have a natural tendency to drift poleward, and eventually are either torn apart by the cyclonic shear of the host belt, or generate stable upper-level anticyclones moving west with the prevailing current. When a major storm approaches the poleward boundary of a belt, its energetic updraft can penetrate into the lateral flow between the upper- and mid-tropospheric convection cells, and be swept into the ascending circulation of the adjacent zone. This occasionally gives birth to a new anticyclonic oval, but more often energizes the overall zonal flow, and feeds into any nearby pre-existing oval.
          The middle troposphere encompasses the principal realm of Jopian life. Hospitable regions are confined to the belts, mainly between 500 and 620 kilurets depth. Here upwelling currents rich in raw nutrients and thermal energy are common. Unlike Earth biochemistry, which is carbon based, Jopian biochemistry is anchored both to carbon and to silicon-oxygen polymers, which stabilize biomolecules against the higher temperatures and pressures on Jopitar. While carbon and oxygen are plentiful throughout the middle troposphere, silicon is not, and must be furnished from below. The tops of ordinary silicate (mainly SiO₂) clouds lie at nearly 990 kilurets depth, where ambient temperatures exceeding 890 nevlu (about 730 degrees Celsius) would destroy any organic molecule. But the hot plumes and associated rivers and springs that surge through the middle troposphere carry silane (SiH₄) upward more than 370 additional kilurets, through the 600 nevlu (400 degrees Celsius) level. Here a fraction of the silane is naturally converted to silicone polymers. Simple, heat-tolerant chemosynthetic and (infrared) photosynthetic microorganisms that swarm along the edges of the upwellings convert these and additional silane to stable biochemical forms, and carry the silicon to still higher levels on the upward flow, as the remaining unstable silane converts to silicon dioxide grit and precipitates away. Warm currents rich in the stuff of life ascend through cooler domains harboring progressively more complex organisms. At some 512 kilurets depth (well below the base of typical water clouds), multicellular organisms flourish in tangled thickets, held aloft by enormous bladders filled with pure hydrogen. Ambient temperatures of 512 nevlu (300 degrees Celsius) and pressures 60 times those on Earth are typical here.
          At much deeper levels, beneath the weak radiative layer, churns the lower-tropospheric convection system, which extends to the base of Jopitar's molecular hydrogen envelope. The circulation here is synchronized with the upper-tropospheric pattern, in that fluids tend to rise in the zones and sink in the belts. As at higher levels, ascending fluids expand and cool, leading to the formation of magnesium-silicate clouds at the loftiest reaches, where temperatures hover near 1,800 nevlu. Condensates include solid Mg₂SiO₄ grains (the mineral forsterite), liquid MgSiO₃ droplets, and (in the coolest vapors) grains of solid MgSiO₃ (the mineral enstatite). If magnesium were not depleted with respect to silicon in Jopitar's troposphere (a larger proportion of magnesium is locked away in the planet's core), almost all the atmospheric silicon would be sequestered in and below these deep clouds, and very little would be available to life at higher levels. At still greater depths, iron clouds form. The deepest cloud layers consist of refractory calcium-aluminum and calcium-titanium oxides. The net ascent in the zones is not large-scale, but carried mainly by the vigorous upward motions in hot plumes. Condensation releases latent heat, which provides some additional buoyancy.
          The deepest fluid convection system occupies the metallic hydrogen ocean, which extends from the base of the troposphere all the way to Jopitar's core. This circulation is synchronized with the mid-tropospheric convection system – fluids tend to rise in the belts and sink in the zones. In particular, hot fluids rise directly from the poles, and sink onto the equator. Hot plumes originate in the abyssal depths nearest the core, where they strip away heavy elements from the sludge that blankets the underlying layers of dirty molten ices, and carry the enriched material upward. The plumes are eventually swept latitudinally to feed the ascending zones of the lower troposphere. Electric currents in the intervening ionic ocean generate an intense magnetic field that enfolds the planet in an immense electromagnetic bubble.

          Some of the principal features of the Jopian atmosphere and interior are listed in a table and depicted in a figure below. In deference to Earth-based readers, the table specifies quantities in terms of both octan and human/simion units of measure (color coded blue and red, respectively). Depths are listed in octan kilurets (krt) and human/simion kilometers (km), and are measured from the average level of the tropopause (the inversion layer dividing the top of the troposphere from the base of the stratosphere) in equatorial regions. Temperatures are listed in nevlu and degrees Celsius (°C). Atmospheric pressures are listed in rabs and bars, where one bar is slightly less than the standard sea-level pressure on Earth. Density is listed only in grams per cubic centimeter (g/cm³), human units. Note that the density of air at sea level on Earth is about 0.0013 g/cm³, while the density of liquid water is 1 g/cm³.
          Numbers listed in the table represent typical values only; there can be significant local deviations. For example, cloud bases may be somewhat deeper (higher) where concentrations of condensable volatiles are greater (smaller) than average. Cloud tops may similarly vary. The crowns of strong water storms may even be pushed above the tropopause. The transitions between vertically adjacent regions may further be gradual to some extent, and not sharply defined. In particular, the boundary between the outer molecular hydrogen envelope and the underlying metallic hydrogen ocean is fragmented and blurred.