Clara (Pacifica)

Clara

Space probe photography of Clara and its only moon (bottom left corner)
Designations
Pronunciation	/ˈklɑːrə/
Orbital characteristics
Aphelion	0.390 AU (58.343 million km)
Perihelion	0.383 AU (57.296 million km)
Semi-major axis	0.387 AU (57.894 million km)
Eccentricity	0.0073
Orbital period	0.24 years (87.660 d)
Average orbital speed	48.20 km/s (29.95 mi/s)
Inclination	3.38° to invariable plane
Argument of perihelion	158°
Known satellites	1
Physical characteristics
Equatorial radius	25,559 km (15,882 mi)
Surface area	8.2091×109 km2 (3.1696×109 sq mi)
Volume	6.99×1015 km3 (1.677×1015 cu mi)
Mass	8.6594×1025 kg (1.9091×1026 lb)
Mean density	1.27 g/cm3 (0.0459 lb/cu in)
Surface gravity	8.85 m/s2 (29.0 ft/s2; 0.902 g0)
Escape velocity	21.3 km/s (13.2 mi/s)
Rotation period	2 month 27 d 14 h, 2.92 months, 87.58 days
Axial tilt	0.98° (to orbit)
Albedo	0.176 (Bond) 0.352 (Geometric)
Surface temp. min mean max 0.1 bar 309 K 398 K 543 K
Apparent magnitude	-8.11 to -6.46
Absolute magnitude (H)	−3.824

Clara is the first planet from the Sun and the third-largest in the Pacifica System. It is a gas giant with an average radius of about four times that of Pacifica. It has only one-sixth the average density of Pacifica, but is over 14.5 times more massive. The planet is the smallest out of all the gas giants of the Pacifica System. It orbits the Sun at a distance of 0.39 AU (58 million km) with an orbital period of 88 days, and its rotational period closely resembles its orbital period due to the planet being tidally locked.

Clara's interior is believed to be composed of a rocky core surrounded by an ocean of metallic hydrogen, while the exterior is composed of a gaseous layer, and an electrical current within the metallic hydrogen layer is thought to give rise to Clara's planetary magnetic field. The outer atmosphere is generally bland and lacking in contrast with bands so faint they are hard to observe with the naked eye, however, upper clouds are visible, and some features can appear spontaneously.

The planet doesn't have an apparent ring system, and it holds gravitational influence over a single moon.

Physical characteristics

Clara is a gas giant composed predominantly of hydrogen and helium. It lacks a definite surface, though it is likely to have a solid core. Clara's rotation causes it to have the shape of an oblate spheroid; that is, it is flattened at the poles and bulges at its equator. Due to the low density of Clara, the effective surface gravity along the equator, 8.85 m/s², is lower than the surface gravity of Pacifica. However, the equatorial escape velocity of 21 km/s is almost two times that of Pacifica.

Internal structure

Despite consisting mostly of hydrogen and helium, most of Clara's mass is not in the gas phase, because hydrogen becomes a non-ideal liquid when the density is above 0.01 g/cm³, which is reached at a radius containing 99.9% of Clara's mass. The temperature, pressure, and density inside Clara all rise steadily toward the core, which causes hydrogen to be a metal in the deeper layers. Standard planetary models suggest that the interior of Clara is similar to that of the rest of gas giants, having a small rocky core surrounded by hydrogen and helium, with trace amounts of various volatiles.

This core is similar in composition to Pacifica's, but is denser. This is surrounded by a liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen that gradually transitions to a gas with increasing altitude. Clara has a hot interior, reaching 7400 °C. Clara's thermal energy can't be explained solely by the Kelvin–Helmholtz mechanism of slow gravitational compression, since it is much less massive than the rest of Jovian planets. An alternative or additional mechanism may be the generation of heat through the "raining out" of droplets of helium deep in Clara's interior. As the droplets descend through the lower-density hydrogen, the process releases heat by friction and leaves Clara's outer layers depleted of helium. These descending droplets may have accumulated into a helium shell surrounding the core. Rainfalls of diamonds have been suggested to occur within Clara, as well as in Visi, Porea and ice giant Xion.

Atmosphere

The outer atmosphere of Clara contains 98.3% molecular hydrogen and 1.25% helium by volume. The proportion of helium is significantly deficient compared to the abundance of this element in the Sun. The quantity of elements heavier than helium (metallicity) is not known precisely, but the proportions are assumed to match the primordial abundances from the formation of the Solar System. A significant fraction of the mass of these elements is thought to be located in Clara's core region. Trace amounts of ammonia, acetylene, ethane, propane, phosphine, and methane have been detected in Clara's atmosphere. The upper clouds are composed of ammonia crystals, while the lower level clouds appear to consist of either ammonium hydrosulfide (NH₄SH) or water. Ultraviolet radiation from the Sun causes methane photolysis in the upper atmosphere, leading to a series of hydrocarbon chemical reactions, with the resulting products being carried downward by eddies and diffusion.

Cloud layers

Clara's atmosphere exhibits a banded pattern similar to the rest of Jovian planets, but Clara's bands are so faint they are almost not noticeable. Clara's finer cloud patterns were not observed until the flybys probe spacecraft during the [placeholder]s. Since then, Pacifica-based telescopy has improved to the point where regular observations can be made.

The composition of the clouds varies with depth and increasing pressure. In the upper cloud layers, with temperatures in the range of 309 to 543 K and pressures extending between [placeholder]–[placeholder] bar, the clouds consist of ammonia ice. Water ice clouds begin at a level where the pressure is about [placeholder] bar and extend down to [placeholder] bar, where temperatures range from [placeholder] to [placeholder] K. Intermixed in this layer is a band of ammonium hydrosulfide ice, lying in the pressure range [placeholder]–[placeholder] bar with temperatures of [placeholder]–[placeholder] K. Finally, the lower layers, where pressures are between [placeholder] and [placeholder] bar and temperatures are [placeholder]–[placeholder] K, contains a region of water droplets with ammonia in aqueous solution.

Magnetosphere

Before the arrival of unmanned spacecraft, no measurements of the Claran magnetosphere had been taken, so its nature remained a mystery. Before [placeholder], scientists had expected the magnetic field of Clara to be closely in line with its poles.

Clara has an intrinsic magnetic field that has a simple, symmetric shape—a magnetic dipole. Its strength at the equator—0.229 gauss (22.9 μT)—is approximately one [placeholder]th of that of the field around Visi and slightly weaker than Pacifica's magnetic field. As a result, Clara's magnetosphere is much smaller than Visi's.

When [placeholder] entered the magnetosphere, the solar wind pressure was high and the magnetosphere extended 85 Claran radii, or 2.17 million km (1.35 million mi). Most probably, the magnetic field is generated similarly to that of the rest of gas giants—by currents in the liquid metallic-hydrogen layer, called a metallic-hydrogen dynamo. This magnetosphere is efficient at deflecting the solar wind particles from the Sun, even at its close orbit to the star. The moon [placeholder] orbits just outside the outer part of Clara's magnetosphere and, as such, is unable to hold on to an atmosphere. Clara's magnetosphere, like Pacifica's, produces aurorae.

See also

• Table of physical properties of planets in the Solar System
• Planet Pacifica