Ocean world

Definitions According to Lunine, "oceans" have been defined as "stable, globe-girdling bodies of liquid water." In addition, "Ocean worlds is the label given to objects in the solar system that host stable, globe-girdling bodies of liquid water," in contrast to the terms "'ocean planet' and 'water world', both of which refer to exoplanets (planets orbiting other stars) with substantial mass fractions of water in their bulk compositions." Europa and Enceladus are considered compelling targets for exploration due to their thin outer crusts and cryovolcanic features. Other bodies in the Solar System are considered candidates to host subsurface oceans based upon a single type of observation or by theoretical modeling, including Ariel, Umbriel, Ceres, Miranda, Kepler-22b, Kepler-62e, Kepler-62f, and the planets of Kepler-11 More recently, the exoplanets TOI-1452 b, Kepler-138c, and Kepler-138d have been found to have densities consistent with large fractions of their mass being composed of water. water accounts for only 0.05% of Earth's mass. An extraterrestrial ocean could be so deep and dense that even at high temperatures the pressure would turn the water into ice. The immense pressures of many thousands of bar in the lower regions of such oceans, could lead to the formation of a mantle of exotic forms of ice such as ice V. This ice would not necessarily be as cold as conventional ice. If the planet is close enough to its star that the water reaches its boiling point, the water will become supercritical and lack a well-defined surface. Even on cooler water-dominated planets, the atmosphere can be much thicker than that of Earth, and composed largely of water vapor, producing a very strong greenhouse effect. Such planets would have to be small enough not to be able to retain a thick envelope of hydrogen and helium, or be close enough to their primary star to be stripped of these light elements. Otherwise, they would form a warmer version of an ice giant instead, like Uranus and Neptune. ==History==

Gravitational calculations suggested by the start of 20th century that Europa's composition was water rich, and Earth ground based observations by Gerard Kuiper revealed 1957 the water ice composition. Important preliminary theoretical work was carried out prior to the planetary missions of the 1970s. In particular, Lewis showed in 1971 that radioactive decay alone was likely sufficient to produce subsurface oceans in large moons, especially if ammonia (Ammonia|) were present. Peale and Cassen figured out in 1979 the important role of tidal heating (aka: tidal flexing) on satellite evolution and structure. Meanwhile, the Kepler space observatory, launched on March 7, 2009, has discovered thousands of exoplanets, about 50 of them of Earth-size in or near habitable zones. Planets of many masses, sizes, and orbits have been detected, illustrating not only the variable nature of planet formation but also a subsequent migration through the circumstellar disc from the planet's place of origin. In August 2022, TOI-1452 b, a super-Earth exoplanet with potential deep oceans that is 99 light-years from Earth, was discovered by the Transiting Exoplanet Survey Satellite. ==Formation==

image of HL Tauri, a protoplanetary disk Planetary objects that form in the outer Solar System begin as a comet-like mixture of roughly half water and half rock by mass, displaying a density lower than that of rocky planets. Icy planets and moons that form near the frost line should contain mostly and silicates. Those that form farther out can acquire ammonia () and methane () as hydrates, together with CO, Nitrogen|, and Carbon dioxide|. Conversely, planets that formed close to their host stars are less likely to have water because the primordial disks of gas and dust are thought to have hot and dry inner regions. So if a water world is found close to a star, it would be strong evidence for migration and ex situ formation, Outward migration may also occur under particular conditions. Inward migration presents the possibility that icy planets could move to orbits where their ice melts into liquid form, turning them into ocean planets. This possibility was first discussed in the astronomical literature by Marc Kuchner in 2003. ==Structure==

The internal structure of an icy astronomical body is generally deduced from measurements of its bulk density, gravity moments, and shape. Determining the moment of inertia of a body can help assess whether it has undergone differentiation (separation into rock-ice layers) or not. Shape or gravity measurements can in some cases be used to infer the moment of inertia – if the body is in hydrostatic equilibrium (i.e. behaving like a fluid on long timescales). Proving that a body is in hydrostatic equilibrium is extremely difficult, but by using a combination of shape and gravity data, the hydrostatic contributions can be deduced. Some of the solid-phase water could be in the form of ice VII. Maintaining a subsurface ocean depends on the rate of internal heating compared with the rate at which heat is removed, and the freezing point of the liquid. Ocean survival and tidal heating are thus intimately linked. Smaller ocean planets would have less dense atmospheres and lower gravity; thus, liquid could evaporate much more easily than on more massive ocean planets. Simulations suggest that planets and satellites of less than one Earth mass could have liquid oceans driven by hydrothermal activity, radiogenic heating, or tidal flexing. Where fluid-rock interactions propagate slowly into a deep brittle layer, thermal energy from serpentinization may be the primary cause of hydrothermal activity in small ocean planets. The dynamics of global oceans beneath tidally flexing ice shells represents a significant set of challenges which have barely begun to be explored. The extent to which cryovolcanism occurs is a subject of some debate, as water, being denser than ice by about 8%, has difficulty erupting under normal circumstances. Nevertheless, imaging data from the Voyager 2, Cassini-Huygens, Galileo and New Horizons spacecraft revealed cryovolcanic surface features on several of the icy bodies in our own solar system. Recent studies suggest that cryovolcanism may occur on ocean planets that harbor internal oceans beneath layers of surface ice as it does on the icy moons Enceladus and Europa in our own solar system. Liquid water oceans on extrasolar planets could be significantly deeper than the Earth's ocean, which has an average depth of 3.7 km. Depending on the planet's gravity and surface conditions, exoplanet oceans could be up to hundreds of times deeper. For example, a planet with a 300 K surface can possess liquid water oceans with depths from 30 to 500 km, depending on its mass and composition. ==Atmospheric models==

, a large ocean world with a hydrogen atmosphere To allow surface water to be liquid for long periods of time, a planet—or moon—must orbit within the habitable zone (HZ), possess a protective magnetic field, and have the gravitational pull needed to retain an ample amount of atmospheric pressure. The amount of water lost seems proportional with the planet mass, since the diffusion-limited hydrogen escape flux is proportional to the planet surface gravity. During a runaway greenhouse effect, water vapor reaches the stratosphere, where it is easily broken down (photolyzed) by ultraviolet radiation (UV). Heating of the upper atmosphere by UV radiation can then drive a hydrodynamic wind that carries the hydrogen (and potentially some of the oxygen) to space, leading to the irreversible loss of a planet's surface water, oxidation of the surface, and possible accumulation of oxygen in the atmosphere. Composition models There are challenges in examining an exoplanetary surface and its atmosphere, as cloud coverage influences the atmospheric temperature, structure as well as the observability of spectral features. However, planets composed of large quantities of water that reside in the habitable zone (HZ) are expected to have distinct geophysics and geochemistry of their surface and atmosphere. The atmospheric structure, as well as the resulting HZ limits, depend on the density of a planet's atmosphere, shifting the HZ outward for lower mass and inward for higher mass planets. Atmospheric models for Kepler-62f show that an atmospheric pressure of between 1.6 bar and 5 bar of are needed to warm the surface temperature above freezing, leading to a scaled surface pressure of 0.56–1.32 times Earth's. == Oceanography ==

It is suggested that strong ocean currents exist in Enceladus, Titan, Ganymede, and Europa. In Enceladus, oceanic heat flux inferred from ice shell thickness suggests the upwelling of warm water at the poles and downwelling of colder water at low latitudes. Europa is predicted to have an equatorial upwelling of warm water with greater heat transfer at low latitudes. Titan and Ganymede are hypothesized to behave as a non-rotating system and have no coherent heat transfer patterns. ==Astrobiology==

The characteristics of ocean worlds or ocean planets provide clues to their history, and the formation and evolution of the Solar System as a whole. Of additional interest is their potential to form and host life. Life as we know it requires liquid water, a source of energy, and nutrients, and all three key requirements can potentially be satisfied within some of these bodies, An ocean world's habitation by Earth-like life is limited if the planet is completely covered by liquid water at the surface, even more restricted if a pressurized, solid ice layer is located between the global ocean and the lower rocky mantle. Simulations of a hypothetical ocean world covered by five Earth oceans' worth of water indicate the water would not contain enough phosphorus and other nutrients for Earth-like oxygen-producing ocean organisms such as plankton to evolve. On Earth, phosphorus is washed into the oceans by rainwater hitting rocks on exposed land, so the mechanism would not work on an ocean world. Simulations of ocean planets with 50 Earth oceans' worth of water indicate the pressure on the sea floor would be so immense that the planet's interior would not sustain plate tectonics to cause volcanism to provide the right chemical environment for terrestrial life. On the other hand, small bodies such as Europa and Enceladus are regarded as particularly habitable environments because the theorized locations of their oceans would almost certainly leave them in direct contact with the underlying silicate core, a potential source of both heat and biologically important chemical elements. Oxygen Molecular oxygen () can be produced by geophysical processes, as well as a byproduct of photosynthesis by life forms, so although encouraging, is not a reliable biosignature. In fact, planets with high concentration of in their atmosphere may be uninhabitable. Abiogenesis in the presence of massive amounts of atmospheric oxygen could be difficult because early organisms relied on the free energy available in redox reactions involving a variety of hydrogen compounds; on an -rich planet, organisms would have to compete with the oxygen for this free energy. == See also ==