Ocean planet
An ocean planet, ocean world, water world, aquaplanet or panthalassic planet is a type of terrestrial planet that contains a substantial amount of water either at its surface or subsurface.[1][2][3][4] The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid,[5] such as lava (the case of Io), ammonia (in a eutectic mixture with water, as is likely the case of Titan's inner ocean) or hydrocarbons like on Titan's surface (which could be the most abundant kind of exosea).[6]
Earth is the only astronomical object known to have bodies of liquid water on its surface, although several exoplanets have been found with the right conditions to support liquid water.[7] For exoplanets, current technology cannot directly observe liquid surface water, so atmospheric water vapor may be used as a proxy.[8] 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 originate and host life.
Contents
Overview[edit]
Water worlds are of extreme interest to astrobiologists for their potential to develop life and sustain biological activity over geological timescales.[4][3] The four best established water worlds in the Solar System include Europa, Enceladus, Ganymede, and Callisto.[3] A host of other bodies in the outer Solar System are inferred by a single type of observation or by theoretical modeling to have subsurface oceans, and these include: Dione, Pluto, Triton, and Ceres,[3][10][11][12][13][14] as well as Mimas,[15][16] Eris,[4][17] and Oberon.[4][17]
History[edit]
Important preliminary theoretical work was carried prior to the planetary missions launched starting in 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 (NH
3) was present. Peale and Cassen figured out in 1979 the important role of tidal heating (aka: tidal flexing) on satellite evolution and structure.[3] The first confirmed detection of an exoplanet was in 1992. Alain Léger et al figured in 2004 that a small number of icy planets that form in the region beyond the snow line can migrate inward to ∼1 AU, where the outer layers subsequently melt.[18][19]
3) was present. Peale and Cassen figured out in 1979 the important role of tidal heating (aka: tidal flexing) on satellite evolution and structure.[3] The first confirmed detection of an exoplanet was in 1992. Alain Léger et al figured in 2004 that a small number of icy planets that form in the region beyond the snow line can migrate inward to ∼1 AU, where the outer layers subsequently melt.[18][19]
The cumulative evidence collected by the Hubble Space Telescope, as well as Pioneer, Galileo, Voyager, Cassini–Huygens, and New Horizons missions, strongly indicate that several outer Solar System bodies harbour internal liquid water oceans under an insulating ice shell.[3][20] Meanwhile, the Kepler space observatory, launched in March 7, 2009, has discovered thousands of exoplanets, about 50 of them of Earth-size in or near habitable zones.[21][22]
Planets of almost all 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.[8] As of 1 October 2019, there are 4,118 confirmed exoplanets in 3,063 systems, with 669 systems having more than one planet.[23]
Formation[edit]
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.[19] Icy planets and moons that form near the frost line should contain mostly H
2O and silicates. Those that form farther out can acquire ammonia (NH
3) and methane (CH
4) as hydrates, together with CO, N
2, and CO
2.[24]
2O and silicates. Those that form farther out can acquire ammonia (NH
3) and methane (CH
4) as hydrates, together with CO, N
2, and CO
2.[24]
Planets that form prior to the dissipation of the gaseous circumstellar disk experience strong torques that can induce rapid inward migration into the habitable zone, especially for planets in the terrestrial mass range.[25][24] Since water is highly soluble in magma, a large fraction of the planet's water content will initially be trapped in the mantle. As the planet cools and the mantle begins to solidify from the bottom up, large amounts of water (between 60% and 99% of the total amount in the mantle) are exsolved to form a steam atmosphere, which may eventually condense to form an ocean.[25] Ocean formation requires differentiation, and a heat source, either radioactive decay, tidal heating, or the early luminosity of the parent body.[3] Unfortunately, the initial conditions following accretion are theoretically incomplete.
Planets that formed in the outer, water-rich regions of a disk and migrated inward are more likely to have abundant water.[26] 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,[27] because insufficient volatiles exist near the star for in situ formation.[2] Simulations of Solar System formation and of extra-solar system formation have shown that planets are likely to migrate inward (i.e., toward the star) as they form.[28][29][30] Outward migration may also occur under particular conditions.[30] 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[24] and Alain Léger in 2004.[31]
Structure[edit]
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). However, 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.[3] Specific techniques to detect inner oceans include magnetic induction, geodesy, librations, axial tilt, tidal response, radar sounding, compositional evidence, and surface features.[3]
A generic icy moon will consist of a water layer sitting atop a silicate core. For a small satellite like Enceladus, an ocean will sit directly above the silicates and below a solid icy shell, but for a larger ice-rich body like Ganymede, pressures are sufficiently high that the ice at depth will transform to higher pressure phases, effectively forming a "water sandwich" with an ocean located between ice shells.[3] An important difference between these two cases is that for the small satellite the ocean is in direct contact with the silicates, which may provide hydrothermal and chemical energy and nutrients to simple life forms.[3] Because of the varying pressure at depth, models of a water world may include "steam, liquid, superfluid, high-pressure ices, and plasma phases" of water.[32] Some of the solid-phase water could be in the form of ice VII.[33]
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.[3] 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.[4] 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.[4] 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.[3]
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