Ocean world
In June 2020, NASA scientists reported that it is likely that exoplanets with oceans are common in the Milky Way galaxy, based on mathematical modeling studies.
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,[14] Titania,[15][16] Umbriel,[17] Ceres,[3] Dione,[18] Mimas,[19][20] Miranda,[14] Oberon,[4][21] Pluto,[22] Triton,[23] Eris,[4][24] and Makemake.
[39] 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,[40] or be close enough to their primary star to be stripped of these light elements.
Marc Kuchner in 2003 and 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.
[41][42] 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.
[46] In June 2020, NASA scientists reported that it is likely that exoplanets with oceans may be common in the Milky Way galaxy, based on mathematical modeling studies.
[49] 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.
The internal structure of an icy astronomical body is generally deduced from measurements of its bulk density, gravity moments, and shape.
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).
[3] Specific techniques to detect inner oceans include magnetic induction, geodesy, librations, axial tilt, tidal response, radar sounding, compositional evidence, and surface features.
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.
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] 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.
[56] 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,[57][58][10] and have the gravitational pull needed to retain an ample amount of atmospheric pressure.
A strong planetary magnetosphere, maintained by internal dynamo action in an electrically conducting fluid layer, is helpful for shielding the upper atmosphere from stellar wind mass loss and retaining water over long geological time scales.
During a runaway greenhouse effect, water vapor reaches the stratosphere, where it is easily broken down (photolyzed) by ultraviolet radiation (UV).
[48] The fate of a given planet's atmosphere strongly depends on the extreme ultraviolet flux, the duration of the runaway regime, the initial water content, and the rate at which oxygen is absorbed by the surface.
[60] 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.
[74] 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.
