- Education and Science
Outer space - Human discoveries
Pluto and its moon
An ice planet is a theoretical type of exoplanet with an icy surface of volatiles such as water, ammonia, and methane. Ice planets consist of a global cryosphere. They are bigger versions of the small icy worlds of the Solar System, which includes the moons Europa, Enceladus, and Triton, the dwarf planets Pluto and Eris, and many other small Solar System bodies such as comets.
Ice planets usually appear nearly white with geometric albedos of more than 0.9.[dubious – discuss] An ice planet's surface can be composed of water, methane, ammonia, carbon dioxide (known as "dry ice"), carbon monoxide, and other volatiles, depending on its surface temperature. Ice planets would have surface temperatures below 260 K (−13°C) if composed primarily of water, below 180 K (−93°C) if primarily composed of CO2 and ammonia, and below 80 K (−193°C) if composed primarily of methane.
On the surface, ice planets are hostile to life forms like those living on Earth because they are extremely cold. Many ice worlds likely have subsurface oceans, warmed by internal heat or tidal forces from another nearby body. Liquid subsurface water would provide habitable conditions for life, including fish, plankton, and microorganisms. Subsurface plants as we know it could not exist because there is no sunlight to use for photosynthesis. Microorganisms can produce nutrients using specific chemicals (chemosynthesis) that may provide food and energy for other organisms. Some planets, if conditions are right, may have significant atmospheres and surface liquids like Saturn's moon Titan, which could be habitable for exotic forms of life.
Icy worlds in the outer solar system could harbour life because of tidal heating from an orbital dance.
It is impossible for water to exist on the surface of the icy worlds beyond the orbit of Neptune, known as Trans-Neptunian Objects (TNOs). On the surface of these worlds, that include the Pluto-Charon and Eris-Dysnomia systems, the surface temperatures are well below 200 degrees Celsius. These objects have densities similar to moons known to have subsurface oceans in the solar system. There is strong evidence for subsurface oceans on Ganymede, Callisto and Europa in orbit around Jupiter, and Enceladus in orbit around Saturn. Triton, a moon of Neptune is also believed to have a subsurface ocean. These moons are prime candidates for the search for extraterrestrial life within the solar system.
While it is highly unlikely that liquid water exists on the surface of the TNOs, research suggests that the subsurface oceans on these remote worlds in the outer reaches of the solar system could still maintain liquid water in their interiors. Any subsurface oceans are believed to exist for a brief period because of the radiation from the interiors of the object, from the time they were formed. Eventually, the radioactive elements become stable, and the oceans freeze. New research suggests that there might be a previously unknown source of heat that could significantly extend the lifespan of subsurface oceans on remote icy worlds.
The increased life span can be because of the heat generated from the gravitational pull. The worlds try to enter into a stable orbit with the parent object, but this is not always possible. The remote worlds are bombarded by collisions with other objects, and each time this occurs, their orbits are disturbed slightly. The moons then try to realign their orbits with the parent object, such as Charon with Pluto, and the attraction due to gravity repeatedly squeezes the moons. This process could potentially generate enough friction to heat up the interiors enough to maintain a subsurface liquid water ocean.
Wade Henning of NASA, and co-author of the study says, "We found that tidal heating can be a tipping point that may have preserved oceans of liquid water beneath the surface of large TNOs like Pluto and Eris to the present day."
The process is known as tidal heating, and the finding could extend the number of possible candidate worlds within the solar system that can harbour life. Spectral analysis of the remote icy worlds reveal the presence of crystalline water ice and ammonia hydrates. These are not expected to survive for long on the surface of these worlds because in extremely cold temperatures, ice is more amorphous than crystalline, and the space radiation breaks down ammonia hydrates. The analysis of the light from these worlds indicate that these materials originate from within the bodies themselves, indicating a subsurface ocean. The researchers hope develop better models to predict the duration for which tidal heating can be expected to maintain subsurface oceans on these icy worlds.
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