Extraterrestrial ocean
An extraterrestrial ocean is an ocean on another celestial body that is either on its surface or in deeper layers. (See also: Aquasphere .)
No such ocean has yet been identified. While the astronomers of earlier centuries assumed oceans on the Earth's moon and on several planets of the solar system , today it is assumed that there are no oceans there. However, among the moons of the solar system there are several that could own an ocean. Outside the solar system, there could also be ocean planets whose surface is entirely covered with water.
The exploration of possible extraterrestrial oceans is an important goal of current space research .
Historical hypotheses
In the early days of lunar research, the dark surfaces of the earth's moon were considered to be seas, which is why they were referred to as maria after Giovanni Riccioli . However, Galileo Galilei had doubts about this thesis as early as 1632 in his book Dialogo sopra i due massimi sistemi del mondo (Dialogue on the two most important world systems, the Ptolemaic and the Copernican). In fact, the Mary of the Moon are only dark plains.
Something similar was thought of Mars . Its dark surface features, which are visible in telescopes, were once mistaken for seas and were given names such as Mare Erythraeum , Mare Sirenum or Aurorae Sinus . However, these names are no longer used today. In the late 18th century, William Herschel first described the rise and fall of the polar caps of Mars with the change of the seasons in the respective hemisphere . As early as the middle of the 19th century, astronomers knew that Mars had some similarities to Earth, such as the length of a Martian day (called a sol) being almost the same as a day on Earth. They also knew that its axis tilt is similar to that of Earth, which means that Mars also has seasons like Earth. However, these take almost twice as long, since a Martian year is much longer with 687 days. In 1854, William Whewell theorized that Mars had oceans, land, and possibly life forms . After telescope observations of the Martian channels , which later turned out to be an optical illusion, speculations about life on Mars exploded at the end of the 19th century. For example, the American astronomer Percival Lowell published his book Mars in 1895 , followed by Mars and its Canals in 1906, in which he suggested that the canals were the works of a civilization long ago.
For a long time Venus was also believed to have oceans of water on it. Early astronomers could only see a world wrapped in clouds. Since clouds were associated with water, it was assumed that under this cloud cover it would have to rain incessantly and consequently there was a lot of water. This belief persisted until the early days of space travel, when the Mariner 2 probe first sent measurement data from Venus to Earth on December 14, 1962, showing that Venus is not a planet with a warm, humid climate. Although there is a large amount of sulfuric acid in Venus' atmosphere, it does not rain down to the surface.
Ice moons
An ocean probably exists, hidden under a mighty, opaque ice crust
- on Jupiter's moons Europa and Ganymede , perhaps also on Callisto ,
- on Saturn's moons Titan , Enceladus and Dione as well as Mimas
- on the Uranus moons Titania and Oberon .
In addition, an ammonia-rich ocean is suspected on the Neptune moon Triton .
In order for water to exist below the surface in liquid form, various factors are decisive:
- Heat: Due to tidal forces , caused by the giant planets and neighboring moons, the lunar bodies are deformed, causing their inner materials to rub against each other. Due to the friction , parts of the kinetic energy are converted into thermal energy . Also radioactive decay in the core of the moon could release heat energy.
- Salts dissolved in the water reduce the melting point.
- High pressure reduces the melting point.
Technologies (e.g. cryobots ) that are being tested on Earth in the subglacial lakes of the Antarctic could be used for future research into such aquasphere . The development of life under similarly extreme conditions can also be researched at these terrestrial subglacial lakes .
In 2015, the Pluto companion Charon was examined by the New Horizons space probe for superficial traces of a former ocean.
Former extraterrestrial oceans
Mars
Mars appears today as a dry desert planet. The results of the Mars missions available so far, however, allow the conclusion that the Martian atmosphere was much denser in the past (billions of years ago) and that there was abundant liquid water on the surface of the planet. Today it has a very thin atmosphere, which means that water can no longer exist in liquid form on the surface of Mars, except for a short time in the deepest areas.
Venus
Infrared measurements from the “ Galileo ” probe from 1990, which were re-evaluated in 2009, suggest that Venus may once have had oceans and tectonic activities.
Outside the solar system
Current observation techniques are not sufficient to detect oceans on planets or moons of other stars. Evidence of water vapor in the atmosphere was only possible in a few exoplanets, such as Gliese 1214b .
See also
Individual evidence
- ↑ Alfred Russel Wallace: Is Mars habitable ?: A critical examination of Professor Percival Lowell's book 'Mars and its canals,' with an alternative explanation . Macmillan, London 1907, OCLC 263175453 .
- ↑ Fraser Cain: Is There Water on Venus? Universe Today, June 29, 2009, accessed September 24, 2014 .
- ↑ Archive link ( Memento of the original from March 15, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Charon, another moon with wetlands . In: zeit.de, accessed on October 3, 2014
- ↑ Were there once continents and oceans on Venus? weltderphysik.de, accessed on September 30, 2015 .
- ↑ F. Taylor, D. Grinspoon: Climate evolution of Venus. In: Journal of Geophysical Research. 114, 2009, doi : 10.1029 / 2008JE003316 .
