Ice giant (astronomy)

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Uranus2.jpg
Uranus , image from Voyager 2 in January 1986
Neptune - Voyager 2 (29347980845) flatten crop.jpg
Neptune , image from Voyager 2 in August 1989

An ice giant is a giant planet that consists mainly of volatile chemical compounds such as water (H 2 O), ammonia (NH 3 ) or methane (CH 4 ) and has a powerful atmosphere of light elements . In the solar system there are two ice giants, Uranus and Neptune . When searching for the hypothetical planet nine , one also assumes an ice giant.

Outside the solar system , the first ice giant was found in 2014. It was given the designation OGLE-2008-BLG-092L Ab . There are also exoplanets that are similar to ice giants, but differ in essential properties: the Hot Neptunes are a subclass of the ice giants, while the Mini- Neptunes are considered a class of their own.

The term "ice giant" is easy to misunderstand: the majority of the material in ice giants is not cold and is not in frozen form, but in the form of a hot supercritical fluid . This is a state of matter that combines properties of gases and liquids under high pressure and high temperature .

terminology

For a relatively long time, all four giant planets of the solar system were considered gas planets or gas giants. It was already clear in the late 1940s that Uranus and Neptune have a significantly different composition than Jupiter and Saturn . Nevertheless, for a long time they were still counted among the gas giants and not one of their own planetary classes.

Jupiter and Saturn consist of more than 90 percent by mass of hydrogen and helium , Uranus and Neptune, on the other hand, only about 20 percent by mass. For this reason, a separate name was coined for this planet class. In astrophysics and planetology , the term “ice” is used to denote volatile chemical compounds whose freezing points are above about 100  Kelvin - such as water , ammonia or methane - regardless of the physical state in which these compounds are present. This shaped the use of calling these planets ice giants. The term also coincides with the fact that the material of the planet during its formation in frozen form or in the form of gas that is trapped in water ice was present.

The term "ice giant" first came up in science fiction literature in the early 1970s . Its first scientific use was likely in a NASA report in 1978 . In the 1990s it was finally widely used.

properties

Internal structure of the four giant planets of the solar system.

Uranus has about 15 times the mass of the earth , Neptune about 17 times. This means that the two ice giants are far more massive than Earth, but significantly less massive than the gas giants Jupiter (about 318 times the mass of the earth) and Saturn (about 95 times the mass of the earth). They are also far larger in diameter than Earth, and significantly smaller than the gas giants.

The ice giants and the gas giants of the solar system have very different properties. In the case of the gas giants, it is assumed that their hydrogen (in metallic, i.e. electrically conductive form ) reaches down to their (presumably liquid) rock cores under enormous pressure. The ice giants, on the other hand, consist mainly of heavier elements. According to the abundance of the chemical elements in the universe , these are most likely oxygen , carbon , nitrogen and sulfur .

Although ice giants also have hydrogen shells, these are much smaller and do not make up most of the planet. However, the hydrogen shells make it difficult to observe the interior of the ice giants, and so current knowledge of their composition and development is limited.

Atmosphere and weather

From the study of the atmospheric patterns of the ice giants, knowledge for atmospheric physics can be gained . Their special composition is the cause of characteristic atmospheric processes. In addition, the ice giants in their distant orbits receive far less sunlight than any of the other planets in the solar system, and for this reason the influence of their internal heat on the weather patterns is greater.

The atmosphere of Uranus gives the planet the appearance of a turquoise, even pearl. Neptune is more bluish and has clearly more recognizable atmospheric structures. The blue-green color of both ice giants comes from methane crystals in the upper layers of the atmosphere. Other possible aerosols in the upper atmosphere are ammonia ice , water ice and ammonium hydrogen sulfide - (NH 4 ) HS -.

In their movement patterns, the gaseous outer shells of the ice giants show many similarities to those of the gas giants. There are long-lasting strong winds in the equatorial zones , polar vortices , large circulatory systems and complex chemical processes that are driven from above by UV radiation and by mixing with the deeper atmosphere. The biggest visible structure on Neptune the Great Dark Spot (English "Great Dark Spot"). It dissolves every few years and then reappears, in contrast to the Great Red Spot of Jupiter, which has existed for centuries.

So far there are no complete mathematical models that explain the atmospheric properties of the ice giants. Understanding these would advance our understanding of giant planetary atmospheres as a whole, and with it our understanding of the atmospheres of hot Jupiters and of exoplanets whose masses and radii lie between those of the giant and rocky planets of the solar system.

internal structure

The assumed inner structure of Uranus and Neptune:
(1) Upper atmosphere, cloud cover
(2) Atmosphere made of hydrogen, helium and methane gas
(3) Coat made of water, ammonia and methane ice
(4) Solid core made of silicates and nickel-iron

The hydrogen-methane atmospheres of the two ice giants of the solar system have no fixed lower limit, but rather go into a supercritical state in which the gas is under such high temperature and pressure that it becomes fluid . Metallic hydrogen cannot exist in this planet, the pressure is not sufficient for that. Most of the planets are covered by a mantle about which little is known. It probably consists of water, ammonia and methane ice in a hot, dense fluid state with high electrical conductivity. Planetologists refer to it as "ice" because of its chemical components, but it is not ice in the usual sense of the word. Some authors call the mantle a water-ammonia ocean.

With increasing depth, the pressure and temperature in the mantle may rise to such an extent that the methane molecules are broken up and diamonds “hail”. According to experiments at the Lawrence Livermore National Laboratory , an ocean of liquid diamond with diamond "icebergs" drifting may also be possible.

At their core, Uranus and Neptune probably have a solid core made of rock ( silicates ) and metal (especially nickel and iron ). The core of Uranus is estimated at 0.55 Earth masses, that of Neptune at 1.2 Earth masses.

Thermal radiation

Of all known giant planets in the solar system, Neptune emits the greatest internal heat per unit of sunlight it absorbs, around 2.6 times as much. The next stronger heat emitter is Saturn with 1.8 times. Uranus emits the least amount of heat, with only a tenth the value of Neptune. This is perhaps due to its extreme axis inclination of 98˚, which ensures completely different seasonal changes than those of the other planets in the solar system.

Emergence

How rocky planets and gas giants are formed is relatively simple and scientifically largely indisputable. The rocky planets of the solar system were created by himself during the compression of the protoplanetary disk increasingly by collisions into larger bodies dust particles have connected . This is how planetesimals arose, and from them planets emerged. For the formation of gas giants like Jupiter and Saturn, it is assumed that solid cores of around 10 times the mass of the earth were initially formed in the same way. Over the course of a few million years, these then accreted gas envelopes from the surrounding solar mist . There are also alternative models for the creation of the gas giants, such as the pebble accretion proposed in 2015 . Some extrasolar gas giants could also have been created by gravitational instabilities in the protoplanetary disk.

Explaining the formation of Uranus and Neptune by a similar process of nuclear accretion is far more problematic. The escape speed of small protoplanets, which are about 20 astronomical units from the Sun, is of a similar order of magnitude as their relative speeds. When such bodies crossed the orbits of Saturn or Jupiter, they would either be thrown out of the solar system, swallowed into the gas giants, or placed in eccentric cometary orbits . Therefore, there is no consensus, but various proposed approaches for the formation of the ice giants.

migration

A simple solution to circumventing the orbital difficulties of protoplanets 20 AU or more from the center is to have Uranus and Neptune formed between the orbits of Jupiter and Saturn and migrate to their outer orbits from there are.

Disc instabilities

According to model calculations, regions in the protoplanetary disk with a slightly higher density - gravitational instabilities - can lead to the formation of clumps in just 1000 years, which become planets at distances between 10 and 30 AU from the center. This is a much faster process than core accretion, which takes 100,000 to 1,000,000 years. There are several suggested scenarios of how such instabilities can arise in a previously stable disk: For example, a close encounter with another protostar can lead to this. A disk that develops magnetically has magnetic dead zones, probably because of fluctuations in the degree of ionization , in which matter accumulates and is moved by magnetic forces. And it can also come about through the way the disc accretes matter.

Photoevaporation

The phenomenon of photoevaporation has been observed in protoplanetary disks in the star cluster Trapez in the Orion Nebula . From Theta 1 Orionis C is extreme ultraviolet radiation from. The action of such high-energy photons robs planets of their atmospheres. This results in another possible mechanism for the formation of ice giants: After protoplanetary gas giants with several Jupiter masses have formed, most of their hydrogen shells are torn away by EUV radiation from a nearby massive star.

In the Carina Nebula, the EUV irradiance is approximately 100 times higher than in the Orion Nebula. Protoplanetary disks are present in both nebulae. The higher EUV radiation in the Carina Nebula should favor the formation of ice giants there.

Exploring ice giants with probes

Past missions

  • Voyager 2 ( NASA probe that visited Uranus 1985–1986 and Neptune in 1989)

Suggested missions

  • Uranus Pathfinder (proposed in 2010)
  • Uranus Orbiter and Probe (proposed in 2011; considered by NASA in 2017)
  • MUSE (Mission to Uranus for Science and Exploration, proposed in 2012; considered by NASA in 2014 and by ESA in 2016)
  • Outer Solar System (proposed in 2012)
  • ODINUS (proposed in 2013, consisting of two separate probes , one of which flies to Uranus and one to Neptune)
  • Triton Hopper (proposed in 2015; NASA considered in 2018)
  • Oceanus (proposed in 2017)

literature

  • Mark Hofstadter: The Atmospheres of the Ice Giants, Uranus and Neptune . White Paper for the Planetary Science Decadal Survey. National Research Council, 2011, p. 1–2 ( nationalacademies.org [accessed January 18, 2015]).

See also

Individual evidence

  1. ↑ Radosław Poleski, Jan Skowron, Andrzej Udalski, Cheongho Han, Szymon Kozłowski, Łukasz Wyrzykowski, Subo Dong, Michał K. Szymański, Marcin Kubiak, Grzegorz Pietrzyński, Igor Soszyński, Krzysztic Ulacrukould- Triple 2008 , Pietruzrolenski Pawezzyk, Pawezrolensk -blg-092l: Binary Stellar System With a Circumprimary Uranus-type Planet . In: The Astrophysical Journal . tape 795 , no. 1 , 2014, ISSN  0004-637X , p. 42 , doi : 10.1088 / 0004-637X / 795/1/42 .
  2. ↑ Astronomers discover first "ice giant" exoplanet. In: nasa.gov. Exoplanet Exploration: Planets Beyond our Solar System, accessed February 19, 2020 .
  3. ↑ a b c d e f g h i j k Mark Hofstadter: The Atmospheres of the Ice Giants, Uranus and Neptune . White Paper for the Planetary Science Decadal Survey. National Research Council, 2011, p. 1–2 ( nationalacademies.org [accessed January 18, 2015]).
  4. ↑ a b Mark Marley, "Not a Heart of Ice", The Planetary Society , April 2nd 2019. Link
  5. ↑ For example in Bova, B. 1971, The Many Worlds of Science Fiction (Boston, MA: EP Dutton)
  6. ↑ James A. Dunne and Eric Burgess, The Voyage of Mariner 10: Mission to Venus and Mercury , Scientific and Technical Information Division, National Aeronautics and Space Administration, 1978, 224 pages, page 2. read
  7. ↑ Karan Molaverdikhani: From Cold to Hot Irradiated Gaseous Exoplanets: Toward an Observation-based Classification Scheme . In: The Astrophysical Journal . 873, No. 1, 2019, p. 32. arxiv : 1809.09629 . bibcode : 2019ApJ ... 873 ... 32M . doi : 10.3847 / 1538-4357 / aafda8 .
  8. ↑ Dava Sobel: The planets . eBook Berlin Verlag, 2010, ISBN 978-3-8270-7238-2 , pp. 122 ( books.google.de ).
  9. ^ S. Atreya, P. Egeler, K. Baines: Water-ammonia ionic ocean on Uranus and Neptune? . In: Geophysical Research Abstracts . 8, 2006, p. 05179.
  10. ↑ Is It Raining Diamonds on Uranus . SpaceDaily.com. October 1, 1999. Retrieved May 17, 2013.
  11. ^ Sarah Kaplan, It rains solid diamonds on Uranus and Neptune . In: The Washington Post , August 25, 2017. Retrieved August 27, 2017. 
  12. ↑ D. Kraus, J. Vorberger, A. Pak, NJ Hartley, LB Fletcher, S. Frydrych, E. Galtier, EJ Gamboa, DO Gericke, SH Glenzer, E. Granados, MJ MacDonald, AJ MacKinnon, EE McBride, I Nam, P. Neumayer, M. Roth, AM Saunders, AK Schuster, P. Sun, T. van Driel, T. Döppner, RW Falcone: Formation of diamonds in laser-compressed hydrocarbons at planetary interior conditions . In: Nature Astronomy . 1, No. 9, September 2017, pp. 606–611. bibcode : 2017NatAs ... 1..606K . doi : 10.1038 / s41550-017-0219-9 .
  13. ^ Outer planets may have oceans of diamond (en-AU) . In: ABC Science , January 18, 2010. Retrieved October 9, 2017. 
  14. ↑ Emily Baldwin: Oceans of diamond possible on Uranus and Neptune Archived from the original on December 3, 2013. In: Astronomy Now . January 21, 2010. Retrieved February 6, 2014.
  15. ↑ JJ Lissauer, O. Hubickyj, G. D'Angelo, P. Bodenheimer: Models of Jupiter's growth Incorporating thermal and hydrodynamic constraints . In: Icarus . 199, No. 2, 2009, pp. 338-350. arxiv : 0810.5186 . bibcode : 2009Icar..199..338L . doi : 10.1016 / j.icarus.2008.10.004 .
  16. ↑ a b Gennaro D'Angelo, Richard H. Durisen, Jack J. Lissauer: Giant Planet Formation . In: Sara Seager (Ed.): Exoplanets . University of Arizona Press, December 2010, ISBN 978-0-8165-2945-2 , pp. 319-346, arxiv : 1006.5486 .
  17. ↑ Harold F. Levison, Katherine A. Kretke, & Martin J. Duncan. Growing the gas giant planets by the gradual accumulation of pebbles. Nature, 2015 DOI: 10.1038 / nature14675
  18. ↑ a b c d e f g h Alan P. Boss: Rapid Formation of Outer Giant Planets by Disk Instability . In: The Astrophysical Journal . 599, No. 1, December 2003, pp. 577-581. bibcode : 2003ApJ ... 599..577B . doi : 10.1086 / 379163 . , §1–2
  19. ↑ Bruno Christophe, LJ Spilker, JD Anderson, N. André, SW Asmar, J. Aurnou, D. Banfield, A. Barucci, O. Bertolami, R. Bingham, P Brown, B. Cecconi, J.-M. Courty, H. Dittus, LN Fletcher, B. Foulon, F. Francisco, PJS Gil, KH Glassmeier, W. Grundy, C. Hansen, J. Helbert, R. Helled, H. Hussmann, B. Lamine, C. Lämmerzahl , L. Lamy, R. Lehoucq, B. Lenoir, A. Levy, et al .: OSS (Outer Solar System): a fundamental and planetary physics mission to Neptune, Triton and the Kuiper Belt . In: Springer (Ed.): Experimental Astronomy . 34, No. 2, July 8, 2012, pp. 203-242. arxiv : 1106.0132 . doi : 10.1007 / s10686-012-9309-y .