Enceladus (moon)

Enceladus
Mosaic of Enceladus in false colors from images of the Cassini spacecraft
Central body	Saturn
Properties of the orbit
Major semi-axis	237,948 km
Periapsis	236,830 km
Apoapsis	239,066 km
eccentricity	0.0047
Orbit inclination	0.019 °
Orbital time	1,370217824 d
Mean orbital velocity	12.6353 km / s
Physical Properties
Albedo	0.81 ± 0.04 (Bondsche), 1.375 ± 0.008 (geometric)
Apparent brightness	11.8 mag
Medium diameter	504.2 ± 0.4 km (513.2 × 502.8 × 496.6) km
Dimensions	1.08022 ± 0.00101 × 10 20 kg
surface	798,648 km 2
Medium density	1.608 ± 0.003 g / cm 3
Sidereal rotation	1.370217824 days
Axis inclination	0 °
Acceleration of gravity on the surface	0.114 m / s 2
Escape speed	239 m / s
Surface temperature	(−240.3 to −198 to −128 ° C) 32.9 to 75 to 145 K
discovery
Explorer	Wilhelm Herschel
Date of discovery	August 28, 1789
Remarks	Enceladus has an atmosphere with <10 −6 Pa
The positions of the inner moons of Saturn in Saturn's ring system, from inside to outside: Pan, Atlas, Prometheus, Pandora, Janus and Epimetheus, Mimas, Enceladus, Tethys, Dione and Rhea

Enceladus (from ancient Greek Ἐγκέλαδος Enkélados ; also Saturn II ) is one of the largest moons of Saturn . In terms of mass and diameter, it is in sixth position and in terms of distance from the central planet Saturn in fourteenth position of the 82 known moons . It is an ice moon and shows cryovolcanic activities, the very high fountains of water ice particles from which create a thin atmosphere in the southern hemisphere. These fountains likely feed Saturn's E ring . In the area of ​​volcanic activity, evidence of liquid water has also been found, making Enceladus one of the possible places in the solar system with favorable conditions for life to arise.

Discovery and naming

Enceladus was discovered on August 28, 1789 by the German-British astronomer Wilhelm Herschel .

Enceladus is the sixth Saturn moon discovered and the twelfth moon discovered in the entire solar system . Due to its orbit, which was the second closest to Saturn at that time, it was designated as the second innermost of the seven previously known large moons of Saturn by the International Astronomical Union (IAU) with the Roman number II .

The moon was named after the giant Enkelados (Latin form: Enceladus) from Greek mythology . The name is stressed on the second syllable.

The name "Enceladus" and names for seven other Saturn moons were proposed by Wilhelm Herschel's son, the astronomer John Herschel , in the 1847 publication Results of Astronomical Observations made at the Cape of Good Hope . They should be named after siblings of the titan Kronos , who corresponds to the Roman Saturn .

Track properties

Orbit

Enceladus orbits Saturn in a prograde , almost perfectly circular orbit at a mean distance of 237,948 km (approx. 3.948 Saturn radii) from its center (or the center of gravity ), i.e. approx. 177,680 km above its cloud ceiling. The orbit eccentricity is 0.0047, the orbit is inclined 0.019 ° to the equator of Saturn , so it is almost in the equatorial plane of the planet. Due to the low eccentricity, the orbit varies in the distance to Saturn by about 2,236 km.

The orbit of the next inner moon Pallene is on average about 25,668 km from the orbit of Enceladus, the distances of the orbits of the next outer moons Tethys and their Trojan moons Telesto and Calypso are on average about 56,671 km.

Enceladus orbits Saturn in 1 day, 8 hours, 53 minutes and 6.82 seconds. Enceladus needs 5 hours and about 12.7 minutes longer than its inner neighbor Pallene to make one revolution.

Railway resonances

Enceladus is in gravitational interaction with its neighborhood. He is currently in a 2: 1 orbit resonance with Dione and close to a 3: 2 resonance with Mimas . In addition, Enceladus runs almost in a 4: 3 orbit resonance with the nearest outer moon, Tethys . In addition, it disturbs its immediate inner neighbor Pallene through its gravitation and causes deviations in their orbit on the order of about 4 km.

E ring

E-ring with ejection from Enceladus

The E-ring, sometimes called the "Enceladus ring", is the outermost of the regular Saturn rings. It is extremely wide compared to the other rings on the planet; it is a very diffuse disk of microscopic ice or dust particles (with silicates , carbon dioxide and ammonia ), which extends roughly from the orbit of Mimas to the orbit of Rhea, although some observations suggest that it even extends up to to the Titan-Bahn, which means a width between 340,000 and 1,040,000 km. According to various mathematical calculations, such a ring is unstable and has a lifespan between 10,000 and a million years, so it must be constantly fed with new material. Enceladus orbit is within the ring, at its narrowest, but also currently densest location. For this reason, it is believed that Enceladus is the main source of the ring particles. This theory was supported by Cassini's flyby. There are two different mechanisms that can feed the ring: The first and possibly most important source are the cryovolcanic regions at the South Pole, which eject material, the majority of which falls back to the surface, but due to Enceladus' low escape speed of 866 km / Particles can escape and enter an orbit around Saturn. The second mechanism is bombardment by micrometeorites that hit the surface of the moon and release dust particles. The latter process is not unique to Enceladus, it affects all moons orbiting Saturn within the E ring.

rotation

The axis of rotation is not inclined towards the orbit, so it is perpendicular to the plane of rotation. The rotation time is the same as the orbital time and Enceladus shows, like the Earth's moon and all the large satellites of the gas giants , a synchronous rotation , which thus also takes place within 1 day, 8 hours, 53 minutes and 6.82 seconds, i.e. always shows same hemisphere to Saturn.

Physical Properties

size

Size comparison: Enceladus and Great Britain (photomontage)

Enceladus is roughly spherical in shape, with a mean diameter of 504.2 km. The exact dimensions are 513.2 km × 502.8 km × 496.6 km. The deviation of about 3% is due to the tidal forces of Saturn, which gives the moon the shape of an ellipsoid . The long axis is aligned with Saturn, the middle axis is between the leading and following hemispheres and the shortest axis is between the poles . Enceladus is the sixth largest moon of Saturn and ranks 17th among all planetary moons in the entire solar system .

In terms of size, Enceladus is most comparable to the second largest main belt - asteroid Vesta or the fifth largest Uranus moon Miranda .

The total area of ​​Enceladus is around 798,650 km², which roughly corresponds to the area of Mozambique or Pakistan . The area can also be compared with that of France and Great Britain together (excluding overseas territories).

internal structure

Enceladus is believed to be composed primarily of water ice . At 1.61 g / cm ³ , it has the third largest density of all of Saturn's large moons (only exceeded by Phoebe and Titan ). In its interior there must therefore be larger proportions of dense material, such as silicate rock; it is therefore a differentiated body.

surface

Close-ups of Cassini show clearly distinguishable terrain.

Enceladus is exceptionally bright because it is extensively covered with pure water ice, which reflects 99% of the sunlight. This is the highest albedo of any celestial body in the solar system; it even exceeds the reflexivity of freshly fallen snow. Due to the high reflection of sunlight, Enceladus usually has temperatures below −200 ° C or below 70  Kelvin .

Various terrains could be made out on its surface. In addition to impact craters , flat plains and pronounced fractures and faults are visible. Part of its surface appears to be relatively young, with an estimated age of 100 million years. This suggests that Enceladus is geologically active. The cause is obviously cryovolcanism (cold volcanism), in which water emerges from the interior of the moon and is distributed over the surface. Enceladus is the smallest known body in the solar system with such geological activity.

The largest named crater on Enceladus, Ali Baba, is only 34 km in diameter. The longest rift system, Samarkand Sulci , extends over 360 km. The names of all formations of the satellite were set by the IAU to be those from the Arabian Nights .

Exploration by the Cassini probe

The Cassini space probe examined the moon in several close pass-by flights from March 2005. It discovered a magnetic field and a thin water vapor atmosphere. Since Enceladus's gravity is too weak to hold the gases for long periods of time, this suggests a permanent source on the moon itself. The gases come either from the surface or from inside the moon. It was thought that they could be ejected by volcanoes , geysers, or other activity. Enceladus is, besides Titan, the second moon of Saturn that has an atmosphere. However, the Enceladus atmosphere appears to be limited to the geologically active southern polar region, as further data from the Cassini mission revealed.

Volcanic activity in the southern polar region

Model of a "cold geyser" on Enceladus

Cryovolcanic activity on Enceladus

Surprisingly, there is a zone of local warming at the south pole of this moon, which heats the surface there by about 20 to 25 K more than would be expected. The energy source for the volcanic processes is unknown. However, different models are being discussed. Enceladus is actually far too small for radioactive decay to cause significant warming in the interior of the moon. It orbits Saturn in a 2: 1 resonance with the moon Dione (like the moons Io and Europa the Jupiter ), whereby tidal forces become effective, which cause friction in the interior of the moon and thus a warming. However, this mechanism is not sufficient to generate enough heat to liquefy water ice. The total heating rate, which results from the sum of possible radioactive decay in the interior and the maximum tidal forces, is only about a tenth of the observed thermal energy. There could be chemicals inside Enceladus that lower the melting point of the ice. The presence of ammonia, which could cause this, is discussed. Waite et al. Published new measurement data from Cassini in 2009, in which ammonia could be detected for the first time.

Temperature profile on the so-called "tiger stripes"

In the geologically active region, the surface is criss-crossed by parallel strips hundreds of kilometers long, consisting of crevices up to 300 meters deep in which crystalline ice penetrates to the surface. The appearance of the surroundings is reminiscent of a temporarily solidified, viscous mass. The ice may move in convection currents beneath the surface and trigger cryovolcanic crevice activity. The effects of the process are reminiscent of the Earth's plate tectonics or comparable activities on Jupiter's moon Europa. The output volume of the tiger stripes fluctuates cyclically. The geysers appear weakest when Enceladus is at the closest point of its orbit to Saturn, and then its activity increases continuously the further the moon moves away from its planet. The emission rate at the point furthest from Saturn is ultimately three to four times as high as at the point closest to Saturn. One explanatory model is that with closer proximity to Saturn and the resulting greater gravitational load, the tiger stripes are literally compressed, whereby the ejection openings are reduced and less material can escape.

The latest research shows that the eruptions usually do not occur at individual points in the crevices, but over almost the entire length of the crevice. Due to optical effects, they are only visible at certain points and not over the entire length of the column.

This region apparently appears to be the source of the very fine E ring of Saturn and also of the thin atmosphere around Enceladus. The ring material cannot stay on its orbit for more than a few thousand years, but the geological activity of the moon ensures a constant supply.

On July 14, 2005, the Cassini space probe, which flew over the moon at a distance of just 175 km, observed countless chunks of ice on the surface the size of a single-family house, the origin of which is not entirely clear. Since these chunks are located in the area of ​​the striped pattern already mentioned, there is a high probability that there is a connection to the cryovolcanic activity in the South Pole region.

Opportunities for life on Enceladus

Depiction of the fountains in the southern hemisphere in a fictional JPL poster

On March 9, 2006, NASA announced that images of Cassini suggest liquid water in the South Pole region of Enceladus. It could reside in chambers that may be just a few meters below the surface and then erupt to the surface like a geyser. The geyser-like fountains in the southern polar region could be observed up to a height of 500 kilometers. Some of the ice particles fall back onto the surface and cause Enceladus to be particularly reflective. Most of the particles that have escaped into space reach the surface of other Saturnian moons, which is why Mimas , Tethys , Dione and Rhea - the satellites in the area of ​​the E ring - are unusually bright compared to other moons.

Ion and Neutral Mass Spectrometer data as of March 12, 2008

On October 9, 2008, Cassini passed Enceladus at a distance of only 25 kilometers. This was the closest distance at flyby of a spacecraft in space history . Cassini flew through freshly ejected particles. Two instruments were in operation at the time: The Cosmic Dust Analyzer and the Ion and Neutral Mass Spectrometer . The measurement results showed a much higher density of volatile gases such as water vapor , carbon dioxide and carbon monoxide than assumed. But organic materials that had already been detected by a spectroscopic analysis of a star eclipse were more common than expected. The particle density was so high that it exerted a measurable torque on the probe. The chemical composition of the ejected particles, which was similar to that of a comet , was a surprise . In contrast to the comet, Enceladus is heated from the inside.

Enceladus thus possesses heat, water and organic chemicals, some of the essential building blocks for the development of life. Cassini also detected hydrogen molecules in the geyser eruptions. This was interpreted as an indication that hot hydrothermal springs exist on the floor of the Enceladus Ocean, analogous to those on the floor of the Earth's oceans . There are hypotheses that the first primitive life forms on earth arose near such hot hydrothermal vents. It is therefore believed that life could have originated on Enceladus as well. Recent studies of Cassini's probe data, according to which molecular hydrogen is present in the outflowing gas plumes, corroborate the theses of hydrothermal activity, the formation of methane and also the possibility of life on Enceladus.

At the FH Aachen , a team of students has been developing the IceMole cryobot since 2010 , which is supposed to drill into a cryovolcano and examine the water inside. Such a mission is only realistic from the year 2040. Furthermore, the Enceladus Life Finder mission was proposed in 2015, but not selected.

Geological structure of Enceladus

Water ocean

Gravimetric measurements indicate that there is not only an ocean of water under the ice of the South Pole region, but that a global extraterrestrial ocean exists. For this purpose, pass-by flights from Cassini were used and evaluated. The mass distribution inside the moon influences the trajectory of the probe, which can be measured by the Doppler shift of its radio signals. In addition, the lunar surface libration was measured. An ocean of water means that the ice crust can rotate independently of the core, which fits the measured values ​​better than a fixed connection with the core. Thus the shell of higher density and lower strength was discovered, which is interpreted as a water ocean with a depth of 10 km under 30 to 40 km of ice. .

There may be hydrothermal springs at the bottom of the ocean. Data collected in the flyby from the gases escaping from Enceladus indicate the existence of molecular hydrogen as well as complex organic molecules, which supports the theory of the ocean of water under the ice and the existence of hydrothermal springs. In addition, the formation of methane in the context of hydrothermal energy as a source for hydrogen emissions has been identified as very likely, which means that the possibility of life in this ocean is again being discussed as possible.

Electrical connection with Saturn

Enceladus is connected to Saturn by an electric current along Saturn's magnetic field lines. Where the electrons hit Saturn's atmosphere, spots that glow in the UV light appear in the polar regions.

literature

• CC Porco, P. Helfenstein, PC Thomas, AP Ingersoll, J. Wisdom, R. West, G. Neukum, T. Denk, R. Wagner, T. Roatsch, S. Kieffer, E. Turtle, A. McEwen, TV Johnson, J. Rathbun, J. Veverka, D. Wilson, J. Perry, J. Spitale, A. Brahic, JA Burns, AD DelGenio, L. Dones, CD Murray, S. Squyres: Cassini Observes the Active South Pole of Enceladus , Science (2006) 311, pp. 1393-1401; doi : 10.1126 / science.1123013 .
• Thorsten Dambeck: Ice moon as a source of dust. In: Image of Science . March 2006, pp. 46-47.
• Saturn moon Enceladus: Mystery of spectacular geysers solved , Spiegel.de, May 17, 2007.
• Jens Lubbadeh: Saturn moon Enceladus: Space probe "Cassini" flies through Enceladus' mysterious fountains , Spiegel.de, March 12, 2008.
• Saturn moon Enceladus: "Cassini" photographs the geyser region of Enceladus , Spiegel.de, August 14, 2008.
• Carolyn Porco: Saturn moon Enceladus: Icy candidate for life , Spiegel.de, June 14, 2009.
• F. Postberg, S. Kempf, J. Schmidt, N. Brilliantov, A. Beinsen, B. Abel, U. Buck & R. Srama: Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus , Nature 459, pp. 1098-1101 (June 25, 2009); doi : 10.1038 / nature08046 .
• Frank Postberg, Ute Kehse: The secret of the tiger stripes. On: Wissenschaft.de from June 24, 2011.
• Paul M. Schenk: Enceladus and the icy moons of Saturn. The University of Arizona Press, Tucson 2018, ISBN 9780816537075 .

Web links

Commons : Enceladus (moon)  - album with pictures, videos and audio files

• Enceladus on the Cassini homepage (English)
• The cometary moon. On: Wissenschaft.de of December 9, 2005. The Saturn moon Enceladus sends a steady stream of fine ice particles into space.
• Cassini discovers geysers spouting water
• Atmosphere at the south pole of Enceladus
• Current Cassini images from Enceladus a picture gallery of the field of planetology and remote sensing at the Free University of Berlin
• Voyager 1 and 2 - Atlas of Six Saturnian Satellites (English, NASA maps of six Saturn moons based on images of the Voyager probes)
• NASA article about the discovery of liquid water (English)
• Enceladus and the Search for Water - Astronomy Picture of the Day of March 10, 2006 (English).
• jo: PLANET RESEARCH, ice moon conjures up Saturn polar lights in ASTROnews, date: April 24, 2011, accessed: April 26, 2011
• astronews.com: Picture of the day: North Pole

Individual evidence

1. ↑ Howett CJA, Spencer JR, Pearl J., Segura, M .: Thermal inertia and bolometric Bond albedo values ​​for Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus as derived from Cassini / CIRS measurements . In: Icarus . 206, No. 2, 2010, pp. 573-593. bibcode : 2010Icar..206..573H . doi : 10.1016 / j.icarus.2009.07.016 .
2. ↑ Verbiscer A., R. French, M. Showalter, Helfenstein P .: Enceladus: Cosmic Graffiti Artist Caught in the Act . In: Science . 315, No. 5813, 2007, p. 815 (supporting online material, table S1). bibcode : 2007Sci ... 315..815V . doi : 10.1126 / science.1134681 . PMID 17289992 .
3. ^ William Herschel: Account of the Discovery of a Sixth and Seventh Satellite of the Planet Saturn; With Remarks on the Construction of Its Ring, Its Atmosphere, Its Rotation on an Axis, and Its Spheroidical Figure. By William Herschel, LL.DFRS Phil. Trans. R. Soc. Lond. January 1, 1790 80: 1-20; doi : 10.1098 / rstl.1790.0001 ( full text )
4. ↑ Enceladus: Crater, craters. ( Memento from January 24, 2015 in the web archive archive.today ) In: Gazetteer of Planetary Nomenclature of the IAU (WGPSN) / USGS . Retrieved February 7, 2016.
5. ↑ Enceladus: Sulcus, sulci. In the Gazetteer of Planetary Nomenclature of the IAU (WGPSN) / USGS . Retrieved February 7, 2016.
6. ↑ Damond Benningfield: Names on Distant Moons. Deutschlandfunk.de, March 1, 2007. Accessed February 7, 2016.
7. ↑ Waite et al. (2009): http://www.nature.com/nature/journal/v460/n7254/full/nature08153.html - July 23, 2009.
8. ↑ Astronews.com: What governs the Enceladus fountains - August 1, 2013.
9. ↑ NASA Jet Propulsion Laboratory: Saturn Moon's Activity Could Be 'Curtain Eruptions'. ( Memento of May 10, 2015 in the Internet Archive ), in NASA Solar System Exploration, Date: May 6, 2015, Accessed: May 28, 2015.
10. ^ Enceladus on the rocks. On: Wissenschaft.de from July 21, 2005.
11. ↑ NASA's Cassini Discovers Potential Liquid Water on Enceladus - Press Release, March 9, 2006.
12. ↑ How Enceladus makes its neighbors shine. On: Wissenschaft.de from February 9, 2007.
13. ↑ Cassini Equinox Mission: Cassini Tastes Organic Material at Saturn's Geyser Moon. January 6, 2010, accessed July 7, 2020 . 
14. ↑ McKay, Christopher P .; Anbar, Ariel D .; et al. (April 15, 2014). "Follow the Plume: The Habitability of Enceladus". Astrobiology. 14 (4): 352-355. Bibcode: 2014AsBio..14..352M. doi: 10.1089 / ast.2014.1158. PMID 24684187
15. ↑ Jonathan Amos: Saturn moon 'able to support life'. BBC News, April 13, 2017, accessed April 14, 2017 . 
16. ↑ J. Hunter Waite, Christopher R. Glein, Rebecca S. Perryman, Ben D. Teolis, Brian A. Magee, Greg Miller, Jacob Grimes, Mark E. Perry, Kelly E. Miller, Alexis Bouquet, Jonathan I. Lunine, Tim Brockwell, Scott J. Bolton: Cassini finds molecular hydrogen in the Enceladus plume: Evidence for hydrothermal processes . In: Science . tape 356 , no. 6334 , p. 155–159 , doi : 10.1126 / science.aai8703 (English, sciencemag.org ).
17. ↑ ^{a b} Jeffrey S. Seewald: Detecting molecular hydrogen on Enceladus. Science 356 (633414) April 2017; Pp. 155-159. doi : 10.1126 / science.aai8703
18. ↑ Robert Gast: Do microbes live under the ice of Enceladus? Spektrum.de, April 13, 2017; Retrieved April 18, 2017.
19. ↑ Ruth-Sophie Taubner, Patricia Pappenreiter, Jennifer Zwicker, Daniel Smrzka, Christian Pruckner, Philipp Kolar, Sébastien Bernacchi, Arne H. Seifert, Alexander Krajete, Wolfgang Bach, Jörn Peckmann, Christian Paulik, Maria G. Firneis, Christa Schleper, Simon K.-MR Rittmann: Biological methane production under putative Enceladus-like conditions . In: Nature Communications . tape 9 , no. 1 , February 27, 2018, ISSN  2041-1723 , p. 748 , doi : 10.1038 / s41467-018-02876-y , PMID 29487311 , PMC 5829080 (free full text) - ( nature.com [accessed December 9, 2018]).
20. ↑ Robot is supposed to search for life on Enceladus , ORF.at, February 23, 2015.
21. ↑ PC Thomas, R. Tajeddine, MS Tiscareno, JA Burns, J. Joseph, TJ Loredo, P. Helfenstein, C. Porco: Enceladus's measured physical libration requires a global subsurface ocean. Icarus, September 2015, doi: 10.1016 / j.icarus.2015.08.037
22. ↑ Press report from September 16, 2015
23. ↑ Jonathan Amos: Saturn's Enceladus moon hides 'great lake' of water . In: BBC News , April 3, 2014. Retrieved April 7, 2014. 
24. ↑ Jane Platt, Brian Bell: NASA Space Assets Detect Ocean inside Saturn Moon . In: NASA . April 3, 2014. Retrieved April 3, 2014.
25. ↑ L. Iess, DJ Stevenson, M. Parisi, D. Hemingway, RA Jacobson, JI Lunine, F. Nimmo, Jw Armstrong: The Gravity Field and Interior Structure of Enceladus . In: Science . 344, No. 6179, April 4, 2014, pp. 78-80. doi : 10.1126 / science.1250551 . Retrieved April 3, 2014.
26. ^ Wall, Mike (May 7, 2015). "Ocean on Saturn Moon Enceladus May Have Potential Energy Source to Support Life". Space.com. Retrieved August 15, 2015
27. ↑ Jet Propulsion Laboratory: Saturn Moon's Ocean May Harbor Hydrothermal Activity ( Memento from December 4, 2015 in the Internet Archive ), in: NASA Solar System Exploration, date: March 11, 2015, May 27, 2015.
28. ↑ ^{a b} J. Hunter Waite, Christopher R. Glein, Rebecca S. Perryman, Ben D. Teolis, Brian A. Magee, Greg Miller, Jacob Grimes, Mark E. Perry, Kelly E. Miller, Alexis Bouquet, Jonathan I. Lunine, Tim Brockwell, Scott J. Bolton: Cassini finds molecular hydrogen in the Enceladus plume: Evidence for hydrothermal processes. Science 356 (633414) April 2017; Pp. 132-133. doi : 10.1126 / science.aan0444
29. ↑ Frank Postberg et al. 2018. Macromolecular organic compounds from the depths of Enceladus. Nature 558: 564-568; doi: 10.1038 / s41586-018-0246-4
30. ↑ Cassini Sees Saturn Electric Link With Enceladus , Date: April 20, 2011, Accessed: April 26, 2011.

further inside	Saturn moons	further outside
Pallene	Semi- major axis  (km) Enceladus 238.100	Tethys