Editing Ithaca Chasma

{{Short description|Major valley system on Tethys}}
{{Use dmy dates|date=May 2024}}
[[File:Tethys PIA07738.jpg|thumb|Tethys and Ithaca Chasma]]
'''Ithaca Chasma''' {{IPAc-en|'|I|T|@|k|@|_|'|k|ae|z|m|@}} is a valley ([[graben]]) of [[Saturn]]'s [[Moons of Saturn|moon]] [[Tethys (moon)|Tethys]], named after the island of [[Ithaca (island)|Ithaca]], in [[Greece]].{{sfn|USGS: Tethys: Ithaca Chasma}} It is up to {{convert|100|km|abbr=on}}  wide, {{convert|3-5|km|abbr=on}} deep and {{convert|2000|km|abbr=on}} long, running approximately three-quarters of the way around Tethys's circumference, making it one of the [[List of largest rifts and valleys in the Solar System|longer valleys in the Solar System]].  Ithaca Chasma is approximately concentric with [[Odysseus (crater)|Odysseus crater]].{{sfn|Moore Schenk et al.|2004|pp=424–30}}

Ithaca Chasma may have originated when the global salt water ocean inside Tethys froze. This would have caused the surface to crack in order to accommodate the resulting increase in volume. Another hypothesis is that Ithaca Chasma is the outermost ring of the Odysseus impact basin; however the chasma is from 4 to 0.4 billion years old, making it slightly older than Odysseus.

== Discovery ==
Ithaca Chasma was discovered by [[Voyager 1]] spacecraft on 12 November 1980 during its flyby of Saturn.{{sfn|Stone|Miner|1981}} However its full extent was realized only in 1981 after the [[Voyager 2]] flyby.{{sfn|Stone|Miner|1982}} It was named after the island of [[Ithaca (island)|Ithaca]], in [[Greece]].{{sfn|USGS: Tethys: Ithaca Chasma}}

== Geology ==
Ithaca Chasma is a giant [[Trough (geology)|trough]] system about 3&nbsp;km deep and approximately confined to a great circle running through the poles of Tethys.{{sfn|Moore Schenk et al.|2004|pp=424–30}}  It is approximately concentric with [[Odysseus (crater)|Odysseus]] impact crater—a pole of Ithaca Chasma lies only approximately 20° from it.{{sfn|Jaumann Clark et al.|2009|pp=645–46}}

The chasma has a rather complex structure consisting of two narrow branches towards the south.{{sfn|Jaumann Clark et al.|2009|p=669}} Its exterior walls are made of multiple sub-parallel [[escarpment|scarp]]s and terraces. At some places the chasma has a rim standing as high as 0.5&nbsp;km about the surrounding cratered plains. Its width varies from only a few kilometers at some places to more than 100&nbsp;km.{{sfn|Moore Schenk et al.|2004|pp=424–30}}

The age of Ithaca Chasma is estimated to be either 4.0 or 0.4–3.3 billion years depending on chosen impact chronology. The crater counts indicate that the chasma is slightly older than Odysseus crater but much younger than the cratered plains.{{sfn|Giese|Wagner|Neukum|Helfenstein|2007}}
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[[File:Ithaca Chasma PIA07734.jpg|thumb|Cassini closeup of the southern end of Ithaca Chasma]]

== Origin ==
There are two basic hypothesis as to how Ithaca Chasma formed. One of them is that it formed as Tethys's internal global liquid salt water ocean solidified, causing the moon to expand and cracking its surface to accommodate the extra volume within.{{sfn|Moore Schenk et al.|2004|pp=424–30}} Earlier [[Impact crater|crater]]s made before Tethys solidified were probably all erased by geological activity before then.{{sfn|Giese|Wagner|Neukum|Helfenstein|2007}}

Tethys's global subsurface ocean may have resulted from a 2:3 [[orbital resonance]] between this moon and [[Dione_(moon)|Dione]] early in the Solar System's history. The resonance would have led to [[orbital eccentricity]] and [[tidal heating]] that may have warmed Tethys's interior enough to form the ocean. Subsequent freezing of the internal ocean after the moons escaped from the resonance may have generated the extensional stresses that created Ithaca Chasma.{{sfn|Chen|Nimmo|2008}}{{sfn|Rincon|2008}}

An alternative [[hypothesis]] is that it was formed at the same time as the large crater Odysseus which lies near a pole of the Ithaca Chasma. When the impact that created Odysseus occurred, the shockwave may have traveled through Tethys and produced a [[circumcircular]] fracture analogous to outer ring graben of multiring impact basins.{{sfn|Moore Schenk et al.|2004|pp=424–30}} However, age determination based on crater counts in high resolution Cassini images showed that Ithaca Chasma is older than Odysseus, making the impact hypothesis unlikely.{{sfn|Jaumann Clark et al.|2009|pp=645–46}}
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== Citations ==

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== References ==
{{refbegin|2}}
* {{cite conference
  | last1 = Chen
  | first1 = E. M. A.
  | last2 = Nimmo
  | first2 = F.
  | date = 10–14 March 2008
  | title = Thermal and Orbital Evolution of Tethys as Constrained by Surface Observations
  | book-title = 39th Lunar and Planetary Science Conference, (Lunar and Planetary Science XXXIX)
  | page = 1968
  | location = League City, Texas
  | id = [[Lunar and Planetary Institute|LPI]] Contribution No. 1391
  | url = http://www.lpi.usra.edu/meetings/lpsc2008/pdf/1968.pdf
  | access-date = 2011-12-12
  }}
* {{Cite journal | last1 = Giese | first1 = B. | last2 = Wagner | first2 = R. | last3 = Neukum | first3 = G. | last4 = Helfenstein | first4 = P. | last5 = Thomas | first5 = P. C. | title = Tethys: Lithospheric thickness and heat flux from flexurally supported topography at Ithaca Chasma | doi = 10.1029/2007GL031467 | bibcode = 2007GeoRL..3421203G| url = http://ciclops.org/media/sp/2007/4702_10268_0.pdf| journal = Geophysical Research Letters | volume = 34 | issue = 21 | pages = 21203 | year = 2007 | doi-access = free }}
* {{cite book| doi = 10.1007/978-1-4020-9217-6_20| last1 = Jaumann| first1 = R.| last2 = Clark| first2 = R. N.| last3 = Nimmo| first3 = F.| last4 = Hendrix| first4 = A. R.| last5 = Buratti| first5 = B. J.| last6 = Denk| first6 = T.| last7 = Moore| first7 = J. M.| last8 = Schenk| first8 = P. M.| last9 = Ostro| first9 = S. J.| last10 = Srama| first10 = Ralf| year = 2009| chapter = Icy Satellites: Geological Evolution and Surface Processes| title = Saturn from Cassini-Huygens| pages = 637–681| isbn = 978-1-4020-9216-9| ref = {{sfnRef|Jaumann Clark et al.|2009}}}}
* {{cite journal| doi = 10.1016/j.icarus.2004.05.009| last1 = Moore| first1 = Jeffrey M.| last2 = Schenk| first2 = Paul M.| last3 = Bruesch| first3 = Lindsey S.| last4 = Asphaug| first4 = Erik| last5 = McKinnon| first5 = William B.| date=October 2004 | title = Large impact features on middle-sized icy satellites| journal = Icarus| volume = 171| issue = 2| pages = 421–443| url = http://planets.oma.be/ISY/pdf/article_Icy.pdf| bibcode = 2004Icar..171..421M| ref = {{sfnRef|Moore Schenk et al.|2004}}
}}
* {{cite web
  | last = Rincon
  | first = Paul
  | title = Saturn moon 'once had ocean'
  | work = news.bbc.co.uk
  | publisher = [[BBC News]]
  | date = 2008-03-14
  | url = http://news.bbc.co.uk/2/hi/science/nature/7297408.stm
  | access-date = 19 December 2011
  }}
* {{cite journal| doi = 10.1126/science.212.4491.159| last1 = Stone| first1 = E. C.| last2 = Miner| first2 = E. D.| date = 10 April 1981| title = Voyager 1 Encounter with the Saturnian System| journal = Science| volume = 212| issue = 4491| pages = 159–163| pmid = 17783826| url = http://authors.library.caltech.edu/43660/1/1685660.pdf| bibcode = 1981Sci...212..159S}}
* {{cite journal| doi = 10.1126/science.215.4532.499| last1 = Stone| first1 = E. C.| last2 = Miner| first2 = E. D.| date = 29 January 1982| title = Voyager 2 Encounter with the Saturnian System| journal = Science| volume = 215| issue = 4532| pages = 499–504| pmid = 17771272| bibcode = 1982Sci...215..499S| s2cid = 33642529| url = https://authors.library.caltech.edu/43660/1/1685660.pdf}}
* {{cite web
  | author = [[USGS]]/[[IAU]]
  | title = Tethys: Ithaca Chasma
  | date = Jun 5, 2008
  | publisher = USGS Astrogeology
  | work = Gazetteer of Planetary Nomenclature
  | url = http://planetarynames.wr.usgs.gov/Feature/2751
  | access-date = 19 December 2011
  | ref = {{sfnRef|USGS: Tethys: Ithaca Chasma}}
  }}
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== External links ==
* {{APOD |date=24 June 2008|title=Ithaca Chasma: The Great Rift on Saturn's Tethys}}

{{Tethys}}

[[Category:Extraterrestrial valleys]]
[[Category:Surface features of Tethys (moon)]]