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Ithaca Chasma

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Tethys and Ithaca Chasma

Ithaca Chasma is a valley on Saturn's moon Tethys, named after the island of Ithaca, in Greece.[1] It is on average 100 km wide, 3 to 5 km deep and 2,000 km long, running approximately three-quarters of the way around Tethys' circumference, making it one of the longer valleys in the Solar System. Ithaca Chasma is approximately concentric with Odysseus crater.[2]

Discovery

Ithaca Chasma was discovered by Voyager 1 spacecraft on 12 November 1980 during its flyby of Saturn.[3] However its full extent was realized only in 1981 after the Voyager 2 flyby.[4] It was named after named after the island of Ithaca, in Greece.[1]

Artist's rendering of Ithaca Chasma

Geology

Ithaca Chasma is a giant trough system about 3 km deep and approximately confined to a great circle running through the poles of Tethys.[2] It is approximately concentric with Odysseus impact crater—a pole of Ithaca Chasma lies only approximately 20° from it.[5]

The chasma has a rather complex structure consisting of two narrow branches towards the south.[6] Its exterior walls are made of multiple sub-parallel scarps and terraces. At some places the chasma has a rim standing as high as 0.5 km about the surrounding cratered planes. Its width varies from only a few kilometers at some places to more than 100 km.[2]

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 younger than Odysseus crater and much younger than the cratered plains.[7]

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' internal liquid water ocean solidified, causing the moon to expand and cracking its surface to accommodate the extra volume within.[2] Earlier craters made before Tethys solidified were probably all erased by geological activity before then.[7]

Tethys' subsurface ocean may have resulted from a 2:3 orbital resonance between Dione and Tethys early in the solar system's history. The resonance would have led to orbital eccentricity and tidal heating that may have warmed Tethys' interior enough to form the ocean. Subsequent freezing of the ocean after the moons escaped from the resonance may have generated the extensional stresses that created Ithaca Chasma.[8][9]

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 analogues to outer ring graben of multiring impact basins.[2] 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.[5]

Citations

  1. ^ a b "Tethys: Ithaca Chasma". Gazetteer of Planetary Nomenclature. USGS Astrogeology. Retrieved 7 September 2011.
  2. ^ a b c d e Moore Schenk 2004, pp. 424–30.
  3. ^ Stone & Miner 1981.
  4. ^ Stone & Miner 1982.
  5. ^ a b Jaumann 2009, pp. 645–46.
  6. ^ Jaumann 2009, p. 669.
  7. ^ a b Giese 2007.
  8. ^ Chen & Nimmo 2008.
  9. ^ Rincon, Paul (2008-03-14). "Saturn moon 'once had ocean'". BBC News web site. BBC. Retrieved 7 September 2011. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)

References

  • Chen, E. M. A.; Nimmo, F. (March 10–14, 2008). "Thermal and Orbital Evolution of Tethys as Constrained by Surface Observations" (PDF). 39th Lunar and Planetary Science Conference, (Lunar and Planetary Science XXXIX). League City, Texas. p. 1968. LPI Contribution No. 1391. Retrieved 2011-12-12. {{cite conference}}: Invalid |ref=harv (help); Unknown parameter |booktitle= ignored (|book-title= suggested) (help); Unknown parameter |unused_data= ignored (help)CS1 maint: date format (link)
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