![Tethys PIA07738](https://upload.wikimedia.org/wikipedia/commons/a/ab/Tethys_PIA07738.jpg)
Ithaca Chasma is a valley (
graben
In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults.
Etymology
''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic contex ...
) of
Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
's
moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
Tethys, named after the island of
Ithaca
Ithaca most commonly refers to:
*Homer's Ithaca, an island featured in Homer's ''Odyssey''
*Ithaca (island), an island in Greece, possibly Homer's Ithaca
*Ithaca, New York, a city, and home of Cornell University and Ithaca College
Ithaca, Ithaka ...
, in
Greece
Greece,, or , romanized: ', officially the Hellenic Republic, is a country in Southeast Europe. It is situated on the southern tip of the Balkans, and is located at the crossroads of Europe, Asia, and Africa. Greece shares land borders with ...
. It is up to wide, deep and 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.
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. The chasma is from 4 to 0.4 billion years old, and slightly older than Odysseus.
Discovery
Ithaca Chasma was discovered by
Voyager 1
''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin ''Voyager 2'', ''Voya ...
spacecraft on 12 November 1980 during its flyby of Saturn. However its full extent was realized only in 1981 after the
Voyager 2
''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, ''Voyager 1'', on a ...
flyby. It was named after the island of
Ithaca
Ithaca most commonly refers to:
*Homer's Ithaca, an island featured in Homer's ''Odyssey''
*Ithaca (island), an island in Greece, possibly Homer's Ithaca
*Ithaca, New York, a city, and home of Cornell University and Ithaca College
Ithaca, Ithaka ...
, in
Greece
Greece,, or , romanized: ', officially the Hellenic Republic, is a country in Southeast Europe. It is situated on the southern tip of the Balkans, and is located at the crossroads of Europe, Asia, and Africa. Greece shares land borders with ...
.
Geology
Ithaca Chasma is a giant
trough
Trough may refer to:
In science
* Trough (geology), a long depression less steep than a trench
* Trough (meteorology), an elongated region of low atmospheric pressure
* Trough (physics), the lowest point on a wave
* Trough level (medicine), the l ...
system about 3 km deep and approximately confined to a great circle running through the poles of Tethys. It is approximately concentric with
Odysseus
Odysseus ( ; grc-gre, Ὀδυσσεύς, Ὀδυσεύς, OdysseúsOdyseús, ), also known by the Latin variant Ulysses ( , ; lat, UlyssesUlixes), is a legendary Greek king of Ithaca and the hero of Homer's epic poem the ''Odyssey''. Odysse ...
impact crater—a pole of Ithaca Chasma lies only approximately 20° from it.
The chasma has a rather complex structure consisting of two narrow branches towards the south. 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 plains. Its width varies from only a few kilometers at some places to more than 100 km.
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.
Origin
There are two basic hypothesis as to how Ithaca Chasma formed. One of them is that it formed as Tethys' internal global liquid salt water ocean solidified, causing the moon to expand and cracking its surface to accommodate the extra volume within. Earlier
craters made before Tethys solidified were probably all erased by geological activity before then.
Tethys' global subsurface ocean may have resulted from a 2:3
orbital resonance
In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
between this moon and
Dione early in the Solar System's history. The resonance would have led to
orbital eccentricity
In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values betwee ...
and
tidal heating
Tidal heating (also known as tidal working or tidal flexing) occurs through the tidal friction processes: orbital and rotational energy is dissipated as heat in either (or both) the surface ocean or interior of a planet or satellite. When an objec ...
that may have warmed Tethys' 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.
An alternative
hypothesis
A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obse ...
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. 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.
Citations
References
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External links
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{{Tethys
Extraterrestrial valleys
Surface features of Tethys (moon)