Ithaca Chasma is a valley on Saturn

Saturn

Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

's moon Tethys

Tethys (moon)

Tethys or Saturn III is a mid-sized moon of Saturn about across. It was discovered by G. D. Cassini in 1684 and is named after titan Tethys of Greek mythology. Tethys is pronounced |Odysseus]] is about 400 km in diameter, while the largest graben—Ithaca Chasma is about 100 km wide and...

, named after the island of Ithaca

Ithaca

Ithaca or Ithaka is an island located in the Ionian Sea, in Greece, with an area of and a little more than three thousand inhabitants. It is also a separate regional unit of the Ionian Islands region, and the only municipality of the regional unit. It lies off the northeast coast of Kefalonia and...

, in Greece

Greece

Greece , officially the Hellenic Republic , and historically Hellas or the Republic of Greece in English, is a country in southeastern Europe....

. 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

Odysseus (crater)

Odysseus is the largest crater on Saturn's moon Tethys. It is 445 km across, more than 2/5 of the moon's diameter, and is one of the larger craters in the Solar System. It is situated in the western part of leading hemisphere of the moon—the latitude and longitude of its center are 32.8°N and...

.

Discovery

Ithaca Chasma was discovered by Voyager 1

Voyager 1

The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...

 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

The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

 flyby. It was named after named after the island of Ithaca

Ithaca

Ithaca or Ithaka is an island located in the Ionian Sea, in Greece, with an area of and a little more than three thousand inhabitants. It is also a separate regional unit of the Ionian Islands region, and the only municipality of the regional unit. It lies off the northeast coast of Kefalonia and...

, in Greece

Greece

Greece , officially the Hellenic Republic , and historically Hellas or the Republic of Greece in English, is a country in southeastern Europe....

.

Geology

Ithaca Chasma is a giant trough

Trough (geology)

In geology, a trough generally refers to a linear structural depression that extends laterally over a distance, while being less steep than a trench.A trough can be a narrow basin or a geologic rift....

 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 (crater)

Odysseus is the largest crater on Saturn's moon Tethys. It is 445 km across, more than 2/5 of the moon's diameter, and is one of the larger craters in the Solar System. It is situated in the western part of leading hemisphere of the moon—the latitude and longitude of its center are 32.8°N and...

 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 scarp

Escarpment

An escarpment is a steep slope or long cliff that occurs from erosion or faulting and separates two relatively level areas of differing elevations.-Description and variants:...

s 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.

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.

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. Earlier crater

Impact crater

In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

s made before Tethys solidified were probably all erased by geological activity before then.

Tethys' subsurface ocean may have resulted from a 2:3 orbital resonance

Orbital resonance

In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

 between Dione and Tethys early in the solar system's history. The resonance would have led to orbital eccentricity

Orbital eccentricity

The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

 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.

An alternative hypothesis

Hypothesis

A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...

 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. 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.