Tirawa (crater)

Tirawa

This Cassini image of Rhea shows Tirawa straddling Rhea's terminator
Feature type	Peak-ring impact basin
Location	Leading hemisphere, Rhea
Coordinates	34°12′N151°42′W / 34.200°N 151.700°W / 34.200; -151.700 [1]
Diameter	~350 km (220 mi) [2]
Depth	~6 km (3.7 mi) [2]
Discoverer	Voyager 1
Eponym	Tirawa

Tirawa is the largest impact basin on Saturn's moon Rhea. It was glimpsed by Voyager 1 during its flyby of the moon and later photographed in greater detail by the Cassini orbiter. Tirawa is elongated in shape and overlaps Mamaldi, a larger and more degraded basin to its southwest. ^[3]

Observation and naming

Tirawa was first observed by the Voyager 1 as it captured imagery of Rhea during its flyby of the Saturn system on 13 November 1980. Though poorly imaged from the Voyager 1 encounter, Tirawa was identified as an impact structure by 1983 and provisionally named Basin A. ^{[4] [3]} Tirawa was later named after the primary deity in Pawnee mythology, Tirawa; the name was officially approved by the International Astronomical Union (IAU) in 1987. ^[1] Tirawa was later observed in greater detail by the Cassini orbiter in multiple flybys of Rhea throughout the late 2000s and the 2010s.

Geology

With a rough diameter of 350 kilometers, Tirawa is the largest identified impact structure on Rhea. Tirawa is a peak-ring impact basin, with a degraded inner ring of ridges and plateaus roughly 180 kilometers across. Individual ridges in the peak ring are typically ~50 kilometers across at their base and rise 1–3 kilometers above the basin floor. Tirawa is slightly elongated into an ellipsoid, with an a/b ratio of ~0.9, indicating that it was created in a low-angle oblique impact. Based on the crater counting method, Tirawa is about 4 billion years old. ^{[2] [4]}

The impact event that created Tirawa ejected massive amounts of material, with up to ~15-20% of the material ejected at speeds exceeding Rhea's escape velocity. ^[2] Any such impact ejecta would've escaped Rhea's influence entirely, directly orbiting Saturn. Some of the ejecta later impacted Rhea, likely accounting for a large amount of Rhea's small craters. ^[5]

See also

• Odysseus – An impact basin on Tethys that is analogous to Tirawa in size and structure

References

1. "Tirawa". Gazetteer of Planetary Nomenclature. USGS Astrogeology Research Program. (Center Latitude: 34.20°, Center Longitude: 151.70°; Planetographic, +West)
2. Moore, Jeffrey M.; Schenk, Paul M.; Bruesch, Lindsey S.; Asphaug, Erik; McKinnon, William B. (October 2004). "Large impact features on middle-sized icy satellites". Icarus. 171 (2): 421–443. Bibcode:2004Icar..171..421M. doi:10.1016/j.icarus.2004.05.009.
3. Wagner, R. J.; Neukum, G.; Giese, B.; Roatsch, T.; Wolf, U. (March 2007). The Global Geology of Rhea: Preliminary Implications from the Cassini ISS Data (PDF). 38th Lunar and Planetary Science Conference. The Woodlands, Texas, United States. 1958. Archived (PDF) from the original on 3 March 2016. Retrieved 22 January 2008.
4. Moore, J. M.; Horner, V. M.; Greeley, R. (February 1985). "The geomorphology of RHEA - Implications for geologic history and surface processes". Journal of Geophysical Research. 90: C785 –C795. Bibcode:1985LPSC...15..785M.
5. Leliwa-Kopystynski, Jacek; Banaszek, Marcin; Wlodarczyk, Ireneusz (January 2012). "Longitudinal asymmetry of craters' density distributions on the icy satellites". Planetary and Space Science. 60 (1): 181–192. Bibcode:2012P&SS...60..181L. doi:10.1016/j.pss.2011.08.002.