Tirawa is the largest impact basinonSaturn's moonRhea. 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]
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 deityinPawnee 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 2000's and the 2010's.
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]
^ ab"Tirawa". Gazetteer of Planetary Nomenclature. USGS Astrogeology Research Program. (Center Latitude: 34.20°, Center Longitude: 151.70°; Planetographic, +West)
^ abcdMoore, 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.
^ abMoore, 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.
^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.
