Tirawa (crater)
Largest impact basin on Saturn's moon Rhea.
Tirawa is the biggest impact basin on Saturn’s moon Rhea. Voyager 1 caught a glimpse of it during its flyby, and later the Cassini orbiter captured much sharper images. Tirawa is elongated in shape and overlaps Mamaldi, a larger and more degraded basin to its southwest. **Observation and naming** Voyager 1 first spotted Tirawa while photographing Rhea during its pass through the Saturn system on 13 November 1980. Although the images from that encounter were fuzzy, scientists had identified it as an impact structure by 1983, giving it the temporary name Basin A. It was later named after Tirawa, the creator god in Pawnee mythology, and the International Astronomical Union officially approved the name in 1987. The Cassini spacecraft later studied Tirawa in much finer detail during several flybys of Rhea in the late 2000s and 2010s. **Geology** Roughly 350 kilometers across, Tirawa is the largest known impact crater on Rhea. It’s a peak-ring basin, with a worn inner ring of ridges and plateaus that spans about 180 kilometers. Individual ridges in that ring are typically around 50 kilometers wide at the base and rise 1–3 kilometers above the basin floor. The basin is slightly stretched into an ellipsoid shape, with an a/b ratio of about 0.9, which suggests it was formed by a low-angle, oblique impact. Based on crater counting, Tirawa is roughly 4 billion years old. The impact that created Tirawa threw out huge amounts of material—about 15–20% of it at speeds high enough to escape Rhea’s gravity entirely and go into orbit around Saturn. Some of that ejected debris later crashed back into Rhea, likely accounting for many of the moon’s smaller craters. **See also** Odysseus – An impact basin on Tethys similar to Tirawa in size and structure.
Quick Facts
- Diameter
- 350 km
- Depth
- 6 km
- Discoverer
- Voyager 1 in 1980
- Eponym
- Tirawa
- Location
- Rhea
- Age
- 4 billion years
Facts from the source article.
Lore & Background
Tirawa was first observed by 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. Tirawa was later named after the creator deity in Pawnee mythology, Tirawa; the name was officially approved by the International Astronomical Union (IAU) in 1987. Tirawa was later observed in greater detail by the Cassini spacecraft in multiple flybys of Rhea throughout the late 2000s and the 2010s. Geologically, 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 around 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. 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. 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.
Reader's Guide
Tirawa's significance lies in its status as the largest impact basin on Rhea, providing key insights into the moon's impact history and geological evolution. Its elongated shape and low-angle oblique impact origin suggest a specific type of collision event. The basin's age of about 4 billion years places it among the oldest features on Rhea, dating back to the early Solar System. The massive ejecta from the Tirawa impact, with up to 15–20% of material escaping Rhea's gravity, highlights its role in shaping the moon's surface and possibly contributing to Saturn's ring system or other moons. The detailed observations by Cassini, following the initial glimpse by Voyager 1, allowed scientists to characterize its peak-ring structure and degraded inner ring. Tirawa is also compared to the Odysseus basin on Tethys, which is analogous in size and structure, aiding comparative planetology studies of Saturn's icy moons.
Did You Know?
- Tirawa was provisionally named Basin A before receiving its official name in 1987.
- Tirawa's impact ejected up to 15–20% of material at speeds exceeding Rhea's escape velocity.
More in Moons of Saturn, Part 2 1-24
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