THE MANY AGES OF TRITON

After the 1989 encounter, the Voyager Imaging Team (including this year’s Gilbert awardee) estimated Triton’s “Oldest cratered surfaces probably about 3.5 billion years old. Youngest crater surfaces probably less than 0.5 billion years old” (Smith et al. 1989). This was of course before recognition of the true extent of the Kuiper belt (KB). Subsequently, Stern & McKinnon (2000) attempted a rigorous application of KB impactor fluxes as then known to Triton’s crater population, specifically estimating surface ages between ~200 and 300 Ma, depending on geological unit, but not attempting to date the enigmatic cantaloupe terrain. They also argued that the true ages were possibly much younger, between 40 and 50 Ma. Later Schenk & Zahnle (2007, SZ07) more firmly concluded that the more cratered regions of Triton were about 70 Ma, with the more lightly cratered, resurfaced Neptune-facing units as young as 10 Ma (tied to a more precise understanding of the KB flux as a function of distance from the apex of motion). These surface ages were upper limits, however, as heliocentric bombardment was assumed, whereas SZ07 actually advocated for a dominantly prograde planetocentric origin based on the high concentration of impacts on Triton’s leading hemisphere. With such a marked reduction in heliocentric contribution, the average age of Triton’s datable terrains plunged to ~10 Ma, making Triton the most globally active icy satellite of all (n.b. potentially explainable by obliquity tidal heating). Mah & Brasser (2019) counterargued from numerical calculations that a prograde debris swarm would be far too concentrated toward the apex of motion to match observations, but their model swarm is very dynamically cold (low inclination), and not a good representation of ejecta generated by a major impact strike on an inner Neptune satellite such as Proteus (advocated by SZ07). Plus, there are clear crater count and stratigraphic age differences between terrains on Triton’s leading hemisphere, especially on the 10-frame highest resolution mosaic (thank you Candy!), which complicates glib interpretations of the crater spatial distribution. The origin of such a geologically recent prograde impactor swarm remains mysterious, however, and generating sufficiently large and fast sesquinary ice blocks is not consistent with observations of secondary crater fields on icy satellites (Singer et al. 2013).