Triton's surface age and impactor population revisited in light of Kuiper belt fluxes: Evidence for small Kuiper belt objects and recent geological activity
ASTRONOMICAL JOURNAL, 119 (2):
945--952 (2000)Abstract
Neptune's largest satellite, Triton, is one of the most fascinating
and enigmatic bodies in the solar system. Among its numerous interesting
traits, Triton appears to have far fewer craters than would be expected
if its surface were primordial. Here we combine the best available
crater count data for Triton with improved estimates of impact rates
by including the Kuiper belt as a source of impactors. We find that
the population of impacters creating the smallest observed craters
on Triton must be subkilometer in scale and that this small-impactor
population can be best fitted by a differential power-law size index
near -3. Such results provide interesting, indirect probes of the
unseen small body population of the Kuiper belt. Based on the modern,
Kuiper belt and Oort cloud impactor flux estimates, we also recalculate
estimated ages for several regions of Triton's surface imaged by
Voyager 2, and find that Triton was probably quite geologically active
on a timescale no greater than 0.1-0.3 Gyr ago (indicating Triton
was still active after some 90% to 98% of the age of the solar system),
and perhaps even more recently. This activity must surpass that explainable
by the surface geysers seen by Voyager 2 by many orders of magnitude.
The time-averaged volumetric resurfacing rate on Triton implied by
these results, 0.01. km(3) yr(-1) or more, is likely second only
to Io and Europa in the outer solar system, and is within an order
of magnitude of estimates for Venus and for the Earth's intraplate
zones. This finding indicates that Triton likely remains a highly
geologically active world at present, some 4.5 Gyr after its formation.
We briefly speculate on how such a situation might obtain.