  Voyager 2 observations revealed that the internal luminosity of Neptune is an
order of magnitude higher than that of Uranus. If the two planets have similar
interior structures and cooling histories, the luminosity of Neptune can only
be explained by invoking some energy source beyond gravitational contraction.
This paper investigates whether Centaur impacts could provide the energy
necessary to produce the luminosity of Neptune. The major findings are (1) that
impacts on both Uranus and Neptune are too infrequent to provide luminosities
of order the observed value for Neptune, even for optimistic impact-rate
estimates, and (2) that Uranus and Neptune rarely have significantly different
impact-generated luminosities at any given time. Uranus and Neptune most likely
have structural differences that force them to cool and contract at different
rates.
