Inverse of the impact frequency (in units of years ago) versus impactor size D on Uranus for two different scenarios: impactors in which the size–number distribution of impactors represent ecliptic comets (red solid line), and nearly isotropic comets such as long-period and Halley-type comets (blue dashed line; see Zahnle et al. 2003, and references therein). Stars show the (D, t_impact) from Table 4 that allows for an optimal match of Herschel CO data in the stratosphere and below the upper limit in the troposphere (Teanby & Irwin 2013).