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tokens: because their terminal velocity increases due to the increasing gravitational field strength at small planetary radii. Only bolides in a very limited size range are large enough that they do not reach the sublimation temperature and yet small enough for their low terminal velocities to render them immune to frictional ablation. Given the uncertainties and limitations (for example, of excluding fragmentation and lateral spread) that enter our modelling, this narrow range may indeed be either somewhat smaller or simply non-existent in a scenario with more modes of ablation. If impacting planetesimals move with hypervelocities beyond the escape velocity of Jupiter of approximately 60 km s −1 , some high φ i bolides can ablate and still escape the upper outer envelope beyond a normalized altitude of 1. Fig. 6 shows the case of bolides impacting at right angles to the local zenith to 126 physical seconds. This time value is 10 times the time required for vaporisation of a 30 m bolide travelling at 100 km s −1 and is a good benchmark time to gauge when all highest velocity planetesimals either escape the system or completely ablate. Planetesimals capable of escaping the Jovian envelope are represented by black stars. Bolides of sizes 30–200 m with ∼80–100 km s −1 completely ablate upon impact, whilst larger bolides with these high kinetic energies manage to escape the Jovian radius (e.g. a 1 km planetesimal retains ∼0.60 of its initial mass in the process). At future times, the region of the plot involving velocities greater than the escape velocity evolves little whilst the low-velocity regime is subject to progressive ablation as in Fig. 5 . The former region does not evolve because we do not model the re-entry of planetesimals that may fall back to the planet due to their decreased velocities. Planetesimals of velocities exceeding Jovian escape offer chemical depositions that should be observable perhaps even in the atmosphere of such planets given the proximity of the 1 bar level and mixing processes. Figure 6. Mass ablation fraction to 126 s for planetesimals incident tangentially with initial velocities exceeding that of Jovian escape. Black stars represent escaped planetesimals. We see that planetesimals of velocities exceeding the Jovian escape should offer chemical depositions that should be observable at the 1 bar level. 5 DISCUSSION Our model is a simple description of planetesimal dynamics with implications for the enrichment of Jovian-like exoplanetary envelopes. There is consequently extensive potential for further improving the sophistication of our model and we here discuss a few examples of potential refinements. First, Nellis ( 1997 ) highlights that there is indeed a continual process of molecular dissociation into a metallic plasma; there is thus no real distinct boundary between molecular layers and monoatomic conductive hydrogen and the band-gap separation is bridged continuously with increasing pressure rather than jumping