The Dust Mantle of Comet 9P/Tempel 1: Dynamical Constraints on Physical Properties

TL;DR

This study uses the evolution of dust plumes from comet 9P/Tempel 1 during NASA's Deep Impact mission to constrain the physical properties of surface dust, revealing a dust mantle of compact aggregates around 20 micrometers in size with low thermal conductivity.

Contribution

It provides the first dynamical constraints on the physical properties of the comet's dust surface based on plume evolution analysis.

Findings

Dust particles have a radiation pressure to gravity ratio of about 0.4.

Surface dust consists of approximately 20 μm-sized compact aggregates.

The comet's dust mantle has low thermal conductivity around 1 erg cm^{-1} K^{-1} s^{-1}.

Abstract

The trajectories of dust particles ejected from a comet are affected by solar radiation pressure as a function of their ratios of radiation pressure cross section to mass. Therefore, a study on the orbital evolution of the particles caused by the radiation pressure reveals the physical properties of dust on the surface of the comet nucleus. In the course of NASA's Deep Impact mission, the ejecta plume evolved under the influence of the radiation pressure. From the evolution and shape of the plume, we have succeeded in obtaining $\beta \approx 0.4$, where $\beta$ is the ratio of the radiation pressure to the solar gravity. Taking into account $\beta \approx 0.4$ as well as the observational constraints of a high color temperature and a small silicate-feature strength, dust particles ejected from the surface of comet 9P/Tempel 1 are likely compact dust aggregates of sizes $\approx 20\,\mu$m (mass $\sim 10^{-8}$\,g). This is comparable to the major dust on the surface of comet 1P/Halley ($\sim 10\mu$m) inferred from in-situ measurements and theoretical considerations. Since such dust aggregates with $\beta \approx 0.4$ must have survived on the surface against jets due to ice sublimation, the temperature of ice in the nucleus must be kept below 145\,K, which is much lower than equilibrium temperature determined by solar irradiation and thermal emission. These facts indicate that 9P/Tempel 1 has a dust mantle composed of $20\,\mu$m-sized dust aggregates with low thermal conductivities $\sim 1 \, {\rm erg\, cm}^{-1} \, {\rm K}^{-1}\,{\rm s}^{-1}$.