Properties of comet 9P/Tempel 1 dust immediately following excavation by deep impact

Abstract

We analyzed Deep Impact High Resolution Instrument (HRI) images acquired within the first seconds after collision of the Deep Impact impactor with the nucleus of comet 9P/Tempel 1.These images reveal an optically thick ejecta plume that casts a shadow on the surface of the nucleus. Using the 3D radiative transfer code HYPERION we simulated light scattering by the ejecta plume, taking into account multiple scattering of light from the ejecta, the surrounding nuclear surface and the actual observational geometry (including an updated plume orientation geometry that accounts for the latest 9P/Tempel 1 shape model). Our primary dust model parameters were the number density of particles, their size distribution and composition. We defined the composition through the density of an individual particle and the ratio of its material constituents, which we considered to be refractories, ice and voids. The results of our modeling indicate a dust/ice mass ratio for the ejecta particles of at least 1. To further constrain the parameters of the model, we checked for consistency between the ejecta mass resulting from our modeling with the ejecta mass estimated by the crater formation modeling. Constraining the particle size distribution by results of other studies of the Deep Impact ejecta, we find the number density of ejecta particles equal to ~10 4 particles/cm 3 at the base of the plume.

Studying the nucleus of comet 9P/Tempel 1 using the structure of the Deep Impact ejecta cloud at the early stages of its development

“…Similar numbers were obtained from ground-based observations by Schleicher et al (2006) and Biver et al (2007). Our estimate (Nagdimunov et al 2014), possible only for a gravity controlled crater, was 3*10 7 kg. Notice also that a strength dominated crater should be formed quite quickly, within several seconds, whereas a gravity dominated crater would require hundreds of second for its formation (Richardson et al 2007) and the last fact is more consistent with the formation of the Deep Impact crater, specifically, in the MRI images (McLaughlin et al 2014b) we see no evidence of detachment of the ejecta from the nucleus surface even 12 minutes after impact, which is more consistent with continuous excavation of the nucleus material for a period longer than a couple of seconds after impact.…”

“…Whereas high albedo may be associated with a higher abundance of ice and low albedo with a low ice abundance or domination of absorbing (perhaps carbonaceous) materials, the inhomogeneities in the optical depth are harder to explain unambiguously. Our radiative transfer modeling reported in Nagdimunov et al (2014) showed that, due to the fact that the optical depth is a function of the extinction cross-section multiplied by the number density, we can find a variety of combinations of extinction cross-section and number density that produce the same fit for the optical depth. The bands of low optical depth or azimuthal inhomogeneities in optical depth can be areas of lower bulk density of the material (due to higher porosity) or areas characterized by dust consisting of particles of smaller extinction crosssection.…”

Light Scattering and Thermal Emission by Primitive Dust Particles in Planetary Systems

“…11 The real image of the ejecta plume from the surface of comet 9P/Tempel 1 and its shadow on the surface taken by the Deep Impact High Resolution Instrument (left) and the simulated image of the ejecta plume and its shadow computed by radiative transfer computations (right). From Nagdimunov et al (2014) dominate the forward-scattering region, have not been incorporated in their model.…”

“…Nagdimunov et al, 2014;Shalima et al, 2015). Nagdimunov et al (2014) have succeeded in modeling both an optically thick ejecta plume and its shadow on the surface of comet 9P/Tempel 1 produced and imaged by the Deep Impact mission (see Fig. 11).…”

“…Recently, radiative transfer computations to model such an ejecta curtain have been performed with assumptions about the phase function, single scattering albedo, and asymmetry parameter of the ejecta particles (cf. Nagdimunov et al, 2014;Shalima et al, 2015). Nagdimunov et al (2014) have succeeded in modeling both an optically thick ejecta plume and its shadow on the surface of comet 9P/Tempel 1 produced and imaged by the Deep Impact mission (see Fig.…”