Comet 9P/Tempel 1: Interpretation with theDeep ImpactResults

Abstract

According to our common understandings, the original surface of a short-period comet nucleus has been lost by sublimation processes during its close approaches to the Sun. Sublimation results in the formation of a dust mantle on the retreated surface and in chemical differentiation of ices over tens or hundreds of meters below the mantle. In the course of NASA's Deep Impact mission, optical and infrared imaging observations of the ejecta plume were conducted by several researchers, but their interpretations of the data came as a big surprise: (1) The nucleus of comet 9P/Tempel 1 is free of a dust mantle but maintains its pristine crust of submicron-sized carbonaceous grains. (2) Primordial materials are accessible already at a depth of several tens of centimeters with abundant silicate grains of submicrometer sizes. In this study, we demonstrate that a standard model of cometary nuclei explains well available observational data: (1) A dust mantle with a thickness of ∼1-2 m builds up on the surface, where compact aggregates larger than tens of micrometers dominate. (2) Large fluffy aggregates are embedded in chemically differentiated layers as well as in the deepest part of the nucleus with primordial materials. We conclude that the Deep Impact results do not need any peculiar view of a comet nucleus.

The morphological, elastic, and electric properties of dust aggregates in comets: A close look at COSIMA/Rosetta’s data on dust in comet 67P/Churyumov-Gerasimenko

“…Moreover, dust particles with a low porosity, namely, a high fractal dimension tend to fall back on the surface of a comet and thus most likely elevate the fractal dimension of dust particles in the surface of the comet as a result of inelastic collisions. Therefore, it is reasonable to assume that dust particles in a dust mantle of a comet are composed of aggregate particles with relatively compact structures, in other words, high fractal dimensions, compared with those in the inner nucleus of the comet (see Yamamoto et al 2008).…”

Collisional process on Comet 9/P Tempel 1: Mass loss of its dust and ice by impacts of asteroidal objects and its collisional history

“…(13), we estimate that a crater with D cr = 100 m is formed at each ∼70 yr, which is lower than the previous estimate (Yamamoto et al, 2008). This is because these authors used the maximum asteroidal population model by Hawkes and Eaton (2004), which was based on an extrapolation from the population of large observed asteroids (>∼100 km) with a single power-law distribution.…”

“…Therefore, primordial dust and ice would remain only local regions well below the surface and well above the center of the nucleus (e.g., Jewitt, 1992;Meech, 2000;Prialnik et al, 2004Prialnik et al, , 2008. This scenario, which is hereafter called the standard model, has turned out to describe well the nucleus of Comet 9P/Tempel 1 (T1), the target of NASA's Deep Impact (DI) mission: (1) a low thermal inertia of its surface estimated from thermal maps of the nucleus indicates the presence of a dust mantle ; (2) the existence of large grains in a dust trail along the trajectory of the comet would result from the preferential elimination of small-sized dust during the formation of a dust mantle (Sykes and Walker, 1992;Reach et al, 2007); (3) the optical and infrared properties of dust excavated by the DI event are explained well by the existence of a dust mantle composed of compact aggregates, below which fluffy aggregates are also embedded with icy volatiles (Yamamoto et al, 2008); (4) near-infrared spectroscopic observations of volatiles excavated by the DI event revealed thermal processing of icy layers below a dust mantle (Mumma et al, 2005); (5) the sublimation rate of 6 × 10 27 water molecules s −1 (∼180 kg s −1 ) from the T1 nucleus before the DI event implies that the top surface of several meters could have been lost simply by the sublimation process (e.g., Schleicher et al, 2006). In addition, the standard model has been supported by numerical studies on thermal evolution of the T1 nucleus: the formation of a dust mantle with chemical differentiations of the T1 nucleus (De Sanctis et al, 2007); a crystalline ice crust with a thickness of 40-240 m below a dust mantle (Bar-Nun et al, 1989).…”

“…model of the asteroidal population (e.g., Hawkes and Eaton, 2004;Gronkowski, 2007;Yamamoto et al, 2008). For example, Hawkes and Eaton (2004) estimated the mass flux of asteroids into the T1 nucleus to range from ∼10 2 to 10 7 kg yr −1 , depending on the model they used.…”

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

“…The solid curves, dotted ones, and dash-dotted ones are the infrared spectral of BCCA (D ≈ 2) particles, BPCA (D ≈ 3) particles, and compact spherical particles, respectively. From Kolokolova et al (2007) Yamamoto et al (2008) used fractal agglomerates to interpret the strength of a silicate emission feature as well as the color temperature observed for the ejecta of comet 9P/Tempel 1 during the Deep Impact mission. By taking into account the formation and evolution of dust mantles on comets, they assumed that the dust mantle of the comet consist of agglomerates with D = 2.5 and the interior of the comet consists of agglomerates with D = 1.9.…”