Interpretation on Deep Impact results: Radial distribution of ejecta and the size distribution of large-sized grains
© The Society of Geomagnetism and Earth, Planetary and Space Sciences, The Seismological Society of Japan 2010
Received: 31 July 2008
Accepted: 10 December 2008
Published: 7 February 2015
Several observations of dust grains ejected from the comet 9P/Tempel 1 by the Deep Impact event strongly suggest that the evaporation and expansion of volatiles occurred and that the vapor accelerated some dust grains. When grains are accelerated by gas, size sorting should occur, that is, larger grains tend to stay closer to the nucleus, while smaller grains is pushed farther away. This means that the light at each distance is emitted from identical-sized grains. Hence, we can estimate the size distribution of grains based on the flux as a function of the distance from the nucleus. A simple evaluation indicates that the size distribution of grains with a size larger than ∼10 μm is expressed as a power law and the index is ∼-4. This is expected to be an alternative method to estimate the size distribution of grains, though detailed analyses and numerical simulations should be necessary to evaluate the error of this method.
The impactor launched from the Deep-Impact (DI) spacecraft collided with the comet 9P/Tempel 1 at 2005/7/4 (UT) (A’Hearn et al., 2005). The large amount of cometary materials was ejected. We conducted mid-infrared (mid-IR) observations before and after its collision using COMICS (the Cooled Mid-IR Camera and Spectrometer), mounted on the Subaru Telescope, and both imaging and spectroscopic measurements were performed with three band-pass filters in the N-band with central wavelengths of 8.8, 10.5, and 12.4 μm (Sugita et al., 2005; Kadono et al., 2007; Ootsubo et al., 2007). The observational data indicate that large-scale dust plume ejected by the impact contained a large mass of ∼106 kg of solid dust grains and formed two wings approximately ±45 degrees from the symmetric center (Sugita et al., 2005). These facts are consistent with the gravity-controlled nature of the DI-induced impact cratering. Also, the image data indicate that there are two regions in the plume. In the outer region, which is far from the nucleus and contains dust grains with higher ejection velocities, the slope of the blackbody continuum is flat and the intensity of 10.5 μm silicate feature is small relative to the continuum, and in the inner region, the slope is steep (the intensity at longer wavelength is large) and the intensity of 10.5 μm is relatively large. We can quantitatively understand this result using a simple dust model (e.g., Ootsubo et al., 2007), and this result and the crater formation mechanism (e.g., Melosh, 1989) indicate that the comet 9P/Tempel 1 has a surface layer with a large fraction of small (sub-micron sized) carbonaceous grains whose thickness is ∼1 m and a subsurface region consisting of a large fraction of sub-micron sized silicate grains (Kadono et al., 2007).
Spectral analyses using the data at the mid-infrared wavelength do not clearly indicate the size distribution at a size larger than ∼10 μm because large ejecta grains are not optically active. However, their contribution to the total mass of DI ejecta plume is very large. Thus, estimation of the size distribution of the large ejecta grains is of extremely high importance. Consequently, the method to obtain the size distribution at larger sizes is necessary for precise estimation of ejecta mass. Size distribution of large grains can be estimated by modeling the effect of radiation pressure on the grains based on the images of dust clouds (e.g., Jorda et al., 2007). In this paper, we propose an alternative method to estimate the size distribution at a size larger than ∼10 μm using the observational results of the DI event, taking advantage of the fact that the ejecta release process from the DI impact was practically instantaneous unlike normal activities.
2. Observational Results
We denote the flux at each wavelength in the N-band as I8.8, I10.5, and I12.4, respectively. For all the results, the flux on July 2 (i.e., two days before the collision) is subtracted as a background at each wavelength.
A classic ejecta curtain model predicts a monotonically decreasing mass distribution as a function of ejection velocity (e.g., Housen et al., 1983); that is, there should not be such a peak in Fig. 1. This discrepancy has been pointed out (e.g., Sugita et al., 2005) and several authors suggest that an acceleration process for dust grains in the inner portion of the plume may occur caused by volatile evaporation and expansion. Küppers et al. (2005) observed a large amount of water vapor, Mg ∼ 4.6 × 106 kg. They also suggested that this vapor accelerates solid grains. Also, analyses of both radial and angular profiles show that, though the outer part of the dust plume is conical fan-shape (this is consistent with experimental results in laboratories), the inner part is strongly influenced by the dynamics of volatile evaporation and expansion (Sugita et al., 2005).
We propose an alternative method to estimate the size distribution at a size larger than ∼10 μm based on the observational data of the DI ejecta in both time and space. We consider the radial distributions of Isi and Icont at P.A. 225. The distribution of Isi has a peak which moves ∼100 m/s. Assuming that the evaporation of volatiles occurs and that the expansion of the vapor accelerates solid dust grains, we obtain an analytical relation between Dp and r . Based on this relation and the observational result of Icont, the sizenumber distribution of larger grains is estimated. The result indicates that the distribution is expressed as a power law with an exponent ∼-4.
Because the contribution of the large grains to the total ejecta mass has been highly uncertain and spectral analyses using the data at the mid-infrared wavelength do not clearly indicate the size distribution at a size larger than ∼10 μm, such an estimate on their size distribution is of great importance, though detailed analyses and numerical simulations should be necessary to evaluate the error of this method.
Finally, it is noted that the analysis given by Kadono et al. (2007) is on the outer part of the ejecta plume, where the effect of gas acceleration is minimal. The present study focuses on the inner part of the ejecta, where the gas acceleration effect can be significant. In this sense, these two studies are complimentary to each other.
This paper is based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan (S05A-042 and S05A-OT11). The authors are grateful to two anonymous reviewers for useful comments. This research is partially supported by Grant in Aid from the Japan Society for the Promotion of Science.
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