Abstract
Context. After the landing of Philae, CONSERT probed the nucleus of 67P/Churyumov-Gerasimenko (67P) and observed no heterogeneities at metric scale within the probed part of the small lobe of 67P. Further studies have then quantified the observed homogeneity in terms of maximum permittivity contrast versus the typical size of heterogeneities. Aims. The aim of this article is to interpret the sensitivity limits of CONSERT measurements in terms of composition, and to provide constraints on the maximum variability in composition, porosity, and local dust-to-ice ratio. Methods. The sensitivity of CONSERT measurements to local variations in density, dust-to-ice ratio, and composition was analyzed using permittivity modeling of mixtures. Results. We interpret the maximum detectable heterogeneity size and contrast in terms of composition and porosity of the nucleus. The sensitivity to porosity is ±10 percent points for heterogeneities with a characteristic length scale of a few meters; the sensitivity to local variations in the composition is limited. Conclusions. In terms of accretion, our results are compatible only with scenarios generating porosity heterogeneities at scales lower than one meter, or with porosity variations smaller than ±10 percent points. This is clearly compatible with an accretion model of a gentle gravitational collapse of a pebble cloud.
Highlights
After the landing of Philae, the Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) on board Rosetta probed the nucleus of 67P/Churyumov-Gerasimenko (67P) and observed no heterogeneities at metric scale within the fathomed part of the small lobe of the comet (Kofman et al 2015)
We used a similar approach to analyze the comet homogeneity seen by CONSERT: our interpretation relies on the same dielectric mixing models, but here we focus on the influence of the local variability, setting aside the constraints on average permittivity, density, and dust-to-ice mass ratio (D/I)
This homogeneity was quantified by limiting the maximum variability range of the local composition and porosity versus the size of the heterogeneities, while the average composition and porosity have to fit the values previously derived from CONSERT measurements (Herique et al 2016)
Summary
After the landing of Philae, the Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) on board Rosetta probed the nucleus of 67P/Churyumov-Gerasimenko (67P) and observed no heterogeneities at metric scale within the fathomed part of the small lobe of the comet (Kofman et al 2015). The collisional properties of dust aggregates have been studied by Bukhari Syed et al (2017) and can be applied to the two planetesimal-formation models to derive the expected amount of inhomogeneity, which can be compared to the results of CONSERT. Homogeneity and characteristic length scales of planetesimals formed by model A: Wahlberg Johansen et al (2017) have applied the dust-aggregate collision model by Bukhari Syed et al (2017) to collapsing pebble clouds and found that for bodies. Homogeneity and characteristic length scales of planetesimals formed by model B: through the increasing collision velocity with increasing size for bodies smaller than meters (Weidenschilling 1997; Davidsson et al 2016), all pebbles will be collisionally destroyed. The two competing planetesimal-formation models predict very different internal morphologies for cometary nuclei, which might be compared with the results from CONSERT and from the optical remote sensing of comet surfaces
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