Size-frequency distribution of boulders ≥10 m on comet 103P/Hartley 2

Abstract

Aims. We derive the size-frequency distribution of boulders on comet 103P/Hartley 2, which are computed from the images taken by the Deep Impact/HRI-V imaging system. We indicate the possible physical processes that lead to these boulder size distributions. Methods. We used images acquired by the High Resolution Imager-Visible CCD camera on 4 November 2010. Boulders ≥10 m were identified and manually extracted from the datasets with the software ArcGIS. We derived the global size-frequency distribution of the illuminated side of the comet (∼50%) and identified the power-law indexes characterizing the two lobes of 103P. The three-pixel sampling detection, together with the shadowing of the surface, enables unequivocally detection of boulders scattered all over the illuminated surface. Results. We identify 332 boulders ≥10 m on the imaged surface of the comet, with a global number density of nearly 140/km 2 and a cumulative size-frequency distribution represented by a power law with index of −2.7 ± 0.2. The two lobes of 103P show similar indexes, i.e., −2.7 ± 0.2 for the bigger lobe (called L1) and −2.6 +0.2/ − 0.5 for the smaller lobe (called L2). The similar power-law indexes and similar maximum boulder sizes derived for the two lobes both point toward a similar fracturing/disintegration phenomena of the boulders as well as similar lifting processes that may occur in L1 and L2. The difference in the number of boulders per km 2 between L1 and L2 suggests that the more diffuse H 2 O sublimation on L1 produce twice the boulders per km 2 with respect to those produced on L2 (primary activity CO 2 driven). The 103P comet has a lower global power-law index (−2.7 vs. −3.6) with respect to 67P. The global differences between the two comets' activities, coupled with a completely different surface geomorphology, make 103P hardly comparable to 67P. A shape distribution analysis of boulders ≥30 m performed on 103P suggests that the cometary boulders show more elongated shapes when compared to collisional laboratory fragments as well as to the boulders present on the surfaces of 25 143 Itokawa and 433 Eros asteroids. Consequently, this supports the interpretation that cometary boulders have different origins with respect to the impact-related asteroidal boulders.

Characterization of the Abydos region through OSIRIS high-resolution images in support of CIVA measurements

“…This comparison will be useful for understanding whether boulders and pebbles are produced by the same fragmentation process or if pebbles have a primordial origin, as suggested by the consolidated material strata, which might be representative of primordial stratification. The average value of the albedo, over the entire region, is 5.8% at λ = 480.7 nm and 7.4% at λ = 649.2 nm, in agreement with the average albedo of the comet equal to 6.4% derived by Fornasier et al (2015) and in particular with the albedo and the variations (6.7 ± 1) measured by La Forgia et al (2015) in the analysis of Philae Agilkia landing site by OSIRIS data. The computed albedo is slightly higher than the one found by CIVA, which typically varies from 3 to 5% .…”

“…Therefore a photometric correction taking into account the topography and the illumination angles (see, for example, Fornasier et al 2015;La Forgia et al 2015) would be necessary to derive the normal albedo correctly. However, the particular geometry of the images did not allow this complex approach.…”

“…However, the particular geometry of the images did not allow this complex approach. We therefore decided to simply correct the observed reflectance by the effect of the average phase curve that was studied by Fornasier et al (2015). Assuming that the averaged parameters of a Hapke model fitted by Fornasier et al (2015) on the overall nucleus are also valid for the our region of interest, we applied the photometric correction using the model by Hapke (1981Hapke ( , 1984Hapke ( , 2002.…”

“…where I F Hapke was computed using the disk-integrated Hapke expression and the parameters reported in Table 3 of Fornasier et al (2015) for the orange and blue filter.…”

The Agilkia boulders/pebbles size–frequency distributions: OSIRIS and ROLIS joint observations of 67P surface

“…to determine the structure of the cometary regolith spanning from pebbles (diameters ≤ 0.25 m) to boulders (diameters > 0.25 m). 1 The comparison between the results obtained from previous works on this topic (Pajola et al 2015(Pajola et al , 2016aPommerol et al 2015;Vincent et al 2016) provides fundamental hints to understand if similar boulder/pebble formation and degradation processes occur in such wide range of dimensions. Moreover, our studied size range overlaps the ones described in the 67P coma by Rotundi et al (2015), Davidsson et al (2015) and Fulle et al (2016a); therefore, comparisons between our SFD analysis and the one inferred for coma particles can yield important information regarding the dust lifting processes occurring on the cometary surface.…”