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

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 (similar to 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 H2O sublimation on L1 produce twice the boulders per km(2) with respect to those produced on L2 (primary activity CO2 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.