First analysis of the size-frequency distribution of boulders ≥ 7m on comet 67P

We present the first size-frequency distribution of boulders ≥ 10 m identified on comet 103P/Hartley 2, computed from the images taken by the Deep Impact/High Resolution Imager - Visible CCD camera on 4 November 2010. We derived the size-frequency distribution of the illuminated side of the comet (˜ 50%), as well as identified the power-law indexes characterizing the two lobes of 103P. 332 boulders larger than 10 m were identified on the imaged surface of the comet, with a global number density of nearly 140/km2 and a cumulative size-frequency distribution represented by a power-law with index of -2.7 ± 0.2. The two lobes of 103P show close indexes, i.e. -2.7 ± 0.2 for the bigger lobe (L1) and -2.6 +0.2/-0.5 for the smaller one (L2). Both the similar power-law indexes and the similar maximum boulder sizes derived for the two lobes point towards a similar fracturing/disintegration phenomena of the boulders as well as similar lifting processes that may occur in L1 and L2. Nonetheless, the significative difference in the number of boulders per km2 between the two lobes suggests that the more diffuse H2O sublimation on the bigger lobe produce twice the boulders per km2 with respect to those produced on the small lobe (primary activity CO2 driven). If we compare the boulder distribution of the hyperactive 103P comet with similar studies performed on 67P (67P/Churyumov-Gerasimenko, i.e. the comet studied by Rosetta spacecraft), we derive that 103P shows a lower global power-law index (-2.7 vs -3.6). The size-frequency boulder trend of 103P is somehow closer to the -2.2 value measured on the neck region of 67P, i.e. the most active region of the northern hemisphere of 67P; nevertheless the hyperactivity of 103P works in a very different way than the normal activity of 67P in the neck/Hapi area. In addition to the global differences between the two comets' activities, the absence of cliffs and walls on 103P show a completely different surface geomorphology between Hartley 2 and Churyumov-Gerasimenko, making them hardly comparable.