Blocks Size Frequency Distribution in the Enceladus South Pole

Enceladus is a heavily cratered icy moon of Saturn. In 2005, the Cassini ISS-NAC camera took high-resolution images of the moons south pole, revealing a craterless, fracture-dominated and geologically active province. Here, cryovolcanic and tectonic activity is evident with icy geysers emanating water vapor plumes from four tension fractures, called tiger stripes (hereafter named TS). In 2008/2009, Cassini performed three Enceladus close flybys. The m-scale NAC images revealed the presence of icy blocks located close to the TS. Martens et al. [1] focused on the spatial density of such features, showing that the variations in number density can be substantial over short spatial scale. Through eight NAC high-resolution images of the TS, we expanded the work of [1] obtaining the SFD of the identifiable blocks in the TS area. We have identified 17070 blocks with sizes ranging from 30 to 366 m [2]. By using the Clauset et al. [3] methodology we derived that the TS blocks SFD can be fitted by a power-law curve, with a power-law of -5.4±0.4. To see if any differences in cumulative SFD per km2 or power-law indices, we decided to split the full dataset of blocks into three main groups [2]: the Damascus, Baghdad and Cairo ones. The same methodology applied on the three TS separately studied returned a power-law index of -5.1±0.4 (Damascus), -5.1±0.3 (Baghdad), and -6.3±0.3 for Cairo. We explain these results considering that different processes concur in the formation and evolution of such blocks, in particular sublimation and cryovolcanic ejection mechanisms, as previously hypothesized by [1]. We also investigated the block diameter versus TS distance trend to test if the cryovolcanic ejection mechanism suggested by [1] can be a possible block formation process. From our analysis, we found that the smallest blocks are present from tens of m to maximum distances of 25 km from the TS, while the largest blocks (100s m wide) are found at much closer range. This result positively supports the vent eruption hypothesis, which foresees that larger blocks usually land closer to the ejection point. This activity has been realized under the ASI-INAF contract 2018-25-HH.0. [1] Martens, H. R., et al., 2015. Icarus, 245, 162. [2] Pajola, M. et al., 2021. Universe, 7, 82. [3] Clauset, A., et al., 2009. SIAM review, 51, 661. [4] Pajola et al., 2015. A&A 583, A37.