**Acknowledgements**

This project was supported by Progetti di Ateneo-University of Padova 2017- N. C93C1800002600 and by MIUR-PNRA (Programma Nazionale Ricerche in Antartide) (PNRA16\_00068). We thank Samuele Cesaro and Ferdinando Polverino de Laureto for the elaboration of the images.
