Structure and elastic parameters of the near surface of Abydos site on comet 67P/Churyumov–Gerasimenko, as obtained by SESAME/CASSE listening to the MUPUS insertion phase

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The accelerometers of the SESAME/CASSE instrument aboard Rosetta's lander Philae recorded surface waves produced during the MUPUS hammering phase. After presenting evidence that all feet of Philae were in contact with the ground, we analyze group arrival time differences between the three feet of Philae and obtain a Rayleigh wave velocity between 79ms−1 and 400ms−1, which translate into a shear modulus μ of 3.6 MPa ≤ μ ≤ 346 MPa, and a Young's modulus E of 7.2 MPa ≤ E ≤ 980 MPa (with the lower bounds being better constrained than the upper bounds). Mixture models of snow and regolith suggest a porosity below 0.74. From the frequency-dependent dispersion of the average signal we conclude that the above values are valid for a surface layer of 10 cm to 50 cm thickness, while rigidity is significantly reduced underneath this layer. Our findings are consistent with the concept of a thin consolidated shell around a less rigid interior.