M. F. A'hearn, M. J. Belton, and W. A. Delamere, Science, vol.332, p.1396, 2011.

C. H. Acton, Planet. Space Sci, vol.44, p.65, 1996.

M. J. Belton, Icarus, vol.210, p.881, 2010.

M. J. Belton, P. Thomas, and J. Veverka, Icarus, vol.187, p.332, 2007.

M. J. Belton, P. D. Feldman, and M. F. Hearn, Icarus, vol.198, p.189, 2008.

J. Blum, B. Gundlach, and S. Mühle, Icarus, vol.235, p.156, 2014.

G. D. Brin, ApJ, vol.237, p.265, 1980.

B. Syal, M. Schultz, P. H. Sunshine, and J. M. , Icarus, vol.222, p.610, 2013.

F. Capaccioni, A. Coradini, and G. Filacchione, Science, vol.347, p.628, 2015.

B. J. Davidsson and H. Rickman, Icarus, vol.243, p.58, 2014.

B. J. Davidsson and Y. V. Skorov, Icarus, vol.159, p.239, 2002.

M. C. De-sanctis, F. Capaccioni, and M. Ciarniello, Nature, vol.525, p.7570, 2015.

M. R. El-maarry, N. Thomas, and L. Giacomini, A&A, vol.583, p.26, 2015.

A. Enzian, Space Sci. Rev, vol.90, p.131, 1999.

T. L. Farnham, D. D. Wellnitz, and D. L. Hampton, Icarus, vol.187, p.26, 2007.

T. L. Farnham, D. Bodewits, and J. Li, Icarus, vol.222, p.540, 2013.

L. M. Feaga, M. F. Hearn, and J. M. Sunshine, Icarus, vol.191, p.134, 2007.

M. Fulle, V. Della-corte, and A. Rotundi, ApJ, vol.802, p.12, 2015.

N. Gortsas, E. Kührt, and U. Motschmann, Icarus, vol.212, p.858, 2011.

S. Gulkis, M. Allen, and P. Von-allmen, Science, vol.347, p.709, 2015.

B. Gundlach and J. Blum, Icarus, vol.219, p.618, 2012.

B. Gundlach, Y. V. Skorov, and J. Blum, Icarus, vol.213, p.710, 2011.

B. Gundlach, J. Blum, and H. U. Keller, A&A, vol.583, p.12, 2015.

P. J. Gutiérrez, J. L. Ortiz, and R. Rodrigo, A&A, vol.374, p.326, 2001.

M. Hässig, K. Altwegg, and H. Balsiger, Science, vol.347, p.276, 2015.

W. F. Huebner, J. Benkhoff, M. Capria, and -. T. , Heat and Gas Diffusion in Comet Nuclei, 2006.

A. V. Ivanova and L. M. Shulman, Adv. Space Res, vol.38, p.1932, 2006.

H. U. Keller, C. Barbieri, and P. Lamy, Space Sci. Rev, vol.128, p.433, 2007.

H. U. Keller, S. Mottola, and B. Davidsson, A&A, vol.583, p.34, 2015.

J. Knollenberg, Z. Lin, and S. Hviid, Icarus, vol.224, p.172, 2013.

E. Kührt, Icarus, vol.60, p.512, 1984.

E. Kührt and H. U. Keller, Icarus, vol.109, p.121, 1994.

J. S. Lagerros, A&A, vol.325, p.1226, 1997.

L. M. Lara, S. Lowry, and J. Vincent, A&A, vol.583, p.9, 2015.

D. Möhlmann, Planet. Space Sci, vol.44, p.541, 1996.

F. Preusker, F. Scholten, and K. Matz, A&A, vol.583, p.33, 2015.

D. Prialnik and A. Bar-nun, ApJ, vol.363, p.274, 1990.

D. Prialnik, J. Benkhoff, and M. Podolak, Comets II, p.359, 2004.

D. Prialnik, M. F. A'hearn, and K. J. Meech, MNRAS, vol.388, p.20, 2008.

A. Rotundi, H. Sierks, and V. Della-corte, Science, vol.347, p.3905, 2015.

H. Sierks, C. Barbieri, and P. L. Lamy, Science, vol.347, p.1044, 2015.

R. Schulz, M. Hilchenbach, and Y. Langevin, Nature, vol.518, p.216, 2015.

Z. Sekanina, D. E. Brownlee, and T. E. Economou, Science, vol.304, p.1769, 2004.

Y. Skorov and J. Blum, Icarus, vol.221, p.1, 2012.

Y. V. Skorov, R. V. Lieshout, and J. Blum, Icarus, vol.212, p.867, 2011.

N. Thomas, B. Davidsson, and M. R. El-maarry, A&A, vol.583, p.17, 2015.

N. Thomas, H. Sierks, and C. Barbieri, Science, vol.347, p.440, 2015.

J. Vincent, N. Oklay, and M. Pajola, A&A, 2016.

. Max-planck, Germany e-mail: shi@mps.mpg.de 2 Department of Astronomy, Justus-von-Liebig-Weg, vol.3, pp.20742-2421

. Braunschweig,

, Physikalisches Institut der Universität Bern, Sidlerstr, vol.5, p.3012

D. Zentrum-für-luft-und-raumfahrt, Institut für Planetenforschung, 12489.

, University of Padova, via Venezia 15, vol.35131, p.92195

C. Rd and ;. Chung-li, Taiwan 9 Department of Physics and Astronomy "G. Galilei, vol.3, p.35122, 32054.

, 13388 Marseille, France 11 Centro de Astrobiología (CSIC-INTA), 28850 Torrejón de Ardoz, Madrid, Spain 12 International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland 13 Scientific Support Office, European Space Research and Technology Centre/ESA, Keplerlaan, vol.7326, p.78280, 2201.

. Guyancourt, France 16 INAF Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy 17 CNR-IFN UOS Padova LUXOR, via Trasea, vol.7, p.35131