Pronounced morphological changes in a southern active zone on comet 67P/Churyumov-Gerasimenko

¹ LESIA, Observatoire de Paris, PSL Research University, CNRS, Université Paris Diderot, Sorbonne Paris Cité, Sorbonne Universités, 5 place J. Janssen, 92195 Meudon Principal Cedex, France
e-mail: hasselmann@on.br
² Department of Astronomy, University of Maryland College Park, MD 20742-2421, USA
³ Center for Technical Physics, Institute of Physics, Vietnam Academy of Science and Technology, Hanoi, Vietnam
⁴ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg, 3, 37077 Göttingen, Germany
⁵ Department of Physics and Astronomy “Galileo Galilei”, University of Padova, Via Marzolo 8, 35131 Padova, Italy
⁶ Center of Studies and Activities for Space (CISAS) “G. Colombo”, University of Padova, Via Venezia 15, 35131 Padova, Italy
⁷ CNR-IFN UOS Padova LUXOR, Via Trasea, 7, 35131 Padova, Italy
⁸ Laboratoire d’Astrophysique de Marseille, UMR 7326 CNRS & Université Aix-Marseille, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
⁹ Centro de Astrobiologia, CSIC-INTA, 28850 Torrejon de Ardoz, Madrid, Spain
¹⁰ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
¹¹ Scientific Support Office, European Space Research and Technology Centre/ESA, Keplerlaan 1, Postbus 299, 2201 AZ Noordwijk ZH, The Netherlands
¹² Jet Propulsion Laboratory, M/S 183-401, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹³ Physics Department, Auburn University, 206 Allison Laboratory, Auburn, AL 36849, USA
¹⁴ INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁵ Department of Industrial Engineering, University of Padova, Via Venezia 1, 35131 Padova, Italy
¹⁶ Faculty of Engineering, University of Trento, Via Mesiano 77, 38121 Trento, Italy
¹⁷ INAF Astronomical Observatory of Trieste, Via Tiepolo 11, 34143 Trieste, Italy
¹⁸ Instituto de Astrofisica de Andalucia (CSIC), c/ Glorieta de la Astronomia s/n, 18008 Granada, Spain
¹⁹ Graduate Institute of Astronomy, National Central University, 300 Chung-Da Rd, Chung-Li 32054, Taiwan
²⁰ Space Science Institute, Macau University of Science and Technology, Avenida Wai Long, Taipa, Macau
²¹ Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany
²² Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, USA

Received: 24 July 2018
Accepted: 23 January 2019

Abstract

A smooth deposit in the southern Khonsu region has been seen in ESA/Rosetta observations as active during the second half of 2015, when the southern summer coincided with the perihelion passage of 67P/Churyumov-Gerasimenko (67P). Image color sequences acquired by the OSIRIS instrument in the period of January 2015 to July 2016, pre- and post-perihelion, show the occurrence of several small transient events as well as three massive outbursts (~10 to 1500 tons). High spatial resolution images taken one year and a half apart allowed us to track a variety of sources: the formation of cavities that are 1.3–14 m deep, ice-enriched patches, scarp retraction, and a second 50 m-wide boulder. We then estimated their masses and the dust mass of their corresponding plumes and outbursts. In particular, the deformation left by that boulder and its lack of talus may provide evidence for the lifting and subsequent falling back to the surface of large blocks. We calculate that a minimum vapor production rate of 1.4 × 10²⁴ m⁻² s⁻¹ is required to lift such an object. The comparison of the masses that are lost in the new cavities to the dust mass of outbursts gives indirect evidence of highly volatile ice pockets underneath. The spectrophotometric analysis and boulder counting also provides evidence for cavities that formed only 30 m apart with different spectral slopes, two long-standing ice patches, and local variations in the boulder-size frequency distribution. All this points to sub-surface ice pockets with different degrees of depth. Finally, the total mass of the morphological changes compared to most recent calculations of the total released mass by activity on 67P is estimated to be between 1.5 and 4.2%. This means that as many as about 25 similar active zones across the nucleus would be enough to sustain the entire cometary activity.