Long-term measurements of the erosion and accretion of dust deposits on comet 67P/Churyumov-Gerasimenko with the OSIRIS instrument

We monitor the seasonal erosion and accretion of dust deposits in the Imhotep, Hatmehit, and Ma'at regions of comet 67P/Churyumov-Gerasimenko with OSIRIS Narrow Angle Camera images. The vertical accuracy of such measurements is 0.2 m and the spatial scale of the images we used is lower than 0.60 mpixel(-1). We calculate the height of 21 boulders by applying a tool that allows to measure the shadow length of a boulder projected on the surrounding dust deposit, assuming that any height variation is not due to boulder intrinsic change. Any boulder height variation provides a direct measurement about the thickness variation of the surrounding dust layer due to the occurring erosion and/or accretion. The analysis concerns the period from 2014 August, inbound to perihelion, to 2016 September, outbound. We measured the erosion in the Ma'at region of 0.6 +/- 0.2 m from 2014 September 12 to December 2, and an erosion of 0.4 +/- 0.3 m from 2014 December 3 to 2015 February 15. Then, we measured a dust deposition of 0.7 +/- 0.3 m during the following perihelion phase, until 2016 May-September. This result confirms the link between the erosion of the Southern hemisphere and the fallout in the northern regions. The Imhotep and Hatmehit regions are characterized by a negligible erosion during the inbound orbit, consistent with pebble-made nucleus models predicting no erosion when the temperature of the nucleus surface is T-s < 205 K.