The QuACK map projection, a novel approach to explore 67P/CG surface

In the last decade, the Rosetta community has struggled with the complexity of mapping the irregular surface of comet 67P/CG [1]. The standard projection techniques derived for spherical bodies fail to give correct renderings of bilobate shapes. As a consequence of the irregular shape and rotation axis orientation [2], the standard longitude-latitude reference system applied to 67P/CG nucleus fails to give a unique correspondence between the position of the points and their geographical coordinates: for some regions of the nucleus, in particular for areas located on the small lobe («head») and in the «neck» areas, the same longitude-latitude value can correspond to up to three different geographical locations. A similar behavior induces degeneration in the maps when using standard mapping techniques and makes annoying their interpretation. To overcome similar limitations, Grieger (2019) [3] has introduced the Quincuncial Adaptive Closed Kohonen (QuACK) map projection method which is applied for the first time to Rosetta / VIRTIS [4] hyperspectral data for which calibrated, and geometry files are available. VIRTIS geometry files, computed through >3 million plates Osiris Shape7 v1.8 model, assign the longitude-latitude values and the ID number of the plates corresponding to the center and the four corners of each VIRTIS pixel. Knowing the cartesian coordinates [x, y, z] of each plate, by applying the QuACK coordinate transformation, it is possible to establish an univocal conversion between each physical point of the surface located on cartesian coordinates [x, y, z] with a new, not-degenerated reference frame based on QuACK coordinates [P, Q], where 0≤P≤1, 0≤Q≤1 [5]. The QuACK projection does not preserve the area nor the shape, but it maintains the tessellation properties of the original shape.

In this work we report on the rendering of key spectral indicators exploited from VIRTIS dataset such as albedo [6], spectral slopes [1], water/carbon dioxide ices [7, 8, 9], and organic matter [10] absorption bands computed on VIRTIS data mosaics collected during different phases of the Rosetta mission. Thanks to the QuACK capability to map the entire surface of 67P without spatial degeneracies, this methodology is particularly efficient in rendering novel global and regional maps from which the composition [11] and diurnal/seasonal evolution [12, 13, 14] can be studied by applying standard VIS-IR spectroscopic techniques.   

References: [1] Filacchione, G., Icarus, 274, 334-349 (2016). [2] Preusker, F. et al., A&A 607, L1 (2017). [3] Grieger, B. A&A 630, A1, (2019). [4] Coradini, A. et al., SSR, 128 (2007). [5] Leon-Dasi, M. A&A 652, A52 (2021). [6] Ciarniello, M. et al., A&A 583, A31 (2015). [7] Filacchione, G. et al., Nature, 529, 368-372 (2016). [8] Barucci, M.A. et al., A&A, 595 (2016). [9] Filacchione, G. et al., Science, 354, 1563-1566 (2016). [10] Raponi, A. et al., Nat. Astro., 4, 500-505 (2020). [11] Filacchione, G. et al., SSR, 215, id.19 (2019) ). [12] De Sanctis, M.C. et al., Nature, 525 (2015). [13] Filacchione, G. et al., Nature, 578, 49-52 (2020). [14] Ciarniello, M. et al., Nat. Astro., 6, 546-553 (2022).