Geological and geomorphological mapping of Martian sedimentary deposits: an attempt to identify current practices in mapping and representation
Authors: Pondrelli Monica , Frigeri Alessandro , Marinangeli Lucia , Di Pietro Ilaria , Pantaloni Marco , Pozzobon Riccardo , Nass Andrea , Rossi Angelo Pio 
Published: October 2020

The quantity, quality, and type of available datasets on Mars have improved in the last couple of decades. Context Camera (CTX) (Malin et al., 2007) on board the NASA Mars Reconnaissance Orbiter (MRO) provides a global coverage with an average resolution of 6 meters/pixel while the High Resolution Imaging Science Experiment (HiRISE) on board MRO (McEwen et al., 2007) allows up to 30 cm/pixel analyses at the local scale. These data allow at places also the DTM generation, but extensive topographic reconstructions at an average scale of 50 meters/pixel are possible using the High Resolution Stereo Camera (HRSC) on board of ESA Mars Express (MEX) (Neukum et al., 2004). Moreover&#160; compositional constraints can be provided by the spectral data coming from Observatoire pour la Min&#233;ralogie, l&#8217;Eau, les Glaces, et l&#8217;Activit&#233; (OMEGA) on board MEX and from Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on board the NASA Mars Reconnaissance Orbiter (MRO) (Murchie et al., 2007).&#160;These relatively recent datasets coupled with the older datasets and Geographical Information Systems (GIS) provide an impressive suite of tools to develop meaningful&#160;planetary geological maps.&#160;In principle, these new data allow to add to the traditional geomorphological or chronostratigraphic mapping approaches, also a geological map approach somewhat akin to the one well-known on Earth, although, a &#8216;true&#8217; geological map should be based on the lithological characters of the mapped units; using tone, texture, absence/presence of sedimentary structure, and, if possible, compositional hints to define the units, may represent an adequate &#8216;planetary perspective&#8217;, obviously in addition to the stratigraphic position within the succession. This map approach has the advantages to be relatively objective (and so potentially more useful for geological context analyses) and to represent the stratigraphic complexity within a region.&#160;This approach is complementary to the more interpretative (because it includes the processes/environments of formation of the different features) one of the geomorphological maps, which has the obvious advantage to describe the environments/processes active in the region.&#160;In the framework of the GMAP (Geologic MApping of Planetary bodies) project, we present here an attempt to merge these two cartographic products taking advantage of GIS-based tools. Moreover, we aim at testing, where possible, the Earth-born symbols designed for the Geological Map of Italy (ISPRA, 2009, 2018) to try to make the &#8216;language&#8217; of geological maps as uniform as possible.&#160;In order to perform these analyses, we selected a series of putative fluvio-lacustrine landforms located in Holden crater, along the south-eastern inner rim (coordinates 26.9&#176;S-33.5&#176;W).&#160;The geological map allowed to distinguish several units separated by unconformities. In particular, the Impact unit, equivalent to the one recognized by Tanaka et al. (2014) is nonconformably covered by the materials located inside the crater and along the crater rim. These materials can be distinguished in two groups separated by a disconformity, the first characterized by a relatively large lateral extension of the units and the second by a scattered appearance of the units. Within the first group, a disconformity separates a lower part from the upper part of the succession.&#160;The geomorphological map allows to genetically interpret these units, defining: i) a first impact stage, correspondent to the emplacement of Holden crater, ii) a &#8216;water-related&#8217; phase (correspondent to the lower group of the geological map), and iii) an aeolian phase made of mega-ripples and dunes (correspondent to the upper group of the geological map). The &#8216;water-related&#8217; phase can be further divided in a fluvio-lacustrine and a glacial phase.We propose the realization of a unique geologic and geomorphologic product that includes a polygon layer with the geological units map described above, a linear layer with the unit contacts&#160; (i.e., stratigraphic characterization), and a linear shapefile with tectonic features and geomorphological interpretations. The polygonal units&#8217; layer&#160;might be also suitable to subdivide the units in lower-order ranked units, using appropriate fields (i.e., formation/members on Earth).&#160;This approach might be best suited for projects developed at the local scale, similar to what is done on Earth, while a chronostratigraphic approach is more fitting and recommended at the regional and global scale. We will identify a possible set of graphical symbols describing the surface features, locating potential limits in current GIS symbology implementation.The GMAP project represents an opportunity to share our experience and to collect current practices in planetary and terrestrial geological and geomorphological mapping, identifying what elements are still lacking and need to be discussed or developed.&#160;&#160;&#160;&#160;&#160;ReferencesISPRA (2009) - Carta Geologica d&#8217;Italia. Guida alla rappresentazione cartografica. Modifiche e integrazioni ai Quaderni 2/1996 e 6/1997. Roma, pp. 166ISPRA (2018) - Carta Geomorfologica d&#8217;Italia. Guida alla rappresentazione cartografica. Modifiche e integrazioni al Quaderno 4/1994. Roma, pp. 95Malin, M. C., J. F. Bell, B. A. Cantor, M. A. Caplinger, W. M. Calvin, T. R. Clancy, K. S. Edgett, L. Edwards, R. M. Haberle, and P. B. James (2007), Context camera investigation on board the Mars Reconnaissance Orbiter, Journal of Geophysical Research: Planets(1991&#8211;2012), 112(E5).McEwen, A. S., E. M. Eliason, J. W. Bergstrom, N. T. Bridges, C. J. Hansen, A. W. Delamere, J. A. Grant, V. C. Gulick, K. E. Herkenhoff, and L. Keszthelyi (2007), Mars reconnaissance orbiter&#8217;s high resolution imaging science experiment (HiRISE), Journal of Geophysical Research: Planets (1991&#8211;2012), 112(E5).Murchie, S. et al. (2009), Evidence for the origin of layered deposits in Candor Chasma, Mars, from mineral composition and hydrologic modeling, Journal of Geophysical Research: Planets, 114, E00D05, doi:10.1029/2009je003343.Neukum, G, R Jaumann, and H. the and Team (2004), HRSC&#8212;The High Resolution Stereo Camera of Mars Express, European Space Agency Special Publication, SP-1240, 17&#8211;35.Tanaka, K., J. Skinner, J. Dohm, R. Irwin, E. Kolb, C. Fortezzo, T. Platz, G. Michael, and T. Hare (2014), Geologic map of Mars, USGS Scientific Investigations Map 3292, doi:10.3133/sim3292.