Ice-hunting in the South Polar Terrains of Ceres
Authors: Frigeri Alessandro , De Sanctis Maria Cristina , Ammannito Eleonora , Raponi Andrea , Ciarniello Mauro , Rousseau Batiste , Carrozzo Filippo Giacomo , Raymond Carol A. , Russell Christopher 
Published: October 2020

Like the Moon and Mercury, Ceres&#8217;s rotation axis is inclined by just a few degrees. &#160; The obliquity of 4 degrees between the normal to the orbital plane and the rotational axis of Ceres implies that Ceres' poles terrains are always slightly angled toward the sun.&#160; The combination of low angle illumination over a cratered surface creates areas never reached by the sun.&#160; In the presence of ice, these permanently shadowed regions (PSRs) create &#8220;cold traps&#8221; protecting ice from sublimation.In 2016 Platz et al. [1] explored the topography and imagery [2,3] of the northern polar regions of Ceres, looking into 634 craters in permanent shadows, finding bright deposits in 10 of them.&#160; Out of this, 1 crater has part of the deposits exposed, so that it has been possible to detect the signature of water thanks to the data returned by Dawn&#8217;s mapping spectrometer VIR [4].&#160;&#160;The last phase of the Dawn mission, which ended in 2017 collected enough imagery to reconstruct the topography of the southern pole of Ceres by stereophotogrammetry [Preusker].&#160; Since then, no evidence of icy terrain from the south polar terrain of Ceres has been reported.&#160; Our study will explore the PSRs in this region for a better understanding of the ice distribution on the dwarf planet.The first step of our study is to locate and characterize the southern PSRs. From the topography, we computed the PSRs using the Integrated Software for Imagers and Spectrometers (ISIS3) version 4.8.3, a suite of applications for the rigorous spatial treatment of planetary data acquired by various missions/instrument, including NASA Dawn at Vesta and Ceres [5].Figure 1 shows the map of PSRs of the southern terrain of Ceres in stereographic projection, latitude grids are spaced by 10 degrees, central is at the top.&#160; The PSRs are reported in blue shades corresponding to their area (see legend), over the shaded relief extracted from the topography.&#160; The hatched area at the south pole locates the place where stereo reconstruction of topography was not possible and was excluded from PSRs computation.&#160; We have located about 950 PSRs with areas ranging from 1 to over 500 square kilometers. The largest PSRs locates in an unnamed 75 km wide crater at 223.6&#176;E 74.9&#176;S. &#160; Figure 2 shows the distribution of the sizes of PSRs. &#160; Most of PSRs are in areas smaller than 10 square km.&#160;The map of PSRs represents the base for observations into the dark areas with Framing Camera and possibly the VIR spectrometer, looking for sings on icy deposits in the south polar regions of Ceres.&#160;Figure 1: The PRSs maps of the south polar terrain of Ceres (stereographic polar projection over a spheroid with a radius of 470km, prime meridian is at top and latitude grid are spaced by 10 degrees).&#160; The blue patches are the area permanently in shadow and color indicates the area in square kilometers.&#160; The etched pattern indicates the area without topographic data.Figure 2:&#160;The frequency distribution of the areal extents of PSRs, which ranges from less than a 1 to more than 500 square kilometers.&#160;References[1] Platz, T. (2017) Nature Astronomy, 1, 7[2] Sierks, H. et al. The Dawn Framing Camera. Space Sci. Rev. 163, 263&#8211;327 (2011).[3] Preusker, F., (2016) LPSC 48 Abstract #1954[4] De Sanctis, (2011) Space Science Reviews, 163, 1-4,329-369[5] Becker, K.J., (2013) LPSC 44 Abstract #2829&#160;&#160;&#160;&#160;&#160;