https://arxiv.org/pdf/1611.05395.pdf How thick are Mercury’s polar water ice deposits? Nov 2016 ABSTRACT An estimate is made of the thickness of the radar-bright deposits in craters near to Mercury’s north pole. To construct an objective set of craters for this measurement, an automated crater finding algorithm is developed and applied to a digital elevation model based on data from the Mercury Laser Altimeter onboard the MESSENGER spacecraft. This produces a catalogue of 663 craters with diameters exceeding 4 km, northwards of latitude +55◦ . A subset of 12 larger, well-sampled and fresh polar craters are selected to search for correlations between topography and radar same-sense backscatter cross-section. It is found that the typical excess height associated with the radar-bright regions within these fresh polar craters is (50 ± 35) m. This puts an approximate upper limit on the total polar water ice deposits on Mercury of ∼ 3 × 1015 kg. 5 DISCUSSION From this study, the typical excess thickness associated with the radar-bright regions in 6 north polar craters with diameters exceeding 20 km is (50 ± 35) m. While this does not represent a statistically significant measurement of a non-zero height increase in the radar-bright regions in polar cold traps, it does provide an upper limit of ∼ 150 m on the depth of the typical ice deposit that may be associated with these regions. This is a factor of ∼ 2 lower than that which was previously available (Talpe et al. 2012). Given the typical, not ice-related, undulations in the surface within craters, it is not feasible to relate a localised change in height within any particular crater with the presence of a thick deposit of water ice. These departures from axisymmetry, even within fresh craters, can be up to ∼ 100 m in amplitude. Consequently, improvements in range measurement accuracy, which is already much smaller than this, will not have a significant impact. More important would be both a more accurate positional matching between radar and DEM data sets, which would lead to reduced scatter in pixel ∆h values caused by positional mismatches, and a denser sampling of the topography. These would permit a reliable extension of the technique developed here to include smaller craters containing radar-bright features. With a larger sample of craters, the difference between radar-bright and control craters could be more accurately determined. To some level, improved laser altimeter sampling is provided by the MLA DR15 data although, as discussed earlier, there are presently some non-negligible glitches in the DR15 GDR that stymie this approach. The BepiColombo Laser Altimeter (Thomas et al. 2007) is scheduled to map Mercury’s surface in the next decade and will have a polar orbit with along-track resolution of ∼ 250 m. At the end of the mission, the cross-track resolution should be better than ∼ 1 km more than 80◦ from the equator. If the elliptical orbit sees BepiColombo flying low over the south pole, then this might double the number of available craters; otherwise, the anticipated lateral sampling will not differ greatly from that provided by the MLA. Taken at face value, if a depth of 50 m of water ice were typical of all the radar-bright regions near the north pole of Mercury, then this would correspond to a total mass of water ice deposited near Mercury’s poles of ∼ 1015 kg, assuming that the south pole contains a similar quantity, the water ice density is 103 kg m−3 and using a value of ∼ 10, 000 km2 for the total north pole radar-bright area (Harmon et al. 2011). If radar-dark permanently shaded regions also host deposits of water ice, then this value would represent an underestimate of the total mass present. Even so, this still amounts to more water ice than could feasibly be delivered to Mercury by micrometeorite bombardment, Halley-type comets or asteroids, according to the estimations of Moses et al. (1999). However, the value lies within their predicted range for water delivery from Jupiter-family comets. Given the uncertainty in the actual typical measured depth, it would be premature to rule out any of the alternative delivery sources on the basis of the results presented here. Furthermore, there are considerable uncertainties in the micrometeorite flux reaching Mercury, with recent studies by Borin et al. (2009); Nesvorny et al. (2010) and Bruck Syal et al. (2015) finding ´ values that are respectively ∼ 60, 30 and 10 times those assumed by Moses et al. (1999). However, this analysis does exclude the possibility of the total water ice deposits exceeding ∼ 3 × 1015 kg, provided that the craters studied have ice depths typical of other regions hosting deposits.
