Joint Analysis of Bulk Water/Chlorine Distribution in the Martian Subsurface along MSL Curiosity Traverse from Comparison between DAN/MSL and other instruments observations onboard Curiosity Rover
Friday, 19 December 2014
Data gathered with the Dynamic Albedo of Neutrons (DAN) instrument onboard Curiosity rover during active neutron spectroscopy observations presents variations of induced subsurface neutron flux due to regional variability of bulk water distribution and elemental composition in martian soil. The duration, shape and amplitude of post pulse neutron emission measured by DAN primarily depend on the subsurface structure and distribution of hydrogen (the most efficient moderation element) but abundance of other elements is also important. In particular, the presence of elements with high thermal neutron absorption cross sections can depress the population of thermal neutrons that leak out of the surface. Several elements such as for example Cl, Fe, Ti, Mn, Gd have large cross-section for thermal neutron absorption. In our analysis we have derived water depth distribution and have also estimated a bulk content of chlorine. It is considered as “chlorine-equivalent” parameter for accounting of all absorbers of thermal neutrons. We have compared our estimations with other instrument observations performed onboard Curiosity in order to test observed variability on different horizontal and vertical scales. First of all, we have tried to correlate our estimations of chlorine-equivalent abundance with elemental composition provided by APXS at several contact science locations along Curiosity path where such a comparison was supported by observable geological context. DAN instrument has a large footprint area with a radius of about 1.5 m under the aft end of the rover. It is significantly larger than the footprint of contact and remote measurements of elemental composition provided by the other science instruments onboard Curiosity. But DAN provides a unique capability to look below the surface as deep as 60 cm and study the depth distribution of water and neutron-absorbing elements. DAN measurements are complementary to other Curiosity observations and could combined with them to provide joint study of subsurface water and chlorine distribution and test compositional heterogeneity in the subsurface.