Local variations of bulk hydrogen and chlorine‐equivalent neutron absorption content measured at the contact between the Sheepbed and Gillespie Lake units in Yellowknife Bay, Gale Crater, using the DAN instrument onboard Curiosity

Abstract

Data gathered with the Dynamic Albedo of Neutron (DAN) instrument onboard rover Curiosity were analyzed for variations in subsurface neutron flux and tested for possible correlation with local geological context. A special DAN observation campaign was executed, in which 18 adjacent DAN active measurements were acquired every 0.75-1.0 m to search for the variations of subsurface hydrogen content along a 15 m traverse across geologic contacts between the Sheepbed and Gillespie Lake members of the Yellowknife Bay formation. It was found that several subunits in Sheepbed and Gillespie Lake could be characterized with different depth distributions of water-equivalent hydrogen (WEH) and different chlorine-equivalent abundance responsible for the distribution of neutron absorption elements. The variations of the average WEH at the top 60 cm of the subsurface are estimated at up to 2-3%. Chlorine-equivalent neutron absorption abundances ranged within 0.8-1.5%. The largest difference in WEH and chlorine-equivalent neutron absorption distribution is found between Sheepbed and Gillespie Lake.

Water and chlorine content in the Martian soil along the first 1900 m of the Curiosity rover traverse as estimated by the DAN instrument

“…Between 17 August 2012 and 12 August 2013, DAN performed 193 individual active measurements in 154 unique locations along the first 1900 m of the MSL traverse path from the landing site. An individual testing spot has a diameter of irradiation of ~3 m and a sensing depth of about 60 cm [see Litvak et al ., ]; therefore, one particular active measurement characterizes the average water content of about 4 m 3 of Martian soil and/or rock below the rover. For one active measurement, the DAN PNG produces pulses at a frequency of 10 Hz for 5 or 15 min depending on the observation.…”

“…Subtracting the count rate values for these late time intervals from the postpulse profiles yields the pure die‐away curves of neutron albedo. Such curves, C tn ( t i ) and C etn ( t i ) for thermal and epithermal neutrons, respectively, in the 64 time bins along the postpulse scale are the observational products from each active measurement [see also Litvak et al , ].…”

“…Before one may use these curves for estimates of subsurface properties at different testing spots, the data must be reduced to account for instrument effects such as variations of the neutron pulse intensity and changes of the sensors efficiency after provision of high voltage (see Litvak et al . [] and Jun et al . [] for details).…”

“…Numerical simulations have shown that DAN CTN and CETN counting rate measured in the earliest time bins of the postpulse emission does not significantly depend on Martian regolith properties but is dominated by initial high‐energy neutrons backscattered inside the rover mechanical structures close to the detectors and/or neutron generator. Making evident assumption that rover properties does not change during surface operations, one may remove the instrumental effects, provided the measured die‐away time profiles of counts of neutron albedo, C tn,etn ( t i ), are normalized to the same values of count rate in the earliest bins <50 µs [ Litvak et al ., ]. These normalized time profiles A tn,etn ( t i ), measured for each testing spot should be compared with the simulated time profiles, F tn,etn ( t i , MG ), calculated for some model group, MG , describing particular type of subsurface.…”

“…To estimate the concentration and distribution of water at a particular location, one has to perform a deconvolution of the neutron data to find the best model fit to the observations (see details in Litvak et al . []). This procedure is based on Monte Carlo simulations of time profiles of leaking thermal and epithermal neutrons for different soil models with varying composition, density, and vertical distribution of hydrogen.…”