MESSENGER Gamma Ray Spectrometer and Epithermal Neutron Hydrogen Data Reveal Compositional Differences Between Mercury's Hot and Cold Poles

Abstract

The presence of hydrogen, most likely in the form of water ice, is well established in Mercury's permanently shaded polar craters. But lower concentrations that may exist away from the poles have not previously been well constrained. In this work we use data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Gamma‐Ray and Neutron Spectrometer to produce the first map of the absolute hydrogen abundance in Mercury's northern hemisphere. We find a mean abundance of ppm and a latitudinal trend that agrees with earlier results showing enhanced hydrogen contained within Mercury's radar bright craters. Additionally, we observe a middle‐ and low‐latitude variation in hydrogen abundance that is correlated most strongly with temperature 20 cm beneath Mercury's surface.

Chemically distinct regions of Venus’s atmosphere revealed by measured N2 concentrations

“…The MCNPX-modeled neutron fluxes were processed using a code that transports neutrons from the top of the atmosphere to the spacecraft, accounting for gravitationally-bound neutrons on ballistic trajectories (Feldman et al, 1989), Doppler-shifting of neutron energy due to the spacecraft velocity relative to Venus (Feldman et al, 1986), and the angle-and energy-dependent neutron detection efficiency of the LG sensors (Lawrence et al, 2010). This simulation toolkit was developed and validated using Mercury flyby (Lawrence et al, 2010) and orbital (Lawrence et al, 2013(Lawrence et al, , 2017Wilson et al, 2019) datasets. Figure 1 plots the time-series of neutron rate measurements made with the NS LG1 and LG2 detectors.…”

Space-based measurements of neutron lifetime: Approaches to resolving the neutron lifetime anomaly

“…The effects varying neutron lifetimes would have on the neutron count rate is shown by simulated count rates (colored traces) in Figure 4c. The details of how these simulated count rates were implemented is described in various prior studies [4,7,11,18,20,21], with the foundational algorithms given by [14]. Neutron lifetime effects manifest as count-rate variations for different tn values when thermal neutrons are detected prior to the rotation.…”

“…In addition, for any given scenario, the exact spacecraft composition and mass distribution may not be fully known. Nevertheless, full simulations of MESSENGER NS data showed that relative uncertainties of ~<0.5% were achievable when neutron arrival angles were restricted to directions that did not travel through the full spacecraft material [7,18,20,21,34]. However, a dedicated space-based neutron lifetime experiment would strive to have a smaller spacecraft that is more easily modeled, thus achieving more uniform accuracies for all orientations.…”

Space-Based Measurement of the Neutron Lifetime using Data from the Neutron Spectrometer on NASA's MESSENGER Mission

“…Many of these elements were mapped by MESSENGER during its 4-year orbital mission [e.g. [29][30][31][32][33]; consequently we have a good understanding of Mercury's composition on large scales. For the Mercury flyby, a set of models with different neutron lifetimes but constant composition were generated.…”