Relationship Between LRO LEND Neutron Flux and Lunar Impact Crater Ages

Introduction: The Lunar Reconnaissance Orbiter spacecraft (LRO) Lunar Exploration Neutron Detector (LEND) permits analysis of the neutron flux [1] for landforms with reasonably well-established surface ages, such as impact craters. In addition, thanks to LRO observations of crater geometric properties that are formation-age dependent, additional factors can be analyzed. Using LEND neutron counting statistics acquired between September 2009 and December 2010 from the primary LRO mapping orbit, we have analyzed the neutron flux signature of a population of fresh Copernican age craters in comparison with a set of “control” craters with greater formation ages (Eratosthenian). Best-available crater age data from published literature [2] has been utilized together with a set of crater geometric properties measured from released LOLA gridded data [3]. The primary objective is to test the hypothesis that neutron flux (and hence bulk Hydrogen content) signatures are related to crater formation ages, at least for impact events in some specific geological settings. Thanks to the high spatial resolving power of current LEND measurements (10-20 km), craters and their surrounding ejecta blankets can be assessed at scales not possible with Lunar Prospector LPNS data. A set of 24 impact crater targets was selected (Table 1) on the basis of crater age, morphology, and LEND sampling statistics. We fully recognize that continuously improving LEND counting statistics as well as calibrations will lead to a continual refinement of these preliminary results. Nonetheless, we believe that the basic trends established herein are noteworthy. LRO LEND Observations: Table 1 illustrates the reference set of 24 impact features selected for analysis. It includes mostly fresh Copernican craters (FCC) with a few Eratosthenian craters as controls. Table 1 highlights the key parameters for each crater from LEND and those derived from geometric analysis of LOLA gridded data using methods developed by Garvin for Mars and Earth [4]. On the basis of the results in [1], LEND neutron flux values < 4.96 counts per second (cps) are associated with appreciable concentrations of hydrogen in the regolith, with values as high as 4 wt. % if a dry layer mantles the underlying H-bearing regolith. On the basis of preliminary analysis, there is no first-order statistical correlation of LEND neutron flux (or related H concentration) and first-order crater age. Young features such as Giordano Bruno and Aristarchus, for example, display neutron flux values that range from 4.98 cps to 5.10 cps, while much older (Eratosthenian) craters have values in a similar range. Thus, exposure of fresh regolith, whether in the mare or the highlands, does not appear to reflect local H concentration, at least in the regions sampled by the 24 craters evaluated in this study (Fig. 1). Some of the most recent but smaller complex craters evaluated, however, do show the greatest level of neutron suppression, suggesting the possibility of enhanced regolith hydrogen (e.g., Kepler, Godin).