Latitudinal Enrichment of Hydrogen in the Lunar Polar Regions: Constraints on Hydrogen Mobility

Transcription

1 Latitudinal Enrichment of Hydrogen in the Lunar Polar Regions: Constraints on Hydrogen Mobility W. V. Boynton, G. F. Droege, K. Harshman, M. A. Schaffner, I. G. Mitrofanov, T. P. McClanahan, and the LEND team LEAG Meeting October 23, 2012

2 Why this part of presentation? Lunar volatiles session at 2012 LPSC Two talks were very critical of the LEND data quality There was no time permitted for a rebuttal Comment to a blog on behindtheblack.com You neglect to mention yet another possibility that this paper and its conclusions are seriously flawed in almost every respect. Paul Spudis We should be beyond this by now.

3 LEND statistics Richard Miller claimed our error bars are too low by a factor of 2. We can test this with the reduced chi squared test page3

4 Reduced chi squared test Reduced chi squared test is a good test to see if the scatter in the data is consistent with estimated uncertainties. If uncertainties are properly estimated, chi squared will be about 1.0 Graph is 14 degrees of freedom, i.e. averaging 15 points. page4

5 LEND uncertainties are proper In making our maps, we needed to average 15 values 3263 times. Histogram of 3263 different determinations of reduced chi squared show that our uncertainties account for all significant scatter in the data. page5

6 If our error bars were too small by a factor of 2, the reduced chi square values would be much smaller. I report, you decide. Error bars too small? page6

7 Eke compared early and late maps First half and second half of data look very different when not processed properly. Dynamic range is 3.8 to 6.2 cps. Our biggest suppression is 0.2 cps, so at this scale we see only noise. page7

8 Smoothed data Suppressions are 0.1 to 0.2 cps; uncertainties are 0.01 to 0.03 cps Count Rate Uncertainty page8

9 Excellent spatial resolution Lawrence and Eke claim the spatial resolution of LEND is not much better than LPNS. Suppression closely follows the LOLA topography of Shoemaker Crater. Relative to terrain 200 km away, this is an 8- sigma suppression page9

10 True collimated count rate? We have to correct for background counts of neutrons from cosmicrays on the spacecraft and from HEE lunar neutrons. We know the spatial distribution of HEE neutrons and we know the transmission of the collimator. We calculate the background as shown and have a net 1.7 cps of collimated neutrons. Eke claims 0.05 cps most likely Lawrence claims 0.15 to 0.18 cps We can test by comparison of suppression with that of LPNS. We must first convert to fractional suppression. page10

11 LEND suppression vs. LPNS We need to know the suppression independent of detector efficiency. LPNS count rate is about 20 cps; Lend is 1.7 cps. We normalize count-rate differences seen on previous trace based on the 1.7 cps. Normalization based on Lawrence or Eke values would give suppressions of around 100% or more. page11

12 LEND polar maps show NSR North South cps cps Neutron Suppressed Regions (NSR) are regions of enhanced H

13 NSR s based on difference map South A Average Count Rate in 1-deg latitude bands South B Cabeus Haworth Shoemaker Faustini Count Rate (cps) South Latitude (deg) A is a difference map. It is the count rate difference relative to non-nsr count rates in the same latitude band B is a plot of the average count rate as a function of latitude band. The map contours are cps Small areas are not significant

14 Background hydrogen increases toward poles independent of NSR 4.95 Average Count Rate in 1-deg latitude bands 4.94 Average Count Rate in 1-deg latitude bands South North Count Rate (cps) Count Rate (cps) South Latitude (deg) North Latitude (deg) Linear decrease in epithermal neutron flux (increase in H content) toward poles even after removing contribution from the NSR. Magnitude of effect is similar at both poles Hard to explain this observation as primarily due to solar wind interaction. The data appear to require a mechanism based on H migration controlled by temperature.

15 Explain increase in polar H? Two sources of hydrogen in the regolith Solar Wind line of sight (6*10 6 kg / y) Impacts omnidirectional (5*10 3 kg / y) (not significant H source) Impacts slightly favored in ecliptic plane but impactor and some of target will vaporize. Vapors globally distributed Distribution of H due to primary implantation? No, it would show reduced H near poles (cosine latitude) H content in soils is in steady state (overall loss = gain) Mechanisms to release H from regolith Sputtering (solar wind) Impact volatilization Thermal desorbing page15

16 Site of H in regolith Bulk Buried deep in grains due to agglutinate formation and gardening due to impacts Surface correlated in outer 1000 Å due to solar wind implantation 4x10 16 molecules/cm 2 is surface correlated (~50%) (DesMarais et al. 1974) Molecular surface Labile H deposited from ballistic trajectories Subject to thermal desorption. (Photo by Brad Jolliff) page16

17 Processes Solar wind Model for migration of H Implantation of H and sputtering (removal of H) Impacts Addition of H to inventory, but small amount Burial of H Thermal Agglutinates (micrometeorites) Gardening (larger meteorites) Vaporization rate a very strong function of temperature Only works on molecular surface page17

18 Vaporization rates Vaporization rate of H 2 O on lunar soils studied by Hibbits et al. (2011) and found residence times of: K K Diviner temperatures: latitude latitude. At low latitudes residence time is short. Vaporization rate of ice. Note strong temperature dependence page18

19 What happens to H? Ballistic trajectories are global in nature Once an H 2 O molecule is vaporized it is lost from the Moon or will return to the molecular surface Stays mobile until it is buried by impact Concentration of H on molecular surface is inversely proportional to vaporization rate. At low latitudes, H on molecular surfaces is not significant compared to total H inventory. At high latitudes, can H on molecular surfaces be significant compared to total H? page19

20 Amount of H on surface? We can calculate amount of H on molecular surface based in increase in neutron suppression We see on the order of 100 to150 ppm excess H (depends on latitude) This is nearly the equivalent of a monolayer of H 2 O page20

21 Conclusions Vaporization of H 2 0 is dominant migration method. Vaporization happens on a rapid time scale Consistent with observations of variations of H 2 0 with time of day (Sunshine et al. 2009) LEND cannot detect small amounts on surface since it averages over tens of centimeters. Enriched H in polar regions (in non-nsrs) is a steady state between molecular surface enhancement (rapid) and gardening (slow). page21

22 Count rate vs. time is constant Reduced chi squared of 0.86 is as expected from statistics. page22