HYDROGEN-BEARING VOLATILES AT THE LUNAR TERMINATOR

Hydration feature

EPOXI: 28–29 May 2008

We observe suppression of epithermal neutron flux at the Lunar Reconnaissance Orbiter (LRO) [1] using the Lunar Exploration Neutron Detector (LEND) [2] collimated and uncollimated epithermal-neutron detectors when LRO is near the dawn terminator at near-equatorial latitude (30°S to 30°N). The maximum flux (least suppression) is at ~14:30 local time. Suppression of epithermal neutron leakage flux is consistent with the presence of hydrogen in the upper regolith near the terminator. The motion of the terminator across the surface, as the Moon rotates, implies a mobile population of hydrogen-bearing volatiles near the terminator that resides transiently in the regolith. The pattern of neutron flux suppression supports the interpretation by Sunshine et al. [3] of mineral hydration observed in reflected light, supporting the assumption that detected hydrogen is in the form of H2O/OH. The difference in suppression contrast between two epithermal-neutron detection systems is qualitatively consistent with hydrogen isolated in the upper part of the ~100 cm depth probed by neutron remote-sensing [4].

Spectroscopic evidence for hydration at the lunar terminator: •  3 µm-wavelength absorption feature deepest near the terminator, interpreted by Sunshine et al. to

indicate relatively desiccated regolith near the subsolar point [3]. •  Low latitude must undergo diurnal hydration/desiccation cycle. •  Sunshine et al. [3] propose that water-group ions migrate from sub-Sun point to terminator. •  Reflectance spectroscopy restricted to surfaces illuminated by Sun. •  Adsorbed H2O/OH may be on hidden grain surfaces that could be detected by techniques that probe

the regolith at depth.

Investigate diurnal variability of equatorial hydrogen with LEND: •  ‘Flat-field’ (normalize) neutron flux in each detector

with the corresponding selenographic map. •  Partition epithermal neutron detection events into

intervals of 5 minutes of local time (1.25° longitude) and ±30° centered on equator.

•  Integrate total counts and total integration time in each interval to compute count rate.

•  Compute average count rate and uncertainty in 2-hour bins from population of 5-minute intervals in each.

•  Use standard error of the mean for conservative uncertainty estimate of measured population.

•  Phasing of two-hour bins selected for maximum peak-to-peak contrast in CSETN.

•  Minimum epithermal neutron flux at dawn in both CSETN and SETN detectors.

•  Maximum epithermal neutron flux in mid-afternoon in both CSETN and SETN detectors, extending to dusk in SETN.

•  CSETN peak-to-peak contrast SNR ~5.3 •  SETN peak-to-peak contrast SNR ~4.8

Modulated Neutron Flux: •  Minimum neutron flux at dawn terminator is consistent with local

enhancement of hydrogen (H2O/OH). •  Flux suppression at dawn is in excess of night-time suppression. •  Flux not as suppressed at dusk as at dawn, inconsistent with

Sunshine et al. dawn-dusk symmetry in hydration signature. •  CSETN flux maximum in afternoon sector is consistent with

desiccation by sunlight, with a phase lag for complete dehydration. Narrow maximum suggests relevant depth of regolith may not be completely desiccated.

•  Broad peak in SETN maximum is consistent with complete desiccation with a phase lag for dehydration and a phase lag for rehydration.

•  Water abundance consistent with observed flux suppression at dawn can be estimated relative to the maximum flux:   CSETN: ~0.041±0.008 wt% at dawn (~192±38 pr-µm)   SETN: ~0.012±0.003 wt% at dawn (~56±14 pr-µm)

•  Sunshine et al. [3] extreme upper limit of 0.5 wt% surface hydration could be accommodated by hydrating to a depth of ~2–8 cm.

•  Lawrence et al. [6] argue for an increase in epithermal flux at low energy from a thin hydrated layer on top of dry regolith. Differing energy sensitivity between SETN and CSETN is consistent with this mechanism to explain differing degrees of contrast in flux suppression if hydrogen is isolated near the surface of the regolith.

Conclusions: •  CSETN biases to higher energy from leakage through collimator

wall. Relative flux-suppression contrast in CSETN and SETN is consistent with with greatest hydration near surface.

•  Proposed interpretation: H2O/OH migrates from subsolar region to cold terminators, where it penetrates into cold near-surface. H2O/OH at dusk terminator freezes into surface and is remobilized by sunlight, migrating toward nearest (dawn) terminator along with ‘new’ H2O/OH, accounting for greater concentration at dawn than at dusk. Finite time required for downward transport of heat and upward/downward transport of H2O/OH out of/into surface accounts for phase lags in desiccation/rehydration; consistent with transport/adsorption model of Schorghofer [7].

•  Loss of H2O/OH to photochemical destruction by over-surface transport and transport along terminator to cryogenic storage at poles must be balanced by delivery of new hydrogen.

References: [1] Vondrak et al. (2010) Space Sci. Rev. 150, 7–22. [2] Mitrofanov, I. G., et al. (2010) Space Sci. Rev. 150, 183–207. [3] Sunshine, J. M., et al. (2009) Science 326, 565–568. [4] Lawrence, D. J., et al. (2010) Astrobiology 10, 183–200. [5] Maurice, S., et al. (2004) JGR-Planets 109, E07S04. [6] Lawrence, D. J., et al. (2011) JGR-Planets 116, E01002. [7] Schorghofer, N. (2012) 43rd LPSC LPI Contribution No. 1659, 1110

•  We use LEND data from Sep 2009 through Jun 2012, in altitude range 51±15 km, accessible from the PDS (LEND DLD format).   Uncollimated SETN detector

senses epithermal-energy neutrons.

  Collimated CSETN detectors combine four epithermal neutron detectors. Leakage through collimator wall biases detected energy spectrum to higher energy than SETN.

•  Detector background flux estimated from modeling mapped flux using linear combination of maps from the Lunar Prospector Neutron Spectrometer (LPNS) [5].

•  Neutron population detected by CSETN biased to higher energy by leakage through collimator wall.

LEND Instrument Description: •  Lunar Exploration Neutron Detector (LEND) on LRO is sensitive to buried hydrogen [2]:

  Galactic cosmic rays eject neutrons from regolith   Epithermal neutrons (~1 eV to 1 MeV) are moderated by H, depleting flux in this interval [4] .   LEND probes H content without solar illumination by measuring epithermal neutron flux.

90°W   0°   90°E  

+90°  

-­‐90°  

-­‐70°  

-­‐50°  

-­‐30°  

-­‐10°  

+70°  

+50°  

+30°  

+10°  

LP  LCROSS  

Red = STN3 thermal neutrons; Green = SETN epithermal neutrons; Blue = CSETN epithermal and fast neutrons

6:30   14:30   6:30   14:30  

Epithermal neutron count rates from 30°S to 30°N, binned to 2-hr local time, for CSETN (top) and SETN (bottom) detectors. Colored bands show estimated 1σ uncertainty.

Estimated water-equivalent hydrogen relative to bin of maximum flux, uncertainty estimated by propagation of error.

T. A. Livengood CRESST/UMD/GSFC

G. Chin NASA/GSFC

I. G. Mitrofanov Institute for Space Research, Moscow

W. V. Boynton LPL University of Arizona

R. Sagdeev University of Maryland

M. Litvak Institute for Space Research, Moscow

T. P. McClanahan NASA/GSFC

A. B. Sanin Institute for Space Research, Moscow