Cold Trapping of Lunar Polar Crater Volatiles: Effects of the Space Environment

W. M. Farrell1, D. M. Hurley2, M. J. Poston3, P. O. Hayne4, J. L. McLain5

1. NASA/Goddard Space Flight Center, Greenbelt MD, 2. Johns Hopkins University/Applied Physics Laboratory, Laurel, MD, 3. Southwest Research Institute, San Antonio, TX, 4. University of Colorado, Boulder, CO, 5. University of Maryland, College Park, MD ‘Frost’ sensed by the LRO/LAMP FUV system

Water Thermal Stability Model FUV signature of water Paige et al 2010 Haynes et al., 2015 White areas= water stable at surface Dark = cold polar crater Orange areas= water stable in first 10 cm Blue= ratio of high and low wavelength bands ‘Frost’ sensed by the LRO/LOLA system

Fisher et al 2017- LOLA’s 1064 nm lidar; Water Thermal Stability Model Anomalously bright pixels possibly from Paige et al 2010 possible water surface frost White areas= water stable at surface Orange areas= water stable in first 10 cm Evaporation Model: Watson et al 1961

Assumes Phase Equilibrium: Gas in equilibrium

Tv with surface. Sublimation and condensation in balance…In this case, gas pressure, Ln P a -constant* Latent Heat/T

-Zhang and Paige (2009) defined a volatility temperature

Tv equal to evaporation rate of 1 mm per billion years for 1 g/cc density (Water Tv ~ 105K) Evaporation from an Icy Surface [Andreas 2007]

1 mm of ice in 1 By

esat,i = Saturation vapor pressure of ice from Murphy and Koot, 2005

Tv Condensation/Sublimation Phase Transition Closed Ice-vapor System

Assume pressure is consistent with condensation-sublimation phase transition

1) Use Clapeyron Equation at phase transition (Vg >> Vice): dP/dT = L/TVg 2) Ideal gas law, P = RT/MWVg Mw = Gram molecular weight of water dP/P = (M L/RT2) dT L = Latent heat at phase transition W P = Pressure T= Temperature of solid and vapor ln (P/Po) ~ -MWL/RT Vg = Gas Volume Derive that P a exp(-constant*L/T) Any assumed surface phase equilibrium is also being disrupted! • Watson et al Model assumes surface in equilibrium at the phase transition – At a given T, sublimation and condensation maintain vapor flux and surface concentrations • For an open system, have added effects: – Have solar wind plasma sputtering and micro- meteoroid vaporization can also release surface water from a 1% icy-regolith concentration – Have disruption of the icy surface by ‘corpuscular’ or discrete injections of energy from the space environment Environmental Losses to Polar Water Frost

Frost is Exposed to the Space Environment: Its Dynamic! These environmental release processes may exceed those from sublimation Zimmerman et al 2012 Expansion of solar wind Ambipolar Expansion of plasma into polar crater Solar Wind into PSRs – Leeward Edge 2D Particle-in-Cell simulation [Zimmerman et al., 2012]

SW Proton Influx

SW proton flux actually reaching the floor of the crater is about 1% of the original solar wind proton flux Ambipolar Expansion into Shoemaker Crater

Solar wind flow

Analytical model of Solar Wind Plasma Inflow into Shoemaker Crater [Farrell et al., 2010; Jackson et al., 2011]

These 1 keV protons are a source of sputtering to remove surface frost Sputtering • 1 keV protons incident on lunar surface Losses • Sputter material via momentum transfer and ionization effects Johnson, 1990

Y-ice ~ 0.7 molecule/ion (liberating) Y-SiO2 ~ 0.03 molecule/ion (collecting/implanting) Irony: The same SW proton source that might form water wants also to destroy water Impact Vaporization in Cold Traps

4000K Maxwellian distribution of IV-released water Redistribution of a cold trap deposit by meteoric impact vaporization Water ‘spilled’ over crater onto topside by meteor impacts and plasma sputtering

1% water ice/regolith (soil) mixture on crater floor Also, get a redistribution of water within the crater itself Farrell et al., 2013 Environmental Release of Water from PSRs with 1% icy regolith mix (LAMP-like situation)

Micro-meteoroid Impact Vaporization: 3 x 1010 molecules released/m2-s [Cintala, 1992] 1% water concentration has water emission rate at 3 x 108 waters/m2-s

Solar wind sputtering: S=YF ~ 2 x 108 waters released/m2-s Yield, Y = 10-2 molecules released per incident ion at 1% water/regolith mix Flux, F ~2 x 1010/m2-s or about 1% of the solar wind gets diverted to crater floor Large variability in ambipolar diverted proton influx to crater floor

Lyman-a stimulated desorption: S = YF ~ 3 x 107 water released/m2-s PSD cross section = 10-23 m2, surface density = 3 x 1017 waters/m2 Photon flux = 1013/m2-s

Farrell et al., 2015 Evaporation from the Surface

Sputtering and IV 10% ice/regolith mix 1% ice/regolith mix Nonthermal processes

Condensate unstable to PS and IV

-100 days -100 yr Sputtering and IV Environmental Losses 10% ice/regolith mix -32 kyr 1% ice/regolith mix -10Myr -3 Byr

Condensate unstable to PS and IV

-100 days -100 yr Sputtering and IV Environmental Losses 10% ice/regolith mix -32 kyr 1% ice/regolith mix -10Myr If frost is everywhere thin, then it all has to be relatively new!!! -3 Byr

Condensate unstable Frost sensed by LAMP in top 0.5 microns of to PS and IV surface is relatively new (< 1 Myr)

Temperature independent processes dominant release over phase equilibrium processes The distribution of icy regolith not completely dependent on temperature effects Evaporation from the Surface Time to evaporate 0.5 microns of ice Water in slow but “dynamic” transport between cold and warmer regions in PSRs -100 days -100 yr Sputtering and IV Environmental Losses 10% ice/regolith mix -32 kyr 1% ice/regolith mix -10Myr -3 Byr

Condensate unstable to PS and IV

Temperature independent processes dominant release over phase equilibrium processes The distribution of icy regolith not completely dependent on temperature effects Thin Layer Sublimation rate: 32 kyrs 100 yrs 100 days | | | (a) Haworth 2 =75 LAMP data from set used by

1 Hayne et al 2015 Off/On Ratio Off/On

0 100 (b) Off/On ratios greater than 1.2 80

% of Values 1.2 > % Values of 20

0 40 60 80 100 120 140 Temperature (K) Thin Layer Sublimation rate: 32 kyrs 100 yrs 100 days One non-unique interpretation: | | | (a) Haworth 2 =75

1 Off/On Ratio Off/On

Space environment erodes this deposit 0 100 (b) Off/On ratios greater than 1.2 80

% of Values 1.2 > % Values of 20

0 40 60 80 100 120 140 Temperature (K) Thin Layer Sublimation rate: 32 kyrs 100 yrs 100 days One non-unique interpretation: | | | (a) Haworth 2 =75

1 Off/On Ratio Off/On Forming a dynamic thin frost in warm Space environment erodes this deposit regions…that slowly migrates back to the 0 100 deposit (b) Off/On ratios greater than 1.2 80

% of Values 1.2 > % Values of 20

0 40 60 80 100 120 140 Temperature (K) Conclusions • The cold trap surfaces are continually disrupted by the injection of corpuscular or discrete space environmental energy events – Space environment may be ‘smearing’ the frost locations – Release of water by harsh environmental processes exceeds evaporation processes below about ~102K – Launched volatiles can even leave the vicinity of the crater – gone! – May explain why the lunar frost in PSRs does not strictly follow temperature [as pointed out by Fisher et al., 2017] • LAMP and LOLA possibly observing a surface frost that is a convolution of condensation (or more likely sorption) process modulated/enveloped by space environmental ‘disruption’ processes