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Mapping of Ice Storage Processes on the Moon with Time-dependent Diviner Data

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Mapping of Ice Storage Processes on the Moon with Time-dependent Diviner Data

  1. 1. Mapping of Ice Storage Processes with Time-dependent Diviner Data Norbert Schorghofer Planetary Science Institute in collaboration with Jean-Pierre Williams (UCLA) AGU Fall Meeting 2020
  2. 2. Theories of water storage on the Moon Theory Variant Reference Cold Traps (negligible sublimation rate) ice exposed on surface Watson, Murray, Brown (1961a,b) micro cold traps Hayne et al. (2020) buried ice Paige et al. (2010) Ice (Vapor) Pump Schorghofer & Aharonson (2014) Adsorption Non-volatile H2O Saal et al. (2008)
  3. 3. • Diviner is an infrared radiometer on the Lunar Reconnaissance Orbiter (LRO) (Paige et al. 2010) • 11 years of continuous surface temperature measurements • Seasonal variations over Draconic year (347 days), solar declination ±1.57° • Williams et al. (2019) used 96 diurnal × 2 seasonal bins • Schorghofer & Williams (2020): 24 diurnal × 6 seasonal bins (subsolar longitude, ecliptic longitude) Diviner Temperature Data
  4. 4. Use information about time-dependency E … sublimation rate Ethres≈100 kg/m2/Gyr≈10cm/Gyr ≈E(109 K) Cold traps are physically defined by meant(E) ≤ Ethres (this work) not by E(maxt(T)) ≤ Ethres (in all previous work)
  5. 5. • Temperatures are binned into 24×6 time bins; typically 91 of 144 are populated • Gaps are filled with downsampled version of the data 12×2 (block averages) • Works well, because missing bins are spread over local time and season • + Frequency-domain filtering Time-Domain Interpolation
  6. 6. Frequency Domain Filtering full symbols … original data empty symbols … interpolated & filtered data; Welch window
  7. 7. Cold Traps
  8. 8. Cold Trap Area 24% difference between maxt(E) and meant(E); little difference between smoothed (Fourier filtered) and unsmoothed data set
  9. 9. Subsurface Temperature Profiles Diviner temperatures on the surface (12 synodic months) - quasi-continuous time series - 346.6/29.53≈11.74 Subsurface temperature from solving heat equation with thermal properties of Hayne et al. (2015). Thermal model run at each of 5 million pixels Loss rate from subsurface: Ess = ℓ/(z+ℓ) meant(E(T(z,t))) ℓ … molecular free path
  10. 10. Area of Subsurface Stability • Area of stability doubles at 2 cm depth • If stable, it is stable within seasonal thermal skin depth • y-axis range is total area considered • result not sensitive to chosen threshold value Note that subsurface stability does not imply the presence of ice, but the lack of subsurface stability implies absence of ice.
  11. 11. Vapor Pumping Water molecules adsorbed on the surface can migrate to greater depth and form ice. (Schorghofer & Taylor 2007; Schorghofer & Aharonson 2014) Pumping differential: ΔE=meant(Eads(surface)) – meant(E(at depth with ice)) Net accumulation if ΔE>0; a small fraction of ΔE will accumulate in the subsurface.
  12. 12. Pumping differential ΔE for supply rate of 1 m/Gyr Total area with ΔE > 0.5 m/Gyr is 96,000 km2 poleward of 80°S (about 5× cold trap area) Vapor Pumping
  13. 13. CO2 Cold Traps (preliminary map) cold trap area: meant(E) <10 kg/m2/Gyr 286 km2 maxt(E) <10 kg/m2/Gyr 79 km2
  14. 14. Conclusions • Processed 11 years of Diviner data; diurnal and seasonal time dependence is included; gaps are filled with time-domain interpolation and frequency-domain filtering • Cold traps are larger and more numerous with meant(E) compared to maxt (E) by ~24% in total area; 17,000 km2 in south polar region • New maps of subsurface stability and ice pumping • There are spatially coherent CO2 (surface) cold traps N. Schorghofer & J.-P. Williams. Planetary Science Journal 1, 54 (2020)

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