Presentation at the AGU Fall Meeting, December 2020

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. 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. • 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. 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. • 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. Frequency Domain Filtering
full symbols … original data
empty symbols … interpolated & filtered data; Welch window

7. Cold Traps

8. Cold Trap
Area
24% difference
between maxt(E) and
meant(E);
little difference
between smoothed
(Fourier filtered) and
unsmoothed data set

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. 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. 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. 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. 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. 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)