1) Water ice is expected to exist on Mercury in gaseous and solid phases, with ice stored in permanently shadowed regions due to water's low sublimation rate. 2) Potential sources of water on Mercury include comets, hydrous asteroids, interstellar clouds, solar wind generation, and outgassing from permanently shadowed regions. 3) Predictions for Mercury include a continuous water exosphere generated by solar wind interaction with the surface, detection of OH bands at polar latitudes, and questions around whether water molecules can hop to be transported to cold traps.

1. Water Group Exospheres
(with relevance to Mercury)
Norbert Schörghofer
Planetary Science Institute, Arizona / Hawaii
June 2021
N. Schorghofer, M. Benna, A.A. Berezhnoy,
B. Greenhagen, B.M. Jones, S. Li, T.M. Orlando,
P. Prem, O.J. Tucker, C. Wohler. Water group
exospheres and surface interactions on the Moon,
Mercury, and Ceres. Space Science Reviews, in
review (2021)

2. Water and Its Significance
H2O expected in gaseous and solid phase.
Among common volatiles, H2O has the lowest sublimation rate → Ice stored
in Permanently Shadowed Regions (PSRs).
Water is the most important resource on the Moon,
but yet we do not know its origin and abundance.
Potential Sources of Water (exogenic / endogenic):
• Comets
• Hydrous Asteroids (e.g. carbonaceous asteroids)
• Interstellar molecular clouds
• Solar wind generation
• Outgassing (PSRs of Mercury are older than the Moon’s, because the
Moon experienced a spin axis excursion about 2 Gyr ago)
Water Group: H2O (molecular water) or OH (hydroxyl)
Volatile and non-volatile H2O

3. How Ice accumulates on the Moon
Watson, Murray, & Brown (1961)
1. Ice is delivered to the Moon from
space (exogenic) or produced on
the surface (endogenic)
2. H2O molecules hop along ballistic
trajectories
3. Ice is trapped and stored in
permanently cold areas near the
poles (permanently shadowed
regions, PSRs)
None of this is confirmed.
Mercury – Earth’s Moon - Ceres: airless bodies, silicate-rich surfaces,
significant escape velocity, permanently shadowed regions (PSRs)

4. Best Evidence for Cold-Trapped Ice
(still) comes from Mercury
Left: Early radar evidence for polar
ice deposits
Top: High-reflectance surface within
Prokofiev crater on Mercury (Chabot
et al. 2014); the radar-bright region
(yellow contour) is located within a
PSR (red).
+ neutrons + modeled temperatures

5. Evidence for water ice on the Moon
 No radar evidence → no massive near-surface ice deposits
 Neutron spectroscopy → polar regions enhanced in hydrogen
(Feldman et al. 2000, Mitrofanov et al. 2010)
 LCROSS Impact (Lunar Crater Observation and Sensing
Satellite); Artificial impact in permanently shaded area (Cabeus
crater); spectral observation of ejecta; Oct 9, 2009; 5.6±2.9%
H2O by mass (Colaprete et al. 2010)
 near-infrared (M3), UV (LAMP), crater aspect ratios, …
→ Distribution of ice on the Moon is uncertain
Why does Mercury have much more ice than Earth’s Moon?

6. Observations of
Exospheres of Molecular Water
 Moon
 Apollo missions, <107 cm-3, one spurious event
 CHACE mass spectrometer (2008), >1010 cm-3, corrected 106 cm-3
(Sridharan et al. 2010, 2015)
 LADEE – water group detection, 0.6-40 cm-3 (Benna et al. 2019)
 Ceres
 OH in UV (A’Hearn & Feldman 1992)
 H2O, Herschel Space Telescope (Küppers et al. 2014); massive
 sporadic, lack of reproducibility, cause unknown
 Mercury
 None
→ More data needed!

7. LADEE – water group release related to
meteoroid streams Benna et al. (2019)
Impactors larger than 0.5cm (0.15g) release indigenous water group species.
LADEE altitude 20-100km.

8. MO
H+
Proton
Implantation
Neutralization/
Reflection
Diffusion
MOH
H2O (g)
H2(g)
H2O (s)
Adsorption/
Desorption
Photodissociation
H + OH
Escape
Recombinative
Desorption
Reaction with
Surface
H
Dissociative
Adsorption
Solar wind induced water cycle
Early work: Zeller et al. (1966); Starukhina (2001)
Jones/Orlando

9. Solar-wind induced production of
OH, H2, and H2O
Chemically bound OH forms from proton implantation (Zeller+ 1966;
Mattern+ 1976; Burke+ 2011; Managadze+ 2011; Ichimura+ 2012, …)
Production of water by recombinative desorption of OH (Jones+ 2018,
Zhu+ 2019). Solar-wind induced reactions with metal oxides:
M-OH + M-OH ￫ M-O-M + H2O (g)
M-OH + M···H ￫ M-O-M + H2 (g)
M … metal; does not work with pure SiO2 (unless T>600 K)
H2/H2O production ratio, current view: H2 H2O
Recombinative desorption is strongly temperature dependent,
exp(-A/kT), and composition dependent; does not occur at peak lunar
surface temperatures (<400 K)
Prediction: Solar-wind production on Mercury higher than on lunar
surface (Jones et al. 2020; ~10% of cold trapped water from this process)

10. The Lunar OH Veneer
Surficial hydroxyl
population
Detection of OH-band
 Pieters et al. (2009) - M3
 Clark (2009) - Cassini flyby
 Sunshine et al. (2009) - EPOXI
 McCord et al. (2011) - M3
 Li & Milliken (2017) - M3
 Wohler et al. (2017) – M3
 Bandfield et al. (2018) – M3
 Hendrix et al. (2012,2019) –
LAMP (UV)
 Honniball et al. (2019) - IRTF
some of the veneer is H2O
(McCord et al. 2011, Honniball et al.
2021)
Latitude-dependent surface hydroxyl
concentration, M3 (near-infrared) (Li &
Milliken 2017); H2O-equivalent

11. More on OH Veneer
Facts & Hypotheses:
1. presence of OH-band is established
2. extent of latitude dependence uncertain → activation energy
3. diurnal variation controversial (M3 vs. M3, physical chemistry
vs. observations), but if it exists it is dawn-dusk symmetric
Interpretations:
• solar-wind induced OH, or
• adsorbed H2O at sub-monolayer coverage (but not supplied by
water exosphere)
Predictions for Mercury:
• solar-wind generated OH at least at polar latitudes.
• Recombinative desorption may be the major source of Hermean
exospheric water.

12. Theory of Water Exospheres
(and it’s just theory, there is not enough data)
• Gases subject to sputtering, photo-ionization, and photo-dissociation,
with the latter being dominant
• Photo-dissociation lifetime of H2O: 22 hr at 1 au, normal sun activity
• Present constituents must be continually resupplied
• Watson, Murray, and Brown (1961): thermalized water exosphere
• But molecules may be chemisorbed on regolith → no hopping, no
transport to cold traps?

13. Thermal Ballistic Hops
(just theory, there is not enough data)
• Mercury: 0.1-1% trapped
• Moon: 10% trapped; typically 100 hops before cold-trapping
• Temporary hiding on night-side

14. Conclusions
Predictions for Mercury:
 Mercury should have a continuous or quasi-continuous exosphere
of molecular water, generated by the interaction of the solar wind
with the surface, more massive and less sporadic than on the Moon
 OH-band expected at polar latitudes
Major Open Questions:
 Definite detection of molecular water in exosphere
 Do water molecules hop? → Transport efficiency of water
exosphere (fraction that ends up in cold traps)
 Main source of cold-trapped ice? In the long-term, solar-wind
generated water may be a minor portion.