Lunar and Planetary Science XLVIII (2017)
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LUNAR WATER SPATIAL DISTRIBUTION AND ITS TEMPORAL VARIATIONS. H. Z. Wang1, J.
Zhang1, Q. Q. Shi1, A. M. Tian1, J. Chen1, J. Liu2, Z. C. Ling1, X. H. Fu1, Y. Wei3, H. Zhang3, W. L. Liu4, S. Y. Fu5,
Q.-G. Zong5, Z. Y. Pu5, 1 Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, School of Space Science and Physics, Shandong University, Weihai, China ([email protected]), 2 Chinese Academy of Sciences, Beijing, China. 3 Geologic and Geophysical Institute, Chinese Academy of Sciences,
Beijing, China. 4 Space Science Institute, School of Astronautics, Beihang University, Beijing, China. 5 School of
Earth and Space Sciences, Peking University, Beijing, China.
Introduction: Various forms of water have been
discovered on the Moon by recent lunar missions: 1)
Polar ice excavated from permanently shadowed regions and detected by Lunar Crater Observation and
Sensing Satellite (LCROSS) experiment in the south
pole [1]; 2) OH/H2O found by Chandrayaan-1 Moon
Mineralogy Mapper (M3), Cassini Visible and Infrared
Mapping Spectrometer (VIMS), and Deep Impact High
Resolution Instrument Infrared Spectrometer (HRI-IR)
[2-4]. The dynamic H2O loss and rehydration cycle
over a lunar day suggests solar wind hydrogen, other
than comets and asteroids, should be an important
source of lunar surface water [4], which is also supported by lunar soil analysis and ion irradiation experiments in laboratory [5-7]. The solar-wind hydrogen
could implant into the top surface of lunar soil grains
and form individual OH and HOH groups by through
the reduction of Fe2+.
As we examined Deep Impact observations [4], it
was noted that one of two events might have occurred
when the Moon passed through Earth's magnetosphere,
which shields solar wind flux from being incident on
the lunar surface. During this period (for 4-5 days every month), proton flux from the Earth's magnetotail
plasma can also provide hydrogen to produce OH/H2O
on the lunar surface [8]. In addition, oxygen ions accompanied with this proton flux might also contribute
to OH/H2O production process [9-11]. Therefore, we
intend to study spatial distribution and temporal variations of lunar water with M3 data, especially those inside/outside magnetotail, attempting to investigate the
influence of the Earth's magnetosphere in the formation of the lunar surface hydration.
Methods: As described in [2], we use M3 spectra
around OH/H2O absorption feature at 2.8 μm to estimate water content. In order to suppress thermal residuals in M3 data, only polar regions are selected due to
their lower temperature. Full-moon times in each
month are taken as zero epoch, then the water contents
at the same day apart from the zero epoch are averaged
over 10°×10° latitude/longitude grid for all months
from Nov. 2008 to Aug. 2009. In each month, 4-5 days
around the full moon time corresponds to the position
of the Moon in the Earth's magnetosphere.
Results and Discussion: As shown in Fig. 1, lunar
surface hydration distribution in one month is closely
related to solar illumination condition, indicating higher abundance along lunar terminator, consistent with
the results by Deep Impact observations [4]. The northsouth asymmetry (Fig. 2) might be caused by residual
artifacts in the M3 data. During the Moon’s passing
through the Earth’s magnetotail (Red rectangle in Fig.
2), solar wind flux incident on the lunar surface almost
vanished due to shielding by the Earth's magnetosphere. However, OH/H2O contents remain nearly the
same order as those outside the magnetotail when the
Moon is exposed to solar wind.
The above findings suggest that solar wind hydrogen is not the sole source to form OH/H2O on the lunar surface, instead proton/oxygen [8-11] flux from the
Earth's magnetotail plasma might dominate this process when the Moon is in the Earth's magnetosphere.
Besides Chandrayaan-1 M3, The Lyman-Alpha Mapping Project (LAMP) onboard the Lunar Reconnaissance Orbiter, still operating and obtained a large
amount of UV spectral data covering the entire lunar
surface since 2009, provides an opportunity for comprehensive studies on lunar water spatial distribution
and temporal variations [12], and can give more evidences on its formation mechanisms.
References: [1] Colaprete A. et al. (2010) Science,
330, 463–468. [2] Pieters C. M. et al. (2009) Science,
326, 568–572. [3] Clark R. N. et al. (2009) Science,
326, 526–564. [4] Sunshine J. M. et al. (2009) Science,
326, 565–568. [5] Liu Y. et al. (2012) Nature Geoscience, 1601, 779–782. [6] Djouadi Z. et al. (2011) A&A,
531, A96. [7] Schaible M. J., and Baragiola R. A.
(2014) JGR, 119, 2017–2028. [8] Starukhina L. and
Shkuratov Y. (2000) Icarus, 147, 585. [9] Seki K. et al.
(2003) Nature, 422, 589–592. [10] Fu S. Y. et al.
(2001) JGR, 106, 683–704. [11] Zong Q. G. et al.
(2001) JGR, 106, 541–556. [12] Hendrix A. R. et al.
(2016) AGU Fall Meeting 2016, poster #P53A-2161.
Lunar and Planetary Science XLVIII (2017)
1831.pdf
Fig. 1. OH/H2O spatial distribution and temporal variations in the lunar northern polar region. Absorption depth
at 2.8 μm is used as indicator of water content. The two red lines indicate lunar terminator (incidence angle of 90°)
variation range.
Fig. 2. OH/H2O spatial distribution and temporal variations in the lunar southern (left) and northern (right) polar
regions. The red rectangles indicate time range when the Moon is inside the Earth’s magnetotail.