Solar wind implantation is thought to be one of the primary mechanisms in the formation of water (OH/H2O) on the surface of the Moon and possibly on the surface of other airless bodies. The lunar nearside spends ~27% of its daytime in Earth’s magnetotail where the solar wind flux is reduced by as much as ~99%. However, no correlated decrease in surficial water content has yet been seen on the lunar nearside. Here we report abundance observations of lunar surficial water on the nearside at different stages during the Moon’s passage through Earth’s magnetotail. We find that the water abundance at lunar mid-latitudes substantially increases in the dusk and dawn magnetosheath when the solar wind flux increases, yet remains nearly constant across the central magnetotail. We suggest that although we have confirmed the importance of the solar wind as a major source of fast water production on the Moon, hitherto unobserved properties of the plasma sheet properties may also play an important role.

1. Nature Astronomy
natureastronomy
https://doi.org/10.1038/s41550-023-02081-y
Article
Formationoflunarsurfacewaterassociated
withhigh-energyelectronsinEarth’s
magnetotail
S. Li 1
, A. R. Poppe2
, T. M. Orlando3
, B. M. Jones 3
, O. J. Tucker 4
,
W. M. Farrell4
& A. R. Hendrix5
Solarwindimplantationisthoughttobeoneoftheprimarymechanismsin
theformationofwater(OH/H2O)onthesurfaceoftheMoonandpossibly
onthesurfaceofotherairlessbodies.Thelunarnearsidespends~27%of
itsdaytimeinEarth’smagnetotailwherethesolarwindfluxisreduced
byasmuchas~99%.However,nocorrelateddecreaseinsurficialwater
contenthasyetbeenseenonthelunarnearside.Herewereportabundance
observationsoflunarsurficialwateronthenearsideatdifferentstages
duringtheMoon’spassagethroughEarth’smagnetotail.Wefindthatthe
waterabundanceatlunarmid-latitudessubstantiallyincreasesinthedusk
anddawnmagnetosheathwhenthesolarwindfluxincreases,yetremains
nearlyconstantacrossthecentralmagnetotail.Wesuggestthatalthough
wehaveconfirmedtheimportanceofthesolarwindasamajorsourceof
fastwaterproductionontheMoon,hithertounobservedpropertiesofthe
plasmasheetpropertiesmayalsoplayanimportantrole.
Solarwindimplantationishypothesizedtobeoneofthemajormecha-
nisms that induce water (OH/H2O, hereinafter referred to as ‘water’)
on the surfaces of the Moon1–7
, Vesta8–10
, Itokawa11
and possibly other
airless bodies12,13
. Previous studies have suggested that lunar surface
water is subject to diurnal variations3,14–16
, which are attributed to a
dailycycleofwaterformationandlossontheMoonthroughsolarwind
implantation and thermal desorption/migration, respectively3,14–19
.
Most of the lunar nearside stays in Earth’s magnetotail for around
4 Earthdays,whichisapproximately27%ofthelunardaytime(around
15 Earth days) (Fig. 1a). The solar wind ion flux on the lunar surface in
the central magnetotail may be reduced by ~99% compared with the
undisturbedsolarwind,asobservedbytheAcceleration,Reconnection,
Turbulence, and Electrodynamics of the Moon’s Interaction with the
Sun(ARTEMIS)mission20
(Fig.1b).Simulationsindicatethatmostofthe
lunarnearsidesurfacereceives~50%lowersolarwindionfluencethan
regions on the farside during magnetotail crossings (Supplementary
Fig. 1). In contrast, other major factors of the space environment that
affect the lunar surface, such as ultraviolet (UV) photon irradiation
andmicrometeoriticbombardment,arenotsubjecttochangesinthe
magnetotail.Consequently,thewaterinstantaneouslyinducedbythe
solarwindonthelunarnearsidesurfaceshoulddropsubstantiallydue
to the suppression of solar wind ions in the magnetotail. In contrast,
themajorlossprocessesofwaterassociatedwithphotodesorptionand
the surface thermal environment that are dominantly affected by the
UVflux18
arenotaffected.However,noprominentwaterdeficiencyon
thelunarnearsidehasbeenobservedintheUV16
orinfrared(IR)21
data
that detect the presence of solar-wind-induced water in the top few
micronstohundredsofmicronsofthelunarregolith,respectively.Note
that the loss of water through solar wind sputtering may be reduced
inthemagnetotail,butsputteringisnotconsideredtobeamajorloss
mechanismoflunarsurfacewateronadailyscale1,12,17
.Althoughsome
studies have suggested that the lunar mare regions on the nearside
showweakerwaterabsorptionfeaturesthanthehighlands14,19,22
,such
differencesareattributedtoacompositionaleffect15,19,22
ratherthanthe
Received: 31 August 2022
Accepted: 11 August 2023
Published online: xx xx xxxx
Check for updates
1
Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI, USA. 2
Space Sciences Laboratory (SSL), University of California at
Berkeley, Berkeley, CA, USA. 3
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA, USA. 4
Goddard Space Flight Center,
Greenbelt, MD, USA. 5
Planetary Science Institute, Tucson, AZ, USA. e-mail: shuaili@hawaii.edu

2. Nature Astronomy
Article https://doi.org/10.1038/s41550-023-02081-y
Thediurnaltrendsforwatercontentsonthelunarfarside(notaffected
by the magnetotail) were used to correct for the effects of local time
on the water contents in regions that were shielded by the magneto-
tail (Methods). Any remaining variations of the water contents in the
nearside regions can be attributed to the effects of the magnetotail.
We analysed M3
data that were acquired in six lunar phase periods for
regionswithinandoutsidethemagnetotail(Methods).
Results
The Moon’s passage through Earth’s magnetotail provides a natural
laboratoryforstudyingtheformationprocessesoflunarsurfacewater
correlated with solar wind implantation. We found ten, two and no
repeatM3
observationsofthesameregionsamongtwo,threeandmore
thanthreelunarphases,respectively(MethodsandTable1).Although
thereisalackofrepeatM3
observationsamongonecontinuouscycleof
sixlunarphaseperiods,12repeatedobservationsamongtwoandthree
ofthosesixlunarphases(Table1andSupplementaryFig.3)shouldbe
sufficient to characterize the trends for water contents on the lunar
surface during the passage of the magnetotail. The mapped water
was binned into three latitude zones to reduce shadowing effects
from rough topographies that exhibit dependence on the local
time, defined as: 30° S to 30° N (low latitudes), 30° to 60° N and S
(mid-latitudes) and 60° to 75° N and S (high latitudes) (Methods and
SupplementaryFig.3).Polarregions(above75°)wereexcludeddueto
strong shadowing effects.
The water contents of 12 regions in low- and high-latitude
zones exhibit a tight correlation with the local time (Supplementary
Fig.3a,c).However,thewatercontentsinregionsinthemagnetosheath
strongly deviate from those in regions in the undisturbed solar wind
in the mid-latitude zone (Supplementary Fig. 3b). To further assess
how the lunar surface water in the magnetosheath deviates from that
in the undisturbed solar wind, we removed the early morning data
(~8.30 a.m.)inSupplementaryFig.3b—noneofwhichwasacquiredin
the magnetosheath—and show the remaining data in Fig. 2. To derive
the diurnal trend of the magnetosheath, we used a quadratic poly-
nomial to fit the five data points in the magnetosheath and two data
points near the magnetosheath boundary (Supplementary Fig. 3b).
The water contents of regions in the magnetosheath and those in the
undisturbed solar wind fall into two distinct diurnal trends (Fig. 2a).
Thediurnaltrendforwaterintheundisturbedsolarwindonthelunar
magnetotail.Furthermore,thehighlandregionsareapproximatelytwo
timesbrighterthanthemareregions23
,andthewaterabsorptionofthe
formerispredictedtobestrongerthanthelatteratthesamewatercon-
tentduetothealbedodependenceofwaterabsorption24
.Thehighland
and mare regions may have similar water contents, and the stronger
water absorption of the highland regions reported in3,19,22,25
may be
mostlybecausetheyhaveahigheralbedothanthemareregions.Itis,
therefore,suggestedthattheremaybeadditionalformationprocesses
ornewsourcesofwaternotdirectlyassociatedwiththeimplantation
of solar wind protons in the magnetotail. For instance, water migra-
tion16
and water sourced from the Earth wind26
have been proposed
as possible new sources responsible for the similar hydration levels
of regions within and outside the Earth’s magnetotail. Our analysis of
water variations on the lunar nearside surface during the passage of
Earth’smagnetotailindeedsuggeststhatwater-formationprocessesare
alteredandmaybedifferentfromthosethoughttobedominantwhen
theMoonisoutsidethemagnetotail.Theseprocessesmayhavebeen
overlookedandarelikelytobeactiveonotherairlessbodiesincluding
but not limited to Mercury.
We analysed the water content of the same regions on the lunar
nearsidemappedfromMoonMineralogyMapper(M3
)reflectancedata
toassesshowitvariesduringthepassageoftheMoonthroughEarth’s
magnetotail(Methods).Ourpreviousstudyshowedthatthelatitude,
opticalmaturityandlocaltimearethreemajorfactorscontrollinglunar
surfacewater15
.Assessingwatervariationsforthesameregionavoids
complicationsfromthefirsttwoofthethreefactors.However,theeffect
of the local time is overprinted on that of the lunar phase in the same
region due to the synchronization between the rotational and orbital
periodsoftheMoon.Thus,wecorrectedthewaterobservedinthesame
regions at different lunar phases and local times to local noon. This
correction also allowed us to correlate the variation of water with the
solarwindionfluxtotheMoonthathadbeenmeasuredbytheARTEMIS
mission. Our previously mapped water contents from the global
lunar surface exhibit no dependence on composition (for example,
mare versus highland)15
. It has been suggested that the pyroclastic
depositsontheMoonretainwaterfromtheinterior27
,whichmayskew
thediurnaltrendsofsolar-wind-inducedwater,sotheseweremasked
inourstudiedregions.Itis,thus,reasonabletoassumethatthewater
variationswithinthemareandhighlandregionswouldfollowthesame
diurnal trends, if they are not affected by the magnetotail (Methods).
Sun
Magnetotail lobe
Magnetotail lobe
Magnetosheath
Magnetosheath
Plasma sheet
–90o
0o
180o
90o
–90 –45 0 45 90
Lunar phase (°)
10
7
10
8
Solar
wind
ion
flux
(particles
per
cm
2
per
s)
Lobes/
plasma sheet
a b
Dawn
Tail centre
Dusk
Solar wind
Solar wind
Not to scale
Lunar orbit
Fig.1|ConfigurationsofEarth’smagnetotailandaprofileofthesolar
windionfluxmeasuredbytheARTEMISmission. a,Side-viewschematic
diagramshowingtheconfigurationofthemagnetotail.TheMoonpasses
throughthemagnetotailinthefollowingsequence:theduskmagnetosheath,
themagnetotaillobesandplasmasheet,andfinally,thedawnmagnetosheath.
b,Solarwindionfluxatdifferentlunarphases(−90°to90°)measuredby
theARTEMISmissionatthesubsolarpointoftheMoon20
.Inbothpanels,the
dark-reddashedlinesmarktheboundarybetweentheregularsolarwindand
themagnetosheath.Thebluedashedlinesindicatetheboundarybetweenthe
magnetosheathandthemagnetotaillobe.Thereddashedlineoutlinesthe
plasmasheet(theredshadedregion).Thepositionoftheplasmasheetishighly
variableinthemagnetotail,andthus,wegroupthemagnetotaillobesandthe
plasmasheetasthe‘magnetotailcentre’.

3. Nature Astronomy
Article https://doi.org/10.1038/s41550-023-02081-y
nearside (Fig. 2a) is identical to that on the farside (Supplementary
Fig.4),whichdemonstratesourassumptionthatthewatervariations
within the nearside mare and farside highland regions follow the
same diurnal trends as if they were not affected by the magnetotail
(Methods).Thewatercontentinregionsinthemagnetosheathisnota-
blyhigherthanthatinregionsintheundisturbedsolarwindatasimilar
localtime(Fig.2a).Figure2bshowsaregionthatwasobservedinthree
lunarphasesbyM3
.Thewatercontentinthisregionincludesdatathat
were collected in the solar wind and in the magnetotail. These fall on
the farside diurnal trend. The water content in the magnetosheath is
much higher than would be predicted in the undisturbed solar wind
despite the data in the magnetotail being collected closer to local
noon (Fig. 2b). M3
observations also show that the water contents in
regionsinthecentralmagnetotailwhenthesolarwindprotonsource
wasshutoffexhibitnosuppressionincomparisontothoseinregions
in the undisturbed solar wind at a similar local time (Supplementary
Fig. 3). Altogether, this indicates that there are distinct formation or
lossprocessesforwaterassociatedwiththemagnetotail.
We averaged the water contents of the 12 different regions
(Table 1 and 12 pairs of coloured symbols in Fig. 3) to assess the water
variation during the Moon’s passage through the magnetotail. The
diurnaltrendsforwatercontentsinthelunarfarside(Supplementary
Fig. 4) were used to correct for the effect of local time on water in
the nearside (Methods). The corrected data were then used to assess
how the lunar surface water varies with the lunar phase at the same
local time. The water content exhibits substantial variations in the
mid-latitudezonebutvarieslittleatlowandhighlatitudes(Fig.3).Ifthe
watercontentvariedbymorethan10,17or32 ppm(partspermillion)
inthelow-,mid-andhigh-latitudezones,respectively,thevariationwas
greater than our mapping uncertainties and, thus, could be detected
by M3
(Methods). Note that the error bars in Fig. 3 represent the
heterogeneityofwatercontentsineachbinnedzoneandarenotobser-
vationuncertainties(Methods).Duetotheheterogeneityofthewater
contentsinthose12regions(SupplementaryFig.3),wecouldnotuse
rigorousstatisticstoassesswhetherthewatervariationwassignificant
(Methods). Thus, the size of the water variations is only a first-order
estimation of the magnetotail effects with unknown uncertainties.
Surprisingly, the water content in the low- and mid-latitude zones
exhibited almost no variations across the central magnetotail (Fig. 3,
from lunar phases −33° to 25°), which apparently deviates from the
significant decrease (~80%–99%) of the solar wind ion flux (Fig. 1b).
The water content did not exhibit substantial changes for around
1–2 Earth days after the Moon exited the magnetotail (lunar phases:
~50°–75°) (Fig. 3). The minimum water content in the low-latitude
zone was near the boundary between the magnetotail lobe and the
dawn magnetosheath (Fig. 3, the lunar phase near 20°), whereas in
comparison, the water content in the mid-latitude zone reached the
lowestpointneartheboundarybetweentheduskmagnetosheathand
themagnetotaillobe(Fig.3,thelunarphasenear−20°),notwithstand-
ingtheinitiallylowlevelofwaterwhilethesurfacewasinthesolarwind
beforetransitingthemagnetotail.Thewatercontentinthelow-latitude
zone was mostly around 40 ppm, and there were strong deviations in
those 12 regions (Fig. 3). The water content in the high-latitude zone
remainedstableataround140 ± 7(2σ) ppm(Fig.3)duringtheMoon’s
passagethroughthemagnetotail,whichisconsistentwithourprevious
findingthatthelunarsurfacewaterexhibitssmallvariationsatlatitudes
higherthanaround60°(ref.15).
A major source of uncertainty in our assessments of water varia-
tions on the lunar surface could be from the correction of local-time
effects. Nevertheless, we found that these uncertainties are insuffi-
cientlylargetoskewthetrendsforlunarsurfacewater.M3
observations
Table 1 | Summary of repeat observations of the surface
water at the same regions made by M3
and acquired at
different lunar phasesa
Dusk
(K)
First half
tail (F)
Second half
tail (S)
Dawn
(N)
1–2days
after (A)
1–2days before (B)
BK BF BS
BKF
Dusk (K) KF KS
First half of the
tail (F)
FS FN
FNA
Second half of
the tail (S)
SN SA
Dawn (N) NA
a
The letter combinations in the table represent two or three lunar phases during which M3
observed the same region. For instance, BK represents two repeat M3
observations of the
same region for the 1–2Earth days before the Moon entered the magnetotail (B) and for when
it was in the dusk magnetosheath (K). FNA represents three repeat observations of the same
region for when the Moon was in the first half of the magnetotail (F), for when it was in the
dawn magnetosheath (N) and for the 1–2Earth days after it exited the magnetotail (A).
Local time (hour)
50
100
150
200
Water
content
(ppm)
a
30° – 60° N and S
Areal extent: 10
6
km
2
Areal extent: 10
5
km
2
Areal extent: 10
4
km
2
In
solar wind
I
n
m
a
g
n
e
t
o
s
h
e
a
t
h
Farside diurnal trend
8 10 12 14
8 10 12 14
Local time (hour)
80
90
100
110
120
130
140
150
Water
content
(ppm)
Farside
diurnal trend
In solar wind
(Lunar phase: –74°)
In magnetosheath
(Lunar phase: –40°)
In magnetotail
(Lunar phase: –22°)
b
Fig.2|Thediurnaltrendforwatercontentsinregionsinthemagnetosheath
stronglydeviatesfromthatinregionsintheundisturbedsolarwindbefore
thelocal-timecorrectioninthemid-latitudezone. a,Watercontentsof
regionsinthemagnetosheath(outlinedwithabluedashedline)andregions
intheundisturbedsolarwind(outlinedwithablackdashedline)onthelunar
nearside.Thesizeofeachsymbolrepresentsthearealextentofthatregion.
b,Variationofwatercontentsinthesameregion(centrednear35° E),whichwas
observedinthreelunarphasesbyM3
.Thedatapointsarethesamegreencircles
asina.Thegreendashedcircle(indicatedbythegreyarrow)markswhatthe
watercontentinthisregionwouldhavebeenifithadfollowedthesamediurnal
trendasthatonthelunarfarside.Symbolsinthesamecolourarefromthesame
regions.Errorbarsindicate1σ frombinningM3
data.

4. Nature Astronomy
Article https://doi.org/10.1038/s41550-023-02081-y
in several regions clearly show that diurnal trends in water contents
between observations in only the solar wind versus in the central
magnetotail versus in the magnetosheath deviate from each other
(Fig. 2 and Supplementary Fig. 3b). Additionally, similar corrections
were performed on the M3
data for the low-, mid- and high-latitude
zones. However, the trends for water contents after the corrections
aredifferentforthosethreelatitudezones(Fig.3).Thestrongestvari-
ationofwaterinthehigh-latitudezoneaftercorrectionisonlyaround
7 ppm(Fig.3),whichisfarlessthanthemappinguncertaintyof32 ppm
(Methods). In comparison, the strongest variations in water content
with lunar phase at low- and mid-latitudes are around 15 and 27 ppm,
respectively(Fig.3),whichishigherthantherespectiveuncertainties
of 10 and 17 ppm (Methods). It is reasonable to believe that the varia-
tions of water in the low- and mid-latitude zones during the passage
of the magnetotail are real and not due to the local-time corrections.
Otherwise,asimilarpatternofwatervariationsshouldalsobeobserved
inthehigh-latitudezone.
Theincreaseofthewatercontentsobservedatmid-latitudesinthe
duskanddawnmagnetosheathrelativetothewatercontentsobserved
in the undisturbed solar wind directly supports the idea that solar
wind implantation is a major contributor to the lunar surface water,
asrevealedbyIRmeasurements.Italsoindicatesthattheformationof
water through solar wind implantation is a fast process, as suggested
in previous studies3,15
. The water contents in the mid-latitude zone
increased by over 20 ppm during the passage of the dusk and dawn
magnetosheath(Fig.3).Suchincreasesofwatercontentscorrespond
promptlywiththeincreasesofthesolarwindionfluxbyaround50%in
theduskanddawnmagnetosheathincomparisontotheundisturbed
solar wind (Fig. 1b). Other known sources, such as meteoritic impact
delivery, interior degassing and terrestrial ionospheric outflow (also
knownastheEarthwind)26,28
,areunlikelytobeabletoaccountforthe
increasesinwatercontentsintheduskanddawnmagnetosheathovera
timescaleof~2 Earthdays.Thelow-latitudezonegainedlesswaterthan
themid-latitudezone(<15 ppmversus>20 ppm)intheduskanddawn
magnetosheath,whichcouldbeduetothemuchhighertemperatures
and,thus,moreefficientlossofwaterinthelow-latituderegions.
Thelowestwatercontentsinthelow-andmid-latitudezoneswere
not observed at the time of the lowest solar wind flux when the Moon
was near zero lunar phase (Fig. 3). The water contents in the low- and
mid-latitude zones near the zero lunar phase are almost the same as
those at other lunar phases when the solar wind flux is much higher
(forexample,theduskmagnetosheath;Fig.3).Thisimpliesthatsome
typeofreplenishmentmaypreventorcounterbalancethedecreaseof
thewatercontentsthatwouldotherwisebecorrelatedwithaminimum
incident ion flux (Fig. 1b). This could be different from the source of
waterinducedbytypicalsolarwindimplantation.
Observations by the electron reflectometer onboard the Lunar
Prospector mission suggest that the Moon spends around 44% of the
duration of the magnetotail crossing in the plasma sheet29
, which is
generallyalongtheeclipticplane30
.ARTEMISmeasurementsdemon-
stratethattheionsandelectronsintheplasmasheetaremuchhotter
and extend to higher energies than those in the solar wind (Fig. 4).
Laboratory experiments of proton irradiation onto SiO2 suggest that
theOHformationyieldisindependentofirradiationenergiesfrom2to
10 keVbeforetheprotonfluencereachessaturationat~3 ⨉ 1016
H+
cm−2
(ref. 31). In comparison, the daytime fluence of the solar wind ions is
around1014
H+
cm−2
and1.5 ⨉ 1014
H+
cm−2
withinandoutsidethewhole
magnetotail (Supplementary Fig. 1), respectively, which is well below
the saturation level. The higher-energy protons from both the solar
windandtheplasmasheetaregenerallyimplanteddeeperintoregolith
grain rims and, thus, may sustain longer lifetimes32
, which may pre-
ventthesignificantlossofthelunarsurfacewaterinthemagnetotail.
–50 0 50
Lunar phase (°)
10
30
50
70
40
60
80
100
120
100
150
200
Corrected
water
content
(ppm)
Dusk Tail centre Dawn
30°–60° N and S
30° N–30° S
Areal extent: 106
km2
Areal extent: 105
km2
Areal extent: 104
km2
60°–75° N and S
Fig.3|Repeatobservationsofwatercontentsinthesameregionsfor
differentlunarphases.Symbolsinthesamecolourarefromthesameregionin
eachlatitudezone.Squares:60°–75°inthenorthernandsouthernhemispheres.
Circles:30°–60°inthenorthernandsouthernhemispheres.Diamonds:30° S
to30° N.Thesizeofeachsymbolrepresentsthearealextentoftherepeated
observationsineachlatitudezone.Theverticaldashedlinesinredandblack
marktheboundariesamongeachsectionofthemagnetotail(Fig.1).Theblack
dottedlinesineachlatitudezonearedoublysmoothedwatercontentsto
illustratethetrend.Thegreybarsontherightsideofeachsubplotrepresent
thetotaluncertaintiesfrommappingandlocal-timecorrections(Methods).
Theerrorbars(1σ)paralleltotheyaxisforeachsymbolareduetobinningof
theheterogeneouswatercontentsinthestudyregions,andtheyshouldbenot
comparedwiththegreybars(seetextformoredetails).
Solar wind
Magnetosheath
Magnetotail lobe
Plasma sheet
10
10
10
8
10
6
10
2
10
4
10
0
1 10 100
Energy (eV)
1,000 10,000
Differential
flux
(s
−1
cm
−2
sr
−1
eV
−1
)
Fig.4|Typicalionandelectrondifferentialnumberfluxesforthesolarwind,
magnetosheath,plasmasheetandmagnetotaillobesasobservedbythe
ARTEMISmission.Solidlinesareforelectronsanddashedlinesareforions.
ARTEMISdataforeachdistributionarefromthesolarwind:7November2020,
18.10to18.40;magnetosheath:26December2020,03.20to03.30;plasmasheet:
30December2020,08.40to09.00andmagnetotaillobe:29December2020,
16.00to16.20.

5. Nature Astronomy
Article https://doi.org/10.1038/s41550-023-02081-y
However, these high-energy ion fluxes within the plasma sheet are
typically around 1%–5% of that in the solar wind (Fig. 4). Thus, the ion
flux in the plasma sheet seems too low to account for the observed
lack of water suppression in the central magnetotail, and thus, there
may be other mechanisms of water formation or retention, which we
considerhere.
We explored a possible mechanism in which the observed
water variations across the magnetotail were due to the diffusion of
implantedsolarwindhydrogen33
.ResultsfromMonteCarlosimulations
inref.33predictedtheincreasesofwatercontentsintheduskanddawn
magnetosheaththatisobservedintheM3
data(Figs.2and3),whichis
attributedtothe~50%increaseofthesolarwindionfluxinthemagne-
tosheath.Thesteady-statewatercontentacrossthecentralmagnetotail
(Fig.3)canbeexplainedbylongerretentionlifetimeswithinlunarrego-
lithgrainsofhigher-bonding-energyhydrogen,evenduringthemaxi-
mumsurfacetemperaturesexperiencednearlocalnoon.Somestrong
variationsofthesurfacewaterinthecentralmagnetotailareobserved
inthelow-andmid-latitudezones.Wethinkthatthesevariationscould
beduetothehighlyvariableplasmaenvironmentassociatedwiththe
plasma sheet (Fig. 1). Note that more than 50% of the M3
observations
inthecentralmagnetotailwerenearlocalnoon(SupplementaryFig.3).
The implanted solar wind hydrogen, which has lower bonding ener-
gies, has shorter lifetimes32,33
and may diffuse out before local noon.
Consequently, the retained high-bonding-energy hydrogen is stable
inthelunarregolithsothatnosubstantialwatervariationisobserved
across the central magnetotail. However, M3
observations during the
local morning (~8.30 a.m.) did not exhibit notable decreases of the
watercontentswhenthesolarwindionsourcewasshutoffinthecentral
magnetotailcomparedtointheundisturbedsolarwind(Supplemen-
taryFig.3andFig.2).Thelowerwatercontentsintheundisturbedsolar
windobservedlaterinthemorning(~9.30 a.m.,SupplementaryFig.3
and Fig. 2) could be due to thermal losses as the surface temperature
rises.Theindiscernibledifferencesinwatercontentsintheearlymorn-
ing,bothwithinandoutsidethecentralmagnetotailinthemid-latitude
zone,maybeexplainedbythelowtemperatures(~300 Kat~8.30 a.m.at
latitude45°;ref.34),andthus,mostofthewaterisretainedduetothe
slow diffusion processes at such low temperatures32
(Supplementary
Fig.3b).However,multipleM3
observationsinthecentralmagnetotail
across the morning in the low-latitude zone (for example, ~8.30 a.m.,
~10.00 a.m. and ~11.30 a.m.; Supplementary Fig. 3c) exhibit no sup-
pression compared with those outside the central magnetotail at a
similarlocaltime.Thediffusionlossofwaterneartheequatorisquite
significantaround10.00 a.m.and11.30 a.m.32
,whichimpliesthatthere
are additional formation processes for lunar surface water that are
different from but otherwise equivalent to the implantation of solar
windprotonsoutsidethemagnetotail.
Note that the electron energies in the plasma sheet are much
higher than those in the solar wind or magnetosheath (Fig. 4). The
electron energies in the solar wind are mostly less than a few tens of
electronvolts,whereasthoseintheplasmasheetcanreachover10 keV
(Fig.4).Thenumberfluxesofelectronsabove20 eVintheplasmasheet
andintheundisturbedsolarwindaresimilarat~1011
cm−2
s−1
(Fig.4;note
that all flux curves were normalized by their energies). Irradiation of
thelunarsurfacebyhigh-energyelectronswillproducedefectsinthe
form of non-bonding oxygen centres35,36
in regolith grains, allowing
any trapped atomic H, either directly from the solar wind or from the
dissociationoftrappedmolecularH2,toreactwiththedefect,resulting
intheformationof–OHorH2O.Previousexperimentsalsosuggestthat
the secondary electrons produced by high-energy electrons can be
trappedingrainrims37–40
.Incontrast,secondaryelectronsgeneratedby
low-energyelectrons(<~100 eV),suchasthoseintheundisturbedsolar
wind, can efficiently leave the regolith grain surface due to their low
energies. The trapped high-energy electrons as well as their induced
secondary electrons can lead to a negative potential, which has been
documentedasaffectingthemobilityofimplantedprotons41
thereby
allowing for the greater retention of implanted hydrogen in the form
of–OH/H2O.High-energyelectronsintheplasmasheetandsolarener-
geticparticlesmayalsoenterthepermanentlyshadedregions42,43
and
contribute to the formation of the observed water ice44
. However, it
is unclear whether these processes are sufficient to account for the
observed formation of water in the central magnetotail and water ice
depositsinpermanentlyshadedregions.
TheaboveassessmentisbasedonirradiationexperimentsonSiO2
and olivine with proton energies of 2–10 keV. The lunar regolith may
haveadistinctyieldofhydroxylgroupsandpossiblyH2Oundersimilar
irradiation conditions45
. In addition, most solar wind ions are within
0.5–2 keVandionsintheplasmasheetarebroadlydistributedinenergy
from0.2to>25 keV(Fig.4).Futureirradiationexperimentsusinglunar
samplesandtherelevantionandelectronenergycharacteristicsofthe
solarwindandplasmasheetarewarrantedtoreassesswhethertheions
andelectronsintheplasmasheetcanaccountfortheobservedwater
formationinthecentralmagnetotail.Theseexperimentsarealsocriti-
calforunderstandingtheprocessesofwaterformationassociatedwith
solarenergeticparticlesthatmayoperatesimilarlyonallairlessbodies
suchastheMoon,Vesta,Mercuryandnear-Earthobjects.
It is surprising to observe that the water content near zero lunar
phasedidnotdecreasewhentheionfluxwasthelowest.Duringcross-
ingsofthecentralmagnetotail,theMoonmayperiodicallyencounter
the plasma sheet, and we hypothesize that there may be additional
water-formation processes associated with the plasma sheet, par-
ticularly the high-energy electrons in it. Simultaneous observations
oflunarsurfacewaterandambientplasmafluxes,includingbothions
andelectrons,tothelunarsurfaceduringtheMoon’spassagethrough
themagnetotailbyfutureorbitalandsurface-basedmissionswillhelp
to constrain how the lunar surface water content varies with ion and
electron fluxes. Additionally, future irradiation experiments using
protons and electrons with energies like those in the plasma sheet
may be necessary to reveal the detailed processes of water formation
thatoperateintheplasmasheetorareduetosolarenergeticparticles.
Methods
Mapping lunar surface water using M3
data
The M3
onboard India’s Chandrayaan-1 mission measured the hyper-
spectral reflectance at the lunar surface from 0.43 to 3.0 μm (ref. 46).
AbsorptionbythefundamentalstretchingofOHandbytheovertoneof
H2Obendingbothoccurnear3 μm,andthisabsorptioncanbedetected
inM3
data.UsingthisM3
dataset,weattemptedtoassessthevariation
ofthewatercontentsonthesurfacewhentheMoonwaseitherwithin
or outside the magnetotail. It is very challenging to discriminate OH
fromH2Obasedontheiroverlappingabsorptionsignalsnear3 μm,so
theyaregroupedtogetheras‘water’15
.M3
dataweregroupedintofive
optical periods (OPs; OP1A, OP1B, OP2A, OP2B and OP2C) according
to the mission status when the data were acquired46
. M3
data for the
former four OPs have a spatial resolution of 140 m per pixel and were
acquired during the 100 km orbit. OP2C data have the best cover-
age of the lunar surface, yet their spatial resolution is downgraded to
~280 mperpixelduetotheascentofthemissionaltitudetothe200 km
orbit46
.StudiessuggestthattheM3
reflectancedatareleasedinnodes
of NASA’s Planetary Data System were under-corrected for thermal
contamination14,47,48
.Thedaytimetemperaturesofthelunarsurfacein
theequatorialregionscanreachover400 Kandthethermalemission
atsuchhightemperaturescansignificantlyreduceandevenmaskany
absorptionbetweenaround2and3 μminM3
data49
.Wedevelopedan
empiricalmodelbasedonthespectralfeaturesoftheApolloandLuna
samples48
and applied it to update the thermal correction of M3
data.
Our modelled lunar surface temperatures were validated with the
bolometrictemperaturesderivedfromthedataacquiredbytheDiviner
radiometer onboard the Lunar Reconnaissance Orbiter50
. Our model
was further validated using the reflectance data returned by China’s
Chang'e 4 rover51
and Chang'e 5 lander52
. M3
radiance (RDN, level 1b),

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Article https://doi.org/10.1038/s41550-023-02081-y
observation (OBS) and location (LOC) files were downloaded from
the NASA Planetary Data System (https://pds-imaging.jpl.nasa.gov/
volumes/m3.html).WeprocessedallM3
imagesinthefiveOPsusingthe
calibrationpipelinedefinedinref.48.Wecalculatedthelunarphaseof
eachM3
imagecubeusingSPICEkernelsprovidedbytheJetPropulsion
LaboratoryinconjunctionwiththeM3
dataacquisitiontime.
In our previous study, we mapped the absolute water content of
each M3
image cube of the global lunar surface from the absorption
strengthnear3 μm15
.Ourpreviouslaboratoryexperimentsusingterres­
trialanaloguesandsynthesizedlunarsamplesshowedthattheabsolute
watercontentslinearlycorrelatewiththeabsorptionstrengthsofour
samplesnear3 μm15
.Wethencharacterizedtheabsorptionstrengths
near3 μminM3
datausingtheHapkeeffectivesingle-particleabsorp-
tion thickness (ESPAT)15,53
and mapped the water contents15
. Our pre-
vious study suggests that the lunar surface water content primarily
depends on the latitude, optical maturity and local time15
. Although
it is suggested that there possibly exist subtle differences among M3
dataacquiredunderdifferentoperationalconditions,suchasoptical
periods and detector temperatures54
, our mapped water abundance
across the global lunar surface exhibits no apparent dependence on
theOPofM3
data15,55
.Thus,weusedM3
datainallOPstoobtainthebest
coverage in this study. The mapping uncertainty was estimated to be
20%ofmappedwaterfromourlaboratoryexperiments15
.
Theboundarybetweentheregionwithinthemagnetotailandthat
outsideitwasempiricallydeterminedfromthesolarwindionfluxpro-
filemeasuredbytheARTEMISmission20
(Fig.1b).Themajorsubregions
ofEarth’smagnetotailincludetheduskmagnetosheath,magnetotail
lobes, plasma sheet and dawn magnetosheath. The dawn and dusk
magnetosheathsareformedbytheshockedsolarwindplasma,which
isdivertedaroundtheobstacleofEarth’sgeomagneticfield.Withinthe
magnetopause, the plasma sheet serves as a boundary layer between
the two magnetotail lobes. However, the boundaries between the
plasmasheetandthemagnetotaillobesarehighlyvariabledepending
ontheseason,upstreamsolarwindandinterplanetarymagneticfield
conditions56
.Weempiricallydeterminedtheboundariesbetweenthe
solarwind,theduskmagnetosheath,themagnetotailcentre(consist-
ing of the magnetotail lobes and plasma sheet together), the dawn
magnetosheath and the solar wind at lunar phases of −63°, −33°, 25°
and50°,respectively(Fig.1b).WethenusedM3
datainsixlunarphase
periodstoinvestigatehowthewatercontentvariesduringtheMoon’s
passage through the magnetotail. There are six lunar phase periods.
The first spans the 1–2 Earth days that the Moon is in the solar wind
beforeenteringthemagnetotail(lunarphases:−88°to−63°).Thenext
four are when the Moon is in the dusk magnetosheath (lunar phases:
−63° to −33°), in the first half of the magnetotail (lunar phases: −33°
to 0°), in the second half of the magnetotail (lunar phases: 0° to 25°)
and in the dawn magnetosheath (lunar phases: 25° to 50°). The sixth
lunar phase period spans the 1–2 Earth days when the Moon is again
in the solar wind after exiting the magnetotail (lunar phases: 50° to
75°)(Fig.1b).Wesplitthemagnetotailcentreintothefirstandsecond
halvestounderstandhowthelunarsurfacewatercontentvarieswithin
thecentralmagnetotail.Eachlunarphaseperiodroughlycovers25°.
ThelunarphaseperiodforanM3
imagecubewaspickedbasedon
the lunar phase when the data were acquired. There are 92, 72, 75, 49,
56 and 72 M3
image cubes that were acquired for the six lunar phase
periods:the1–2 EarthdaysbeforetheMoonenteredthemagnetotail,
whenitwasintheduskmagnetosheath,whenitwasinthefirsthalfof
themagnetotailcentre,whenitwasinthesecondhalfofthemagneto-
tail centre, when it was in the dawn magnetosheath and the 1–2 Earth
daysafteritexitedthemagnetotail.ThenamesofthoseM3
imagecubes
arelistedinSupplementaryTable1.
The water content of each M3
image cube was derived from band
83 (2.85 μm) using the method in ref. 15. We found that the absorp-
tion centres of water in our analogue and synthesized lunar samples
were consistently near 2.85 μm, that is band 83 in the M3
data15
. We
used the maximum reflectance between 2.5 and 2.7 μm in the M3
data
(band73–78)tocastaflatlinecontinuum,whichwaslimitedbytheM3
spectral range, as that covers only half of the water absorptions from
around2.6to4 μm(ref.15).WeconvertedM3
reflectancedataforbands
73–78 and 83 into single-scattering albedo using Hapke’s model15,53
.
The continuum was then removed for the single-scattering albedo in
band 83. The ESPAT parameter at 2.85 μm was then calculated15
. The
water content was finally derived using this empirical relationship:
H2O (ppm) = 5,000 ⨉ ESPAT2.85 μm (ref. 15). We mosaiced the mapped
waterfromM3
imagecubesineachlunarphaseperiodusingthesimple
cylindricalmethodataresolutionof280 mperpixel(Supplementary
Figs. 5–10). M3
pixels with the minimum phase angles were used for
the overlapping observations in the mosaicking, which like what we
didinourpreviouswork2,15
.Therespectivelocaltime(Supplementary
Figs. 11–16) and lunar phases (Supplementary Figs. 17–22) were also
mosaicedinthesameway.
Weusedquadraticpolynomialsempiricallyderivedforthelunar
farsideinthelow-,mid-andhigh-latitudezones(SupplementaryFig.4)
tocorrectthelocal-timeeffectsofourmappedwateronthenearside,
as the water contents may have been affected by the magnetotail. We
foundthatthelocaltimeofM3
datainourstudiedregionswasmostly
around8 a.m.,10 a.m.,12 p.m.and2 p.m.(SupplementaryFigs.11–16).
M3
image cubes acquired at similar local times on the lunar farside
were chosen and mosaiced in the same way as those in the magneto-
tail. We then searched repeat M3
observations of the same region to
empirically determine the diurnal trend of the water contents on the
lunar farside (Supplementary Fig. 4). The two major water reservoirs
on the lunar surface are solar-wind-induced water in regolith grain
rims11,57–59
andwatertrappedinagglutinates7
andvolcanicglasses60–62
.
Solar-wind-induced water in the space-weathered rims of the lunar
regolithmainlycontributestotheobserveddiurnalvariations,whereas
water trapped in agglutinates and glasses provides a background
that may be less affected by the lunar surface thermal environment
and mostly stable across the daytime. Irradiation experiments using
protonsshowthattheyieldofwaterisindependentofthecomposition
of the host grains31
. Thus, it is reasonable to assume that the diurnal
trends for solar-wind-induced water in regolith grains are similar on
the lunar nearside and on the farside, whereas the diurnal trends for
background water trapped in impact and volcanic glasses can differ
duetothedistincthistoriesofimpactgardeningandvolcanicactivities
(forexample,lavaflows).Regionalpyroclasticdepositsweremasked.
Thecorrectionwasperformedintwosteps.Thedifferenceintheback-
groundwatercontentindifferentregionsisreflectedintheintercepts
oftheempiricalquadraticpolynomialswiththeyaxis(Supplementary
Fig.4).ForeachM3
pixel,wefirstcalculatedthenewinterceptforeach
latitudezone(b1,b2 andb3)ofthequadraticpolynomialforthediurnal
trendofourstudiedregions:
b1 = w − 3.9 × LT
2
+ 95.2 × LT , if a pixel is between 30° S and
30° N b2 = w − 6.4 × LT
2
+ 155.8 × LT , if a pixel is within 30° S to 60° S
or 30° N to 60° Nb3 = w − 5.2 × LT
2
+ 124.2 × LT , if a pixel is within
60° S to 75° S or 60° N to 75° N, where w is our mapped water
contentofeachM3
pixelandLTisthelocaltimeofthatM3
pixel.
We then corrected the water content of each M3
pixel to the
local noon (12 p.m., wnew): wnew = 3.9 × 12
2
− 95.2 × 12 + b1 , if a pixel is
between 30° S and 30° N;wnew = 6.4 × 12
2
− 155.8 × 12 + b2, if a pixel is
within30° Sto60° Sor30° Nto60° N wnew = 5.2 × 12
2
− 124.2 × 12 + b3,
ifapixeliswithin60° Sto75° Sor60° Nto75° N
Wesearchedforregionsthathadrepeatobservationsacrossmore
thanonelunarphase.Thereare15,20,15,6and1region(s)withrepeat
observationsacrosstwo,three,four,fiveandsixlunarphaseperiods,
respectively.Toensurethatourresultswerestatisticallysignificant,we
usedonlyrepeatobservationscoveringatleast0.5longitudedegrees.
Inourfinalresults,thereweretenobservationsthathadrepeatcover-
age of a single region over two lunar phases, two observations that
hadrepeatcoverageofasingleregionoverthreelunarphases,andno

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Article https://doi.org/10.1038/s41550-023-02081-y
observationsofanyregionsthathadrepeatcoverageoverfour,fiveor
sixlunarphases(Table1).TherepeatM3
observationsinaregionafter
the local-time correction were binned into three latitude zones and
plotted in Fig. 3. To better illustrate the trend for the water contents,
weusedthemovingaveragealgorithmtosmoothourdata.Wedoubly
smoothed (smoothed twice) the mean water contents to reduce the
effect of spiky data points for the three latitude zones in Fig. 3 using
a moving average window of 25°. A trend line was derived for each
latitude zone (the black dashed lines in Fig. 3). The binned M3
data
withoutanycorrectionareshowninSupplementaryFig.3.
Two main sources of uncertainty are considered in this study.
Oneisthemappinguncertainty,whichwasestimatedtobe20%ofthe
mappedwatercontent15
.Theotherisfromtheresidualofthelocal-time
correction. We used the root mean squared errors of the quadratic
polynomial fits of the diurnal trends (Supplementary Fig. 4) as the
uncertainties of the local-time correction. The root mean squared
errors for the low-, mid- and high-latitude zones are 3, 1 and 3 ppm,
respectively. We used the mean water contents of the low-, mid- and
high-latitudezonestoestimateourmappinguncertainties.Themap-
pinguncertaintiesare7 ppm(37 ppm ⨉ 20%),16 ppm(79 ppm ⨉ 20%)
and29 ppm(143 ppm ⨉ 20%)forthelow-,mid-andhigh-latitudezones,
respectively.So,thetotaluncertaintiesinourmappedwatercontents
afterthelocal-timecorrectionare10,17and32 ppmforthelow-,mid-
andhigh-latitudezones,respectively.
Interpretingtheerrorbarsinthebinnedwatercontentdata
The error bars in the binned water data (Figs. 2 and 3 and Supplemen-
taryFigs.3and4)representtheheterogeneityofthewatercontentsin
each binned zone, which is due to the dependence of the water distri-
bution on the latitude, optical maturity and local time15
. They are not
observationuncertainties.Itismeaninglesstocomparetheerrorbars
andwaterheterogeneitywiththemappinguncertainties.Weillustrate
thedifferencebetweenthetwoparametersinSupplementaryFig.23.
Thefigureshowsthatthedifferenceinthewatercontentbetweenlunar
phasesAandBexceedstheassumedmappinguncertaintyof10 ppmin
allpixelsinthisbinnedlatitudezone,whichmeansthattheincreasein
watercontentcanbedetectedbytheinstrumentinallpixels.Themeans
and standard deviations of the water contents for the same region in
lunarphasesAandBare:32 ± 14 ppmand44 ± 14 ppm,respectively.In
thisstudy,wecomparedthemeanvaluesforlunarphasesAandBand
foundthattheincreaseofthewatercontentis12 ppm,whichisgreater
than the mapping uncertainty of 10 ppm and, thus, can be detected.
There is no reason to compare the standard deviations of the water
contents in each lunar phase with the mapping uncertainty due to
thedistinctphysicalmeaningsthatthesetwoparametersrepresent.
Whycalculatingstatisticsforthebinnedwatercontentisnot
appropriate
Weaveragedthebinnedwatercontentineachlunarphaseperiod(Fig.3)
toassessthevariationinthewatercontentduringtheMoon’spassage
throughthemagnetotail.However,thereisalackofM3
coverageforthe
entirecycleofsixlunarphaseperiods.Wefoundonlytenandtworepeat
observations among two and three of those six lunar phase periods,
respectively(Table1).Thus,weaveragedthewatercontentsfromdif-
ferentregionswhenassessinghowthewatervariesfromonetoanother
lunarphase.Thewatercontentofoneregioncanbeverydifferentfrom
anotherduetovariationsinthesurfaceopticalmaturity15
(forexample,
foralarge,young,impactregionversusatypical,mature,mareregion),
althoughthetworegionscouldbeinthesamelatitudezoneandwere
correctedforthelocal-timeeffect.Forinstance,assumethatthereare
four regions (1, 2, 3 and 4) in lunar phase A and that their mean water
contentsare35,38,41and37 ppm,respectively.However,inlunarphase
B,onlyregions1,2and3arethesameasthoseinlunarphaseAandtheir
watercontentsare48,49and52.AnewregioninlunarphaseBisfrom
a giant, young impact (for example, Tycho) and the water content in
this region is only 24 ppm. We can see that the increase of the water
content in all regions 1–3 exceeds our assumed mapping uncertainty
of10 ppm.Themeansandstandarddeviationsofthewatercontentsin
lunarphasesAandBare30 ± 3 ppmand43 ± 13 ppm,respectively.The
increaseofthemeanwatercontentis13 ppm,whichisgreaterthanthe
mappinguncertainty.However,thestandarddeviationforlunarphase
Bisgreaterthanthemappinguncertainty,whichisstronglyskewedby
theheterogeneityofthewatercontentinthenewregion,whichhasno
repeat observations. This simplified case explains why we could not
rigorously calculate statistics for the binned water contents of those
12regionsinFig.3and,thus,assesswhetherthevariationofthewater
content from one lunar phase to another is significant. However, any
variation of the water content greater than the mapping uncertainty
canbedetectedbytheM3
instrument.
Dataavailability
The Moon Mineralogy Mapper L1B data are available at https://
pds-imaging.jpl.nasa.gov/volumes/m3.html.Thederivedwatermaps
and their associated local time and lunar phases are archived to the
NASAPlanetaryDataSystemCartographyandImagingSciencesnode
athttps://doi.org/10.17189/gmce-w279.
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Acknowledgements
S.L. acknowledges the support of the Lunar Data Analysis programme
(Grant No. 80NSSC19K0369). A.R.P. was supported by the NASA
SSERVI LEADER team (Grant No. 80NSSC20M0060). We acknowledge
NASA contract NAS5-02099 and V. Angelopoulos for use of data
from the THEMIS-ARTEMIS Mission, specifically C.W. Carlson and J.P.
McFadden for use of ESA data. B. Jones and T. Orlando were supported
by the NASA Solar System Exploration Research Virtual Institute
(SSERVI) under cooperative agreement number 80ARC017M0007
(REVEALS).
Authorcontributions
S.L. conceived the project and performed the data analysis and
interpretation. A.R.P. prepared the ARTEMIS data. A.R.P., T.M.O., B.M.J.,
O.J.T., W.M.F. and A.R.H. contributed to the data analysis. T.M.O. and
B.M.J. contributed to the discussion of the role of electrons on the
formation of water. O.J.T. and W.M.F. contributed to the discussion
of the diffusion of the implanted solar wind hydrogen. S.L. wrote the
paper. All coauthors read, commented and agreed on the submitted
paper.
Competinginterests
The authors declare no competing interests.
Additionalinformation
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material available at https://doi.org/10.1038/s41550-023-02081-y.
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