GEOPHYSICAL RESEARCH LETTERS, VOL. 18, NO. 11, PAGES 2117-2120, NOVEMBER 1991
LARGE
SCALE LUNAR
HORIZON
GLOW
AND A HIGH
ALTYIIJDE
LUNAR
DUST EXOSPHERE
HerbertA. Zook andJamesE. McCoy
SN3, NASA JohnsonSpaceCenter
Abstract.All threeApollo 17 astronautssketcheda "lunar
horizon glow," seen from orbit above the Moon. It is shown
that the shapeof the glow is compatiblewith scatteringof sunlight off of gasor dustat high altitudesabovethe Moon. Our
mathematical modeling best simulatesthe glow with submicron dust grainswhosespatialdensityvaries with altitude
abovethe Moon as exp(-H/Hs), whereHs is in the range of 5
to 20 km. Thesedustgrainsare probablyelectricallycharged
andejectedabovethe lunarsurfaceby localelectricfields.
CDR SOLAR CORON•,•K
Introduction
The Moon, it was assumed,shouldprovide an ideal shield
againstthe bright sun and permit photographsto be takenof
the coronaand zodiacallight (CZL) at smallelongationangles
to the Sun. These could be takenwhen the Apollo Command
Module, during its orbit, was occultedfrom the Sun by the
Moon. One would not need to worry abouttrying to separate
out a "horizonglow," suchas must be done when in orbit
aboutthe Earth, as the Moon's atmospherewas consideredto
be negligible.To this end, a vigorouscampaignof CZL photographywas carried out from lunar orbit with both 35 mm
cameras[Mercer et al., 1973] and 70 mm cameras[MacQueen
et al., 1973], using high speedKodak 2485 film.
In additionto photographs,all three Apollo 17 astronauts
(E. A. Cernan, H. H. Schmitt, and R. E. Evans) also sketched
CZL andotherdim light features.Figure 1 is a reproductionof
five sketchesby Cernanfrom the Apollo CommandModule
showingCZL and other featuresand their developmentwith
time beforeorbitalsunrise.Besidesthe CZL centralbulge,two
other featuresare shown:Straight line "streamers"that first
appearabout2 minutesbefore orbital sunrise,and a "horizon
Fig. 1.Fivesketches
drawnbyE. A. Ceman(Commander)
of
glow" that first appearsas a "shoulder"on the CZL about 3
sunrise
as viewedfromlunarorbitduringthe Apollo17
minutesbeforeorbitalsunriseandthat spreadswith time along
mission.
Thetimesin minutes(i.e. T-6, T-3, T-2, andT-l)
the lunar horizon. A sketch by Evans [see McCoy and
and seconds(i.e. T-5 sec) refer to the time before first
Criswell, 1974] suggestsspreadingof 30ø to 40ø either sideof
appearanceof the sun.
the centralbulge.
Although the streamers were initially thought to be
Moon at an altitudeh = 110 km. The Z-axis pointsin the antiextensionsof the K-corona, McCoy and Criswell [1974]
established
that they were of near-lunarorigin and suggested sundirection.The origin of the primed systemis placedat the
that the "rayedpattern"was causedby mountainsat the lunar
locationof the observer(in the spacecraft),
andthe Z' andX'
terminatorcastingshadowson sunlightscatteredfrom high
axeshavebeenrotatedby anangle90-oiabouttheY-axis,so
altitude lunar dust. McCoy [1976] later establishedthat a
that the sunand the spacecraftbothlie in the X'Z' and the XZ
considerablefraction of the light in the presumedCZL bulge
planesand Z' is the local vertical. The "line of sight"has an
was sometimesof local origin, and he arguedthat this same
elevationangle• with respectto the X'Y' plane,and its
dustcloudalso causedthe additional"CZL." In this paperwe
projectionin the X'Y' planeis at angleq•(azimuthangle)with
turnto an analysisof the lunarhorizonglow;we ignorefor
now the streamers and the "excess" CZL.
respectto the X'-axis. The lunarhorizonis at • = •o = -19.9ø
Analysis
Figure2 defines
thelightscattering
geometry
for an
andthe sunis at • = -tz andq•= 0.
Considersunlightscatteredoff of dustgrainsof radiuss, of
spatialdensityn(H), at heightH abovethe lunar surface,and
observerin a spacecraftorbiting over the dark side of the
at alldistances
A alongthelineof sight.It is scattered
toan
Thispaperis not subjectto U.S. copyright.Publishedin 1991by theAmericanGeophysical
Union.
observeron the spacecraft
locatedat distanceRm + h alongthe
Z' axis. AlthoughA andH are not shownin Figure 2, they are
easilyobtainedand are relatedby
H = [A2+ (Rm+h)2
+ 2(Rm+h)Asin/lt]0.5
_Rm,
Papernumber91GL02235
2117
(1)
2118
Zook andMcCoy:LunarHorizonGlow andDust
We also have that
X 2+ y2 = R2
m
(7)
astheprojection
ofthe• vector
ontheXYplane
isjustequal
to theradiusof the lunar shadowcylinder.
Equation (7) resultsin a quadraticequationfor Am that is
givenby
Am= [-b + (b2-4ac)l/2]/(2a),
(8)
I
1%o-a
•
where
'---Z]1_
LUNAR
CYI.. I SH^DO"
NDER
+2
sin0;
cos0;
sin•tcos•tcos•)+cos2•
sin2•),
a=sin
20;
COS2Xl/COS2•)
+COS20;
sin
211/
b=
2(Rm+
h)(cos0;sin0;
cos•cos•)+
sin•t),
I(9)
c= (Rm+h)2cos20;R2
m'
In equations(8) and(9) all quantitiesareknown,or assumed,
SUN
-
•
as0 and• arethelocalazimuth
andelevation
lookdirections
ANTISUN
•Fig,:2.Geometryand anglesusedfor observations
from orbit.
where Rm is the radius of the Moon. (Variables shown
withouta vectorsymbolrepresent
the scalarmagnitudeof that
vector.)Am is the smallestvalue of A thatis in sunlight.The
brightness
of sunlightscattered
by all dustparticlesalongA is
thengivenby
B(0)
=Io(xs2)fs(0)
•Amn(H)dA,
(2)
whereIo is the solarintensityat 1 AU andrs(0) is the fraction
of interceptedlight that is scatteredby grainsof radiuss per
steradianin direction0. The angle0 throughwhichsunlightis
scattered remains essentially constant throughout the
integrationalongA. 0 is obtainedfrom
cos0- cos0;cos• cos0- sin0;sin•,
(3)
and it is assumedthat n(H) = noexp(-H/Hs),where Hs is the
assumed
exponentialscaleheightof thedust.We first obtain
Am,the initial value of A in the integralin Eq. (2).
Thevector
7•misgiven
by
7•m= Am(•'cos•cos
0 + •' cos•sint•
+ •c'sin•),(4)
where1•, •, and1•areunitvectors
along
theX', Y', andZ'
axes.
Thevector• (fromthecenter
oftheMoontotheend
point
ofthevector
7•m)canbewritten
^
^
,
i$,=•X+jY+kZ = Rm+h)+7•
m.
(5)
Expressing
Amin unprimedcoordinates,
we obtain
X = (R m+ h)cos0;
+ Am(Sin0;
coswco• + cos0;sin•t),
Y = AmcosWSin0,
Z = (R m+ h)sin0;
+ A m(Sin0;
sinw-cos0;cos•t cos9).
(6)
relative to the X' "up sun" direction, and 0; is the angle
between the radius vector to the spacecraftand the local
terminator. Each look direction given by (0,•) maps to a
locationon a photographor, approximately,to a locationin
one of the sketchesin Figure 1. When 0; = 20.1%the solar
edgejustappears
overthehorizon(at•o = - 19.9ø).
Equation(2) is numericallyintegratedfor eachof several
values of the scale height Hs and angle 0;, as well as many
valuesof 0 and•. The resultsare shownin Figure3 for Hs =
10 km, 0; = 23, 26, 29, and 32 degrees (solar depression
angle= •o + 0;). What is shownare isophotesof calculated
brightness
(in arbitraryunits)for eachof 4 spacecraft
locations
from 1 minuteto 4 minutesbeforeorbital sunrise(the Apollo
spacecrafttravelstoward the terminatorat about3ø per
minute).Anglesabovethe horizonare plottedon the vertical
scalein eachcase(0øcorresponds
to •o = -19.9ø) andazimuth
angles are plotted horizontally along the lunar horizon (0
angles).For theseplots, we use fs(0) - 1/(4•). The actual
scattering
functionrs(0)will dependuponwhethergasor dust
is doing the scatteringand, if dust, on its grain size and
composition. As this information is not yet known, an
isotropicscatteringfunctionrepresentsa first step,and the
easiestfor thereaderto adjustto any otherscatteringfunction.
It is seen at once that there is a striking resemblance
between Cernan's sketches and our model results, if one
allowsfor the fact that lunar curvatureand the CZL bulge are
not shown in Figure 3. One notes a very steep fall-off of
intensity with elevation angle above the surface.When we
modeledvariousshapesof contaminantcloudsthat might be
generated
by the spacecraft
in orbit,noneof thesecloudscould
be madeto resembleanythinglike the horizonglow sketched
by the astronauts.Thus there is no doubt that this glow is
indigenousto theMoon itself.
EstimatedBrightness
It is difficult to accuratelyestimatethe absolutebrightness
of the lunar horizonglow from the astronautsketches,primar-
ZookandMcCoy:LunarHorizonGlowandDust
Modeled
Lunar Horizon
2119
Glow
Scale Height = 10 km
e•'5
•
xxx•.•.•.•.
•.•.• •
r''"" '" *"'
"••::::••:<•::••
•''" *'"" ' ' ''
"•-••<<•-•••••
•0
.................•
•
•lar
d•ress•n
ang•
=3•r•s•-1)•
•'•:-••:<<-•••••:-•<<-'•••• .................y ................•................•••••••••:•
• 5•
10
•1•d•r•n •g•=6•r• (T-2)
•
• 5•
10
•1=d•r•ion
•g•=9•r•sif-3)
•5
• •
10
Sol•
d•ression
•gle- 12degrees
(T4)•
•
m
50
40
30
•
10
Angle alo•
0 .........
•'5............. '• ..............55..............'• ...............•5............ 60
horizon (degrees)
Fig.3. Themathematicflly
m•elled brightnesses
(in •bitra• units)of the1• horizonglowthatshouldbeobse•edfrom
lun• orbitat 1, 2, 3, •d 4 •nutes beforeorbitflsu•se. •e ve•ic• •is is theangleabovethehorizonandthehofizon• a•s
isthea•muthfl•gle in eachcase(seetext).•e c•ed lines=e cflculat• isophmes.
ily becausewe aren't sure of the scale on the sketches.In a
sketchby Schmitt[McCoy and Criswell, 1974] the planet
Jupiteris shownnear25ø elongationfrom the sun.From this
sketch,which alsodepictsthe horizonglow,we estimatethat
the astronauts
havesketchedthe CZL bulgeout to 15ø to 20ø
elongation.AcceptedCZL brightnesses
at thoseelongations
wouldbe 10-12Boto 2 x 10-12Bo
[MacQueen
et al., 1973],
where BO is the mean brightnessof the sun. If, further, we
assumethat Cernan'ssketchesdepict isophotesof CZL and
horizonglow brightness,then the horizonglow would be
aboutthatsamebrightness.
No evidenceof "horizonglow", however,has yet been
detected
in the70 mm and35 mm photography.
Accordingto
MacQueenet al. [ 1973] the 10 secondexposures
shouldhave
recordedall brightnesses
above2.1 x 10-12Bo.
We have
T-1 minutes.With this slow change,it would appearthat one
shouldexpect to see the glow before T-3. This is not observed.Also, with a 50 km scaleheight,the brightness,at an
elevation angle above the lunar limb of 12ø, would only be
about a factor of 4 less than that at the limb. Such a horizon
glow would probablyappearto be rather "thicker"than the
glow that was sketched.According to Potter and Morgan
[ 1988a, 1988b], the sodiumvaporbrightnessin the lunarexo-
sphere
isabout3.8kilorayleighs
(9.2x104photons/cm2deg2s)
just abovethe lunar limb and has scaleheightsof about 120
and 500 km. The potassiumbrightnessis 1.8 kilorayleighs
with a scaleheightof 90 km. Thus the sodiumandpotassium
scaleheightsdo not seemto resultin goodqualitativefits to
the sketches.
Second,
3.8kilorayleighs
is probably
belowthethreshold
reexamined the Kodak 2485 film calibration uncertainties and
of visibility of the unaidedeye. For very dim light, the wavesuspectthat the absolutephotographicthresholdbrightness lengthat maximumsensitivityof theeye shiftstowardtheblue
couldbe up to 4 timeshigher.
(Purkinje Shift) with the peak sensitivityoccuringat about
Severnyet al. [1975] measureda vertical brightnessof
0.51 gm [Linksz, 1952]. The sensitivity for the sodium D
9900 S10units (tenthmagnitudesolartype starsper square lines is about 10% of the peak. The eye is even lesssensitive
degree)from the Lunokhod2 lunar rover just after sunset, to potassiumlight. Therefore,bothfor reasonsof scaleheight
whenthe sunwas aboutonedegreebelow the horizon.9900
and glow intensity,gasdoesnot seemlikely to be the source
S10unitscorresponds
to a verticalbrightness
of 4.5x10-12Bo. of "horizon glow." This leaves sunlightscatteredfrom exoBecausethe near-horizonbrightness,as observedfrom an
sphericdustgrainsas the probablecauseof horizonglow.
orbiting spacecraft, should be several times the vertical
We can imagine two processesthat could put dust onto
brightness,
theSevernydatais muchbrighterandin apparent trajectoriesabovethe lunar surface:Impact ejectionof lunar
contradiction
withtheorbitalphotographic
data.It is possible grainsby meteoroids,andelectrostatic
ejectionof chargeddust
that either the Apollo photographicor the Lunokhod
grainsfrom a like-chargedlunarsurface.We stateherewithout
photometer
calibrations
arein error.Anotherpossibility
is that proofthatthe meteoroidfluxesgivenin Zook et al. [1970] and
thehorizonglowbrightness
variesfromplaceto place.For
in Grtin et al. (1985) do not appearat all adequateto eject
these reasons we remain somewhat uncertain about the true
enoughlunar grainshigh enoughto producethe observed
brightnesses
sketchedby theastronauts.
scatteredfight.
That leaves electrostatic ejection of dust to consider.
Gas versus Dust
What causes the horizon glow? Dust or gas? Like
Kozlowskiet al. [1990],oneof us(HAZ) initiallythought
that
sodiumgasmightbecausing
theglow,andstarted
modelling
on that basis. However we now believe that it is dust.
First, we get a best"qualitative"matchwith the astronaut
sketches
witha scaleheightHsof about10km,although
scale
heights
of 5 to 20 km couldalsofit thethesketched
shape.
Witha scaleheightof 50 km,thebrightness
nearthehorizon
is calculatedto be only abouta factorof two lessat T-3 thanat
Criswell
[1972]andRennilson
andCriswell
[1974]argue
that
only electrostatictransportof lunardustgrainscouldaccount
for the "horizonglow" observedwith the Surveyorcameras
after sunset.Although the scale height for the Surveyor
horizon glow was deduced to be only several tens of
centimeters,the Surveyordata provide strongevidencefor
electrostatictransportof duston the Moon. It is possiblethat
the main differencebetweenthe horizonglow sketchedby the
Apollo astronauts
and thatrecordedby Surveyoris the sizeof
the lunar grains involved--giving rise to different scale
heights. The samelocal fields requiredto acceleratelarge
2120
ZookandMcCoy:LunarHorizonGlowandDust
grainsto 10 cm would acceleratesmallergrainsto muchhigher
velocities,launchingtheminto 10 km ballistictrajectories.
Berg et al. [1974; 1976] found that the dust particle flux
strikingthe Apollo 17 LEAM (Lunar Ejecta And Meteoroid)
experimentincreaseddramaticallysometensof hoursbefore
sunrise.They argued that this was due to electrostatically
transported dust. Finally, Page and Carruthers [1978]
attributedareasof scattering
of far ultravioletlight in Apollo 16
cameraphotosto lunardustelectrostatically
suspended
above
Berg, O.E., H. Wolf, and J. Rhee,Lunar soil movement
registeredby theApollo 17 cosmicdustexperiment.,
in:InterplanetaryDustandZodiacalLight (H. Els•isser
andH. Fechtig,eds.), Springer-Verlag,New York,
233-237, 1976.
Criswell, D.R., Lunar dust motion, in: Proc. 3rd Lunar Sci.
Conf., The MIT Press,Cambridge,MA, 2671-2680, 1972.
Freeman, J.W. and M. Ibrahim, Lunar electric fields,
surfacepotentialandassociated
plasmasheaths,
The
Moon 14, 103-114, 1975.
the surface.
Consideringthe expectedsolarwind interactionsdiscussed
by Siscoe and Goldstein [1973], and the surfacepotential
measurements
of FreemanandIbrahim [ 1975], it appearsto us
that, for electrostaticforcesto eject dust grainsto altitudesin
excessof 10 km, the dustgrainsare probablysmallerthan0.1
gm in radius. Such small grains should scatterlight with a
Rayleigh
scattering
function,
whichbrings
a (1+ cos20)
factor
into fs(0). This factorwould only slightlymodify the shapeof
the isophotesdepictedin Figure 3. Such a small grain size
could make it possibleto discernthis "horizon glow" from
Earth-basedtelescopes.
Near 0 = 90ø (nearquarterMoon), the
light scatteredby the dustshouldbe nearly 100% polarized.It
shouldalso be very blue comparedto moonlight.These two
characteristics
may make it feasibleto separatethis light from
the background earthshine on the Moon and from
atmosphericallyscatteredlight from the bright part of the
Moon. Similarlyfor light scattered
insidethetelescope.
Grtin, E., H.A. Zook, H. Fechtig, and R.H. Giese,
Collisionalbalanceof themeteoriticcomplex,Icarus
62, 244-272, 1985.
Kozlowski, R.W.H., A.L. Sprague,and D.M. Hunten,
Observations of Potassium in the tenuous lunar
atmosphere,Geophys.Res. Lett. 17, 2253-2256,
1990.
Linksz, A., Vision: Physiologyof the Eye, Vol. 2, Grune &
Stratton, New York, 1952.
MacQueen,R.M., C.L. Ross,and T.K. Mattingly,
Observations
fromspaceof thesolarcorona/inner
zodiacal
light,Planet.SpaceSci.21, 2173-2179,1973.
McCoy,J.E.andD.R. Criswell,Evidencefor a highaltitude
distributionof lunar dust, in: Proc. 5th Lunar Conf.,
PergamonPress,New York, 2991-3005,1974.
McCoy,J.E.,Photometric
studiesof light scattering
abovethe
lunarterminatorfrom Apollo solarcoronaphotography,
in:
Proc. 7th Lunar Sci. Conf., PergamonPress,New York,
1087-1112, 1976.
Conclusions
Our primaryconclusions
arethefollowing:
1) The lunar horizon glow sketched by the astronauts
representslight scatteredby a lunar exospherewith a scale
heightof about10 km.
2) The wide extent of the horizon glow in azimuth around
the lunar horizonindicatesthat the light scatterers
do not have
a strongforward scatteringcomponent.
3) Thelightscatterers
areconsistent
withsmall(less0.1 gm
in radius) dust grains that are chargedand are electrically
ejectedfrom the lunarsurface.
4) The lunarhorizonglow may be detectablefrom terrestrial
telescopes.
Acknowledgements. We thank the following: The
astronautsfor their sketches (which depicted phenomena
recordedin no otherway) and for valuablediscussions;
Alan
Sternfor suggestingin a lecturethat the lunar sodiumglow
might be seenwith the unaidedeye; FranciscoCapella for
notingan errorin our first versionof Figure2.
References
Berg, O.E., F.F. Richardson,J.W. Rhee, and S. Auer,
Preliminaryresultsof a cosmicdustexperimenton the
Moon, Geophys.Res. Lett. 1, 289-290, 1974.
Mercer, R.D., L. Dunkelman, and R.E. Evans, Zodiacal light
photography,
in: Apollo17 PreliminarvScienceReport,
NASA SP-330,pp. 34-1 to 34-4, 1973.
Page,T. andG.R. Carruthers,
S201far ultravioletatlasof the
largemagellaniccloud,NRL Report8206, 1978.
Potter,A.E. andT.H. Morgan,Discoveryof Sodiumand
Potassium
vaporin theatmosphere
oftheMoon,Science
241, 675-680, 1988a.
Potter,A.E. andT.H. Morgan,ExtendedSodiumexosphere
of the Moon, Geophys.Res. Lett. 15, 1515-1518, 1988b.
Rennilson,J.J.andD.R. Criswell, Surveyorobservations
of
lunarhorizonglow, The Moon 10, 121-142, 1974.
Severny,A.B., E.I. Terez, andA.M. Zvereva,The
measurements
of skybrightness
on Lunokhod-2,The
Moon 14, 123-128, 1975.
Siscoe, G.L. and B. Goldstein, Solar wind interactionswith
lunarmagneticfields,J. Geophys.Res.78, 6741-6748,
1973.
Zook, H.A., R.E. Flaherty, and D.J. Kessler,Meteoroid
impactson theGeminiwindows,Planet.SpaceSci. 18,
953-964, 1970.
Herbert A. Zook and JamesE. McCoy, SN3, NASA
JohnsonSpaceCenter, Houston,Texas77058.
(Received June 3, 1991;
AcceptedAugust6, 1991)