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1972LPSC....3.2671C

Proceedings of the Third Lunar Science Conference (Supplement 3, Geochimica et Cosmochimica Acta)

Vol. 3, pp. 2671-2680 The M.I.T. Press, 1972

Lunar dust motion

DAVID R. CRISWELL The Lunar Science Institute,

3303 NASA Road 1, Houston, Texas 77058

Abstract-Surveyor 7 photographed a bright glow along the western lunar horizon one hour after local sunset. This horizon glow must result from the forward scattering of sunlight by a swarm of dust grains extending 3 to 30 centimeters above the local horizon with a column density of 5 grains/ cm2 • The observable glow is evoked by grains approximately 6 microns in radius. An annual churning rate of 10- 3 gr/cm2 is implied. A model is presented for dust cloud production in terms of electrostatic levitation of lunar surface fines.

HORIZON-GLOW

FIGURE 1, a composite of one daytime (lower portion) and two postsunset (upper portion) photographs of the lunar horizon 200 m west of the Surveyor 7 spacecraft, clearly shows a bright strip of light referred to as horizon glow (HG). This example of HG extended 2° to 3° on each side of the sunset line, was much brighter than the solar corona, and persisted for 90 min after local sunset (Gault et al., 1968a, b; Rennilson, 1968). This HG silhouetted the rocks and surface irregularities that con-stituted the horizon (Allen, 1968). The HG could be mapped to a scale of 3-4 cm, which was the Surveyor 7 television resolution at 200 m. Horizon-HG vertical separation was introduced in the preparation of Fig. 1. Polarization was not apparent in the analog photography (Shoemaker et al., 1968). An extended analysis of the Surveyor images will be presented by Criswell and Rennilson (1972).

Figure 2 is a digitized representation of section (a) in Fig. 1. Each number (PV) is proportional to the image photometric brightness (B) at the corresponding location (pixel) on the television photocathode. Extensive preflight calibrations (Rennilson, 1971) established B (foot-lamberts) = 3.364 x (PV) for the normal operating mode (0.150 sec exposure) of the camera as digitized in this observation. In cgs units, B (candles/cm2 ) = 1.14 x 10- 3 x (PV).

HG has a clearly resolved vertical extent of 3 to 30 cm (approximately 1 to 7 pixels) which is not a systems artifact due to spread of a bright image. This is demon-strated by the insert which is a digitized segment of a Surveyor 7 photograph (time exposure of 3 sec) of an earth-based laser (Alley and Currey, 1968). The central numbers (63's) indicate saturation of the photocathode. However, the PV values decrease to at least 50% of maximum ( < 32) within one to two pixels of the saturated pixels. Conversely, the HG does not saturate the television system and is in many places 4 to 7 pixels in width with constant brightness (PV 10-19). Three to 30 cm vertical extensions rule out scattering by surface grains (Gault et al., 1968a), residual gas molecules (Rozenberg, 1970), and secondary meteorite ejecta (Gault et al., 1963) as sources.

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1972LPSC....3.2671C

2672

Obscured Solar Disk

(a)

DAVID R. CRISWELL

Fig. 1. Composite photograph of horizon west of Surveyor 7 (6.4° angular width). Slight horizontal scale differences are present in the composites. However, note that the horizon glow (upper bright lines) faithfully silhouettes the horizon features. The dashed line indicates the approximate apparent path of the sun. HG is occurring along two different ridges. The HG gap is the shadow zone of a background ridge (right) on the foreground ridge. The upper limb of the sun is approximately 1.3 solar diameters

beneath the horizon as indicated by the black circle.

The proposed source mechanism is presented in Fig. 3. Electrically charged dust grains are levitated into the sunlight by an electrostatic field. The field is induced about partially illuminated rocks due to the ejection of photoelectrons by soft solar x-rays and the subsequent accretion of a portion of these photoelectrons in adjacent dark areas. The grains scatter sunlight to the shadowed Surveyor spacecraft. Large sphere diffraction applies producing a brightness described by (Van de Hulst, 1957)

B (candles/cm2) = ND x 2a2 (2J1(x sin 0))2

I0 (lm/cm2) 4 x sin 0

where I0 = 13.7 lm/cm2 (solar intensity in cgs visual units); a 1'I grain radius (cm); N 1'I # scatters/cm3 ; D 1'I cloud depth along line of sight (cm); A 0.5 x 10- 4 cm (wavelength for peak response of Surveyor television system); x = 2na/ A; 0 1'I angle between sun-Surveyor and sun-HG lines; and 1 1 1'/ first-order Bessel function with 11 (3.832) = 0. Assuming HG brightness approaches zero at 11 (3.832) and 0 = 3°, we have a = 6 x 10- 4 cm = 6µ. Other particles are certainly present. However, B 1'I a4 ; thus smaller particles scatter much less light which Surveyor can detect. Conversely, larger particles have diffraction limits 0 < 3° and will not contribute scattered light at 0 = 3°.

Using 0 I O and PV = 18 for the brightness at that angle away from the sun, we get

N (#/cm3) x D (cm) = 4.7 grains/cm2



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Lunar dust motion 2673

3 3 3 3 4 I 5 9 16 18 19 19 15 5) 0 11 12 17 23 18 23 13 19 20 15 23 3 3 4 4 4 J 6 2 18 19 19 19 12 /1 0 12 13 12 18 9 23 26 18 23 18 22 3 3 3 4 r~ 9 16 18 18 19 18 10 1 0 0 16 13 12 13 17 24 24 15 20 16 16 3 4 3 3

' 7 3 18 18 18 18 19 1 / 2 0

18 23 15 15 20 r n1±_6 21 20 18 19 3 3 3 3 ( 5 12 17 17 17 18 17 81 1 0 16 30 I 59 4 77 11 11 16 19 3 4 3

;;,, 17 15 15 3 '-5 13 17 17 18 17 10 /1 1 2 17 26 17 14[38!63 63!30 13 17 11 3 3 3 3 4) 14 16 17 17 12 1 2 1 1 20 18 25 14 20 25 23 12 11 18 12 3 2 3 3 rs 5 10 15 16 17 13{ 3 0 1 11 15 12 19 13 10 20 2 21 21 15 3 3 3 3 / 5 8 14 17 17 16 91 1 1 2 14 15 13 14 11 12 8 17 13 11 11 3 3 3 y 5 10 16 17 16 15 8 I 0 0 2 11 1 11 10 18 13 12 13 15 14 8 3 3 3 /5 7 12 16 16 16 15 7/ 0 1 2

5 10 4 6 13 13 15 9 15 20 9 3 3 (s 1 11 16 17 17 16 14 /3 0 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 5 9 15 17 16 17 16 1 'I 2 0 2 3 2 3 3 3 3 3 3 3 3 3 3 3 3 I 5 9 16 15 16 16 13 1'1 1 1 2 3 3 2 2 3 3 3 2 3 3 3 3 3 4./ 7 1 15 15 16 15 9 I 1 1 2 2 3 2 2 3 2 2 3 3 3 2 3 3 3 (5 7 14 15 15 15 15 6 / 0 0 2 2 2 -:,, 3 3 2 2 3 2 2 2 2 3 3 3 8 14 15 15 15 14 /4 0 1 1 1 3 2 2 2 2 3 3 3 3 3 3 3 3 4 \ 9 14 14 14 14 9 Io 0 1 2 2 2 3 2 2 2 2 3 3 3 3 2 3 3 3 '{ 13 15 14 14 1 / 1 0 1 2 2 3 3 2 2 2 3 2 2 3 3 3 3 3 3 3) 7 2 14 14 10 3 1 2 2 3 3 2 2 2 3 3 3 3 3 3 2 3 3 3 4 5 7 9 9 61 3 3 3 3 3 3 3 3 3 2 2 3 3 3 3 3 3 3 4_,,{ 7 9 10 9 3 3 3 3 3 3 3 2 3 2 3 3 3 3 3 3 3 3 /5 7 11 14 14 9 /4 1 3 3 3 3 3 3 3 2 3 3 2 2 3 3 3 3 !, 5 0 14 15 14 12 I 4 1 1 3 3 3 3 3 3 3 2 2 2 3 3 3 3 3 /5 9 14 15 14 14 8 / 1 1 2 2 3 3 3 3 3 3 3 3 3 3 2 3 4 3 I 5 14 14 14 13 5j o 1 2 3 2 3 3 2 2 3 2 2 3 3 2 3 3 3 I 1 13 14 14 14 8 /1 0 1 2 2 2 3 3 3 3 3 3 2 3 3 3 3 3 3} 1 14 14 14 14 8 Io 0 2 3 2 2 3 3 I 2 2 2 2 3 2 2 2 3 4 3/ 5 10 14 14 1 6/ 0 1 2 2 3 2 3 3 3 2 3 3 2 2 3 3 4 /67 5 5 10 12 8....,..,,....3 2 2 2 2 3 3 3 2 3 3 3 2 3 2 3 3 3 I B~ /2 3 -3 3 3 3 3 3 3 3 3 3 3 2 2 3 3 2 3 3 4 \ 6 12 11 / 3 2 3 3 3 3 3 3 3 3 3 3 3 3 3 2 3 2 3 3 3 3 }) 5 5/ 4 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 4 .. /5-J/_,.J-4 4 3 3 3 3 3 3 3 3 3 3 3 3 3 2 3 3 3 3 /"5-6 7 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 2 3 3 3 3 3 I 5 9 9 5\ 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 I 5 9@ 5 J 1 2 3 3 3 3 3 3 3 2 3 3 3 3 3 2 2 2 3 3 4) 5 6 6 5/ 3 2 2 3 3 3 3 3 3 3 3 3 3 3 3 2 3 3 3 /56 8 7 5 /3 3 3 3 3 2 3 2 3 3 3 3 3 3 3 3 3 3 3 3 \6 11 14 10 5 I 2 2 3 3 2 2 3 3 2 3 3 2 3 2 3 3 2 3 3 3 4 11 14 13 1 I o 1 3 3 2 3 3 3 3 3 3 3 3 3 3 3 3 2 2 3 4 10 13 13 7 / 2 2 3 3 3 3 3 2 3 3 3 3 2 3 3 3 3 3 2 3 3 4 ,5 _ _§__~ 4 3 3 3 3 3 3 3 3 3 3 3 3 3 3 2 3 3 3 3 3 3 3 4 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 2 3 3 3 3 3 4 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 2 3 3 3 3 4 3 3 3 3 3 3 3 2 3 3 3 3 3 3 3 3 3 3

Fig. 2. Digitized representation of area (a) in Fig. 1 with the digital representation rotated approximately 60° counterclockwise from the photograph. Zero is black and sixty-three is white. The insert is the digitized segment of an earlier Surveyor 7 photograph of a laser signal which saturated (63 and 63) two pixels. Different calibration coefficients

apply to the PV values of the HG and the insert.

Assuming D d we have N (#/cm3) 0.16 (d == 30 cm) to 1.6 (d == 3 cm). The

corresponding column mass density is Mc 1.3 x 10- 8 gr/cm2 assuming D == d. An approximate columnar mass flow rate of Mc 2 x 10-s gr/cm2 sec is obtained by assuming the grains accelerate at 10% lunar gravity (gz), d == IO cm, Mc M/2/t), and t == (2d/. lgz)½. HG persists the order of 5.4 x 103 sec, which implies a total churning per sunset of 10- 4 gr/cm2 and per yr of 10- 3 gr/cm2 • An annual churning depth de 6 x 10- 4 cm or 6 microns/yr is implied for a soil bulk density of 1.5 gr/cm3 , where 50% porosity is assumed.



1972LPSC....3.2671C

2674 DAVID R. CRISWELL

DARK REGION

REFLECTION AREA

Fig. 3. Levitation of a dust grain about a partially illuminated rock located in the sunset terminator results in forward scattering sunlight to the completely shadowed Surveyor spacecraft. Large sphere diffraction describes the scattering. The distance D is the HG depth along the TV line of sight (LOS); d is the vertical height of the HG.

Surface bombardment by 10- 5 gr micrometeoroids (Hartung et al., 1972) at 6 x 10- 17 gr/cm2 sec (Dohnanyi, 1971) producing 6 x 10- 12 primary impacts/cm2

sec will eject approximately 2 x 10-s secondaries/cm2 sec in the 3-8 micron range (Braslau, 1970; Gold et al., 1971). This is 105 times too low a secondary production rate to evoke HG. The production rate of tertiary ejecta is not known; however, tertiary particles should not be present in significant quantities due to the energy dissipation in primary and secondary impacts.

Shoemaker et al. (1970) predict 102 to 103 yr turn over times for the top 10 µ of regolith due to micrometeorites and all their ejecta. This is 50 to 500 times slower than required for HG production.

LEVITATION CONDITION

Levitation will occur when the electrical force on loose, charged grains exceeds the gravitation force,

4 QE > - na3 pg1 3

where the mass density oflunar grains p 3 gr/cm3, Q = (na2 )[(5.5 x 105 electron/ cm2) x (E)] is the charge on a grain of radius a, and Eis the surface electric field in volts/cm. It is assumed the surface electrical charge is uniformly distributed over small



1972LPSC....3.2671C


areas ( < 1 cm) and that a given grain simply acquires a net charge proportional to its circular cross-sectional area. Combining these equations we obtain the levitating field strength

E (volts/cm) 270[ a(µ)J½

A dust grain of a = 5 µ will levitate in a field of 606 volts/cm. A charge imbalance of 262 electrons will be present on the grain. Levitation of a wide range of dust sizes will occur due to the square-root dependence of Eon a(µ). The 5 µ grains will transport bound electrons at the rate Q 2100 electrons/cm2 sec between dark and light areas for the HG observed by Surveyor 7.

GENERATION OF THE ELECTRIC FIELD

Gold (1955), Heffner (1965), and Singer and Walker (1962) have suggested and examined various processes to evoke electrostatic transport of dust on the fully sunlit lunar surface. The suggested processes will not work because the pervasive dayside electric fields will be the order of a few volts/cm (Reasoner and Burke, 1972). The mechanism proposed in this paper operates only in the terminator zone. Reference to Fig. 3 will assist in following the explanation.

The rock is only partially illuminated due to shadowing by the western ridge. Soft solar x-rays of energy Bo > 500 eV evoke the ejection of photoelectrons (B 500 eV) at the rate}~ <l>(B0 > 500 eV) x Y(>B0 ) where <I> is the integral solar flux (photons/ cm2 sec) for Bo > 500 eV or A ;:5 25 A and Y(>B0 ) is the yield of photoelectrons with energies near B0 . Spicer (1971) estimates Y(>500 eV) 10- 2 for the observed elemental abundances of lunar material (Wink and Ojanpera, 1970). The value <1>(>500 eV) 5 x 107 photons/cm2 sec, while the HG was detected, was derived from data and analyses of Drake et al. (1969), Teske (1970), and Wende (1972). Thus,}(> 500 eV) 5 x 105 photoelectrons/cm2 sec seems a reasonable ejection rate from the sunlit rock surface.

Photoelectrons will escape the rock until a sufficient positive charge (q) remains to retain newly ejected photoelectrons in the vicinity of the rock. A fraction (/) of j will accrete on the dark areas of the rock and remain fixed due to the rock's low elec-trical conductivity.

A computer model was developed to trace the motion of monoenergetic electrons (B) sequentially ejected in random directions from randomly chosen ejection points on one hemisphere of an insulated sphere. The sphere was placed just above an infinite, nonconductive plane. Electrons moved under the influence of a smoothed, net positive charge on the ejection (sunlit) hemisphere and the isolated negative charges which had previously accreted on the opposite (dark) hemisphere and plane (lunar surface). The fraction/was calculated versus the equivalent electrical potential (U = q/4nB0 d) of a conductive sphere. Let U0 = B/e. It was found that U asymptotically approached 1.2 U0 and that/(U = U0 ) 0.05,f(U = I.I U0 ) 0.01, and/(U 1.2 U0 ) 0.

Applying these results to the previously calculated} we obtain an accretion current density jd 5-25 x 103 electrons/cm2 sec. A potential difference U l.1 U0 > 550 volts should be produced between the light and dark hemispheres. E U/d will be



1972LPSC....3.2671C


greatest near the light-dark boundary and should exceed 550 volts/cm for d 1 cm due to charge concentration near both sides of the boundary.

The expected charging rate exceeds the discharging rate ( 2 x 103 electrons/cm 2

sec) due to the motion of charged dust grains between the light and dark areas. Current densities in this process, the order of 10- 15 amps/cm2 , are exceedingly

small. The solar wind plasma, photoelectrons produced by scattered light, and con-duction current through the rock must be considered as discharging agents. Solar wind does not directly impact the sunset terminator due to the (1 °-4°) velocity abbera-tion of the wind resulting from the earth-moon system's orbital motion about the sun. In addition, the accretion of solar wind electrons, which is driven by the 10 e V thermal energy of the electrons, will build up a negative charge layer in dark areas, thus pre-venting the entry of further solar wind electrons. The 1 ke V solar wind protons cannot reach the dark negatively charged surfaces since these protons are traveling in essen-tially straight lines. The 10-15 e V plasma effects will not be significant because levi-tating electric fields are generated over scale lengths (d few em's) much smaller than the local Debye length(> 103 cm).

The monopole field will not occur if low energy photoelectrons can :flow in from the dark region just below the (Fig. 3) directly illuminated area. Such an electron :flow will replace escaping 500 to 1500 e V photoelectrons. An electron influx to the primary reflection area from the directly illuminated rock, westward sunlit areas, or the residual solar wind will complete the overall current loop in this neutralizing case. Neutralizing photoelectrons are evoked in greatest number by solar ultraviolet photons with ener-gies of approximately e0 when the product <l>(>e0) x Y(e0 ) is a maximum. Figure 3 helps one to visualize the light scattering pattern about the partially illuminated rock. Photons are scattered from the sunlit surface of the rock, to the primary reflection area in front of the rock, and then back to the dark region of the rock. Less than 2 x 10- 8 of the directly incident photons reach the dark region of the rock. This "attenua-tion" is calculated on the assumptions of lambert scattering at both surfaces, the top quarter face of the rock being illuminated, and dust and rock albedos of 0.01 in the mid-ultraviolet. Lebedinsky, et al. (1968) reported overall lunar backside albedos of 0.01-0.015 for 2200 A.

R < I (equation (1)) must occur for the discharging photoelectron flux to be negligible.

(A . ) [ Y(t:0 w)<l>(e0 w) ] R = ttenuabon

Y(e0 500 eV)<l>(e0 ;c; 500 eV) (1)

Feuerbacher et al. (1972) report Y(w = 12 eV) 5 x 10- 2 and Y(w = 5 eV) 10- 7 for dust samples. Using <1>(>500 eV) = 5 x 10+ 1 /cm2 sec, <1>(>12 eV) = 5.6 x 1010, <I>(> 5 eV) = 3 x 1014, and Y(> 500 eV) = 10- 2 , we get R(12 eV) 10-4

and R(5 eV) 10- 6 • Even these approximate estimates indicate that secondarily evoked photoelectrons will not discharge the sunlit surface.

Conduction current is the final discharging mechanism. The effective electrical conductivity (r,) of the rock must be r, < 10- 18/ohm-cm. This is consistent with the stability over 10 days of surface charges induced triboelectrically on the fresh surface oflunar rocks cleaved in a vacuum as observed by Grossman et al. (1970). Lunar rocks



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are heterogeneous assemblages of many small grains of differing mineralogies. The mineralogy indicates that water was not present when the rocks achieved their present state, a~d no water is present in the in situ rocks (Schmitt et al., 1970). Review of Parkhomenko (1967) indicates that, for the lunar rocks, surface charge will be trapped along the many interfaces between fused grains and mineral phases. This results in the creation of an internal polarization electric field rather than conduction currents.

Schwerer et al. (1971) have experimentally determined 11(~300°K) 10- 10 to 10- 9 /ohm-cm for an igneous rock and a microbreccia. These results are probably not applicable for two reasons. The samples were very thin ( 2 mm). High conductivity paths, such as an iron-rich phase, through the sample could produce a high apparent conductivity. These paths would not be connected over 1 to 20 cm lengths of the actual rock. More serious is permanent sample contamination and modification by trace levels of water during preparation of the thin sections and set-up of the experi-ment. In situ measurements may be necessary to determine 17. It should be noted that Chung and Westphal (this volume) using lunar samples obtained 17 10- 12/ohm-cm at 100 Hz for temperatures between 80°K and 250°K. Presumably the zero-frequency conductivity should be even lower.

DISCUSSION

The levitation mechanism will tend to preferentially remove dust from rocks and smooth the dust about rocks. Surface charge density, and therefore the levitation force, will be greater and more uniform on the sunlit than on surrounding dark areas because the negative charge can accrete over a larger dark area than exists on the sunlit rock surface. This is consistent with comments by Holt and Rennilson (1968) and Gold (1971 b) on the lack of dust layers on rocks, even those which protrude only a few centimeters above the soil. It is also consistent with the lack of impact scars in the soil about secondary ejecta. However, careful study of the lifetimes of rocks in known surface orientations will be necessary to confirm this contention.

The mechanism will tend to selectively levitate the smallest grains ( < 5 micron radii) from the topmost soil layer near crests and on illuminated slopes. Thus, these smallest grains will have- enhanced exposure to the higher energy solar wind ions and solar cosmic rays. Levitation is enhanced when the sun is active. This is consistent with observations of enhanced particle track densities(~ 1011 /cm2 ) in the outermost surface of micron-sized grains (Kreplin, 1970; Barber et al., 1971).

Bibring et al. (1972) contend the lunar surface albedo decreases with increased lunar surface coverage by these intensely track-damaged grains. Conversely, Holt and Rennilson (1968) note the undisturbed lunar surface about Surveyor 7 is brighter than soil disturbed by Surveyor. In addition, sloping surfaces about Surveyor 7 have higher albedo than adjacent flat surfaces. The playas about the Surveyor 7 site are the lowest and darkest terrain. A speculative but plausible explanation is possible. The sloping areas are illuminated at either sunset or sunrise and are thus positively charged. The low areas are dark and negatively charged. The dark, micron-sized grains are preferen-tially extracted from the top 10 microns of the sloping surfaces and are slowly migrated toward the lower, negatively charged areas, where they gently come to rest on the



1972LPSC....3.2671C


surface. Thus, slopes are lightened while flat areas are darkened. The idea can be verified. The albedo of a highland playa of area AP should be proportional to AP/ As where As is the slope area which is the source of the darkening grains. Micrometeorites should stir up both areas at the same rate and not selectively move one size range. Thus, the downslope electrostatic drifting of the smallest grains would be the differen-tial, separating mechanism. This should apply for AP 1 km2 .

Gold (1971a) suggested a different dust motion mechanism driven by the differen-tial electrical charging of adjacent surface grains. Magneto tail electrons (0. 5-1. 5 ke V) striking the earthward lunar surface evoke secondary electron emission at the rate r. The rate r varies with primary electron energy and grain chemistry (Anderegg et al., 1972). If r < 1 a negative grain charge results, while r > 1 evokes a positive grain charge. This mechanism will not directly produce the observed HG because there is a zero net surface charge over centimeter areas. Thus, the grains cannot be levitated the few centimeters necessary to produce HG. In addition, the Surveyor 7 longitude was 12°W, which makes it very unlikely that the moon was in the earth's magnetotail when the HG was observed.

Acknowledgments-I am especially grateful to Mr. J. J. Rennilson (California Institute of Technology) and Mr. J. N. Lindsley (Jet Propulsion Laboratory) for assistance in digitization of the Surveyor 7 pictures and critical discussions of the Surveyor camera systems. Credit must be extended to the scientific and engineering teams which developed the Surveyor television systems as precision photo-metric instruments. Miss Jo Ann Birchett (Manned Spacecraft Center-Computational and Analysis Division) programmed the computer model of electron accretion. This research was conducted at The Lunar Science Institute, which is operated by the Universities Space Research Association and supported by Contract NSR 09-051-001 with the National Aeronautics and Space Administration. This is Lunar Science Institute Contribution No. 95.

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