Observations of Sodium in the Lunar Atmosphere during International Lunar Atmosphere Week, 1995

ICARUS

131, 372–381 (1998) IS975848

ARTICLE NO.

Observations of Sodium in the Lunar Atmosphere during International Lunar Atmosphere Week, 1995 A. L. Sprague and D. M. Hunten Lunar and Planetary Laboratory, The University of Arizona, Tucson, Arizona 85721 E-mail: [email protected]

R. W. H. Kozlowski and F. A. Grosse Susquehanna University, Selinsgrove, Pennsylvania 17870

and R. E. Hill and R. L. Morris Lunar and Planetary Laboratory, The University of Arizona, Tucson, Arizona 85721 Received May 27, 1997; revised August 25, 1997

We report results of a part of an organized effort of five observing groups to simultaneously observe sodium in the lunar atmosphere. International Lunar Atmosphere Week spanned the week of September 15–22, 1995. Of the seven nights, we experienced four nights with good viewing conditions. We used the Mt. Lemmon Lunar Coronagraph (MLLC) and DARRK spectrograph. Both are especially designed for lunar atmospheric measurements from the surface extending to an altitude of approximately 1 lunar radius or p1700 km (860 km geopotential height). Emission rates for Na were compatible with those previously reported for relatively large phase angles (968 to 1298) with the average total of D2 and D1 emission rate p6.9 kRayleigh. A thermal component was observed only on the night of September 17, when the solar zenith angle at the limb was only 68. We observed a factor of p2 greater column abundances over the north pole than over the sunlit equatorial limb on September 18 while on September 19, the equatorial bright limb column abundance was a factor of p5 higher than the north polar. Apparent geopotential scale heights varied from 279 to 435 km, indicating an extended atmosphere. If the scale heights are represented by a temperature, values are 985 to 1470 K. The data set appears to support the idea that the dominant source during this period was meteoritic impact volatilization.  1998 Academic Press Key Words: lunar atmosphere; sodium atmosphere; meteoritic impact; exospheres.

INTRODUCTION

The Na atmosphere is easy to observe and difficult to explain. It is easy to observe because the neutral Na atom 372 0019-1035/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.

has a large resonance scatter probability, two bright lines in visible wavelengths, and the Moon is available every month for viewing. Despite a growing body of good observations, little general consensus on these important issues has emerged (Potter and Morgan 1988a, 1991, 1994, Tyler et al. 1988, Kozlowski et al. 1990, Mendillo et al. 1991, Sprague et al. 1992, Flynn and Mendillo 1993, Stern and Flynn 1995, Mendillo and Baumgardner 1995, Contarini et al. 1996, and Cremonese and Verani 1997) and several papers on sources, sinks, and recycling (e.g., Potter and Morgan 1988a; 1994, Tyler et al. 1988, Morgan and Shemansky 1991, Sprague et al. 1992, Flynn and Mendillo 1995, Potter and Morgan 1994, Smyth and Marconi 1995, Mendillo and Baumgardner 1995, Flynn and Stern 1996, and Stern et al. 1997). The most widely observed component of the lunar atmosphere consists of suprathermal atoms in a velocity distribution resembling a temperature around 1000 K (Potter and Morgan 1988b, Kozlowski et al. 1990 (for K), Sprague et al. 1992, Contarini et al. 1996, Cremonese and Verani 1997). Meteoritic volatilization of the surface materials and the meteorite itself is one dominant source of suprathermal atoms (Morgan et al. 1989, Morgan and Shemansky 1991, Cremonese and Verani 1997, Hunten et al. 1988). Local enhancements in Na and K emission caused by meteoritic impact should be discernible. Such an event may have been seen by Hunten et al. (1991) when the abundance of Na in the southern hemisphere increased by a factor of 40% over a 2 day period while the equatorial abundance remained nearly constant. Thermal atoms have been observed by Tyler et al. (1988),

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TABLE I Participants in International Lunar Atmosphere Week Stern, Flynn, Emerson Mendillo, Baumgardner Hunten, Sprague, Hill Watanabe, Oshima, Ayani Potter and Morgan Barbieri, Cremonese, Verani

Keeler Obs.; 0.25 m McDonald Obs.; 10 cm coronagraph Mt. Lemmon Lunar Coronagraph Bisei Astronomical Obs.; 1.01 m McMath-Pierce Solar Telescope Asiago-Cima Ekar, Padova

One good night One good night Reported here Clouded out Time not awarded Clouded out

Moon. The scale heights of thermal and suprathermal Na are of order 80 and 200 km and their proper delineation requires a considerably greater field of view. We tried concatenation of three exposures, but variations in observing conditions introduced additional uncertainties. We therefore designed and built the Mount Lemmon Lunar Coronagraph system, consisting of a classic Lyot coronagraph and a smaller grating spectrograph (described in the next section). The field of view is 10 times greater and gives coverage from the lunar limb to a height of p1 lunar radius. No other facility has these capabilities. OBSERVATIONS WITH THE MLLC

Potter and Morgan (1988a,b), Kozlowski et al. 1990 (for K), Sprague et al. (1992), and Stern and Flynn (1995) and in the data presented here. They are observed only when the line of sight passes close to the subsolar point, a fact explained by Sprague et al. (1992) as caused by the relatively warm temperatures that enhance the rate of thermal evaporation. They proposed a model of competing release mechanisms that accounted nicely for the dependence of the velocity distribution on solar zenith angle. In accord with earlier observations, recent work by Cremonese and Verani (1997) did not find a signature from thermal atoms from 143 to 480 km above the lunar surface and at a local solar zenith angle of 198. The extension of the suprathermal component to altitudes of several Earth radii has been studied by filter imaging (Flynn and Mendillo 1993, Mendillo et al. 1993, Mendillo and Baumgardner 1995). These data show a complex dependence on solar zenith angle, sometimes including a tail apparently caused by solar radiation pressure. Searches for atoms and ions (Mg, Si, Li, Ca, Si, etc.) thought to be produced by charged particle sputtering have resulted in strict upper limits but no detections (Flynn and Stern 1996, Stern et al. 1997). Stern et al. suggest that the low metallic species abundances indicate nonstoichiometric charged particle sputtering as the source of the atmosphere, although volatilization and a recycling mechanism like that suggested by Sprague et al. (1992) could also explain the observations, an explanation we prefer. International lunar atmosphere week. Principal observers of the lunar atmosphere united to organize the International Lunar Atmosphere Week. During this week, each group planned nightly observations of lunar atmospheric Na. It was hoped that a combination of all data would help to settle the uncertainties and differences described above. Table I gives observing team and telescope location of all participants, along with a summary of the results. Our previous observations were made with our large e´chelle spectrograph and a 153-cm telescope, giving a field of view somewhat less than 200 km at the distance of the

Our team had four nights when weather permitted lunar atmospheric measurements. Table II gives the date, starting exposure time, and spectrograph slit location for each measurement. Instrumentation. The MLLC (Mount Lemmon Lunar Coronagraph) was designed especially for observing the lunar atmosphere by Hunten, Kozlowski, and Sprague. We took advantage of an empty dome on Mt. Lemmon at 9328 ft (in the same range as Mt. Bigelow) to install a Lyot coronagraph with the same f/13.5 focal ratio as the 153 cm but approximately 1/10 the aperture and focal length (Fig. 1). The 10-mm slit length gives a 2000 km field of view. The material of the objective lens is laser-quality crown glass; optical work was done by the Optical Sciences Center of the University of Arizona. The diameter is 17.8 cm (7 inches) and the clear aperture is 16.5 cm (6.5 inches); the focal length is 203 cm (80 inches). The equatorial drive is patterned after the automated telescopes described by Genet and Hayes (1989). It is powered by small stepper motors and drive electronics, taken from a floppy-disk drive. Speed reduction is by sprockets and timing belts,

TABLE II Observations Date September, 1995 17 18

19

20

Image #

Starting exposure time (UT) hm

Exposure duration (seconds)

Slit location equatorial limb (eql) north polar limb (npl)

11 14 9 14 18 19 7 8 14 8 9

10 40 11 18 9 35 10 43 11 12 11 19 10 39 11 12 12 20 10 40 11 16

1800 1800 900 1800 900 900 1800 1800 900 1800 1800

eql eql eql eql npl npl eql eql npl eql eql

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FIG. 1. Sketch of the lunar coronagraph with DARRK spectrograph and CCD dewar. The knife-edge mirror and guiding TV camera are mounted on a turntable which permits the knife-edge to be placed tangent to the limb at any position angle. Not shown are the polar axis and the mounting stand.

and the RA drive operates by friction on a large disk. The timing pulses are generated by a board plugged into an old PC, controlled by a program that also has an autoguiding function. At the prime focus, a turntable carries the diagonal mirror and guiding CCD camera which views the image of the Moon. Because of the chromatic aberration, the guiding beam is provided with an interference filter cen˚ . The output of the camera is taken to the tered at 6000 A guiding computer and then to a monitor. Autoguiding is provided with the aid of an interface board that is able to select the intensities of three specified pixels. Two pixels are placed at limbs (N or S and E or W), and the third near the center, to provide a reference intensity. The RA and Dec track rates are adjusted up or down as required to keep the intensities of the limb pixels equal to half that of the reference one. In consonance with the classical Lyot design, a field lens produces an image of the objective on a stop slightly smaller than the image; the stop is followed by another lens that images the lunar atmosphere on the spectrograph slit. Between is a Dove prism used to rotate the image. The original intention was to produce a coude´ beam for the e´chelle spectrograph, but we were unable to maintain the required alignments. We therefore designed and built a smaller grating instrument, which we call DARRK for

the initials of the designers (four of the authors and Dr. Kent Wells). As illustrated in Fig. 1, it uses high-quality achromatic lenses for the collimator and camera and is folded once to keep it within the available space. The 600 line/mm grating has a blaze angle of 498, and in 4th order has a theoretical resolving power of around 50,000, or a ˚ at the sodium wavelength. In practice, linewidth of 0.12 A ˚ wide at half we use a slitwidth that gives a line almost 1 A maximum. The spectrum is measured with our cryogenically cooled 15 em pixel CCD camera from Photometrics Ltd, operated at 2838C with a 1024 3 1024 CCD by Loral. The readout noise level is about 5 electrons per pixel. With ˚ per the equipment as described, the dispersion is 0.117 A pixel. A small video CCD camera observes the light reflected from the slit jaws and allows us to place the lunar image exactly at the desired location on the slit. DATA REDUCTION, PREPARATION AND ANALYSIS

Data were obtained in two-dimensional, spatial, and spectral images in standard FITS format and transferred to our SUN station for reduction with IRAF from the National Optical Astronomy Observatories and its many reduction tasks suited to spectroscopic imaging. We first remove the voltage offset by ‘‘bias-subtraction.’’ At 2908C

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the dark current is negligible: only a fraction of a DN (data number) accumulates in an exposure lasting 900–1800 s. To correct for uneven illumination in the optics and patterns on the CCD chip (bad columns or pixels) we take several high S/N flat fields using a special flat field lamp on the telescope dome. These are normalized and divided into each data frame. The images are then all corrected for nonlinear dispersion and rectified into vertical height above the lunar limb and spectral dimension. In the atmospheric data frame, DN in the Na emission lines is from both true Na emission from atoms in the lunar atmosphere and light scattered into the line by the Earth’s atmosphere and the optics. It is thus necessary to subtract this scattered light to obtain the true Na emission. We do this in the standard method, using a ‘‘solar spectrum’’ in the region of the emission lines to fit the continuum, scaled to the scattered light intensity in each frame at each line. The solar continuum is obtained from a collapse of several lines on a bright image from the surface at a known location. By using a surface reflectance spectrum for our ‘‘solar spectrum’’ we are certain that the resolution is the same and any internal scattering in the optics is preserved in both the atmospheric data frame and the solar spectrum that is subtracted from the atmosphere spectrum. We also use the lunar surface spectrum to calibrate the sensitivity of our equipment. As is standard practice for lunar and mercurian work, we use a Hapke rough surface model (Hapke 1986) to obtain the rough reflectance factor (rR) for a high S/N section of the surface. Recognizing that such calibration information is important in comparing one data set to another we include here a description of our calibration procedure using the lunar surface as our standard. The surface reflectance factor is obtained by making Hapke surface models for each night’s observing. Because our data traces across the lunar surface show brightness variations corresponding to bright and dark terrains (highlands and maria) we made Hapke models using Hapke parameters for both bright and dark lunar terrains of Veverka et al. (1988). Traces were taken across the Hapke model and compared to the actual lunar surface data trace. A good correspondence to both models was found. Figure 2 shows data from September 18 plotted along with the Hapke traces of rR for each solution set. The rR have been multiplied by 10,000 for ease in plotting and viewing. We chose a lunar mare location (Oceanus Procellarum) for calibration. An arrow locates it in the figure. Counts or data numbers (DN) from that location on the Moon are then converted to surface brightness by the method detailed in Sprague et al. (1996) using the rR chosen in the manner described above. As pointed out by Domingue et al. (1997), calibrations using spatially resolved data can vary substantially if the Hapke parameter set chosen does not realistically describe the surface terrain. We encourage a standardized method of calibration for future lunar atmo-

FIG. 2. Traces across our Hapke models for the reflected light from the lunar surface are compared to our data trace across the illuminated crescent Moon the night of September 18, 1995. Two different solution sets of scattering parameters were used from Veverka et al. (1988). One solution set (dotted) was found for dark lunar terrains (typically maria) and the other (dashed) for bright (highlands and fresh craters). Our data correspond well to the models generated for each type of terrain with the region crossing Mare Procellarum coinciding with the ‘‘dark’’ curve and the highlands of the limb coinciding with the ‘‘bright’’ curve. The Hapke rough reflectance factors rR have been multiplied by 10,000 for ease in viewing. The data trace has been scaled to fit between the Hapke solution traces for display purposes.

spheric observations by all observers to permit more accurate intercomparison of data. Of the four nights of observations, September 17 had some clouds, though not too close to the Moon, and September 18–20 were cloudless. Table III gives the instrument sensitivity and the relevant geometrical and illumination parameters for each night’s viewing. Because of changing illumination, the region of the surface chosen for intensity calibration was slightly different each night, though generally within Oceanus Procellarum. The Hapke reflectances therefore are also different, though they show the general decrease expected as the phase changes from close to the quarter toward new. The instrument sensitivity

TABLE III Observational Details Date in Sept., 1995 Heliocentric dist. (AU) Fraction illuminated Phase angle Diameter (arsec) Hapke reflectance Lunar brightness (MR/A) Instrument sensitivity (kR s/DN) g-factor D2 (s21)

17 1.0049 0.45 96 1774 0.0050 3.3 29.3

18 1.0041 0.35 107 1778 0.0040 2.6 13.7

19 1.0034 0.27 118 1787 0.0050 3.3 19.6

20 1.0027 0.18 129 1801 0.0037 2.4 12.0

0.58

0.58

0.58

0.58

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than 50 R. The S/N is highest for spectral traces a few hundred km above the surface where emission is still large and noise from the continuum is negligible; it becomes decreasingly lower, but still good, in the 919 km measurement. The best S/N is around 30 for a single pixel; since there are around 5 pixels per line width this increases to 70 in a reasonable average. RESULTS

Montages of each night’s data along with the least squares regression best fit are shown in Figs. 4–7. Emission brightness is plotted in Rayleighs as a function of the geopotential height above the limb. Because emissions are faint, we obtain a better S/N if we bin several lines (km above the limb) into one flux measurement. We can get 7–20 bins from the limb to just above 1 lunar radius, the extent of our slit. The image scale is 2.8 km/pixel. The height above the limb is calculated using the pixel number at the midpoint of the bin used in the average. The results from all images are shown in Table IV. Lineof-sight abundance is obtained directly from the emission

FIG. 3. A fully processed spectral image showing the Na emission lines extending from the lunar surface to 1800 km in altitude (p900 km geopotential height) above the limb. Brightness close to the surface is p600 R for D2 and 400 R for D1. Data are from September 20, 1995. During this observing period the emission was indicative of an extended atmosphere of relatively low Na abundance (see Table IV for more information).

dropped after the 17th because a narrower slit was used, and the other variations are presumably due to changing sky conditions and air mass. After frames are prepared as described above, the emission lines can be seen as lines rising above the lunar continuum as in Fig. 3 (September 20, equatorial limb) after removal of scattered light; the D2 and D1 emission lines are seen in their relative intensity ratio of p1.7. The spectral dimension is along the bottom of the image and the wave˚ ) and the D1 (5895.92 A ˚) lengths of the D2 (5889.95 A have been labeled. The bright emission lines can be seen extending from the lunar surface across the frame to row height 879 and geopotential height 919 km. Brightness values vary from p660 R near the surface (D2) to less

FIG. 4. Montage of data and single-component least squares regression fits (solid lines) to the data for images #11 and #14 of September 17 when the Moon’s phase angle was p968 (both from equatorial sunlit limb). Also shown by a dotted line is the two-component fit to each image. The abundances used for each component can be found in Table IV.

LUNAR ATMOSPHERIC SODIUM

377

FIG. 5. Montage of data and single-component least squares regression fit to the data for images #9, #14, #18, and #19 of September 18. Equatorial bright limb and north polar measurements are shown. The zenith column abundances and other information can be found in Table IV.

rate (kR). The actual conversion to vertical column abundance involves the use of a model. Because the lunar atmosphere is extended and contains at least two velocity distributions (thermal and suprathermal) some method of obtaining a vertical column from a long line-of-sight view almost tangent to the limb is required. The models used vary from group to group (cf. Flynn and Mendillo 1993, Smyth and Marconi 1995 (Monte Carlo), Potter and Morgan 1988b, 1991, 1994, Sprague et al. 1992, Stern and Flynn 1995 (Chamberlain theory)). As in Sprague et al. (1992) we model data after methods in Chamberlain and Hunten (1987) described in the appendix. Unlike Sprague et al. (1992), however, who assumed one temperature (2000 K) for the suprathermal component of their two-component fits, we fix the thermal component at 350 K and then fit the remainder of the data by least squares, allowing the suprathermal temperature and abundance to be determined by the data. Our results are shown in Table IV which gives the date, frame number, slit location, local solar zenith angle (LSZA), apparent geopotential scale height (H*), the scale height (H) and the temperature (T ) from the Chamberlain temperature fit, the brightness in Rayleighs, the Na number density at the surface, and the zenith column abundance. September 17 has three entries

for each of frame #11 and #14 to show both the onecomponent fit and the two-component fits. As described above, the surface temperature was assumed for the thermal population and the suprathermal population temperature was obtained by fitting the residual emission once the thermal component was subtracted. Because the D2 and D1 emission lines were nearly parallel in all frames we decided to only do fits to the brighter D2 line. Table IV thus shows only the brightness for D2. The measurements for the D1 line can be found in Figs. 4–7. As in our previous work (Sprague et al. 1992) and the discovery frame by Potter and Morgan (1988a), we see evidence for a thermal component only in the frame closest (68) to the subsolar point, and even here a single-component fit, as shown by solid lines in Fig. 4, is reasonably satisfactory. The average total brightness (D2 1 D1) is 6.9 kR at the equatorial sunlit limb for all nights. These values are somewhat larger than previous measurements for the same height above the lunar limb at phase angles (96 to 129 degrees) near the quarter. (Potter and Morgan 1988a, 1991, 1994, Sprague et al. 1992, Contarini et al. 1996, Cremonese and Verani 1997). Notable in Figs. 5 and 6 is the contrast between polar

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FIG. 6. Montage of data and single-component least squares regression fit to the data for images #7, #8, and #14 of September 19. Equatorial bright limb and north polar measurements are shown. The zenith column abundances and other information can be found in Table IV.

number and direction of meteors impacting the Moon as suggested by Hunten et al. (1991). The increasing scale height as a function of increasing local solar zenith angle (Sprague et al. 1992) is not observed in this data set. There is, however, a continuous increase in apparent scale height with decreasing phase angle with the exception of September 17 #14 which does not fit the trend. Our previous data set (Sprague et al. 1992) seemed to show polar Na abundances weaker than equatorial ones, and our explanation was that a larger fraction of the atoms was on the cold surface and fewer in the atmosphere. But this idea is contradicted by the present results for September 18, and what we now observe seems better explained by a variable meteoritic source. Attempts to correlate lunar brightenings with meteor showers are frustrated because a low-velocity shower can be an important source at the Moon but undetectable as meteors (e.g., Hunten et al. 1991). The data points for September 19, #14 show a consistent deviation from the least-squares fitted straight line. There is a strong curvature with a maximum deviation at p500 km geopotential height. Examination of the original data shows no obvious errors that might explain this deviation, which therefore appears to be real. We suggest that the explanation is a discrete event, either a dropout of the Na source or an increase caused by an unusually large impact or shower. The free-fall times at the higher altitudes observed are around half an hour, and any fluctuations on

and equatorial brightnesses: the pole is almost twice as bright on the 18th and 4 or 5 times fainter on the 19th. CONCLUSIONS

We conclude that the lunar atmosphere is variable in brightness and abundance. We also conclude that the source is variable. No one solar zenith angle source can fit both the high and low altitude data. No one correlation with any source or release mechanism fits all the data sets to date. In this data set two consecutive nights show fluctuations of factors of 4 from equatorial limb to north pole. Similar behavior was seen by Potter and Morgan (1991) who observed 50 km above the lunar surface and saw a factor of 4 difference in the D2 emission rate between observations made at the N, S, E, and W cardinal points close to full Moon. Above the west limb (terminator limb) the emission rate was about 400 R. N, E, and S values were about 1200, 1500, and 1900 R, respectively. This result, along with ours, seems to support the conclusion of Cremonese and Verani (1997) who suggest meteoritic volatilization as one of the ‘‘competing release mechanisms.’’ One possible cause of variations is a fluctuation in the

FIG. 7. Montage of data and single-component least squares regression fit to the data for images #8 and #9 of September 20. Equatorial bright limb measurements are shown. The zenith column abundances and other information can be found in Table IV.

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TABLE IV Summary of Results Date Sept., 1995 17

Frame #, Slit location #11, eql

LSZA p6

#14, eql

18

19

20

#9, #14, #18, #19, #7, #8, #14, #8, #9,

eql eql npl npl eql eql npl eql eql

p17 p88 p28 p88 p39

Scale height H*

Scale height H

279 350 70 407 350 70 334 378 394 374 394 393 430 422 435

220 267 66 307 267 66 257 287 298 284 298 297 324 318 328

this time scale should cause a significant deviation from the steady state that is a necessary assumption of models of atmospheric structure. DISCUSSION

We suggest that there are three regimes to the lunar Na and K atmosphere: (a) close to the surface (less than half a lunar radius) in relatively warm areas, (b) a lower atmosphere (surface to one lunar radius), and, (c) an extended coma (up to 6 lunar radii). Different physical processes control the abundance and distribution of the Na and K in these three regimes. Most of these processes have already been discussed in the literature but not in a way to associate different processes with the three different regimes (a, b, c) nor to differentiate in which regime each process is important. Looking at the entire Na and K data set as a body, we now attempt such an association of processes and differentiation of regimes. (a) Less than half a lunar radius, surface T . 300 K. Thermal atoms have been observed by Tyler et al. (1988), Potter and Morgan (1988a,b), Kozlowski et al. (1990) (for K), and Stern and Flynn (1995) and in the data presented here. Thermal atoms can be produced from outgassing from freshly exposed Na-bearing regolith (source) or released from the surface by thermal desorption (recycled). The atoms take on a velocity distribution similar to a Maxwellian with an exponential drop off in zenith column abundance that is fitted with a scale height commensurate to a temperature near the surface temperature. Recent observations by Cremonese and Verani (1997), from 143 to 480 km above the lunar surface and at a local solar zenith angle of 198, did not find a signature from thermal atoms.

T(K)

4fI Rayleighs

n atoms cm23

N zenith 108 atoms cm22

985 1047 295 1376 1047 295 1149 1284 1334 1271 1334 1331 1451 1424 1468

4244 3660 2750 3370 3210 3940 2880 3540 6130 6130 5450 4830 1060 5080 4400

40 28 56 31 24 71 22 28 47 48 42 38 8 38 32

12 11 4.0 14 9.2 5.1 8.4 12 20 20 18 16 3.6 17 15

(b) Surface to one lunar radius, surface T , 300 K. Suprathermal atoms have been observed in this region by Potter and Morgan (1988b), Kozlowski et al. (1990) (for K), Sprague et al. (1992), Contarini et al. (1996), and Cremonese and Verani (1997). Almost no thermal atoms have been seen close to the surface at high latitudes or near the equator near crescent or gibbous phases (high local solar zenith angle). As described in Sprague et al. (1992), recycling of adsorbed atoms back into the ambient atmosphere from all but the warmest regions involves physical processes other than thermal release. Meteoritic volatilization of the surface materials and the meteorite itself is certainly one dominant process (Morgan et al. 1989, Morgan and Shemansky 1991, Cremonese and Verani 1997, Hunten et al. 1988) and is a source of suprathermal atoms. Local enhancements in Na and K emission caused by meteoritic impact should be discernible. Such an event may have been seen by Hunten et al. (1991) when the abundance of Na in the southern hemisphere increased by a factor of 40% over a 2 day period while the equatorial abundance remained nearly constant. But Sprague et al. (1992) could fit a large body of Na and K data with two-component (thermal and suprathermal) models that partitioned relative numbers in each population with local solar zenith angle. Because volatilization cannot explain this phenomenon, they proposed a recycling mechanism with ‘‘competing release mechanisms’’ that takes source atoms and redistributes them into populations of thermal and suprathermal atoms according to the relative efficiency of thermal- and photo-desorption. Photo-desorption, as used in this model, is different from photo-sputtering. In the first case, visible light breaks a moderate strength bond with an adsorbed atom and imparts suprathermal velocity. In the second

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case, a higher energy photon enters the lattice structure of a tightly bound atom, breaks the bond, and imparts a suprathermal velocity to the atom. Not all observations have supported the strict interpretation of a steady state atmosphere dominated by recycling with ‘‘competing release mechanisms.’’ The emission rate in the D2 line was observed to vary by a factor of 4 between N, E, S, and W cardinal points close to full Moon at 50 km above the surface on February 21, 1989 (Potter and Morgan 1991). All of the local solar zenith angles were close to 908. Observations from 10 to 70 km above the lunar surface at low phase angles (23.68 to 298) and high local solar zenith angles (observations were over the equatorial illuminated limb) showed D2 emission rates varying from 0.02 to 1.16 kR (Potter and Morgan 1994). Eight of the 19 measurements fell below 0.5 kR. These were lower than other measurements previously reported by any group. This seemed to support the idea that low abundances were a result of the lunar surface being shielded from solar-wind particle sputtering when the Moon was in the Earth’s magnetosheath. Other observations have not supported this (Mendillo and Baumgardner 1995) but their observations were made looking higher in the atmosphere in regime (c). (c) The lunar corona, between 1 and 6 lunar radii. Twodimensional imaging (Flynn and Mendillo 1993, Mendillo et al. 1993, Mendillo and Baumgardner 1995) has shown that the suprathermal nature of this region is inarguable, but its cause is not. The high altitude Na data set is small but one common aspect is the highest emission rate on the sunlit equatorial limb with decreasing emission toward both high latitude limbs. Much attention has been focused on finding a source that will give a solar zenith angle distribution. Modeling a distribution of atoms that is highly dependent on the local solar zenith angle can fit observations in this region (Flynn and Mendillo 1995). However, it is likely that the loss of Na is highly solar zenith angle dependent. One loss mechanism that fits this description is the freezing out of Na and K onto the surface (adsorption), a process that is much more efficient at high latitudes where the solar zenith angle is highest. The effect of this adsorption onto the surface is to remove much of the Na atoms from the column above cold ground and leave Na in the atmosphere above warmer ground. The effects of ground temperature differences combined with the solar zenith angle release process of photo-desorption were modeled by Sprague et al. (1992). In this distant regime, some atoms may be in direct escape as suggested by Morgan and Shemansky (1991), but not in great amount relative to the total abundance because on the sunward side the atomic densities are in a r22.6– 24 distribution (Sprague et al. 1992, Mendillo et al. 1993). Either photo-desorption, photo-sputtering, or charged particle sputtering could be controlling the distribution of atoms in this regime. An undetermined fraction of the atoms are affected by radia-

tion pressure and moved tailward (Ip 1991, Smyth and Marconi 1995) but not every image shows a streaming tail (Flynn and Mendillo 1993). Because observations are few, the variable character of the extended tail is not known. ACKNOWLEDGMENTS Support was provided by NASA’s Planetary Astronomy program Grants NAGW-4987 and NAGW-346590 and a NASA JOVE grant to Kozlowski. We thank the Steward Observatory Mountain Operations and their extremely helpful and competent staff, especially Bob Peterson, Jim Grantham, Kirk Laughlin, and Chris Weddle for their help and good humor in all kinds of weather. Steve Bell designed the board that captures pixel values from a television camera for autoguiding.

REFERENCES Chamberlain, J. W., and D. M. Hunten 1987. Theory of Planetary Atmospheres. Academic Press, New York. Contarini, C., G. Barbieri, G. Corrain, G. Cremonese, and R. Vio 1996. Spectroscopic observations of the sodium atmosphere of the Moon. Planet. Space Sci. 44, 417–420. Cremonese, G., and S. Verani 1997. High resolution observations of the sodium emission from the Moon. Adv. Space Res. 19(10), 1561–1569. Domingue, D. L., A. L. Sprague, and D. M. Hunten 1997. Dependence of mercurian atmospheric column abundance estimations on surface reflectance modeling. Icarus 128, 75–82. Flynn, B. C., and S. A. Stern 1996. A spectroscopic survey of metallic species abundances in the lunar atmosphere. Icarus 124, 530–536. Flynn, B., and M. Mendillo 1993. A picture of the Moon’s atmosphere. Science 261, 184–186. Flynn, B., and M. Mendillo 1995. Simulations of the lunar sodium atmosphere. J. Geophys. Res. 100, 23,271–23,278. Genet, R. M., and D. S. Hayes 1989. Robotic Observatories. AutoScope Corporation, Mesa, Arizona. Hapke, B. 1986. Bi-directional reflectance spectroscopy 4. The extinction coefficient and the opposition effect. Icarus 67, 264–280. Hunten, D. M. 1967. Spectroscopic studies of the twilight airglow. Space Sci. Revs. 6, 493–573. Hunten, D.M. 1992. The equilibrium of atmospheric sodium. Planet. Space Sci. 40, 1607–1614. Hunten, D. M., R. W. H. Kozlowski, and A. L. Sprague 1991. A possible meteor shower on the Moon. Geophys. Res. Lett. 18, 2101–2104. Hunten, D. M., T. H. Morgan, and D. Shemansky 1988. The Mercury atmosphere. In Mercury (Vilas, F., Chapman, C.R., and Matthews, M.S., Eds.) p. 794. Univ. of Arizona Press, Tucson. Ip, W. H. 1991. The atomic sodium exosphere/coma of the Moon. Geophys. Res. Lett. 18, 2093–2096. Kozlowski, R. W. H., A. L. Sprague, and D. M. Hunten 1990. Observations of potassium in the tenuous lunar atmosphere. Geophys. Res. Lett. 17, 2253–2256. Mendillo, M., and J. Baumgardner 1995. Constraints on the origin of the Moon’s atmosphere from observations during a lunar eclipse. Nature 377, 404–406. Mendillo, M., J. Baumgardner, and B. Flynn 1991. Imaging observations of the extended sodium atmosphere of the Moon. Geophys. Res. Lett. 18, 2097–2100. Mendillo M., B. Flynn, and J. Baumgardner 1993. Imaging experiments

LUNAR ATMOSPHERIC SODIUM

381

to detect an extended sodium atmosphere on the Moon. Adv. Space Res. 13, 313–319.

Sprague, A. L., D. M. Hunten, and F. A. Grosse 1996. Upper limit for Lithium in Mercury’s Atmosphere. Icarus 123, 345–349.

Morgan, T. H., and D. E. Shemansky 1991. Limits to the lunar atmosphere. J. Geophys. Res. 96, 1351–1367. Morgan, T. H., H. A. Zook, and A. E. Potter 1989. Production of sodium vapor from exposed regolith in the inner Solar System. Proc. Lunar Plan. Sci. Conf. 19th, 297–304. Potter, A. E., and T. H. Morgan 1988a. Discovery of sodium and potassium vapor in the atmosphere of the Moon. Science 241, 675–680. Potter, A. E., and T. H. Morgan 1988b. Extended sodium exosphere of the Moon. Geophys. Res. Lett. 15, 1515–1518. Potter, A. E., and T. H. Morgan 1991. Observations of the lunar sodium exosphere. Geophys. Res. Lett. 18, 2089–2092. Potter, A. E., and T. H. Morgan 1994. Variation of lunar sodium emission intensity with phase angle. Geophys. Res. Lett. 21, 2263–2266. Smyth, W. H., and M. L. Marconi 1995. Theoretical overview and modeling of the sodium and potassium atmospheres of the moon. Astrophys. J. 443, 371–392.

Sprague, A. L., R. W. H. Kozlowski, D. M. Hunten, W. K. Wells, and F. A. Grosse 1992. The sodium and potassium atmosphere of the Moon and its interaction with the surface. Icarus 96, 27–42. Stern, S. A., and B. C. Flynn 1995. Narrow-field imaging of the lunar sodium exosphere. Astron. J. 109, 835–841. Stern, S. A., J. W. Parker, T. H. Morgan, B. C. Flynn, D. M. Hunten, A. L. Sprague, M. Mendillo, and M. C. Festou 1997. An HST search for magnesium in the lunar atmosphere. Icarus 127, 523– 526. Tyler, A. L., R. W. H. Kozlowski, and D. M. Hunten 1988. Observations of sodium in the tenuous lunar atmosphere, Geophys. Res. Lett. 15, 1141–1144. Veverka, J., P. Helfenstein, B. Hapke, and J. D. Goguen 1988. Photometry and polarimetry of Mercury. In Mercury (F. Vilas, C. R. Chapman, and M. S. Matthews, Eds.) p. 794. Univ. of Arizona Press, Tucson.