- Email: [email protected]
Spectroscopic observations of the sodium atmosphere of the Moon
Pl1rr1er.Spllcv sci.. Vol. 44. No. 5. pp. 1 I7 470. IO96 Copyright 8~;’1996 Published by Ekvier Science Ltd Printed in Great Britain. All rights reserved OO3? X)6.7! Y)6 s IC.00 + (1.01l
Pergamon
0032-0633(95)00
I 18-2
Spectroscopic observations of the sodium atmosphere of the Moon (;.Contarini,’
C. Barbieri,’ G. Corrain,’ G. Cremonesej and R. Vio’
‘CISAS, University of Padova. Padova. Italy ‘Department of Astronomy. University of Padova. Padova. Ital) ’ 2stronomical Observatory of Padova, Padova. Italy Received for prlbluk~n
I4 September
1905
Abstract. The first results are presented of a program
aimed at the study of the Moon’s atmosphere in the Na lines carried out with the Asiago-Cima Ekar echelle spectrograph. The observations obtained in July 1993 show the feasibility of the program and give quantitative information about the line brightness. Copyright 0 1996 Published by Elsevier Science Ltd
into this problem. we have started ;I program of high resolution spectroscopy at the Asiago Cima Ekar observatory. using the same method successfully applied to IO and Jupiter studies (Cremonese c’t al.. 1992). We report here the first results of this program, that needs further refinements both in observing strategy and in the d;ltu reduction. but that has already proven to he fruitful.
2. Observations I. Introduction The presence of a tenuous atmosphere on the Moon was discovered in the 1970s by the in situ measurements of the Apollo missions obtained serving the nightside of the satellite. According to these observations the basic composition of the atmosphere consisted of argon, neon. helium and some traces of other elements. In the last few years interest for the lunar atmosphere is growing after the discovery of sodium and potassium in the atmosphere of Mercury (Potter and Morgan, 1988a). Given the analogy between the two objects, regarding the composition and the atmosphere source processes, observations of the Moon have been intensified leading to the discovery of the same elements also in the lunar atmosphere (Potter and Morgan, IY8Xa). The observations reported have been focused on the sodium emissions because of their high efficiency in scattering sunlight. The measurements revealed a sodium density of 50 atoms per cubic centimeter within 100 km from the lunar surface. More recently. the field had a considerable growth thanks to the imaging technique devised by Mendillo c/ trl. (199 I) who were able to obtain a map of the sodium distribution out to several Moon’s radii. With a different technique. Stern and Flynn (1995) were able to obtain Images very close to the lunar surface (some 5(1 km above the surface). In order to gain further insight
The first spectra of the Moon‘s atmosphere and disk were obtained in July 1993. For all observations we used the REOSC echelle spectrograph at the f 9 Cassegrain focus of the Cima Ekar 182 cm telescope (Asiago Observatory). See also Table 1 for instrumental parameters. The data were obtained during the night between 7 and 8 July 1993 (three days after full Moon. average renith distance 57 deg) placing the slit parallel to the solar direction at the bright limb of the Moon (namely always on the sunward side and at the position angle of the bright limb given in the Astronomical Almanac for the date. namely 65 deg). In order to know at which distance from the Moon each slit element was placed, we kept the Moon limb iusr in one corner of the slit. The telescope has a scale of 17.56 arcsec mm- ’ on the focal plane so that the 6 mm slit length used in this program covers 75.4 arcsec on the sky, corresponding to _ 150 km at the mean distance of the
Table 1. Instrumental
parameters
slit width slit length angular scale on focal plane angular scale on detector plane echelle dispersion resolution CCD (Thomson) pixel size
I SO/cm h mm I I!.56 ;IrcWc‘ mm ’ 29.37 ilrcscc mm ’ 0.19 A pixel j _ 15.000 I 100 x I 100 pixel ~ 19
~“11
G. Contarini rr al. : Spectroscopic observations
418
5840
5860
5880
5920
5900
Wavelenght
5940
5860
5880
5860
t...o
5880 Wovelenght
5900
5900
Wavelenght
t...,.,.,...,...,.,.j 5840
5840
[A]
5920
5940
[A]
5840
5920
I
..1._.,,..,
5860
5880 Wavelenght
5940
[A]
5900
5920
5940
[A]
Fig. 1. Profiles obtained collapsing the atmosphere bidimensionat spectrum along the spatial direction, after the Moon-scattered light subtraction
Moon. The adopted useful region was actually 80 km. The reason for excluding the first 70 km from the edge of the slit was two-fold : the first 20 km fall inside the satellite, while the additional 50 km are affected by heavy scattered light. The exposure time for the atmosphere spectra was 10 min. Longer exposures were avoided because of the strong brightness of Moon limb. In the future we plan to guide the telescope in a different way and expose for longer durations. Four spectra were thus obtained, and are analyzed below.
3. Data reduction and calibration The main problem of these observations is the brilliant light from the Moon’s surface scattered by the Earth’s atmosphere and also by the telescope itself, which must be subtracted before any emission from the thin sodium atmosphere may be detected. To reach this goal we have devised the following strategy of analysis : l The standard operations of order identification, background subtraction, spectrum extraction and wavelength calibration were performed using a special set of procedures developed under an IDL environment. l The bidimensional spectrum thus obtained was rebinned along the spatial direction over the whole range of about 50-130 km. Figure 1 shows the spectra obtained after these operations. Sodium emission lines are clearly
seen above the noise. Solar absorptions are very well cancelled out. The mean brightness detected is 3.4 kR. l To check the trend of brightness with the distance to the surface, the atmosphere spectrum was divided in bins of 10 pixels along the spatial direction (about 11 km). For our data, this value appears as the ‘best’ compromise between the resolution and the noisiness of the final results. In any case, with a number of bins in the range 5 -~ 15, the results we obtained are very similar. Smaller bins have not been considered because typically the seeing in Asaigo is about 3 arcsec corresponding to 5 pixels. l Each bin was collapsed in a single unidimensional spectrum in order to improve the signal-to-noise ratio. l Each of these spectra was corrected for the solar contribution by subtracting an averaged and appropriately normalized spectrum of the Moon disk. Absolute brightness calibration has been performed by comparing the emission spectra to the disk of Jupiter. The center of the planet disk is assumed to have a surface brightness of 5.5 MR.&’ (Brown and Schneider, 1981). In the brightness calibration operation we have corrected our data for differential extinction.
4. Data analysis In planetary atmospheres studies, fundamental parameters are the scale height H and the abundance at the critical level H,. In the following we describe the technique adopted to get these quantities.
G. Contarini 01 II/. : Spectroscopic observations
41 ‘)
Given its very low density, the Moon’s atmosphere can be assimilated to an exosphere. In the simplest case of a one-component barometric model, the abundance n(r) at the distance I’ from the center of the body, is given by (Chamberlain and Hunten. 19X7) t/(r) = II, e “L “‘,‘(i.).
(1)
l /l is a specified direction: ,D = 0 for line-of-sight 01 transverse direction. ,I = I for the radial or ;renith direction. l K(,Y,~.) is the elective path length. For /r = 0 and I. the values of this function are calculated in the tables reproduced in Chamberlain and Hunten ( 19X7) for Larious i, and i.
Here 2 = potential perature I‘rom the l
;.(I.) is the absolute value of the gravitational energy written in units of kT, CT,is the temof the critical level or exobase) at the distance I satellite
In our case of observations made outside the atmosphere. the line-of-sight traverses an integrated density of 2N(/c = 0.2). Furthermore, we are observrng verv close to the surface thus we can use the approximation .
center
II, is the abundance at the exobase. c(i) is a partition function. which will describe how particles are partitioned into the three different classes or orbits : ballistic. escaping or satellite. In our case. satellite I>articles. which derive from collisions. are neglected. Thus l
l
; = ih.$,+ s,,.,.
(3)
The values of < are reproduced in Chamberlain tables for i,arious i,. All these quantities are not directly obtainable from observations. Actually from the data we are able get the photon brightness 1. and to derive the line-of-sight column density N from the relationship 4Ttl, = Jj, * :I!
(4)
where ,y is the !/-f&or. Since the 1 km s-l Sun-Moon Doppler shift is lower than the resolution of our spectrograph, the quantity g has been computed without this correction. The ~q-value is 8.80 for the sum of D, and II, :tt I AU with no Doppler shift and 1,‘3 of this for D2alone (Chamberlain and Hunten, 19X7). Thus we have used :I, = 0.27 and $/> = 0.53 photons ’ atom--‘. Table 2 shows the brightness and the column densities obtained for each of the considered regions for our spectra. The relation between the abundance and the column density is (Chamberlain and Hunten. 1987) !V(/Li.) = n(r,)HK(pI)
zz
,,,
e
lx,
In the distance range we have used (50 I30 km from the limb), the function,f(r) shows a variation of7”&, which is smaller than the uncertainty of our data. Thus we have assumed f(r) as a constant and, for N(0.j.). the same exponential law ofn(l). From equation (71, WC obtain logN(O./.)
= C,(7;).
:*t
(‘,(//,.7.,)
(X)
where z* is a function of the distance from the ccntcr 01‘ the Moon. Figure 2 shows that this expectation is satisfied with discrete accuracy by our data. The temperature r, (and consequently the true scale height H = AT,g,Af,,,) and the value N,, (the column density at the surface) are obtained by means of the slope and intercept of a least squares straight line. Our results indicate a value of the temperature in the range 1000-1400 K, and a value for the column densit) at the surface N,, in the range X.3--9.2 x IO” atoms cm ‘. Through the scale height definition and equation (7). ;I scale height in the range 224-3 13 km and a number density II, in the range 44-~58 atoms cm ’ arc deri\,ed.
(5)
where l
H is the scale height calculated
5. Discussion at the distance
I’ from
1he center of the Moon :
Potter and Morgan (19XXb) and Sprague t’/ ~1. ( 1992) assume that the sodium atmosphere can be represented by two Maxwellian distributions. one having a ten-
Table 2. Measurements
of mean brightness.
U.T. (hh : mm) Spectrum Spectrum Spectrum Spectrum
I
00 : 02
2 3 4
00: 55 01 :06 01144
“y;g
0
3.31(0.31) 3.32(0.29) 3.59(0.31) 3.50(0.?1)
temperatures.
scale heights tind number dens~t!
T,( kc) (K)
Win) (km)
I/,( IL 0) atoms cm
1004(242) 1403 (273)
224(54) 313(61)
5X.4( 11.7) 44 7 (6.9)
I ll7(301)
749 (67)
57.1( I.%51
G. Contarini
420
23.0
23.0 F---‘lj
22.2. 40
. . . 1 60 Distance
. from
I . . . 1 . .
a0
1 . * .
100 120 the Moon surface [km]
22.2 t... 40
’
Distance
a0 from
’
1
observations 1 ,
.
22.2 . . 1 I . . , I . . , , . , . , , I , 40 60 a0 100 140 120
140
60 Distance
’
et ~1. : Spectroscopic
100 120 the Moan surface [km]
from
the Moon
surface
[km]
140
Fig. 2. Trend of the log of the O2 column density
perature of about 400 K, the thermal component, and another at a temperature much larger defined as the suprathermal component. The former is likely due to the thermal desorption and dominates the atmosphere close to the surface in the sub-solar region and the latter is mainly due to the micrometeoroid impact vaporization and the solar wind sputtering. From the limited spatial extent of our data, we cannot perform a meaningful twocomponent analysis. However, applying the single term model. we find a temperature significantly higher than that of the surface. A possible explanation is that the contribution of the suprathermal Na is already dominant close to the surface. This conclusion is consistent with Potter and Morgan (1988b) and Sprague et al. (1992). However. the situation is still controversial. In fact more recently Stern and Flynn, besides the standard twocomponent model, proposed the alternative model of a continuum of temperatures ranging from the local one up to or exceeding the escape energies. As a consequence, we feel that additional investigations are essential to establish the thermal structure of the Na lunar atmosphere.
Acknowledgements. We would like to thank Michael Mendillo for his useful suggestions and comments. Support was received
with a contract (ASI).
to C. Barbieri
from Agenzia
Spaziale
Italiana
References Brown and Schneider, Sodium remote from IO. Iccr~u 48, 519. 1981. Chamberlain, J. W. and Hunten, D. M., Tllro~~. of’ Pl~n~/uq~ ,4trno.$t~~. Academic Press. Orlando, 1987. Cremonese, G., Thomas, N., Barbieri, C. and Pernecbele, C., High resolution spectra of lo’s neutral sodium cloud. AS/IXUI. .4strop/zys. 256, 286, 1992. Flynn, B. and Mendillo, M., A picture of the Moon’s atmosphere. Science 261, 184, 1993. Mendillo, M., Baumgardner, J. and Flynn B.. Imaging observations of the extended sodium atmosphere of the Moon. Geoph?~. Rcs. Lett. 18, 2097-2100. 1991. Potter, A. E. and Morgan, T. H., Discovery of sodium potassium vapor in the atmosphere of the Moon. .S&rzc,c~ 675-6X0, 1988a. Potter, A. E. and Morgan, T. H.. Extended sodium atmosphere of the Moon. Geoph_r.s. Rev. Left. 18, 208992097. I988b. Sprague, A. L., Kozlowsky, R. W. H., Hunten, D. M. and Wells, W. K.. The sodium and potassium atmosphere of the Moon and its interaction with the surface. Ictrrus 96, 17 47. 1997. Stern, S. A. and Flynn, B. C., Narrow-field imaging of the lunar sodium exosphere. Astrm. J. 109(2). 835841. 1995.
Pergamon
0032-0633(95)00
I 18-2
Spectroscopic observations of the sodium atmosphere of the Moon (;.Contarini,’
C. Barbieri,’ G. Corrain,’ G. Cremonesej and R. Vio’
‘CISAS, University of Padova. Padova. Italy ‘Department of Astronomy. University of Padova. Padova. Ital) ’ 2stronomical Observatory of Padova, Padova. Italy Received for prlbluk~n
I4 September
1905
Abstract. The first results are presented of a program
aimed at the study of the Moon’s atmosphere in the Na lines carried out with the Asiago-Cima Ekar echelle spectrograph. The observations obtained in July 1993 show the feasibility of the program and give quantitative information about the line brightness. Copyright 0 1996 Published by Elsevier Science Ltd
into this problem. we have started ;I program of high resolution spectroscopy at the Asiago Cima Ekar observatory. using the same method successfully applied to IO and Jupiter studies (Cremonese c’t al.. 1992). We report here the first results of this program, that needs further refinements both in observing strategy and in the d;ltu reduction. but that has already proven to he fruitful.
2. Observations I. Introduction The presence of a tenuous atmosphere on the Moon was discovered in the 1970s by the in situ measurements of the Apollo missions obtained serving the nightside of the satellite. According to these observations the basic composition of the atmosphere consisted of argon, neon. helium and some traces of other elements. In the last few years interest for the lunar atmosphere is growing after the discovery of sodium and potassium in the atmosphere of Mercury (Potter and Morgan, 1988a). Given the analogy between the two objects, regarding the composition and the atmosphere source processes, observations of the Moon have been intensified leading to the discovery of the same elements also in the lunar atmosphere (Potter and Morgan, IY8Xa). The observations reported have been focused on the sodium emissions because of their high efficiency in scattering sunlight. The measurements revealed a sodium density of 50 atoms per cubic centimeter within 100 km from the lunar surface. More recently. the field had a considerable growth thanks to the imaging technique devised by Mendillo c/ trl. (199 I) who were able to obtain a map of the sodium distribution out to several Moon’s radii. With a different technique. Stern and Flynn (1995) were able to obtain Images very close to the lunar surface (some 5(1 km above the surface). In order to gain further insight
The first spectra of the Moon‘s atmosphere and disk were obtained in July 1993. For all observations we used the REOSC echelle spectrograph at the f 9 Cassegrain focus of the Cima Ekar 182 cm telescope (Asiago Observatory). See also Table 1 for instrumental parameters. The data were obtained during the night between 7 and 8 July 1993 (three days after full Moon. average renith distance 57 deg) placing the slit parallel to the solar direction at the bright limb of the Moon (namely always on the sunward side and at the position angle of the bright limb given in the Astronomical Almanac for the date. namely 65 deg). In order to know at which distance from the Moon each slit element was placed, we kept the Moon limb iusr in one corner of the slit. The telescope has a scale of 17.56 arcsec mm- ’ on the focal plane so that the 6 mm slit length used in this program covers 75.4 arcsec on the sky, corresponding to _ 150 km at the mean distance of the
Table 1. Instrumental
parameters
slit width slit length angular scale on focal plane angular scale on detector plane echelle dispersion resolution CCD (Thomson) pixel size
I SO/cm h mm I I!.56 ;IrcWc‘ mm ’ 29.37 ilrcscc mm ’ 0.19 A pixel j _ 15.000 I 100 x I 100 pixel ~ 19
~“11
G. Contarini rr al. : Spectroscopic observations
418
5840
5860
5880
5920
5900
Wavelenght
5940
5860
5880
5860
t...o
5880 Wovelenght
5900
5900
Wavelenght
t...,.,.,...,...,.,.j 5840
5840
[A]
5920
5940
[A]
5840
5920
I
..1._.,,..,
5860
5880 Wavelenght
5940
[A]
5900
5920
5940
[A]
Fig. 1. Profiles obtained collapsing the atmosphere bidimensionat spectrum along the spatial direction, after the Moon-scattered light subtraction
Moon. The adopted useful region was actually 80 km. The reason for excluding the first 70 km from the edge of the slit was two-fold : the first 20 km fall inside the satellite, while the additional 50 km are affected by heavy scattered light. The exposure time for the atmosphere spectra was 10 min. Longer exposures were avoided because of the strong brightness of Moon limb. In the future we plan to guide the telescope in a different way and expose for longer durations. Four spectra were thus obtained, and are analyzed below.
3. Data reduction and calibration The main problem of these observations is the brilliant light from the Moon’s surface scattered by the Earth’s atmosphere and also by the telescope itself, which must be subtracted before any emission from the thin sodium atmosphere may be detected. To reach this goal we have devised the following strategy of analysis : l The standard operations of order identification, background subtraction, spectrum extraction and wavelength calibration were performed using a special set of procedures developed under an IDL environment. l The bidimensional spectrum thus obtained was rebinned along the spatial direction over the whole range of about 50-130 km. Figure 1 shows the spectra obtained after these operations. Sodium emission lines are clearly
seen above the noise. Solar absorptions are very well cancelled out. The mean brightness detected is 3.4 kR. l To check the trend of brightness with the distance to the surface, the atmosphere spectrum was divided in bins of 10 pixels along the spatial direction (about 11 km). For our data, this value appears as the ‘best’ compromise between the resolution and the noisiness of the final results. In any case, with a number of bins in the range 5 -~ 15, the results we obtained are very similar. Smaller bins have not been considered because typically the seeing in Asaigo is about 3 arcsec corresponding to 5 pixels. l Each bin was collapsed in a single unidimensional spectrum in order to improve the signal-to-noise ratio. l Each of these spectra was corrected for the solar contribution by subtracting an averaged and appropriately normalized spectrum of the Moon disk. Absolute brightness calibration has been performed by comparing the emission spectra to the disk of Jupiter. The center of the planet disk is assumed to have a surface brightness of 5.5 MR.&’ (Brown and Schneider, 1981). In the brightness calibration operation we have corrected our data for differential extinction.
4. Data analysis In planetary atmospheres studies, fundamental parameters are the scale height H and the abundance at the critical level H,. In the following we describe the technique adopted to get these quantities.
G. Contarini 01 II/. : Spectroscopic observations
41 ‘)
Given its very low density, the Moon’s atmosphere can be assimilated to an exosphere. In the simplest case of a one-component barometric model, the abundance n(r) at the distance I’ from the center of the body, is given by (Chamberlain and Hunten. 19X7) t/(r) = II, e “L “‘,‘(i.).
(1)
l /l is a specified direction: ,D = 0 for line-of-sight 01 transverse direction. ,I = I for the radial or ;renith direction. l K(,Y,~.) is the elective path length. For /r = 0 and I. the values of this function are calculated in the tables reproduced in Chamberlain and Hunten ( 19X7) for Larious i, and i.
Here 2 = potential perature I‘rom the l
;.(I.) is the absolute value of the gravitational energy written in units of kT, CT,is the temof the critical level or exobase) at the distance I satellite
In our case of observations made outside the atmosphere. the line-of-sight traverses an integrated density of 2N(/c = 0.2). Furthermore, we are observrng verv close to the surface thus we can use the approximation .
center
II, is the abundance at the exobase. c(i) is a partition function. which will describe how particles are partitioned into the three different classes or orbits : ballistic. escaping or satellite. In our case. satellite I>articles. which derive from collisions. are neglected. Thus l
l
; = ih.$,+ s,,.,.
(3)
The values of < are reproduced in Chamberlain tables for i,arious i,. All these quantities are not directly obtainable from observations. Actually from the data we are able get the photon brightness 1. and to derive the line-of-sight column density N from the relationship 4Ttl, = Jj, * :I!
(4)
where ,y is the !/-f&or. Since the 1 km s-l Sun-Moon Doppler shift is lower than the resolution of our spectrograph, the quantity g has been computed without this correction. The ~q-value is 8.80 for the sum of D, and II, :tt I AU with no Doppler shift and 1,‘3 of this for D2alone (Chamberlain and Hunten, 19X7). Thus we have used :I, = 0.27 and $/> = 0.53 photons ’ atom--‘. Table 2 shows the brightness and the column densities obtained for each of the considered regions for our spectra. The relation between the abundance and the column density is (Chamberlain and Hunten. 1987) !V(/Li.) = n(r,)HK(pI)
zz
,,,
e
lx,
In the distance range we have used (50 I30 km from the limb), the function,f(r) shows a variation of7”&, which is smaller than the uncertainty of our data. Thus we have assumed f(r) as a constant and, for N(0.j.). the same exponential law ofn(l). From equation (71, WC obtain logN(O./.)
= C,(7;).
:*t
(‘,(//,.7.,)
(X)
where z* is a function of the distance from the ccntcr 01‘ the Moon. Figure 2 shows that this expectation is satisfied with discrete accuracy by our data. The temperature r, (and consequently the true scale height H = AT,g,Af,,,) and the value N,, (the column density at the surface) are obtained by means of the slope and intercept of a least squares straight line. Our results indicate a value of the temperature in the range 1000-1400 K, and a value for the column densit) at the surface N,, in the range X.3--9.2 x IO” atoms cm ‘. Through the scale height definition and equation (7). ;I scale height in the range 224-3 13 km and a number density II, in the range 44-~58 atoms cm ’ arc deri\,ed.
(5)
where l
H is the scale height calculated
5. Discussion at the distance
I’ from
1he center of the Moon :
Potter and Morgan (19XXb) and Sprague t’/ ~1. ( 1992) assume that the sodium atmosphere can be represented by two Maxwellian distributions. one having a ten-
Table 2. Measurements
of mean brightness.
U.T. (hh : mm) Spectrum Spectrum Spectrum Spectrum
I
00 : 02
2 3 4
00: 55 01 :06 01144
“y;g
0
3.31(0.31) 3.32(0.29) 3.59(0.31) 3.50(0.?1)
temperatures.
scale heights tind number dens~t!
T,( kc) (K)
Win) (km)
I/,( IL 0) atoms cm
1004(242) 1403 (273)
224(54) 313(61)
5X.4( 11.7) 44 7 (6.9)
I ll7(301)
749 (67)
57.1( I.%51
G. Contarini
420
23.0
23.0 F---‘lj
22.2. 40
. . . 1 60 Distance
. from
I . . . 1 . .
a0
1 . * .
100 120 the Moon surface [km]
22.2 t... 40
’
Distance
a0 from
’
1
observations 1 ,
.
22.2 . . 1 I . . , I . . , , . , . , , I , 40 60 a0 100 140 120
140
60 Distance
’
et ~1. : Spectroscopic
100 120 the Moan surface [km]
from
the Moon
surface
[km]
140
Fig. 2. Trend of the log of the O2 column density
perature of about 400 K, the thermal component, and another at a temperature much larger defined as the suprathermal component. The former is likely due to the thermal desorption and dominates the atmosphere close to the surface in the sub-solar region and the latter is mainly due to the micrometeoroid impact vaporization and the solar wind sputtering. From the limited spatial extent of our data, we cannot perform a meaningful twocomponent analysis. However, applying the single term model. we find a temperature significantly higher than that of the surface. A possible explanation is that the contribution of the suprathermal Na is already dominant close to the surface. This conclusion is consistent with Potter and Morgan (1988b) and Sprague et al. (1992). However. the situation is still controversial. In fact more recently Stern and Flynn, besides the standard twocomponent model, proposed the alternative model of a continuum of temperatures ranging from the local one up to or exceeding the escape energies. As a consequence, we feel that additional investigations are essential to establish the thermal structure of the Na lunar atmosphere.
Acknowledgements. We would like to thank Michael Mendillo for his useful suggestions and comments. Support was received
with a contract (ASI).
to C. Barbieri
from Agenzia
Spaziale
Italiana
References Brown and Schneider, Sodium remote from IO. Iccr~u 48, 519. 1981. Chamberlain, J. W. and Hunten, D. M., Tllro~~. of’ Pl~n~/uq~ ,4trno.$t~~. Academic Press. Orlando, 1987. Cremonese, G., Thomas, N., Barbieri, C. and Pernecbele, C., High resolution spectra of lo’s neutral sodium cloud. AS/IXUI. .4strop/zys. 256, 286, 1992. Flynn, B. and Mendillo, M., A picture of the Moon’s atmosphere. Science 261, 184, 1993. Mendillo, M., Baumgardner, J. and Flynn B.. Imaging observations of the extended sodium atmosphere of the Moon. Geoph?~. Rcs. Lett. 18, 2097-2100. 1991. Potter, A. E. and Morgan, T. H., Discovery of sodium potassium vapor in the atmosphere of the Moon. .S&rzc,c~ 675-6X0, 1988a. Potter, A. E. and Morgan, T. H.. Extended sodium atmosphere of the Moon. Geoph_r.s. Rev. Left. 18, 208992097. I988b. Sprague, A. L., Kozlowsky, R. W. H., Hunten, D. M. and Wells, W. K.. The sodium and potassium atmosphere of the Moon and its interaction with the surface. Ictrrus 96, 17 47. 1997. Stern, S. A. and Flynn, B. C., Narrow-field imaging of the lunar sodium exosphere. Astrm. J. 109(2). 835841. 1995.