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Sodium and potassium atmospheres of Mercury
Planet. Space Sci., Vol. 45, NO. 1, pp. 95-100, 1997
Pergamon
Published by Elsevier Science Ltd Printed in Great Britain 0032-0633/97 $17.00+0.00 PII: S00320633(96)00100-6
Sodium and potassium atmospheres of Mercury A. E. Potter’ and T. H. Morgan’ ‘SN3, NASA Johnson Space Center, Houston, TX 77058, U.S.A. ‘Southwest Research Institute, San Antonio, Texas, U.S.A. Received 31 October 1995; revised 29 March 1996; accepted 29 March 1996
Potassium atoms in the atmosphere of Mercury can also be detected by scattering of sunlight from resonant atomic transitions (Potter and Morgan, 1986 ; Sprague et al., 1990). Since sodium and potassium are similar in their chemical and physical properties, we expected that the sodium and potassium atmospheres would be similar. However, there are significant differences between sodium and potassium in volatility, atomic weight, and chemical reactivity. It was thought that comparison of sodium and potassium images might give some insight into the processes that maintain these atmospheres. For that reason, we attempted to obtain images of both sodium and potassium emissions, taken as close together in time as possible.
Observations Introduction Sodium in the atmosphere of Mercury can be detected by sunlight scattered at the D, and D2 resonance lines (Potter and Morgan, 1985). The sodium emission is sufficiently intense that it was found possible to produce images of the planetary distribution of sodium emission (Potter and Morgan, 1990). These images showed that sodium was concentrated at high latitudes, and changed significantly from day to day. The concentration at high latitudes and relatively rapid rate of change of the sodium emission suggested that sodium was generated from sputtering by magnetospheric particles precipitated to the surface. Recently, Potter (1995) has proposed that the sputtering process responsible for sodium generation is probably chemical sputtering, whereby sodium atoms are generated by chemical reaction of magnetospheric protons with sodium minerals at the surface to produce sodium vapor and water.
Correspondence to: A. E. Potter
The image slicer technique as described previously (Potter and Morgan, 1990) was applied to obtain images of the sodium and potassium distributions on Mercury over a period of 5 days, from December 6 to 10, 1990. A brief description of the technique follows : the image slicer views an area 5 arcsec on a side. This area is decomposed in the slicer optics into ten 0.5 x 5.0 arcsec slices that are spread end-to-end along the spectrograph slit. The slit is imaged through the spectrograph on to a CCD. Each slice is divided into 12 elements by the CCD pixels. The spectrum from each pixel is analyzed to extract the sodium or potassium intensity as a function of position along the slit. Following this step, the image is recomposed to yield an image of sodium or potassium intensity having 10 x 12 = 120 resolution elements. Because the angular size of the planet exceeded the angular area seen by the image slicer, it was necessary to collect several such images displaced a few arcseconds from one another, and then register and add these images together (averaging them in overlap regions) to get an image of the whole planet. A computer-controlled image stabilizer that allowed precise positioning of the planet was an essential part of this procedure.
96
A. E. Potter and T. H. Morgan: Sodium and potassium atmospheres of Mercury
Table 1. Observation conditions for Mercury
Date in 1990 December December December December December
6 7 8 9 10
Table 2. Sodium and potassium peak emission intensities
Magnitude
Polar diameter (arcsec)
Phase angle (deg)
Heliocentric distance (AU)
-0.4 -0.4 -0.3 -0.3 -0.2
6.72 6.88 7.08 7.23 7.42
76.9 80.5 84.4 88.5 92.9
0.3648 0.3590 0.3534 0.3478 0.3424
The sodium emission is intense enough so that images in both D, (5896 A) and Dz (5890 A) wavelengths is possible. However, imaging the potassium emission is difficult, because the potassium emission intensity is much less intense than sodium. Potassium scatters sunlight at resonance traOnsitions located at 7699 A (D,) and 7664 A (D2). The 7699 A transition is clear of atmospheric absorption:, but is too weak to give satisfactory images. The 7664A transition is twice as strong, but ordinarily is masked by a strong oxygen absorption line in the Earth’s atmosphere. However, there are short periods when the Mercury-Sun Doppler shift is large, so as to move the line out of the bottom of the solar potassium Fraunhofer line, and simultaneously, the Mercury-Earth Doppler shift is large enough to move the line out from under the terrestrial oxygen line. This combination occurs two or three times a year. When it does occur, then the potassium emission is intense enough so that an image can be generated. This condition existed during the period December 6-10, 1990, so that it was possible to image Mercury in the light of the potassium Dz transition at 7664A. Even so, it was necessary to average four resolution elements to get a satisfactory signal-to-noise ratio for the potassium emission. The conditions of Mercury during these observations are listed in Table 1. All observations were done during daylight hours at the National Solar Observatory McMath solar telescope, located at Kitt Peak National Observatory, Arizona.
Observational
results
Figure 1 shows a sodium D, emissionimage along with an image of the intensity of 59OCL59 10 A sunlight reflected from the Mercury surface as observed on December 7, 1990. An outline of the planet is superimposed over the images to provide a sense of scale and direction. The intensity of the emission is color coded, with red as maximum and blue as minimum, as shown in the color bar at the bottom of the figure. Figure 2 is a similar illustration, that shows the potassium and surface reflection images for the same date. After making allowance for the larger potassium pixels, the two images are generally similar. In both, most of the emission is concentrated in the southern hemisphere, with lesser amounts in the northern hemisphere. While not shown, the sodium and potassium images on the other dates are also similar in their planetary distribution. The peak intensities of sodium and potassium emissions for each of the obser-
Sodium, D, Sodium, D, Potassium, D2 peak intensity peak intensity peak intensity Date in 1990 (megarayleighs) (megarayleighs) (megarayleighs) December December December December December
6 7 8 9 10
2.9 4.2 3.2 5.1 5.7
4.1 6.1 4.4 7.1 7.9
0.23 0.32 0.26 0.40 0.53
vation dates are given in Table 2. The apparent intensity of sunlight reflected from the Mercury surface outside the sodium and potassium lines was used as a calibration source to calculate the atom emission intensities, as described previously by Potter and Morgan (1990). The peak emission rates for potassium Dz reported in Table 2 are comparable to, but generally larger than those previously reported by Sprague et al. (1990), who gave peak values ranging from 0.049 to 0.32megarayleighs. The daily variations of sodium and potassium emissions are compared in Fig. 3, where the peak intensities are plotted against the date. There are substantial daily changes in all the emissions, and there appears to be a secular increase of all intensities with time. It is apparent from this plot that all the emissions are strongly correlated with one another, suggesting a common origin. This is shown clearly in Fig. 4, where sodium D2 emission intensity is plotted against potassium Dz emission intensity. On average, the ratio of sodium D, to potassium Dz intensity is 17. The intensity data can be used to estimate the peak column abundance of sodium and potassium. This is straightforward for potassium, which is optically thin. Multiplication of the Rayleigh intensity (photons per square centimeter column) by the number of solar photons scattered by each potassium atom per second yields the column abundance in atoms per square centimeter column. A reasonable estimate for the uncertainty of the potassium abundance is about T 10%. The case of sodium is not so simple, since it is optically thick on Mercury, at least for the most intense emission areas. The sodium abundance was calculated from the ratio of the Dz and Dr sodium emission line strengths, using radiation transfer theory for an optically thick atmosphere (Chamberlain, 1978). The result depends on the temperature of the sodium, which is not well known. A mean temperature of 500K was used, based on measurements of the Doppler width of the sodium emission line reported by Potter and Morgan (1987). With the uncertainties of the two individual measurements, a reasonable estimate of the uncertainty of the sodium abundance is about T 20%. Results of the column density calculations are given in Table 3, and plotted in Fig. 5. The table and the plot show that the metal atom abundance increased substantially during the period of observation. The solar radiation intensity increased by a factor of about 15% during this period, but the metal atom abundance increased by almost a factor of two, suggesting that a factor other than solar radiation intensity was at work. The data in Table 3 yield an average ratio of sodium to potassium abundance (Na/K) of 190. This is larger by
A. E. Potter and T. H. Morgan:
Sodium and potassium
atmospheres
of Mercury
Fig. 1. Images of Mercury in 5900-5920 .& sunlight reflected from the surface and in sunlight scattered at 5890 A from sodium vapor in the Mercury atmosphere. The measurements were done on December 7,199o
97
98
A. E. Potter and T. H. Morgan: Sodium and potassium atmospheres of Mercury
Fig. 2. @ages of Mercury in 767&7680 A sunlight reflected from the surface and in sunlight scattered at 7664A from potassium vapor in the Mercury atmosphere. The measurements were done on December 7, 1990
99
A. E. Potter and T. H. Morgan: Sodium and potassium atmospheres of Mercury 10000
I
5000 2 ‘g
,
1
t
I
$242”;
7000
3000
2 0 22 d 2 2 -r 5 ..-2 5 Y I a
1000 700 500 300
I
1
1
I
I
6
7
8
9
10
100 5
11
Day of Month, December, 1990
5
Fig. 3. The peak emission intensities for sodipm D, (5896A), sodium D, (5890 A), and potassium D2 (7664 A) from Mercury over the period from December 6 to 10, 1990. These peak intensities were extracted from the images similar to those shown in Figs 1 and 2. The fact that the intensities vary substantially from day to day is clearly evident
3000
4000
5000
6000
7000
1:
8000
9000
Sodium 5890 A Emission, kilorayleighs
Fig. 4. The sodium and potassium D, peak intensities are compared in this figure. They are closely correlated to one another, which suggests that they have a common origin
6
7
8
Date, December,
9
10
11
1990
Fig. 5. The column abundances of sodium and potassium are compared in this plot. There is a secular increase of abundance with time that is much larger than can be accounted for by changes in solar radiation intensity. The fluctuations of potassium abundance seen in this plot are probably real
about a factor of two than the result of 80 reported earlier for November 16, 1985 (Potter and Morgan, 1987). The ratio in both cases is much larger than expected from the cosmic abundance (20) in lunar rocks (2-7), and in meteorites (average of 13). As noted previously (Potter and Morgan, 1986), photoionization is the dominant loss process for both sodium and potassium. Since the photoionization rate for potassium is 40% larger than for sodium, we expect that potassium will be lost at a greater rate than sodium, resulting in an increase in the atmospheric sodium/potassium ratio above the value of the ratio in the source materials. The November 16, 1985 measurement was done near solar minimum, when the F10.7 index was near 80. The December 6-10, 1990 measurements were done near solar maximum, when the F10.7 index was approximately 230. The F10.7 index is a measure of the ultraviolet activity of the Sun. We suggest that increased solar ultraviolet flux is the cause of the increased Na/K ratio observed during December 6-10, 1990 relative to the Na/K ratio observed on November 16, 1985.
Concluding remarks Table 3. Column densities of sodium and potassium
Date in 1990 December December December December December
6 7 8 9 10
Sodium density (atoms cm-’ column x IO-“)
Potassium density (atoms cm-’ column x 10m9)
4.5 4.5 6.0 5.5 6.0
2.1 2.9 2.3 3.5 4.6
The general nature of the sodium images is like that previously observed. The sodium emission is most prominent at high southern and northern latitudes, and changes from day to day. For the period of these observations, the emission was most intense at high southern latitudes. The potassium images are similar, in that the emission is most prominent at high southern latitudes, and changes from day to day. The sodium and potassium images are closely correlated both in their intensity and distribution. This result supports the view that they are both produced by
100
A. E. Potter and T. H. Morgan:
the same process. The latitude distribution and daily changes of intensity for both sodium and potassium suggest that they are both generated by particles impacting on the Mercury surface in the magnetospheric cusps. The peak values for potassium column densities were in the same range of peak values previously reported by Sprague et al. (1990) around the Caloris Basin near 200 deg longitude and its antipode. The peak values for our observations were observed at longitudes ranging from 70 to 90 deg. There are no geologic features of major size or importance in this region, which weakens the case for the suggestion that sodium and potassium intensities are correlated with major geologic features (see also Killen et al., 1990, 1991; Shemansky and Morgan, 1991). The differences of physical and chemical properties of sodium and potassium seem to have little effect, except on the relative abundance of the species. Potassium is more rapidly removed from the atmosphere than sodium by photoionization. The result of this is that the potassium abundance relative to the sodium abundance in the atmosphere depends on the flux of ionizing solar radiation, decreasing with increasing radiation flux. This effect could explain the increased values of the Na/K ratio observed near solar maximum relative to those observed near solar minimum.
Sodium and potassium
atmospheres
of Mercury
References Chamberlain, J. W. (1978) Theory of Planetary Atmospheres. Academic Press, New York. Killen, R. M., Potter, A. E. and Morgan, T. H. (1990) Spatial distribution of sodium vapor in the atmosphere of Mercury.
Icarus 85,145-167. Killen, R. M., Potter, A. E. and Morgan, T. H. (1991) Detecting potassium on Mercury. Science 262, 974-975. Potter, A. E. (1995) Chemical sputtering could produce sodium vapor and water on Mercury. Geophys. Res. Lett. 22, 3289-
3292. Potter, A. E. and Morgan, T. H. (1985) Discovery of sodium in the atmosphere of Mercury. Science 229,651-653. Potter, A. E. and Morgan, T. H. (1986) Potassium in the atmosphere of Mercury. Icarus 67, 336-340. Potter, A. E. and Morgan, T. H. (1987) Variation of sodium on Mercury with solar radiation pressure. Icarus 71,472477. Potter, A. E. and Morgan, T. H. (1990) Evidence for magnetospheric effects on the sodium atmosphere of Mercury. Science 248, 835-838. Shemansky, D. E. and Morgan, T. H. (1991) Source processes for the alkali metals in the atmosphere of Mercury. Geophys. Res. Lett. 18, 1659-1662. Sprague, A. L., Kozlowski, R. W. H. and Hunten, D. M. (1990) Caloris basin: an enhanced source for potassium in Mercury’s atmosphere. Science 249, 1140-l 143.
Pergamon
Published by Elsevier Science Ltd Printed in Great Britain 0032-0633/97 $17.00+0.00 PII: S00320633(96)00100-6
Sodium and potassium atmospheres of Mercury A. E. Potter’ and T. H. Morgan’ ‘SN3, NASA Johnson Space Center, Houston, TX 77058, U.S.A. ‘Southwest Research Institute, San Antonio, Texas, U.S.A. Received 31 October 1995; revised 29 March 1996; accepted 29 March 1996
Potassium atoms in the atmosphere of Mercury can also be detected by scattering of sunlight from resonant atomic transitions (Potter and Morgan, 1986 ; Sprague et al., 1990). Since sodium and potassium are similar in their chemical and physical properties, we expected that the sodium and potassium atmospheres would be similar. However, there are significant differences between sodium and potassium in volatility, atomic weight, and chemical reactivity. It was thought that comparison of sodium and potassium images might give some insight into the processes that maintain these atmospheres. For that reason, we attempted to obtain images of both sodium and potassium emissions, taken as close together in time as possible.
Observations Introduction Sodium in the atmosphere of Mercury can be detected by sunlight scattered at the D, and D2 resonance lines (Potter and Morgan, 1985). The sodium emission is sufficiently intense that it was found possible to produce images of the planetary distribution of sodium emission (Potter and Morgan, 1990). These images showed that sodium was concentrated at high latitudes, and changed significantly from day to day. The concentration at high latitudes and relatively rapid rate of change of the sodium emission suggested that sodium was generated from sputtering by magnetospheric particles precipitated to the surface. Recently, Potter (1995) has proposed that the sputtering process responsible for sodium generation is probably chemical sputtering, whereby sodium atoms are generated by chemical reaction of magnetospheric protons with sodium minerals at the surface to produce sodium vapor and water.
Correspondence to: A. E. Potter
The image slicer technique as described previously (Potter and Morgan, 1990) was applied to obtain images of the sodium and potassium distributions on Mercury over a period of 5 days, from December 6 to 10, 1990. A brief description of the technique follows : the image slicer views an area 5 arcsec on a side. This area is decomposed in the slicer optics into ten 0.5 x 5.0 arcsec slices that are spread end-to-end along the spectrograph slit. The slit is imaged through the spectrograph on to a CCD. Each slice is divided into 12 elements by the CCD pixels. The spectrum from each pixel is analyzed to extract the sodium or potassium intensity as a function of position along the slit. Following this step, the image is recomposed to yield an image of sodium or potassium intensity having 10 x 12 = 120 resolution elements. Because the angular size of the planet exceeded the angular area seen by the image slicer, it was necessary to collect several such images displaced a few arcseconds from one another, and then register and add these images together (averaging them in overlap regions) to get an image of the whole planet. A computer-controlled image stabilizer that allowed precise positioning of the planet was an essential part of this procedure.
96
A. E. Potter and T. H. Morgan: Sodium and potassium atmospheres of Mercury
Table 1. Observation conditions for Mercury
Date in 1990 December December December December December
6 7 8 9 10
Table 2. Sodium and potassium peak emission intensities
Magnitude
Polar diameter (arcsec)
Phase angle (deg)
Heliocentric distance (AU)
-0.4 -0.4 -0.3 -0.3 -0.2
6.72 6.88 7.08 7.23 7.42
76.9 80.5 84.4 88.5 92.9
0.3648 0.3590 0.3534 0.3478 0.3424
The sodium emission is intense enough so that images in both D, (5896 A) and Dz (5890 A) wavelengths is possible. However, imaging the potassium emission is difficult, because the potassium emission intensity is much less intense than sodium. Potassium scatters sunlight at resonance traOnsitions located at 7699 A (D,) and 7664 A (D2). The 7699 A transition is clear of atmospheric absorption:, but is too weak to give satisfactory images. The 7664A transition is twice as strong, but ordinarily is masked by a strong oxygen absorption line in the Earth’s atmosphere. However, there are short periods when the Mercury-Sun Doppler shift is large, so as to move the line out of the bottom of the solar potassium Fraunhofer line, and simultaneously, the Mercury-Earth Doppler shift is large enough to move the line out from under the terrestrial oxygen line. This combination occurs two or three times a year. When it does occur, then the potassium emission is intense enough so that an image can be generated. This condition existed during the period December 6-10, 1990, so that it was possible to image Mercury in the light of the potassium Dz transition at 7664A. Even so, it was necessary to average four resolution elements to get a satisfactory signal-to-noise ratio for the potassium emission. The conditions of Mercury during these observations are listed in Table 1. All observations were done during daylight hours at the National Solar Observatory McMath solar telescope, located at Kitt Peak National Observatory, Arizona.
Observational
results
Figure 1 shows a sodium D, emissionimage along with an image of the intensity of 59OCL59 10 A sunlight reflected from the Mercury surface as observed on December 7, 1990. An outline of the planet is superimposed over the images to provide a sense of scale and direction. The intensity of the emission is color coded, with red as maximum and blue as minimum, as shown in the color bar at the bottom of the figure. Figure 2 is a similar illustration, that shows the potassium and surface reflection images for the same date. After making allowance for the larger potassium pixels, the two images are generally similar. In both, most of the emission is concentrated in the southern hemisphere, with lesser amounts in the northern hemisphere. While not shown, the sodium and potassium images on the other dates are also similar in their planetary distribution. The peak intensities of sodium and potassium emissions for each of the obser-
Sodium, D, Sodium, D, Potassium, D2 peak intensity peak intensity peak intensity Date in 1990 (megarayleighs) (megarayleighs) (megarayleighs) December December December December December
6 7 8 9 10
2.9 4.2 3.2 5.1 5.7
4.1 6.1 4.4 7.1 7.9
0.23 0.32 0.26 0.40 0.53
vation dates are given in Table 2. The apparent intensity of sunlight reflected from the Mercury surface outside the sodium and potassium lines was used as a calibration source to calculate the atom emission intensities, as described previously by Potter and Morgan (1990). The peak emission rates for potassium Dz reported in Table 2 are comparable to, but generally larger than those previously reported by Sprague et al. (1990), who gave peak values ranging from 0.049 to 0.32megarayleighs. The daily variations of sodium and potassium emissions are compared in Fig. 3, where the peak intensities are plotted against the date. There are substantial daily changes in all the emissions, and there appears to be a secular increase of all intensities with time. It is apparent from this plot that all the emissions are strongly correlated with one another, suggesting a common origin. This is shown clearly in Fig. 4, where sodium D2 emission intensity is plotted against potassium Dz emission intensity. On average, the ratio of sodium D, to potassium Dz intensity is 17. The intensity data can be used to estimate the peak column abundance of sodium and potassium. This is straightforward for potassium, which is optically thin. Multiplication of the Rayleigh intensity (photons per square centimeter column) by the number of solar photons scattered by each potassium atom per second yields the column abundance in atoms per square centimeter column. A reasonable estimate for the uncertainty of the potassium abundance is about T 10%. The case of sodium is not so simple, since it is optically thick on Mercury, at least for the most intense emission areas. The sodium abundance was calculated from the ratio of the Dz and Dr sodium emission line strengths, using radiation transfer theory for an optically thick atmosphere (Chamberlain, 1978). The result depends on the temperature of the sodium, which is not well known. A mean temperature of 500K was used, based on measurements of the Doppler width of the sodium emission line reported by Potter and Morgan (1987). With the uncertainties of the two individual measurements, a reasonable estimate of the uncertainty of the sodium abundance is about T 20%. Results of the column density calculations are given in Table 3, and plotted in Fig. 5. The table and the plot show that the metal atom abundance increased substantially during the period of observation. The solar radiation intensity increased by a factor of about 15% during this period, but the metal atom abundance increased by almost a factor of two, suggesting that a factor other than solar radiation intensity was at work. The data in Table 3 yield an average ratio of sodium to potassium abundance (Na/K) of 190. This is larger by
A. E. Potter and T. H. Morgan:
Sodium and potassium
atmospheres
of Mercury
Fig. 1. Images of Mercury in 5900-5920 .& sunlight reflected from the surface and in sunlight scattered at 5890 A from sodium vapor in the Mercury atmosphere. The measurements were done on December 7,199o
97
98
A. E. Potter and T. H. Morgan: Sodium and potassium atmospheres of Mercury
Fig. 2. @ages of Mercury in 767&7680 A sunlight reflected from the surface and in sunlight scattered at 7664A from potassium vapor in the Mercury atmosphere. The measurements were done on December 7, 1990
99
A. E. Potter and T. H. Morgan: Sodium and potassium atmospheres of Mercury 10000
I
5000 2 ‘g
,
1
t
I
$242”;
7000
3000
2 0 22 d 2 2 -r 5 ..-2 5 Y I a
1000 700 500 300
I
1
1
I
I
6
7
8
9
10
100 5
11
Day of Month, December, 1990
5
Fig. 3. The peak emission intensities for sodipm D, (5896A), sodium D, (5890 A), and potassium D2 (7664 A) from Mercury over the period from December 6 to 10, 1990. These peak intensities were extracted from the images similar to those shown in Figs 1 and 2. The fact that the intensities vary substantially from day to day is clearly evident
3000
4000
5000
6000
7000
1:
8000
9000
Sodium 5890 A Emission, kilorayleighs
Fig. 4. The sodium and potassium D, peak intensities are compared in this figure. They are closely correlated to one another, which suggests that they have a common origin
6
7
8
Date, December,
9
10
11
1990
Fig. 5. The column abundances of sodium and potassium are compared in this plot. There is a secular increase of abundance with time that is much larger than can be accounted for by changes in solar radiation intensity. The fluctuations of potassium abundance seen in this plot are probably real
about a factor of two than the result of 80 reported earlier for November 16, 1985 (Potter and Morgan, 1987). The ratio in both cases is much larger than expected from the cosmic abundance (20) in lunar rocks (2-7), and in meteorites (average of 13). As noted previously (Potter and Morgan, 1986), photoionization is the dominant loss process for both sodium and potassium. Since the photoionization rate for potassium is 40% larger than for sodium, we expect that potassium will be lost at a greater rate than sodium, resulting in an increase in the atmospheric sodium/potassium ratio above the value of the ratio in the source materials. The November 16, 1985 measurement was done near solar minimum, when the F10.7 index was near 80. The December 6-10, 1990 measurements were done near solar maximum, when the F10.7 index was approximately 230. The F10.7 index is a measure of the ultraviolet activity of the Sun. We suggest that increased solar ultraviolet flux is the cause of the increased Na/K ratio observed during December 6-10, 1990 relative to the Na/K ratio observed on November 16, 1985.
Concluding remarks Table 3. Column densities of sodium and potassium
Date in 1990 December December December December December
6 7 8 9 10
Sodium density (atoms cm-’ column x IO-“)
Potassium density (atoms cm-’ column x 10m9)
4.5 4.5 6.0 5.5 6.0
2.1 2.9 2.3 3.5 4.6
The general nature of the sodium images is like that previously observed. The sodium emission is most prominent at high southern and northern latitudes, and changes from day to day. For the period of these observations, the emission was most intense at high southern latitudes. The potassium images are similar, in that the emission is most prominent at high southern latitudes, and changes from day to day. The sodium and potassium images are closely correlated both in their intensity and distribution. This result supports the view that they are both produced by
100
A. E. Potter and T. H. Morgan:
the same process. The latitude distribution and daily changes of intensity for both sodium and potassium suggest that they are both generated by particles impacting on the Mercury surface in the magnetospheric cusps. The peak values for potassium column densities were in the same range of peak values previously reported by Sprague et al. (1990) around the Caloris Basin near 200 deg longitude and its antipode. The peak values for our observations were observed at longitudes ranging from 70 to 90 deg. There are no geologic features of major size or importance in this region, which weakens the case for the suggestion that sodium and potassium intensities are correlated with major geologic features (see also Killen et al., 1990, 1991; Shemansky and Morgan, 1991). The differences of physical and chemical properties of sodium and potassium seem to have little effect, except on the relative abundance of the species. Potassium is more rapidly removed from the atmosphere than sodium by photoionization. The result of this is that the potassium abundance relative to the sodium abundance in the atmosphere depends on the flux of ionizing solar radiation, decreasing with increasing radiation flux. This effect could explain the increased values of the Na/K ratio observed near solar maximum relative to those observed near solar minimum.
Sodium and potassium
atmospheres
of Mercury
References Chamberlain, J. W. (1978) Theory of Planetary Atmospheres. Academic Press, New York. Killen, R. M., Potter, A. E. and Morgan, T. H. (1990) Spatial distribution of sodium vapor in the atmosphere of Mercury.
Icarus 85,145-167. Killen, R. M., Potter, A. E. and Morgan, T. H. (1991) Detecting potassium on Mercury. Science 262, 974-975. Potter, A. E. (1995) Chemical sputtering could produce sodium vapor and water on Mercury. Geophys. Res. Lett. 22, 3289-
3292. Potter, A. E. and Morgan, T. H. (1985) Discovery of sodium in the atmosphere of Mercury. Science 229,651-653. Potter, A. E. and Morgan, T. H. (1986) Potassium in the atmosphere of Mercury. Icarus 67, 336-340. Potter, A. E. and Morgan, T. H. (1987) Variation of sodium on Mercury with solar radiation pressure. Icarus 71,472477. Potter, A. E. and Morgan, T. H. (1990) Evidence for magnetospheric effects on the sodium atmosphere of Mercury. Science 248, 835-838. Shemansky, D. E. and Morgan, T. H. (1991) Source processes for the alkali metals in the atmosphere of Mercury. Geophys. Res. Lett. 18, 1659-1662. Sprague, A. L., Kozlowski, R. W. H. and Hunten, D. M. (1990) Caloris basin: an enhanced source for potassium in Mercury’s atmosphere. Science 249, 1140-l 143.