Distribution and Abundance of Sodium in Mercury's Atmosphere, 1985–1988

Sprague, A. L.; Kozlowski, R. W. H.; Hunten, D. M.; Schneider, Nicholas M.; Domingue, D. L.; Wells, W. K.; Schmitt, W.; Fink, Uwe

doi:10.1006/icar.1997.5784

Cited by 103 publications

(73 citation statements)

References 30 publications

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Section: Ii-3 Resultsmentioning

confidence: 99%

“…As explained by Sprague et al (1997), seeing can be accounted for by convolving the simulated 2D emission brightness with a Gaussian distribution:…”

Section: Ii-3 Resultsmentioning

confidence: 99%

“…A dawn/ dusk asymmetry in the sodium emission from Mercury was first suggested by Hunten and Sprague (1997) using ground based observations and by analogy with the Moon's argon cycle (Hodges 1974). However, the presence of such an asymmetry in Mercury's sodium exosphere was debated for a long time because of a lack of clear observational evidence.…”

Section: Introductionmentioning

confidence: 99%

“…This determines the efficiency of a particle to scatter solar photons (the g-factor) and is based on the same Before comparing simulated 2D images of the brightness with observation, the effect of the Earth atmospheric seeing is included. As explained by Sprague et al (1997), seeing can be accounted for by convolving the simulated 2D emission brightness with a Gaussian distribution:where r is the distance to the position where the emission brightness is being calculated, f 0 is a normalization factor, and σ is the one-sigma value of the seeing effect (the full width at half maximum, FWHM, being equal to ln2 σ 2 × × ). Comparing panels a, d, e and f also provides an illustration of the effect of the atmospheric seeing on Mercury's exospheric emission.…”

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confidence: 99%

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Mercury exosphere I. Global circulation model of its sodium component

Leblanc

Johnson

2010

Icarus

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Our understanding of Mercury's sodium exosphere has improved considerably in the last 5 years thanks to new observations (Schleicher et al. 2004) and to the publication of a summary of the large set of ground based observations (Potter et al. 2008;2007;. In particular, the non-uniformity in longitude of the dayside sodium distribution (the dawn/dusk asymmetry) has now been clearly observed. This suggests that Mercury's sodium exosphere is partly driven by a global day to night side migration of the volatiles. One of the key questions remaining is the nature of the prevailing sodium ejection mechanisms. Because of the uncertain parameters for each ejection mechanisms, solving this problem has been difficult as indicated by the numerous papers over the last 15 years with very different conclusions. In addition, the variation of the size and of the spatial distribution of the surface reservoir varies with distance from the sun affecting the importance of each ejection mechanism on Mercury's orbital position We here present an updated version of the model. We take into account the two populations of sodium in the surface reservoir , one ambient population (physisorbed in the regolith with low binding energy) and one source population (chemisorbed coming from grain interior or from fresh dust brought to the surface and characterized by a higher binding energy). We also incorporate a better description of the solar wind sputtering variation with solar conditions. The results of a large number of simulations of the sodium exosphere are compared with the measured annual cycle of Mercury sodium emission brightness. These measurements were obtained from the published data by Potter et al. (2007) as well as from our own data obtained during the last two years using THEMIS solar telescope. These data show that: the annual cycle in the emission brightness is roughly the same from one year to another; there are significant discrepancies ACCEPTED MANUSCRIPT3 between what would be observed if the exospheric content were constant; and t the annual cycle of Mercury's sodium exosphere strongly depends on its position in its orbit so that there are seasons in Mercury's exosphere.Based on these comparisons we derived the principal signatures for each ejection mechanism during a Mercury year and show that none of the ejection mechanisms dominates over the whole year. Rather, particular features of the annual cycle of the sodium intensity appear to be induced by one, temporarily dominant, ejection mechanism. Based on this analysis, we are able to roughly explain the annual cycle of Mercury's exospheric sodium emission brightness.We also derive a set of parameters defining those ejection mechanisms which best reproduce this cycle. For our best case, Mercury's exosphere content varies from ~1.6±0.1×10 28 Na atoms at TAA=140° and 70° respectively to ~4.5±0.3×10 28 Na atoms at TAA=180° and 0°. In addition, Mercury's exospheric surface reservoir contains ~1×10 31 Na atoms at TAA=300° and at TAA=170° with up to three times more so...

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Section: Ii-3 Resultsmentioning

confidence: 99%

“…As explained by Sprague et al (1997), seeing can be accounted for by convolving the simulated 2D emission brightness with a Gaussian distribution:…”

Section: Ii-3 Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

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Mercury exosphere I. Global circulation model of its sodium component

Leblanc

Johnson

2010

Icarus

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“…Since the sodium resonance emissions were discovered, hundreds of observations have been made (for example, Sprague, et al 1997). The average column density of sodium is of the order of 10 11 atoms/cm 2 , varying in an apparently random fashion by a factor of two or three throughout the Mercury year, with a tendency to be largest at perihelion and aphelion, and smallest in the parts of the orbit where heliocentric velocity and the corresponding radiation pressure on sodium…”

Section: Sodiummentioning

confidence: 99%

The Surface-Bounded Exosphere of Mercury

Potter

2005

Highlights astron.

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Almost thirty years ago, the instruments on the Mariner 10 spacecraft detected traces of hydrogen, helium, and possibly oxygen in the atmosphere of Mercury. There the matter rested until the mid-80’s, when emission lines of sodium (Potter & Morgan 1985) and potassium (Potter & Morgan 1986) were found in the spectrum of Mercury, resulting from resonance scattering of sunlight by metal atoms in the atmosphere. Searches for other metals in the Mercury atmosphere were fruitless until recently, when an emission line attributed to calcium has been observed (Bida, Killen & Morgan 2000). The densities of all these species are so low that gas phase collisions are negligible, so that the Mercury atmosphere is an exosphere bounded at its base by the surface of the planet. The fact that the exosphere is bounded by the Mercury surface means that interactions with the surface must occur. Energy exchange, surface absorption and desorption, and ion neutralization can take place, and these must be accounted for in models of the exosphere. None of these atmospheric species can survive long on Mercury, being lost mainly by photoionization followed by trapping in the solar wind and partly by solar radiation acceleration. Consequently, the species that we observe must be in a steady state, being generated as fast as they are lost to space. The metals must originate from the surface of the regolith, with the supply of fresh surface material maintained by gardening of the surface by meteoroid impact. The metals are released from the surface into the exosphere by photon-stimulated desorption (Madey et al. 1998), particle sputtering (McGrath, Johnson & Lanzerotti 1986), and meteoroid impact vaporization (Morgan, Zook & Potter 1988). Thermal evaporation of species condensed on the cold side may be important for determining their distribution over the planet (Hunten & Sprague 2002). The interaction and relative importance of all these processes has been discussed by Killen & Morgan (1993). Much can be said about these processes, but this review is focused primarily on the current status of observations of sodium, potassium, and calcium.

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A cold‐pole enhancement in Mercury's sodium exosphere

Cassidy

McClintock

Killen

et al. 2016

Geophysical Research Letters

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The ultraviolet and visible spectrometer component of the Mercury Atmospheric and Surface Composition Spectrometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft characterized the local‐time distribution of the sodium exosphere over the course of its orbital mission. The observations show that the sodium exosphere is enhanced above Mercury's cold‐pole longitudes. Based on previously published sodium exosphere models, we infer that these regions act as night side surface reservoirs, temporary sinks to the exosphere that collect sodium atoms transported antisunward. The reservoirs are revealed as exospheric enhancements when they are exposed to sunlight. As in the models the reservoir is depleted as the cold poles rotate from dawn to dusk, but unlike the models the depletion is only partial. The persistence of the reservoir means that it could, over the course of geologically long periods of time, contribute to an increase in the bulk concentration of sodium near the cold‐pole longitudes.

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Distribution and Abundance of Sodium in Mercury's Atmosphere, 1985–1988

Cited by 103 publications

References 30 publications

Mercury exosphere I. Global circulation model of its sodium component

Mercury exosphere I. Global circulation model of its sodium component

The Surface-Bounded Exosphere of Mercury

A cold‐pole enhancement in Mercury's sodium exosphere

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