During the first MESSENGER flyby of Mercury, the UltraViolet and Visible Spectrometer channel of ... more During the first MESSENGER flyby of Mercury, the UltraViolet and Visible Spectrometer channel of the Mercury Atmospheric and Surface Composition Spectrometer observed Mercury's sodium tail and exosphere. Flybys provide opportunities for extended tail observations.
Ground-based observations of Mercury's sodium exospheric emission were obtained at the McMath... more Ground-based observations of Mercury's sodium exospheric emission were obtained at the McMath-Pierce Solar Telescope at Kitt Peak, Arizona, concurrently with the observations from the Ultraviolet and Visible Spectrometer (UVVS) on the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument onboard the MESSENGER spacecraft during the first flyby of the planet, January 14, 2008, and three weeks after the second flyby
Mercury's exosphere is composed of material that originates at the planet's surface, whet... more Mercury's exosphere is composed of material that originates at the planet's surface, whether that material is native or delivered by the solar wind and micrometeoroids. Many exospheric species have been detected by remote sensing, including H and He by Mariner 10, Na, K, and Ca by ground-based observations, and H, Na, Ca, Mg, and Ca+ by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. Other exospheric species, including Fe, Al, Si, O, S, Mn, Cl, Ti, OH, and their ions, are expected to be present on the basis of MESSENGER surface measurements and models of Mercury's surface chemistry. Here we report on searches for these species made with the Ultraviolet and Visible Spectrometer (UVVS) channel of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS). No obvious signatures of the listed species have yet been observed in Mercury's exosphere by the UVVS as of this writing. It is possible that detections are elusive ...
When Io emerges from eclipse by Jupiter, the sodium D-line emissions are weak, partly because of ... more When Io emerges from eclipse by Jupiter, the sodium D-line emissions are weak, partly because of the small Doppler shift of Io relative to the Sun, but also because the neutral sodium density is a factor of two to four times smaller than when Io has been out of the shadow and exposed to sunshine for an hour or two. We have observed four cases of post-eclipse brightening of the sodium emission, using a 10 arc second aperture image slicer to obtain images of sodium around Io. These show that initially, most of the sodium is confined to the "banana cloud" near Io. As time passes, the sodium density close to Io increases, and after and hour or two, most of the sodium is found surrounding Io. We have thought of two possible explanations for the post-eclipse sodium growth. One has to do with condensation of sulfur dioxide during the eclipse, covering particles of NaCl, which are thought to be the source of sodium (Hunten, 2003). As Io emerges from eclipse, the sulfur dioxide sub...
High resolution line profiles of the Na D2 emission from Mercury's atmosphere were obtained ... more High resolution line profiles of the Na D2 emission from Mercury's atmosphere were obtained at the Anglo-Australian Telescope June 6, 1998, at approximately 3 mA resolution. The spatial resolution (not including atmospheric smearing) was 1"x1.5", or about 1/3 planetary radius. Line profiles were obtained at high N, high S and equatorial latitudes, respectively. If interpreted in terms of a gas in thermal equilibrium, the gas temperature would vary from 1500 K at the equator to 750 K and 550 K at high N and high S latitudes, respectively. In addition, at high latitudes 1 - 10 hot component as source atoms ejected by an energetic process such as meteoritic vaporization or sputtering. The gas is nowhere in thermal equilibrium with the surface but remains hotter than the surface temperature. This implies that an energetic process such as chemistry or photon-stimulated desorption dominates the surface interaction. The difference in derived temperature at the N and S polar ...
We obtained images of the sodium emission surrounding Io at high spectral resolution (R= 150,000)... more We obtained images of the sodium emission surrounding Io at high spectral resolution (R= 150,000), using the National Solar Observatory McMath-Pierce stellar spectrograph and a 10x10 arc second image slicer. At this resolution, we can distinguish different velocity components in the sodium emission line profile. The intensity of these components varies, depending on local magnetospheric conditions at Io. In some cases, we can separate these components and generate images of each. The sodium emission was unusually bright on November 20, 1999. This, combined with excellent seeing conditions, allowed us to generate images for that night of different populations of sodium, each having distinctly different velocities. We observed the changes in these populations over a period of about three hours following emergence of Io from eclipse by Jupiter. These observations were part of our continuing observations of the increases observed in sodium emissions following emergence of Io from eclips...
The ratio of sodium to potassium in the exosphere of Mercury is highly variable and averages abou... more The ratio of sodium to potassium in the exosphere of Mercury is highly variable and averages about 100, considerably larger than for the Moon. We considered the possibility that this high value results from potassium loss rates that are larger than sodium loss rates. The principal loss processes are the sweeping of hot atoms off the planet by solar radiation acceleration and the capture of metal photoions by the solar wind. We found that radiation acceleration losses are not sufficiently different for sodium and potassium to explain the high ratio. Capture of sodium and potassium photoions by the solar wind is thought to be the major loss process. However, there is a competing process for the ions, which is collision with the surface, followed by neutralization. These atoms can return to the exosphere. The net steady state density of sodium and potassium in the exosphere depends on the relative efficiencies of solar wind capture and surface recycling. We propose that the photoion loss and recycling rates are sufficiently different for sodium and potassium to account for the high ratio. The higher mass of the potassium atom relative to sodium results in a smaller scale height and a larger gyroradius for potassium relative to sodium, which may result in more rapid net loss of potassium.
Mercury has a tenuous exosphere created by the combined effects of solar radiation and micrometeo... more Mercury has a tenuous exosphere created by the combined effects of solar radiation and micrometeoroid bombardment on the surface and the interaction of the solar wind with Mercury's magnetic field and surface. Observations of this exosphere provide essential data necessary for understanding the composition and evolution of Mercury's surface, as well as the interaction between Mercury's magnetosphere and the solar wind. The exosphere was observed during the MESSENGER flybys with the Mercury Atmospheric and Surface Composition Spectrometer (MASCS). The detections of sodium and calcium during the first two flybys, as well as magnesium during the second flyby, have been previously reported (McClintock et al. 2008,2009). Sodium and calcium showed significantly different distributions in both the tail and near-surface component of the exosphere suggesting they are ejected by different processes and from different regions. We present a Monte-Carlo model of sodium and calcium in...
Abstract— We propose that argon‐40 measured in the lunar atmosphere and that in Mercury's atm... more Abstract— We propose that argon‐40 measured in the lunar atmosphere and that in Mercury's atmosphere is due to current diffusion into connected pore space within the crust. Higher temperatures at Mercury, along with more rapid loss from the atmosphere, will lead to a similar or smaller column abundance of argon at Mercury than at the Moon, given the same crustal abundance of potassium. Because the noble gas abundance in the mercurian atmosphere represents current effusion, it is a direct measure of the crustal potassium abundance. We assume a fractal distribution of distance to a connected pore space, with the shortest distance increasing with depth. Given this “rock size” distribution, we show that the diffusive flux is not a unique function of temperature. Even though the diffusion coefficient is an exponential function of temperature, the flux to the surface is fairly insensitive to the temperature.
An analysis is presented of the Voyager and IUE lyman alpha spectra of the Jovian equatorial emis... more An analysis is presented of the Voyager and IUE lyman alpha spectra of the Jovian equatorial emission in which was derived a zonal asymmetry in the hydrogen column abundance. Using two estimates of the fraction of Lyman alpha which is due to direct excitation by charged particle precipitation from the ionosphere, upper and lower limits were derived to the H column abundance within and without the perturbed region. That the asymmetry in H abundance may be due to localized heating near the homopause with a consequent rise in scale height is shown. The derived exospheric temperature remains fairly constant with longitude. The required additional heat input over the bulge region, 0.02 erg/cm/s, is supplied by an additional flux of magnetospheric electrons due to Jupiter's magnetic anomaly.
The surface composition of various planetary bodies has been measured by spectrometers in the inf... more The surface composition of various planetary bodies has been measured by spectrometers in the infrared, UV, x-ray and gamma-ray region of the spectrum (e.g. Deep Space 1, NEAR, Odyssey), and others are planned in the near future (CONTOUR, Messenger, Bepi-Colombo). The extent to which the surface composition reflects the composition of the crust or regolith as a whole is an important issue. For example, the observtions with the x-ray and gamma-ray spectrometers onboard the NEAR spacecraft indicated that the extreme surface of Eros is depleted in sulfur [1]. The surface of the Moon, although depleted in sulfur, is enhanced in most volatiles. It is important to understand the reasons for these extreme surface anomalies. We have considered selective vaporization of volatiles on airless bodies due to impact vaporization. We use the planar impact approximation to hyper-velocity impact [2], and we scale the critical pressure for vaporization by the enthalpy of the species in question [3]. ...
We present the dependence of the intensity in the sodium D1 and D2 lines on the planetary phase a... more We present the dependence of the intensity in the sodium D1 and D2 lines on the planetary phase and position on the disk. Theoretical intensities are convolved with an atmospheric point spread function with FWHM of 0″.5 and 2″.0 for planetary phases 60° and 90°. We show that although the absolute intensities are extremely dependent on seeing, the I(D2)/I(D1) ratio remains a reliable indicator of column abundance regardless of the seeing function. For a uniform distribution of sodium, limb brightening will be apparent for a seeing disk with FWHM of 0″.5 but will not be apparent for a FWHM of 2″.0.
NEAR XRS measurements of 433 Eros are most consistent with chondrites, but important compositiona... more NEAR XRS measurements of 433 Eros are most consistent with chondrites, but important compositional differences remain, particularly in regard to S abundance. S depletion appears explainable, in terms of impact volatilization processes.
We have developed a technique to obtain thermal IR spectra of Mercury in the 8-13 micron region i... more We have developed a technique to obtain thermal IR spectra of Mercury in the 8-13 micron region in daylight [1]. We observed a smooth spectral curve for Mercury, with the exception of minor features near 9.6 microns and 12.7 microns. There was no evidence of a Christiansen feature down to the 8-micron limit of our spectra. This is in general
40Ar in the atmospheres of the planets is a measure of potassium abundance in the interiors since... more 40Ar in the atmospheres of the planets is a measure of potassium abundance in the interiors since 40Ar is a product of radiogenic decay of 40K by electron capture with the subsequent emission of a 1.46 eV g-ray. Although the 40Ar in the earth's atmosphere is expected to have accumulated since the late bombardment, 40Ar in the atmospheres of Mercury and the Moon is eroded quickly by photoionization and electron impact ionization. Thus, the argon content in the exospheres of the Moon and Mercury is representative of current effusion rather than accumulation over the lifetime of the planet. We reconsider the source and loss processes for the argon atmospheres of Mercury and the Moon in order to investigate what these atmospheres tell us about the structure and composition of the megaregolith. Argon is especially important because it does not engage in chemistry. In order to interpret measurements of argon in Mercury's atmosphere, we must have a model including sources from th...
During the first MESSENGER flyby of Mercury, the UltraViolet and Visible Spectrometer channel of ... more During the first MESSENGER flyby of Mercury, the UltraViolet and Visible Spectrometer channel of the Mercury Atmospheric and Surface Composition Spectrometer observed Mercury's sodium tail and exosphere. Flybys provide opportunities for extended tail observations.
Ground-based observations of Mercury's sodium exospheric emission were obtained at the McMath... more Ground-based observations of Mercury's sodium exospheric emission were obtained at the McMath-Pierce Solar Telescope at Kitt Peak, Arizona, concurrently with the observations from the Ultraviolet and Visible Spectrometer (UVVS) on the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument onboard the MESSENGER spacecraft during the first flyby of the planet, January 14, 2008, and three weeks after the second flyby
Mercury's exosphere is composed of material that originates at the planet's surface, whet... more Mercury's exosphere is composed of material that originates at the planet's surface, whether that material is native or delivered by the solar wind and micrometeoroids. Many exospheric species have been detected by remote sensing, including H and He by Mariner 10, Na, K, and Ca by ground-based observations, and H, Na, Ca, Mg, and Ca+ by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. Other exospheric species, including Fe, Al, Si, O, S, Mn, Cl, Ti, OH, and their ions, are expected to be present on the basis of MESSENGER surface measurements and models of Mercury's surface chemistry. Here we report on searches for these species made with the Ultraviolet and Visible Spectrometer (UVVS) channel of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS). No obvious signatures of the listed species have yet been observed in Mercury's exosphere by the UVVS as of this writing. It is possible that detections are elusive ...
When Io emerges from eclipse by Jupiter, the sodium D-line emissions are weak, partly because of ... more When Io emerges from eclipse by Jupiter, the sodium D-line emissions are weak, partly because of the small Doppler shift of Io relative to the Sun, but also because the neutral sodium density is a factor of two to four times smaller than when Io has been out of the shadow and exposed to sunshine for an hour or two. We have observed four cases of post-eclipse brightening of the sodium emission, using a 10 arc second aperture image slicer to obtain images of sodium around Io. These show that initially, most of the sodium is confined to the "banana cloud" near Io. As time passes, the sodium density close to Io increases, and after and hour or two, most of the sodium is found surrounding Io. We have thought of two possible explanations for the post-eclipse sodium growth. One has to do with condensation of sulfur dioxide during the eclipse, covering particles of NaCl, which are thought to be the source of sodium (Hunten, 2003). As Io emerges from eclipse, the sulfur dioxide sub...
High resolution line profiles of the Na D2 emission from Mercury's atmosphere were obtained ... more High resolution line profiles of the Na D2 emission from Mercury's atmosphere were obtained at the Anglo-Australian Telescope June 6, 1998, at approximately 3 mA resolution. The spatial resolution (not including atmospheric smearing) was 1"x1.5", or about 1/3 planetary radius. Line profiles were obtained at high N, high S and equatorial latitudes, respectively. If interpreted in terms of a gas in thermal equilibrium, the gas temperature would vary from 1500 K at the equator to 750 K and 550 K at high N and high S latitudes, respectively. In addition, at high latitudes 1 - 10 hot component as source atoms ejected by an energetic process such as meteoritic vaporization or sputtering. The gas is nowhere in thermal equilibrium with the surface but remains hotter than the surface temperature. This implies that an energetic process such as chemistry or photon-stimulated desorption dominates the surface interaction. The difference in derived temperature at the N and S polar ...
We obtained images of the sodium emission surrounding Io at high spectral resolution (R= 150,000)... more We obtained images of the sodium emission surrounding Io at high spectral resolution (R= 150,000), using the National Solar Observatory McMath-Pierce stellar spectrograph and a 10x10 arc second image slicer. At this resolution, we can distinguish different velocity components in the sodium emission line profile. The intensity of these components varies, depending on local magnetospheric conditions at Io. In some cases, we can separate these components and generate images of each. The sodium emission was unusually bright on November 20, 1999. This, combined with excellent seeing conditions, allowed us to generate images for that night of different populations of sodium, each having distinctly different velocities. We observed the changes in these populations over a period of about three hours following emergence of Io from eclipse by Jupiter. These observations were part of our continuing observations of the increases observed in sodium emissions following emergence of Io from eclips...
The ratio of sodium to potassium in the exosphere of Mercury is highly variable and averages abou... more The ratio of sodium to potassium in the exosphere of Mercury is highly variable and averages about 100, considerably larger than for the Moon. We considered the possibility that this high value results from potassium loss rates that are larger than sodium loss rates. The principal loss processes are the sweeping of hot atoms off the planet by solar radiation acceleration and the capture of metal photoions by the solar wind. We found that radiation acceleration losses are not sufficiently different for sodium and potassium to explain the high ratio. Capture of sodium and potassium photoions by the solar wind is thought to be the major loss process. However, there is a competing process for the ions, which is collision with the surface, followed by neutralization. These atoms can return to the exosphere. The net steady state density of sodium and potassium in the exosphere depends on the relative efficiencies of solar wind capture and surface recycling. We propose that the photoion loss and recycling rates are sufficiently different for sodium and potassium to account for the high ratio. The higher mass of the potassium atom relative to sodium results in a smaller scale height and a larger gyroradius for potassium relative to sodium, which may result in more rapid net loss of potassium.
Mercury has a tenuous exosphere created by the combined effects of solar radiation and micrometeo... more Mercury has a tenuous exosphere created by the combined effects of solar radiation and micrometeoroid bombardment on the surface and the interaction of the solar wind with Mercury's magnetic field and surface. Observations of this exosphere provide essential data necessary for understanding the composition and evolution of Mercury's surface, as well as the interaction between Mercury's magnetosphere and the solar wind. The exosphere was observed during the MESSENGER flybys with the Mercury Atmospheric and Surface Composition Spectrometer (MASCS). The detections of sodium and calcium during the first two flybys, as well as magnesium during the second flyby, have been previously reported (McClintock et al. 2008,2009). Sodium and calcium showed significantly different distributions in both the tail and near-surface component of the exosphere suggesting they are ejected by different processes and from different regions. We present a Monte-Carlo model of sodium and calcium in...
Abstract— We propose that argon‐40 measured in the lunar atmosphere and that in Mercury's atm... more Abstract— We propose that argon‐40 measured in the lunar atmosphere and that in Mercury's atmosphere is due to current diffusion into connected pore space within the crust. Higher temperatures at Mercury, along with more rapid loss from the atmosphere, will lead to a similar or smaller column abundance of argon at Mercury than at the Moon, given the same crustal abundance of potassium. Because the noble gas abundance in the mercurian atmosphere represents current effusion, it is a direct measure of the crustal potassium abundance. We assume a fractal distribution of distance to a connected pore space, with the shortest distance increasing with depth. Given this “rock size” distribution, we show that the diffusive flux is not a unique function of temperature. Even though the diffusion coefficient is an exponential function of temperature, the flux to the surface is fairly insensitive to the temperature.
An analysis is presented of the Voyager and IUE lyman alpha spectra of the Jovian equatorial emis... more An analysis is presented of the Voyager and IUE lyman alpha spectra of the Jovian equatorial emission in which was derived a zonal asymmetry in the hydrogen column abundance. Using two estimates of the fraction of Lyman alpha which is due to direct excitation by charged particle precipitation from the ionosphere, upper and lower limits were derived to the H column abundance within and without the perturbed region. That the asymmetry in H abundance may be due to localized heating near the homopause with a consequent rise in scale height is shown. The derived exospheric temperature remains fairly constant with longitude. The required additional heat input over the bulge region, 0.02 erg/cm/s, is supplied by an additional flux of magnetospheric electrons due to Jupiter's magnetic anomaly.
The surface composition of various planetary bodies has been measured by spectrometers in the inf... more The surface composition of various planetary bodies has been measured by spectrometers in the infrared, UV, x-ray and gamma-ray region of the spectrum (e.g. Deep Space 1, NEAR, Odyssey), and others are planned in the near future (CONTOUR, Messenger, Bepi-Colombo). The extent to which the surface composition reflects the composition of the crust or regolith as a whole is an important issue. For example, the observtions with the x-ray and gamma-ray spectrometers onboard the NEAR spacecraft indicated that the extreme surface of Eros is depleted in sulfur [1]. The surface of the Moon, although depleted in sulfur, is enhanced in most volatiles. It is important to understand the reasons for these extreme surface anomalies. We have considered selective vaporization of volatiles on airless bodies due to impact vaporization. We use the planar impact approximation to hyper-velocity impact [2], and we scale the critical pressure for vaporization by the enthalpy of the species in question [3]. ...
We present the dependence of the intensity in the sodium D1 and D2 lines on the planetary phase a... more We present the dependence of the intensity in the sodium D1 and D2 lines on the planetary phase and position on the disk. Theoretical intensities are convolved with an atmospheric point spread function with FWHM of 0″.5 and 2″.0 for planetary phases 60° and 90°. We show that although the absolute intensities are extremely dependent on seeing, the I(D2)/I(D1) ratio remains a reliable indicator of column abundance regardless of the seeing function. For a uniform distribution of sodium, limb brightening will be apparent for a seeing disk with FWHM of 0″.5 but will not be apparent for a FWHM of 2″.0.
NEAR XRS measurements of 433 Eros are most consistent with chondrites, but important compositiona... more NEAR XRS measurements of 433 Eros are most consistent with chondrites, but important compositional differences remain, particularly in regard to S abundance. S depletion appears explainable, in terms of impact volatilization processes.
We have developed a technique to obtain thermal IR spectra of Mercury in the 8-13 micron region i... more We have developed a technique to obtain thermal IR spectra of Mercury in the 8-13 micron region in daylight [1]. We observed a smooth spectral curve for Mercury, with the exception of minor features near 9.6 microns and 12.7 microns. There was no evidence of a Christiansen feature down to the 8-micron limit of our spectra. This is in general
40Ar in the atmospheres of the planets is a measure of potassium abundance in the interiors since... more 40Ar in the atmospheres of the planets is a measure of potassium abundance in the interiors since 40Ar is a product of radiogenic decay of 40K by electron capture with the subsequent emission of a 1.46 eV g-ray. Although the 40Ar in the earth's atmosphere is expected to have accumulated since the late bombardment, 40Ar in the atmospheres of Mercury and the Moon is eroded quickly by photoionization and electron impact ionization. Thus, the argon content in the exospheres of the Moon and Mercury is representative of current effusion rather than accumulation over the lifetime of the planet. We reconsider the source and loss processes for the argon atmospheres of Mercury and the Moon in order to investigate what these atmospheres tell us about the structure and composition of the megaregolith. Argon is especially important because it does not engage in chemistry. In order to interpret measurements of argon in Mercury's atmosphere, we must have a model including sources from th...
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