After decades of speculation and fruitless searches, the lunar atmosphere was first observed by Apollo surface and orbital instruments between 1970 and 1972. With the demise of Apollo in 1972, and the termination of funding for Apollo lunar ground station studies in 1977, the field withered for many years, but has recently enjoyed a renaissance. This reflowering has been driven by the discovery and exploration of sodium and potassium in the lunar exosphere by groundbased observers, the detection of metal ions derived from the Moon in interplanetary space, the possible discoveries of H2O ice at the poles of the Moon and Mercury, and the detections of tenuous atmospheres around more remote sites in the solar system, including Mercury and the Galilean satellites. In this review we summarize the present state of knowledge about the lunar atmosphere, describe the important physical processes taking place within it, and then discuss related topics including a comparison of the lunar atmosphere to other surface boundary exospheres in the solar system.

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The lunar atmosphere: History, status, current problems, and context

The composition of cometary ices provides key information on the chemical and physical properties of the outer solar nebula where comets formed, 4.6 G.y. ago. This chapter summarizes our current knowledge of the volatile composition of cometary nuclei, based on spectroscopic observations and in situ measurements of parent molecules and noble gases in cometary comae. The processes that govern the excitation and emission of parent molecules in the radio, infrared (IR), and ultraviolet (UV) wavelength regions are reviewed. The techniques used to convert line or band fluxes into molecular production rates are described. More than two dozen parent molecules have been identified, and we describe how each is investigated. The spatial distribution of some of these molecules has been studied by in situ measurements, long-slit IR and UV spectroscopy, and millimeter wave mapping, including interferometry. The spatial distributions of CO, H2CO, and OCS differ from that expected during direct sublimation from the nucleus, which suggests that these species are produced, at least partly, from extended sources in the coma. Abundance determinations for parent molecules are reviewed, and the evidence for chemical diversity among comets is discussed.

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The composition of cometary volatiles

String theory is the most promising approach to the long-sought unified description of the four forces of nature and the elementary particles1, but direct evidence supporting it is lacking. The theory requires six extra spatial dimensions beyond the three that we observe; it is usually supposed that these extra dimensions are curled up into small spaces. This ‘compactification’ induces ‘moduli’ fields, which describe the size and shape of the compact dimensions at each point in space-time. These moduli fields generate forces with strengths comparable to gravity, which according to some recent predictions2,3,4,5,6,7 might be detected on length scales of about 100 µm. Here we report a search for gravitational-strength forces using planar oscillators separated by a gap of 108 µm. No new forces are observed, ruling out a substantial portion of the previously allowed parameter space4 for the strange and gluon moduli forces, and setting a new upper limit on the range of the string dilaton2,3 and radion5,6,7 forces.

Upper limits to submillimetre-range forces from extra space-time dimensions

31008, 2012. Sternovsky, Z., et al., Dust spectroscopy of the Jovian satellites, 43rd Lunar and Planetary Science Conference, Abstract #2929, 2012. Stevens, M.H., et al., Bright polar mesospheric clouds formed by main engine exhaust from the Space Shuttle’s final launch, J. Geophys. Res., 117, D19206, doi:10.1029/2012JD017638, 2012. Stroud, R.M., et al., Constraining the origin of impact craters on A1 foils from the Stardust Interstellar Dust Collectors, LPI Science Conference Abstracts, 43, 2001-2012. Strub, P., et al., Dynamics of interstellar dust in the solar system: From in-situ measurements to models, European Planetary Science Congress 2012, 60-, 2012.

Laboratory for Atmospheric and Space Physics

The gas-to-dust mass ratios found for interstellar dust within the solar system, versus values determined astronomically for the cloud around the solar system, suggest that large and small interstellar grains have separate histories and that large interstellar grains preferentially detected by spacecraft are not formed exclusively by mass exchange with nearby interstellar gas. Observations by the Ulysses and Galileo satellites of the mass spectrum and flux rate of interstellar dust within the heliosphere are combined with information about the density, composition, and relative flow speed and direction of interstellar gas in the cloud surrounding the solar system to derive an in situ value for the gas-to-dust mass ratio, Rg/d = 94. This ratio is dominated by the larger near-micron-sized grains. Including an estimate for the mass of smaller grains, which do not penetrate the heliosphere owing to charged grain interactions with heliosheath and solar wind plasmas, and including estimates for the mass of the larger population of interstellar micrometeorites, the total gas-to-dust mass ratio in the cloud surrounding the solar system is half this value. Based on in situ data, interstellar dust grains in the 10-14 to 10-13 g mass range are underabundant in the solar system, compared to a Mathis, Rumple, & Nordsiek mass distribution scaled to the local interstellar gas density, because such small grains do not penetrate the heliosphere. The gas-to-dust mass ratios are also derived by combining spectroscopic observations of the gas-phase abundances in the nearest interstellar clouds. Measurements of interstellar absorption lines formed in the cloud around the solar system, as seen in the direction of CMa, give Rg/d = 427 for assumed solar reference abundances and Rg/d = 551 for assumed B star reference abundances. These values exceed the in situ value suggesting either that grain mixing or grain histories are not correctly understood or that sweptup stardust is present. Such high values for diffuse interstellar clouds are strongly supported by diffuse cloud data seen toward λ Sco and 23 Ori, provided B star reference abundances apply. If solar reference abundances prevail, however, the surrounding cloud is seen to have greater than normal dust destruction compared to higher column density diffuse clouds. The cloud surrounding the solar system exhibits enhanced gas-phase abundances of refractory elements such as Fe+ and Mg+, indicating the destruction of dust grains by shock fronts. The good correlation locally between Fe+ and Mg+ indicates that the gas-phase abundances of these elements are dominated by grain destruction, while the poor correlation between Fe+ and H0 indicates either variable gas ionization or the decoupling of neutral gas and dust over parsec scale lengths. These abundances, combined with grain destruction models, indicate that the nearest interstellar material has been shocked with shocks of velocity ~150 km s-1. If solar reference abundances are correct, the low Rg/d value toward λ Sco may indicate that at least one cloud component in this direction contains dust grains that have retained their silicate mantles and are responsible for the polarization of the light from nearby stars seen in this general region. Weak frictional coupling between gas and dust in nearby low density gas permit inhomogeneities to be present.

https://iopscience.iop.org/article/10.1086/307869/pdf

Dust in the local interstellar wind

An image of comet Hale-Bopp (C/1995 O1) in soft x-rays reveals a central emission offset from the nucleus, as well as an extended emission feature that does not correlate with the dust jets seen at optical wavelengths Neon was found to be depleted in the cometary ice by more than a factor of 25 relative to solar abundance, which suggests that ices in Hale-Bopp formed at (or later experienced) temperatures higher than 25 kelvin A helium line emission at a wavelength of 584 angstroms was detected and may be attributable to charge transfer of solar wind α particles in the cometary coma Ionized oxygen and another helium line contribute to an emission observed at 538 angstroms

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Detection of Soft X-rays and a Sensitive Search for Noble Gases in Comet Hale-Bopp (C/1995 O1)

THE detection of a cloud of neutral sodium near Jupiter's moon Io1 has led to the use of sodium as a tracer of processes in the jovian environment. Although relatively rare in the Io–Jupiter system, sodium atoms are easily detected because of their high efficiency for scattering sunlight at wavelengths of ∼5,890 A. Direct imaging of the sodium cloud2 has suggested that sodium atoms are a common feature close to Io (at distances of about six Io radii, RIo) and detection of high-speed sodium jets3 suggested that sodium is present only sporadically at ∼30/RIo (ref. 4). Sodium emission has been reported at greater distances5, even as far as 60RIo (ref. 6) but these observations have been controversial in view of suggestions7 that the detection of sodium beyond ∼10RIo was implausible on theoretical grounds and probably indistinguishable from terrestrial sodium airglow. Here we report on the detection of sodium to distances beyond ∼400 RI, an observation that requires the ejection rate of sodium atoms to be increased. By relating the shape of this great nebula to conditions in the plasma torus surrounding Jupiter, we show that ground-based imaging techniques can provide information about distant planetary magnetospheres.

The extended sodium nebula of Jupiter

Monochromatic imaging instrumentation has been developed that uses narrow-band [12-A full width at half maximum (FWHP)] interference filters or plane reflection gratings for 2-D imaging and imaging spectrograph applications. By changing the optics in front of the filter or grating, the field of view of the instruments can be varied from 180 deg (all sky) to 6 deg (narrow field). In the case of the 2-D monochromatic imager, the 12-mm-diam. filtered image is formed at ~f/1 on the input photocathode of an intensified CCD camera (380 x 488 pixels). Regardless of the field of view of the instrument, none of the rays passing through the filter exceeds 4.5 deg to the filter normal. In the spectrograph mode, a slit is placed on the field lens (at the focus of the objective lens) and the 4-in. interference filter is replaced with a reflection grating. The spectrum is imaged at f/1 onto the previously described detector with the spatial dimension of 380 pixels and the wavelength dimension of 488 pixels. The sensitivities of the systems are approximately 50 to 100 rayleigh seconds (Rs) (SNR~2). Examples of data taken with both of these instruments include detection and mapping of Jupiter's sodium magnetonebula and stable auroral red (SAR) arcs in the terrestrial ionosphere.

Monochromatic imaging instrumentation for applications in aeronomy of the earth and planets

The Dual Sources of Io's Sodium Clouds

Atomic sodium is a useful tracer of the tenuous lunar atmosphere because of its high efficiency in scattering sunlight at the D1 (5896 angstroms) and D2 (5890 angstroms) wavelengths. In 1988, Earth-based instruments revealed the presence of sodium at a density of less than 50 atoms per cubic centimeter at lunar altitudes below 100 kilometers. Telescopic observations that are made with a coronograph technique to block out the disk of the moon allow a true picture of the circumlunar atmosphere to be obtained and show the presence of sodium out to a distance of several lunar radii. The distribution of sodium has a solar zenith angle dependence, suggesting that most of the sodium that reaches great altitudes is liberated from the moon's surface by solar photons (by heating or sputtering) or by solar wind impact, in contrast to a source driven by uniform micrometeor bombardment.

https://science.sciencemag.org/content/sci/261/5118/184.full.pdf

Brian Flynn

Papers

Detection of Soft X-rays and a Sensitive Search for Noble Gases in Comet Hale-Bopp (C/1995 O1)

The extended sodium nebula of Jupiter

Monochromatic imaging instrumentation for applications in aeronomy of the earth and planets

The Dual Sources of Io's Sodium Clouds

A Picture of the Moon's Atmosphere