A. I. F. Stewart

Enceladus' water vapor plume.

The Cassini spacecraft flew close to Saturns small moon Enceladus three times in 2005. Cassinis UltraViolet Imaging Spectrograph observed stellar occultations on two flybys and confirmed the existence, composition, and regionally confined nature of a water vapor plume in the south polar region of Enceladus. This plume provides an adequate amount of water to resupply losses from Saturns E ring and to be the dominant source of the neutral OH and atomic oxygen that fill the Saturnian system.

Ultraviolet spectroscopy of venus: initial results from the pioneer venus orbiter.

Ultraviolet spectroscopy of the Venus cloud tops reveals absorption features attributed to sulfur dioxide in the atmosphere above the cloud tops. Measurements of scattered sunlight at 2663 angstroms show evidence for horizontal and vertical inhomogeneities in cloud structure. Images of the planet at SO2 absorption wavelengths show albedo features similar to those seen at 3650 angstroms from Mariner 10. Airglowv emissions are consistent with an exospheric temperatuire of ∼275 K, and a night airglows emission has been detected, indicating the precipitation of energy into the dark thermosphere.

Water vapour jets inside the plume of gas leaving Enceladus

A plume of water vapour escapes from fissures crossing the south polar region of the Saturnian moon Enceladus. Tidal deformation of a thin surface crust above an internal ocean could result in tensile and compressive stresses that would affect the width of the fissures; therefore, the quantity of water vapour released at different locations in Enceladus’ eccentric orbit is a crucial measurement of tidal control of venting. Here we report observations of an occultation of a star by the plume on 24 October 2007 that revealed four high-density gas jets superimposed on the background plume. The gas jet positions coincide with those of dust jets reported elsewhere inside the plume. The maximum water column density in the plume is about twice the density reported earlier. The density ratio does not agree with predictions—we should have seen less water than was observed in 2005. The ratio of the jets’ bulk vertical velocities to their thermal velocities is 1.5 ± 0.2, which supports the hypothesis that the source of the plume is liquid water, with gas accelerated to supersonic velocity in nozzle-like channels.

MAVEN observations of the response of Mars to an interplanetary coronal mass ejection

Coupling between the lower and upper atmosphere, combined with loss of gas from the upper atmosphere to space, likely contributed to the thin, cold, dry atmosphere of modern Mars. To help understand ongoing ion loss to space, the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft made comprehensive measurements of the Mars upper atmosphere, ionosphere, and interactions with the Sun and solar wind during an interplanetary coronal mass ejection impact in March 2015. Responses include changes in the bow shock and magnetosheath, formation of widespread diffuse aurora, and enhancement of pick-up ions. Observations and models both show an enhancement in escape rate of ions to space during the event. Ion loss during solar events early in Mars history may have been a major contributor to the long-term evolution of the Mars atmosphere.

Solar cycle study of interplanetary Lyman-alpha variations - Pioneer Venus Orbiter sky background results

PVO observations of the interplanetary Ly-alpha (IPL) background, obtained over an entire solar cycle (SC) from 1979 to 1985, are compiled and analyzed statistically, along with data from other instruments and earlier solar cycles. The results are presented in extensive tables and graphs and characterized in detail. Findings reported include SC variation of 1.8 for the longitudinally averaged IPL intensity (in agreement with the variation of the 27-d disk-averaged integrated solar Ly-alpha flux), yearly averaged ecliptic H-atom lifetime at 1 AU equal to 1.0 Ms at solar minimum and 1.5 Ms at solar maximum, interplanetary H density equal to 0.07 + or - 0.01/cu cm, and interplanetary H/He within the heliopause but far from the sun of 7 + or - 3. 74 references.

Mariner 9 Ultraviolet Spectrometer Experiment: Mars Airglow Spectroscopy and Variations in Lyman Alpha

Abstract Mariner 9 ultraviolet spectrometer observations show the Mars airglow consists principally of emissions that arise from the interaction of solar ultraviolet radiation with carbon dioxide, the principal constituent of the Mars atmosphere. Two minor constituents, atomic hydrogen and atomic oxygen, also produce airglow emissions. The airglow measurements show that ionized carbon dioxide is only a minor constituent of the ionosphere. Using the airglow measurements of atomic oxygen, it is possible to infer that the major ion is ionized molecular oxygen. The escape rate of atomic hydrogen measured by Mariner 9 is approximately the same as that measured two years earlier by Mariner 6 and 7. If the current escape rate has been operating for 4.5 billion years and if water vapor is the ultimate source, an amount of oxygen has been generated that is far in excess of that observed at present. Mariner 9 observations of Mars Lyman alpha emission over a period of 120 days show variations of 20%.

Mariner 9 ultraviolet spectrometer experiment: Structure of Mars' upper atmosphere

Analysis of 18 observations of the limb intensity profile of the CO Cameron bands in the Martian airglow shows that the equivalent subsolar zenith intensity, ICAM, is related to the Ottawa 10.7 cm radio flux index, F10.7, by the expression ICAM = 0.062(74 + F10.7)kR, with a correlation coefficient of 0.80. Comparison of averaged limb intensities of the CO2+ doublet and the Cameron bands on four favorable occasions is consistent with the intensities being directly proportional, in the ratio 0.24:1. The mean of 18 Cameron band topside scale heights is 17.8 km, corresponding to an exospheric temperature of 325°K, and the largest and smallest values observed differ by 9.5 km. These observations are in accord with theoretical predictions within the uncertainties in the latter. However, the solar EUV flux used in these predictions is a factor of at least two too weak to produce the electron densities measured by the S-band occultation experiment.

Mariner 9 ultraviolet spectrometer experiment - Initial results.

The ultraviolet airglow spectrum of Mars has been measured from an orbiting spacecraft during a 30-day period in November-December 1971. The emission rates of the carbon monoxide Cameron and fourth positive bands, the atomic oxygen 1304-angstrom line and the atomic hydrogen 1216-angstrom line have been measured as a function of altitude. Significant variations in the scale height of the CO Cameron band airglow have been observed during a period of variable solar activity; however, the atomic oxygen and hydrogen airglow lines are present during all the observations. Measurements of the reflectance of the lower atmosphere of Mars show the spectral characteristics of particle scattering and a magnitude that is about 50 percent of that measured during the Mariner 6 and 7 experiments in 1969. The variation of reflectance across the planet may be represented by a model in which the dominant scatterer is dust that absorbs in the ultraviolet and has an optical depth greater than 1. The atmosphere above the polar region is clearer than over the rest of the planet.

Atomic oxygen in the Martian thermosphere

Modern models of thermospheric composition and temperature and of excitation and radiative transfer processes are used to simulate the O I 130-nm emission from Mars measured by the Mariner 9 ultraviolet spectrometer. We use the Mars thermospheric general circulation model calculations (MTGCM) of Bougher et al. (1988a) and the Monte Carlo partial frequency redistribution multiple scattering code of Meier and Lee (1982). We find that the decline in atomic oxygen through the daylight hours predicted by the MTGCM cannot be reconciled with the excess afternoon brightness seen in the data. Oxygen concentrations inferred from the data show a positive gradient through the day, in agreement with the original analysis by Strickland et al. (1973), although the absolute amounts are somewhat less because we use a larger photoelectron impact excitation and a somewhat larger solar flux in the 130-nm triplet. In addition, the data suggest that the oxygen abundance increases toward high southerly latitudes, in contrast with the MTGCM prediction of high values in the northern (winter) hemisphere. It appears that solar forcing alone cannot account for the observed characteristics of the Martian thermosphere and that wave and tidal effects may profoundly affect the structure, winds, and composition.

Galileo ultraviolet spectrometer observations of atomic hydrogen in the atmosphere of Ganymede

Atomic hydrogen Lyman alpha radiation (121.6 nm) has been measured in emission from the atmosphere of Ganymede with the Galileo ultraviolet spectrometer. An exospheric model with the following parameters has been fit to the observational data: atomic hydrogen density directly above the surface (radius 2634 km) equal to 1.5 × 104 atoms cm−3 scale height 2634 km, exospheric temperature 450 K. A model calculation of the photodissociation of water vapor from surface ice at 146 K is used to obtain the photodissociation rate necessary to supply the hydrogen atoms that are escaping from the exosphere of Ganymede. The calculated escape flux of atomic hydrogen is 7 × 108 atoms/cm² sec. Two alternate but speculative sources of the atomic hydrogen escaping from Ganymede are photodesorption of water ice by ultraviolet photons in the wavelength range 120.5–186.0 nm and sputtering of water ice by Jupiters magnetospheric ion plasma.