William M. Sinton

Ground-Based Near-Infrared Imaging Observations of Venus During the Galileo Encounter

Near-infrared images of Venus, obtained from a global network of ground-based observatories during January and February 1990, document the morphology and motions of the night-side near-infrared markings before, during, and after the Galileo Venus encounter. A dark cloud extended halfway around the planet at low latitudes (>�40�) and persisted throughout the observing program. It had a rotation period of 5.5 � 0.15 days. The remainder of this latitude band was characterized by small-scale (400 to 1000 kilometers) dark and bright markings with rotation periods of 7.4 � 1 days. The different rotation periods for the large dark cloud and the smaller markings suggests that they are produced at different altitudes. Mid-latitudes (�40� to 60�) were usually occupied by bright east-west bands. The highest observable latitudes (�60� to 70�) were always dark and featureless, indicating greater cloud opacity. Maps of the water vapor distribution show no evidence for large horizontal gradients in the lower atmosphere of Venus.

The nature of the near-infrared features on the venus night side.

Near-infrared images of the Venus night side show bright contrast features that move from east to west, in the direction of the cloud-top atmospheric superrotation. Recently acquired images of the Venus night side along with earlier spectroscopic observations allow identification of the mechanisms that produce these features, their level of formation, and the wind velocities at those levels. The features are detectable only at wavelengths near 1.74 and 2.3 micrometers, in narrow atmospheric windows between the CO2 and H2O bands. The brightest features have brightness temperatures near 480 Kelvin, whereas the darkest features are more than 50 Kelvin cooler. Several factors suggest that this radiation is emitted by hot gases at altitudes below 35 kilometers in the Venus atmosphere. The feature contrasts are produced as this thermal radiation passes through a higher, cooler, atmospheric layer that has horizontal variations in transparency. The 6.5-day east-west rotation period of the features indicates that equatorial wind speeds are near 70 meters per second in this upper layer. Similar wind speeds have been measured by entry probes and balloons at altitudes between 50 and 55 kilometers in the middle cloud layer. The bright features indicate that there are partial clearings in this cloud deck. The presence of these clearings could decrease the efficiency of the atmospheric greenhouse that maintains the high surface temperatures on Venus.

Infrared observations of eclipses of Io, its thermophysical parameters, and the thermal radiation of the Loki volcano and environs

Abstract We report 1981–1984 thermal infrared observations of 10 Io eclipse reappearances and three eclipse disappearances. Absolute calibration errors were estimated from measurements of Callisto during nine of the eclipses with the following values: 5% at 8.7, 10.2, 12.5, and 20 μm; and 15% at 30 μm. The longer wavelength (8.7, 10.2, 12.5, 20, and 30 μm) eclipse data were used to determine not only the thermal radiation from the volcanoes, but also to find the best thermophysical parameters by a least-squares fitting procedure. No simple thermophysical model capable of fitting all wavelengths simultaneously was found. We found that adequate fitting of the eclipse cooling and heating curves requires not only vertically inhomogeneous models, but also models that are horizontally inhomogeneous at least in the sense of requiring two different albedo regimes. The best model required a variation of the thermophysical parameters between the light and dark albedo components. The thermal inertia of the bright component was nearly 10 times that of the dark component and required a vertically inhomogeneous surface. The dark components surface was, however, homogeneous. It is shown from the improvement in the sum of the squares of the residuals that the differences of parameters between light and dark regions are significant at the 99.9% level. A major result of this paper is the accurate determination of the volcanic heat radiation for the sub-Jupiter hemisphere. We were able to find solutions for the temperature, effective radiating area, and longitude separately for a high-temperature and a low-temperature volcanic source. The longitudes of either of these sources, when the data were sufficient to make significant determinations, lie within a few degrees of the Loki volcano. The area of the low-temperature source is approximately that of the lava lake seen in Voyager photographs. We believe that Loki is the primary source of excess thermal radiation seen in eclipses of Io, but based on Voyager results it, most likely, is not the only source contributing to the eclipse flux. We have performed a careful evaluation of errors and find that the statistical uncertainty of the total volcanic flux from the Loki region can be as low as 3 to 9% when multiple eclipses, including both disappearances and reappearances, are observed. When comparing one years data with another, the uncertainty associated with the flux calibration is not involved and only the statistical uncertainty is applicable to the comparison. We have studied the variability of the thermal emission of the volcanoes seen on the sub-Jupiter hemisphere and find that a solution based on the data of W. Sinton, A. Tokunaga, E. Becklin, I. Gatley, T. Lee, and C. Lonsdale (1980, Science 210, 1015–1017) and on the data of D. Morrison and C. Telesco (1980, Icarus 44, 226–233) gives a total flux that is considerably more than that which we find for 1981, 1982, 1983, and 1984 or the Voyager results. Our data for the last four enumerated years give little evidence, by themselves, for variability of the total volcanic flux from the sib-Jupiter hemisphere.

Io: The near-infrared monitoring program, 1979–1981

Abstract More than two years of observations of Io at 2.2, 3.8, and 4.8 μm show that , while Ios 2.2-μm flux can be explained very well by conventional albedo analysis, at 3.8 and 4.8 μm Io has large intrinsic variations presumably associated with volcanism. In contrast to 2.2 μm where Io is generally brighter on its leading hemisphere, at 3.8 μm and especially at 4.8 μm, Io appears brighter or more active on its trailing hemisphere. It is determined from this data that the variable 3.8- and 4.8-μm components have possibly significant terms at the Jupiter corotational periods of 13 and 6.5 hr. These were particularly significant for observations of the trailing hemisphere and have a marked peak when Io is in the active sector. However, these data also show that the variability occurs in events or outbursts. Eight such outbursts have been found. Four were studied at both 3.8 and 4.8 μm, and from these, color temperatures near 700°K were derived. The color temperature decayed to ∼300°K as observations progressed through a period of several hours. Because the peaking of excess flux in the active sector was produced by only four outbursts, it is argued that the apparent peak and the significance of the corotational terms may be fortuitous. Continued observations for several years should resolve this question. A very strange event may have been observed on February 24, 1981 in the midst of an outburst, at which time the light reflected at 3.8 μm decreased threefold for 1 1 2 min .

Does Io have an ammonia atmosphere

Abstract An atmosphere containing 0.5 cm atm of ammonia is assumed on Io. Such an atmosphere will be frozen at the unilluminated pole during the solstices, but will evaporate at the equinoctial seasons. The ammonia atmosphere will explain: (1) the posteclipse brightenings and their observed times of occurrence and nonocurrence; (2) the observed departure from a two-layer model beating curve upon emergence from eclipse; (3) the discordant temperatures obtained at 10 and 20 μm; and (4) discordant temperatures obtained at 10 and 20 μm during the total phase of an eclipse by Jupiter. In order to explain items 3 and 4 above, a proton flux in Jupiters magnetosphere of 1.1 × 109 cm−2s− at an energy of 0.5MeV at ios distance from Jupiter is assumed. This flux is 40 times the flux in Divines (1972) “upper-limit” model of the Jovian radiation belts, while the proton energy is eight times less. The proton flux, plus the solar ultraviolet and infrared flux absorbed by the ammonia, will heat the atmosphere to 245 ± 10°K. At this temperature the occultation atmospheric upper limit allows the addition of 4 cmatm of nitrogen.

Characterization of Io's volcanic activity by infrared polarimetry

The thermal emission from Ios volcanic hot spots is linearly polarized. Infrared measurements at 4.76 micrometers show disk-integrated polarization as large as 1.6 percent. The degree and position angle of linear polarization vary with Ios rotation in a manner characteristic of emission from a small number of hot spots. A model incorporating three hot spots best fits the data. The largest of these hot spots lies to the northeast of Loki Patera, as mapped from Voyager, and the other spot on the trailing hemisphere is near Ra Patera. The hot spot on the leading hemisphere corresponds to no named feature on the Voyager maps. The value determined for the index of refraction of the emitting surface is a lower bound; it is similar to that of terrestrial basalts and is somewhat less than that of sulfur.

Uranus: the rings are black.

An upper limit of 0.05 is established for the geometric albedo of the newly discovered rings of Uranus. In view of this very low albedo, the particles of the rings cannot be ice-covered as are those of rings A and B of Saturn.

Near-infrared photometry of the Galilean satellites

Abstract Near-infrared lightcurves and phase coefficients at 2.2, 3.8, and 4.8 ωm for the Galilean satellites are presented. The geometric albedo of Io, as determined here, includes emission from volcanoes, especially at 3.8 and 4.8 ωm. No major outbursts were detected during the period of observation 1982–1983, covered by this paper. A broad peak in the 4.8-ωm albedo at longitude ≈270–280° is attributed primarily to volcanoes other than Loki. Results for Europa and Ganymede, showing a trend of decreasing albedo with increasing wavelength, are consistent with icy surfaces for these satellites. For Ganymede, the phase coefficient depends strongly on the wavelength in the near infrared, and has the extremely high value of 0.083 mag/deg at 4.8 ωm. Results for Callisto are consistent with observations of other dark solar system objects at visual wavelengths.

Observational tests for sulfur allotropes on Io

Abstract One of the intrinsic properties of particulate sulfur allotropes is a change in UV-visible reflectivity with temperature change of the material. The surface of Io experiences temperature changes during eclipse which are sufficient to cause a detectable change in the spectral reflectivity of sulfur; thus, if the surface of Io is composed primarily of sulfur allotropes, a change in reflectivity at certain wavelengths should be observable shortly after eclipse reappearance. We observed four eclipse reappearances during July and August of 1983 and saw no posteclipse brightening effects in filter bands selected for sensitivity to color changes in sulfur. Our model of the brightness change for S 8 (“yellow” sulfur) implies that this material covers less than 50% of Ios surface. Negative posteclipse brightening observations were also obtained with a filter chosen for the high contrast between SO 2 frost and the average albedo of the surface of Io at that wavelength. We conclude that no significant condensation of optically thick SO 2 occurred on the surface of Io during these eclipses.

The brightness temperatures of Saturn and its rings at 39 microns

We have resolved the relative rings-to-disk brightness (specific intensity) of Saturn at 39 μm (δλ ≃ 8 μm) using the 224-cm telecscope at Mauna Kea Oservatory, and have also measured the total flux of Saturn relative to Jupiter in the same bandpass from the NASA Learjet Observatory. These two measurements, which were made in early 1975 with Saturns rings near maximum inclination (b′ ≃ 25°), determine the disk and average ring (A and B) brightness in terms of an absolute flux calibration of Jupiter in the same bandpass. While present uncertainties in Jupiters absolute calibration make it possible to compare existing measurementsunambiguously, it is nevertheless possible to conclude the following: (1) observations between 20 and 40 μm are all compatible (within 2σ) of a disk brightness temperature of 94°K, and do not agree with the radiative equilibrium models of Trafton; (2) the rings at large tilt contribute a flux component comparable to that of the planet itself for λ ≲ 40 μm and (3) there is a decrease of ∼22% in the relative ring: disk brightness between effective wavelengths of 33.5 and 39 μm.