Glenn J. Veeder

A refined “standard” thermal model for asteroids based on observations of 1 Ceres and 2 Pallas

Abstract We present ground-based thermal infrared observations of asteroids 1 Ceres and 2 Pallas made over a period of 2 years. By analysing these data in light of the recently determined occultation diameter of Ceres (933–945 km) and Pallas (538 km) and their known small-amplitude lightcurves, we have determined a new value for the infrared beaming parameter used in the “standard” thermal emission model for asteroids. The new value is significantly lower than that previously used, and when applied in the reduction of thermal infrared observations of other asteroids, should yield model diameters that are closer to actual diameters. In our formulation, we also incorporate the recently adopted IAU magnitude convention for asteroids, which uses the zero-phase magnitudes (including the opposition effect) the same as is used for satellites.

Io's heat flow from infrared radiometry: 1983–1993

We report the following results from a decade of infrared radiometry of Io: (1) The average global heat flow is more than ∼2.5 W m−2, (2) large warm (≤200 K) volcanic regions dominate the global heat flow, (3) small high-temperature (≥300 K) “hotspots” contribute little to the average heat flow, (4) thermal anomalies on the leading hemisphere contribute about half of the heat flow, (5) a substantial amount of heat is radiated during Ios night, (6) high-temperature (≥600 K) “outbursts” occurred during ∼4% of the nights we observed, (7) “Loki” is the brightest, persistent, infrared emission feature, and (8) some excess emission is always present at the longitude of Loki, but its intensity and other characteristics change between apparitions. Observations of Io at M (4.8 μm), 8.7 μm, N (10 μm), and Q (20 μm) with the Infrared Telescope Facility presented here were collected during nine apparitions between 1983 and 1993. These measurements provide full longitudinal coverage as well as an eclipse observation and the detection of two outbursts. Reflected sunlight, passive thermal emission, and radiation from thermal anomalies all contribute to the observed flux densities. We find that a new thermophysical model is required to match all the data. Two key elements of this model are (1) a “thermal reservoir” unit which lowers daytime temperatures, and (2) the “thermal pedestal effect” which shifts to shorter wave-lengths the spectral emission due to the reradiation of solar energy absorbed by the thermal anomalies. The thermal anomalies are modeled with a total of 10 source components at five locations. Ios heat flow is the sum of the power from these components.

Radiometry of near-earth asteroids

We report 10 micrometers infrared photometry for 22 Aten, Apollo, and Amor asteroids. Thermal models are used to derive the corresponding radiometric albedos and diameters. Several of these asteroids appear to have surfaces of relatively high thermal inertia due to the exposure of bare rock or a coarse regolith. The Apollo asteroid 3103, 1982 BB, is recognized as class E. The Jupiter-crossing Amor asteroid 3552, 1983 SA, is confirmed as class D, but low albedos remain rare for near-Earth asteroids.

A three-parameter asteroid taxonomy

Broadband U, V, and x photometry together with IRAS asteroid albedos have been used to construct an asteroid classification system. The system is based on three parameters (U-V and v-x color indices and visual geometric albedo), and it is able to place 96 percent of the present sample of 357 asteroids into 11 taxonomic classes. It is noted that all but one of these classes are analogous to those previously found using other classification schemes. The algorithm is shown to account for the observational uncertainties in each of the classification parameters.

The dust coma of periodic Comet Churyumov-Gerasimenko (1982 VIII)

Abstract The dust coma of Comet P/Churyumov-Gerasimenko was monitored in the infrared (1–20 μm) from September 1982 to March 1983. Maximum dust production rate of ∼2 × 10 5 g/sec occured in December, 1 month postperihelion. The ratio of dust/gas production was higher than that in other short-period comets. No silicate feature was visible in the 8- to 13-μm spectrum on 23 October. The mean geometric albedo of the grains was ∼0.04 at 1.25 μm and ∼0.05 at 2.2 μm.

Spectral geometric albedos of the Galilean satellites from 0.24 to 0.34 micrometers - Observations with the International Ultraviolet Explorer

Abstract We present the results of an 8-year program of spectrophometry of the Galilean satellites of Jupiter that was undertaken using the International Ultraviolet Explorer (IUE) Spacecraft. The ultraviolet geometric albedos of all four satellites are low. This is consistent with the hypothesis that sulfurous materials escaping from the surface of Io are being distributed by magnetospheric processes to the surfaces of the other three objects. Although iron bearing silicates may also cause UV darkening, these materials also have spectral features in the visible region of the spectrum which are not found in the spectra of the Galilean satellites. For Io, we find that the ultraviolet geometric albedo is very low ( P uv ∼ 0.04). The trailing hemisphere has an albedo that is higher than that of the leading hemisphere. This is opposite of what is observed at visual wavelengths. The decrease of albedo shortward of 0.33 μm is consistent with groundbased observations ( Nelson and Hapke, 1978 ) and the laboratory reflection spectrum of sulfur dioxide frost. The hemispheric albedo asymmetry is consistent with a variable distribution of the frost, it being present in greater abundance on Ios leading hemisphere. The strenght of this feature has not changed with respect to longitude over the8 years of this study. The phase coefficients and opposition surges at ultraviolet wavelenghts indicate that Ios surface regolith is very porous. Europa has the highest ultraviolet albedo of all the Galilean satellites ( P uv ∼ 0.2). This not inconsistent with the hypothesis of recent resurfacing. However, this albedo is not high enough to be consistent with a surface of pure water ice. We confirm a previously reported ultraviolet spectral asymmetry between Europas leading and trailing hemispheres. The new data are consistent with the previous analyses which interpreted this as the spectral signature of sulfur ions from the Jovian magnetosphere which had been embedded preferentially on the trailing side of Europas predominately water ice.surface. The opposition surge observed for Europas trailing side is greater than that for the leading side. This implies that the trailing side is less compact than the leading side, perhaps due to gardening from the ion implantation process. Ganymedes ultraviolet albedo ( P uv ∼ 0.10) is lower than Europas. Ganymede has an ultraviolet spectral asymmetry that is similar to Europas for wavelenghts longer than 0.28 μm. However, at wavelengths shorter than 0.28 μm, the two objects have different opposite hemispherical spectral ratios, indicating that the same mechanism cannot be used to explain the ultraviolet spectral albedo of both objects. One possible explanation is that ozone is present in addition to sulfur embedded on Ganymedes surface. The ultraviolet albedo and opposite hemispherical spectral ratio of Calisto is spectrally flat, indicating that the surface is covered by a material that is spectrally absorbing in the ultraviolet but has no change in absorption at the ultraviolet wavelenghts. The orbital phase variation in the ultraviolet indicates that the absorber is assymmetrically distributed in longitude.

Io - Evidence for silicate volcanism in 1986

Infrared observations of Io during the 1986 apparition of Jupiter indicate that a large eruptive event occurred on the leading side of Io on 7 August 1986, Universal Time. Measurements made at 4.8, 8.7, and 20 micrometers suggest that the source of the event was about 15 kilometers in radius with a model temperature of ∼900 Kelvin. Together with previously reported events, these measurements indicate that high-temperature volcanic activity on the leading side of Io may be more frequent than previously thought. The inferred temperature is significantly above the boiling point of sulfur in a vacuum(715 Kelvin) and thus constitutes strong evidence for active silicate volcanism on the surface of Io.

Stealth plumes on Io

We suggest that Ios eruptive activity may include a class of previously undetected SO2 geysers. The thermodynamic models for the eruptive plumes discovered by Voyager involve low to moderate entropy SO2 eruptions. The resulting plumes are a mixture of solid and gas which emerge from the vent and follow essentially ballistic trajectories. We show that intrusion of silicate magma into buried SO2 deposits can create the required conditions for high entropy eruptions which proceed entirely in the vapor phase. These purely gaseous plumes would have been invisible to Voyagers instruments. Hence, we call them “stealth” plumes. Such eruptions could explain the “patchy” SO2 atmosphere inferred from recent UV and microwave spectral observations. The magma intrusion rate required to support the required gas production for these plumes is a negligible fraction of estimated global magma intrusion rates.

Volcanic hotspots on Io: Stability and longitudinal distribution

We report the first results of a program to determine the longitudinal distribution of volcanic activity on Jupiters satellite Io. Infrared measurements at 8.7, 10, and 20 micrometers have been taken at a variety of orbital longitudes: strong variation in the 8.7- and 10-micrometer flux with longitude demonstrates that infrared emission arising from volcanic hotspots on Io is strongly concentrated in a few locations. Analysis of these data suggests that the active volcanic regions observed by the Voyager experimenters are still active, particularly the region around the feature known as Loki. Another source of flux, although of somewhat smaller magnitude, is indicated on the opposite hemisphere. If these sources are the only major volcanic centers on Io, then current global heat flow estimates must be revised downward. However, heat flow from as yet unobserved longitudes, hotspots at high latitudes, and conducted heat flow must still be measured.

Visual and radiometric photometry of 1580 Betulia

Abstract We obtained broadband visual and 10.6-μm photometry of 1580 Betulia during its close approach to Earth in May 1976. We analyzed our photometry by using the “radiometric method” to derive the radius (2.10 ± 0.40 km) and albedo (0.108 ± 0.012) of Betulia. Radar and polarimetric results indicate a radius greater than 3.0 km and a geometric albedo of about 0.05. To be compatible with these results we also modeled Betulia as having a surface with the thermal characteristics of bare rock rather than those of the “lunar” regolith model used for previous analysis of radiometry of other asteroids. A 3.7-km radius and a geometric albedo of ∼0.04 are compatible with all available observations. Betulia is the first Mars-crossing asteroid found to have such a low albedo, which may be indicative of carbonaceous surface material.