Laurence M. Trafton

The role of H3+in planetary atmospheres

Spectroscopic studies of the upper atmospheres of the giant planets using infrared wavelengths sensitive to the H3+ molecular ion show that this species plays a critical role in determining the physical conditions there. For Jupiter, we propose that the recently detected H3+ electrojet holds the key to the mechanism by which the equatorial plasma sheet is kept in (partial) co–rotation with the planet, and that this mechanism also provides a previously unconsidered source of energy that helps explain why the jovian thermosphere is considerably hotter than expected. For Saturn, we show that the H3+ auroral emission is ca. 1% of that of Jupiter because of the lower ionospheric/thermospheric temperature and the lower flux of ionizing particles precipitated there; it is probably unnecessary to invoke additional chemistry in the auroral/polar regions. For Uranus, we report further evidence that its emission intensity is controlled by the cycle of solar activity. And we propose that H3+ emission may just be detectable using current technology from some of the giant extra–solar planets that have been detected orbiting nearby stars, such as Tau Bootes.

Detection of H3(+) from Uranus

The detection of H3(+) in Uranus is reported. Using the CGS4 spectrometer on the UKIRT telescope, we clearly detected 11 emission features of the H3(+) fundamental vibration-rotation band between 3.89 and 4.09 microns. These features are composed primarily of lines from the Q-branch; the strongest of them is the Q(3) blend at 3.985 microns. Analysis of these features indicates a rotational temperature of 740 +/- 25 K, an ortho-H3(+) fraction of 0.51 +/- 0.03, and a disk-averaged H3(+) column abundance of 6.5 x 10 exp 10 (+/- 10 percent) molecules/sq cm. Comparison is made with Jupiter, Saturn, and Neptune. A detection of the H2 1-0 S(1) line in Uranus and an upper limit to H3(+) emission from Neptune also are reported. The rate of energy deposition into Uranus appears to be significantly higher than the rate reported during the Voyager 2 flyby in January of 1986.

Why is Pluto bright? Implications of the albedo and lightcurve behavior of Pluto

Abstract Methane is abundant on Pluto, however, CH 4 rapidly darkens in the Plutonian solar insolation and charged particle environment. We therefore call attention to the fact that Plutos high albedo is at odds with the observation of methane frost on Plutos surface. We examine a variety of mechanisms to resolve this dilemma, and conclude that Plutos surface is being replenished with fresh (bright) volatile frosts. We propose that this replenishment is due to orbitally driven sublimation and freezout of volatiles in the atmosphere. Thus, Plutos high albedo adds to the case for an atmosphere, and argues for annual volatile transport cycles. Orbitally driven replenishment can also account for the observed secular changes in Plutos lightcurve. We show that thermally driven sublimation is capable of replacing volatiles lost to escape and photolysis. Our model is consistent with present data on the surface composition, albedo, aldebo distribution, and surface color of Pluto, and presents an explanation of the time variability of Plutos lightcurve. Charons darker albedo is consistent with our model because Charons surface is today devoid of volatiles. We predict that the secular variation in Plutos rotational lightcurve should reverse 7–17 years after perihelion due to the thermal inertia of Plutos surface. Based on the minimum mass of CH 4 required to cover newly created dark hydrocarbons each Pluto year, we develop a lower limit of 7–70 cm-am on Plutos maximum atmospheric abundance. Based on the time-lag constraint, we develop a lower limit of between 16 and 45 cm-am on Plutos present atmospheric abundance. Finally, we discuss a number of observational tests for our model.

The Goddard High Resolution Spectrograph: In-orbit performance

The in-orbit performance of the Goddard High Resolution Spectrograph onboard the Hubble Space Telescope (HST) is presented. This report covers the pre-COSTAR period, when instrument performance was limited by the effects of spherical aberration of the telescopes primary mirror. The digicon detectors provide a linear response to count rates spanning over six orders of magnitude, ranging from the normal background flux of 0.01 counts diode -1 s-1 to values larger than 104 counts diode-1 s-1. Scattered light from the first-order gratings is small and can be removed by standard background subtraction techniques. Scattered light in the echelle mode is more complex in origin, but it also can be accurately removed. Data have been obtained over a wavelength range from below 1100 A to 3300 A, at spectral resolutions as high as R = lambda/delta-lambda = 90,000. The wavelength scale is influenced by spectrograph temperature, outgassing of the optical bench, and interaction of the magnetic field within the detector with the earths magnetic field. Models of these effects lead to a default wavelength scale with an accuracy better than 1 diode, corresponding to 3 km s-1 in the echelle mode. With care, the wavelength scale can be determined to an accuracy of 0.2 diodes. Calibration of the instrument sensitivity functions is tied into the HST flux calibration through observations of spectrophotometric standard stars. The measurements of vignetting and the echelle blaze function provide relative photometric precision to about 5% or better. The effects of fixed-pattern noise have been investigated, and techniques have been devised for recognizing and removing it from the data. The ultimate signal-to-noise ratio achievable with the spectrograph is essentially limited only by counting statistics, and values approaching 1000:1 have been obtained.

High-resolution spectra of Jupiter's northern auroral ultraviolet emission with the Hubble Space Telescope

The first spectroscopic observations of planetary aurora with the Hubble Space Telescope (HST) are reported. These include spectral regions centered on the H2 Lyman and Werner bands of a region of Jupiters northern aurora. The observations were made with the Goddard High Resolution Spectrograph (GHRS) using the Large Science Aperture as part of a campaign to study Jupiter at the time of the Ulysses flyby. The individual rotational-vibrational bands are resolved and the observed emissions are essentially all from H2. A rotational-vibrational temperature for H2 of 530 +/- 100 K is derived, a value significantly less than the 850-1100 K reported for Jovian H3(+) in the near-infrared but consistent with the temperature reported for fundamental-band quadrupole H2 emission. Comparison with the Faint Object Camera (FOC) images shows that the observed region was not one of the hot spots of the aurora. The results are interpreted in trms of electron impact excitation of H2 from secondary particles generated by primaries precipitating into Jupiters atmsophere from the magnetosphere. In the region of the aurora observed, the homopause level is found to be significantly hotter but not necessarily higher than observed at nonauroral latitudes. The equatorial H2 dayglow spectrum was also detected; its intensity was 3.2 kR or 13% of the strength of the observed auroral emission.

The Goddard High Resolution Spectrograph: Instrument, goals, and science results

The Goddard High Resolution Spectrograph (GHRS), currently in Earth orbit on the Hubble Space Telescope (HST), operates in the wavelength range of 1150-3200A with spectral resolutions (lambda/delta-lambda) of approximately 2 X 103, 2 X 104, and 1 X 105. This paper describes the instrument and its development from inception, its current status, the approach to operations, representative results in the major areas of the scientific goals, and prospects for the future.

HST High-Resolution Images and Maps of Pluto

We have obtained Hubble Space Telescope (HST) images of Pluto at 410 nm and 278 nm which resolve numerous distinct albedo provinces on this planet. Our images were obtained using the Faint Object Camera (FOC) of the Hubble Space Telescope between 20 June and 01 July 1994. This dataset is the rst longitudinally-complete, rotationallyresolved direct image dataset on Pluto. We have combined the various images that HST obtained to make maps of the planet. These images reveal that Pluto has (i) a highly variegated surface, (ii) extensive, bright, asymmetric polar regions, (iii) large midlatitude and equatorial spots, and (iv) possible linear features hundreds of kilometers in extent. The dynamic range of albedo features across the planet detected at the FOCs resolution in both the 410 nm and 278 nm bandpasses exceeds 5:1. We also present and discuss the multiplicative product of the HST 410 nm and 278 nm maps, which allows us to infer the location of where the cleanest, and therefore the presumably freshest ice deposits, lie. Toward the end of this report, we make some initial comparisons between the HST-derived maps and previously published Pluto maps derived from the inversion of groundbased lightcurves of Pluto. Although more-sophisticated HST-map inversions are planned, the data products presented here provide important inputs to modellers interested in volatile transport, and comparative studies of Pluto and Triton. 1

Pioneer 10 Infrared Radiometer Experiment: Preliminary Results

Thermal maps of Jupiter at 20 and 40 micrometers show structure closely related to the visual appearance of the planet. Peak brightness temperatures of 126� and 145�K have been measured on the South Equatorial Belt, for the 20- and 40-micrometer channels, respectively. Corresponding values for the South Tropical Zone are 120� and 138�K. No asymmetries between the illuminated sunlit and nonilluminated parts of the disk were found. A preliminary discussion of the data, in terms of simple radiative equilibrium models, is presented. The net thermal energy of the planet as a whole is twice the solar energy input.

Pioneer 11 Infrared Radiometer Experiment: The Global Heat Balance of Jupiter

Data obtained by the infrared radiometers on the Pioneer 10 and Pioneer 11 spacecraft, over a large range of emission angles, have indicated an effective temperature for Jupiter of 125� � 3�K. The implied ratio of planetary thermal emission to solar energy absorbed is 1.9�0.2, a value not significantly different from the earth-based estimate of 2.5�0.5.

The H3+ Latitudinal Profile of Saturn

We present an H latitudinal profile of Saturn, obtained in 1998 October using the CSHELL spectrometer on the NASA Infrared Telescope Facility. The profile, measured at 3.953 μm, shows that the majority of the emission is concentrated in the auroral ovals, making Saturn similar to Jupiter and different from Uranus. The spatial resolution is sufficient to resolve the southern auroral oval, currently fully displayed around the south pole, into two peaks separated by 12. At the time of the observations reported here, the emission flux in the H line is 8.3 (±1.7) × 10-18 W m-2 for the intensity integrated over a 10 swath along the southern aurora and 5.8 (±1.3) × 10-18 W m-2 for the northern aurora. There may also be some mid- to low-latitude emission, similar to that on Jupiter. We suggest that planetwide H emission from Saturn is between 1.2 and 3.6 × 1011 W.