L. Trafton

Large seasonal variations in Triton's atmosphere

Tritons seasons differ materially from those of Pluto owing to four important differences in the governing physics: First, the obliquity of Triton is significantly less than Plutos obliquity. Second, Tritons inclined orbit precesses rapidly about Neptune so that a complicated seasonal variation in the latitude of the Sun occurs for Triton. Third, Neptunes orbit is much more circular than Plutos orbit so that the sunlight intercepted by Tritons disk does not vary seasonally. Finally, Tritons atmosphere cannot be saturated at the lower latitudes so that the mass of the atmosphere is controlled by the temperature of the high-latitude ices or liquids (polar caps), as for CO2 on Mars. The consequences of Tritons entire surface being covered with volatile substances have been examined. It is found that the circularity of Neptunes orbit then implies that Triton would have hardly any seasonal variation at all in surface temperature or atmospheric bulk, in spite of the complicated precessional effects of Tritons orbit. The only seasonal effect would be the migration of surface ices and liquids. This scenario is ruled out because it implies a column CH4 abundance much higher than that observed and because it quickly depletes the lower latitudes of volatiles. It is concluded that Tritons most volatile surface substances are probably relegated to latitudes higher than 35° and probably form polar caps. The temperature of the polar caps should be nearly equal, even during midwinter/midsummer when the insolation of the summer pole is greatest. If the summer pole completely sublimates during one of the “major” summers, Tritons atmosphere may begin to freeze out over the winter caps. It is therefore expected that Tritons atmosphere undergoes large and complex seasonal variations. Triton is currently approaching a “maximum southern summer”, and over the remainder of this century, a dramatic increase in CH4 abundance above the current upper limit of 1 m-Am may be witnessed.

The Jovian SII torus: Its longitudinal asymmetry

Abstract Spectra of SII emission obtained from early 1976 to mid-1979 in the vicinity of Jupiter show bright emission near the plane of the magnetic equator in the neighborhood of Ios orbit. The emission drops off sharply beyond 6.66 R u from Jupiter, just outside Ios orbit at 5.87 R u . The SII emission is asymmetric in magnetic longitude, being brightest at λ III (1965) = 260°. It is about five times fainter 180° from this position. The longitude of brightest emission is close to the “active sector” where enhanced ionizing currents are expected to exist along Ios flux tube. Rather large variations in the electron density over the SII cloud are reported but the average value is close to previous determinations. The brightest region observed near the ansa of the emission torus had 3.92 T e n e = 4.2 [cm −3 ] amd column abundance of SII = 1.5 × 10 13 cm −2 . The biggest long-term change appears to be in the clouds sulfur content rather than in its electron temperature or electron density.

The Pluto-Charon system: The escape of charon's primordial atmosphere☆

Abstract Assuming that Charon formed in the outer Solar System, we expect it to have formed with an atmosphere consisting of CH4 and other gases supported by volatile ice deposits (i.e., solid materials on the surface having atmospherically significant vapor pressures at Charons temperature). The large obliquity of Pluto and Charon implies that volatile ices which originally existed on the surface at higher latitudes have been transported to latitudes below 32°. However, seasonally transient polar caps are possible when an atmosphere is present. Charon appears to have lost its atmosphere and surface volatiles. Owing to the lack of heating sufficient to cause widespread melting and separation of the lighter and heavier nonvolatile materials, Charons outer layers probably retain their primordial mix of nonvolatiles. Therefore, spectroscopically determined relative abundances for Charons surface should be representative for Charons total mass (even if deep differentiation has occured) and thus may hold the key to understanding Charons origin. The study of Charons exposed nonvolatile ices (water ice, clathrate hydrates, any NH3 hydrate, and other surface materials with atmospherically insignificant vapor pressures at Charons temperature) may distinguish whether the Pluto-Charon system condensed densed directly out of the solar nebula or whether it condensed out of a protoplanetary nebula, as would be the case if these bodies originally formed as satellites of another planet.

Does pluto have a substantial atmosphere

The presence of CH4 ice on Pluto implies that Pluto may have a substantial atmosphere consisting of heavy gases. Without such an atmosphere, sublimation of the CH4 ice would be so rapid on a cosmogonic time scale that either such an atmosphere would soon develop through the exposure of gases trapped in the CH4 ice or else the surface CH4 ice would soon be all sublimated away as other, more stable, ices became exposed. If such stable ices were present from the beginning, the existence of CH4 frosts would also imply that Plutos present atmosphere contains a remnant of its primordial atmosphere.

Long-term changes in Saturn's troposphere

Abstract We report the results of monitoring Saturns H2 quadrupole and CH4 band absorptions outside of the equatorial zone over one-half of Saturns year. This interval covers most of the perihelion half of Saturns elliptical orbit, which happens to be approximately bounded by the equinoxes. Marked long-term changes occur in the CH4 absorption accompanied by weakly opposite changes in the H2 absorption. Around the 1980 equinox, the H2 and CH4 absorptions in the northern hemisphere appear to be discontinuous with those in the southern hemisphere. This discontinuity and the temporal variation of the absorptions are evidence for seasonal changes. The absorption variations can be attributed to a variable haze in Saturns troposphere, responding to changes in temperature and insolation through the processes of sublimation and freezing. Condensed or frozen CH4 is very unlikely to contribute any haze. The temporal variation of the absorption in the strong CH4 bands at south temperate latitudes is consistent with a theoretically expected phase lag of 60° between the tropopause temperature and the seasonally variable insolation. We model the vertical haze distribution of Saturns south temperature latitudes during 1971–1977 in terms of a distribution having a particle scale height equal to a fraction of the atmospheric scale height. The results are a CH4/H2 mixing ratio of (4.2 ± 0.4) × 10−3, a haze particle albedo of ω = 0.995 ± 0.003 , and a range of variation in the particle to gas scale-height ratio of 0.6 ± 0.2. The haze was lowest near the time of maximum temperature. We also report spatial measurements of the absorption in the 6450 A NH3 band made annually since the 1980 equinox. A 20 ± 4% increase in the NH3 absorption at south temperate latitudes has occurred since 1973–1976 and the NH3 absorption at high northern latitudes has increased during spring. Increasing insolation, and the resulting net sublimation of NH3 crystals, is probably the cause. Significant long-term changes apparently extend to the deepest visible parts of Saturns atmosphere. An apparently anomalous ortho-para H2 ratio in 1978 suggests that the southern temperate latitudes experienced an unusual upwelling during that time. This may have signaled a rise in the radiative-convective boundary from deep levels following maximum tropospheric temperature and the associated maximum radiative stability. This would be further evidence that the deep, visible atmosphere is governed by processes such as dynamics and the thermodynamics of phase changes, which have response times much shorter than the radiative time constant.

The abundances of ammonia in the atmospheres of Jupiter, Saturn, and Titan

An investigation of low-resolution ratio spectra of Jupiter, Saturn, and Titan in the region 5400–6500 A has permitted new evaluations of ammonia absorption bands. The distribution of ammonia over the disk of Jupiter is very inhomogeneous. The carbon-to-nitrogen ratio is distinctly different from the solar value, but this is probably a result of uneven mixing of methane and ammonia, as suggested previously by Kuiper, rather than a compositional anomaly. The abundance of ammonia on Saturn also shows spatial variations, but appears constant in time over a 3-yr period. Two weak, unidentified absorptions were discovered in the red region of Titans spectrum, in the absence of any detectable ammonia. The new upper limit is ηN < 120 cm-am.

The DH ratio in the atmosphere of Uranus: Detection of the R5(1) line of HD

Observations of Uranus during the 1975, 1976, and 1978 apparitions reveal a weak absorption at the wavelength of the R5(1) line of HD with equivalent width 1.0 + or - 0.4 mA. The D/H ratio in Uranus atmosphere implied by this line and other published spectra is (0.000048 + or - 0.000015), and may not be significantly different from that in the atmospheres of Jupiter and Saturn. In addition, the spectra exhibit two weak absorptions at 6044.76 + or - 0.02 and 6045.54 + or 0.02 A which were not identifiable. No trace of absorption is visible near these wavelengths or near the HD wavelength in a laboratory spectrum of 4.92 km-am CH4 which was obtained in an attempt to identify these absorption features and to verify that the HD feature does not arise from CH4.

Uranus' (3-0) H2 quadrupole line profiles

Abstract We have obtained spectra of Uranus S 3 (0) and S 3 (1) H 2 quadrupole lines during the apparitions from 1978 through 1980 (one-third of a Uranian season before the Voyager flyby) and have analyzed the profile shapes in terms of several structural models. Two bracketing temperature-pressure profiles were employed: J. F. Applebys (1986, Icarus 65 , 383–405) model c and the preliminary Voyager 2 radio occultation profile. Significant modification of the K. H. Baines and J. T. Bergstralh (1986, Icarus 65, 406–441) standard model is required to fit these observations: An additional haze layer, one which contains strongly absorbing particles, is required above the 16-km-am H 2 level (0.15 bar). For a Rayleigh phase function, such a haze uniformly mixed with the gas above this level would have an absorption optical depth of 0.16 and a scattering optical depth of 0.067 to give a single scattering particle albedo of ω = 0.30. These optical depths would be somewhat smaller if the added haze where confined to still higher altitudes. The parameters of this haze layer vary little over the range of temperature profiles employed, but the depth of the cloud top is affected. This modification implies a fraction of normal H 2 equal to 0.25 ± 0.10, in good agreement with Baines and Bergstralhs standard model; but complex haze distributions have not been considered which might affect this result. Regardless of the haze distribution, a deep cloud is required by our observations. Further constraint of the maximum CH 4 /H 2 ratio must await better laboratory measurements of the CH 4 absorption coefficient at low temperatures. V. G. Teifels (1983, Icarus 53 , 389–398) proposed model with a high haze layer having a scattering optical depth of 0.3 – 0.5 is ruled out by the (3-0) H 2 observations.

On the distribution of sodium in the vicinity of Io

Abstract We investigate the contribution of scattering in the telescope to our measurements of the size of Ios sodium cloud and to the distribution of emission intensity in the cloud. The brightest regions, within 30″ of Io near opposition and along the equatorial plane, are relatively undistorted but regions further than 45″ away and not close to the equatorial plane are very likely to consist of mainly scattered light. Portions of the cloud in the vicinity of the magnetic equator are also mostly scattered light when Io is near extreme magnetic latitude. The equatorial torus, however, extends up to 20 arcmin from Jupiter. The large size of the cloud is thus confirmed. High-resolution line profile shapes indicate that sodium streams from Io preferentially in the forward direction with velocities distributed up to 18 km sec −1 . The observed wavelength shifts of the peak intensities from Ios rest frame are compatible with a cloud streaming through a bound atmospheric component but they could also be caused by a velocity distribution peaked at very low velocities.

A new class of absorption feature in Io's near-infrared spectrum.

We report our discovery of an absorption feature in the infrared spectrum of Io centered at 2.1253 micrometers (4705.2 cm-1). This band is marginally resolved at resolving power 1200 with a deconvolved full width at half-maximum (FWHM) of about 4 cm-1. This contrasts with the 30- to 50-cm-1 widths of the broad absorption features previously detected at longer wavelengths which arise from mixtures of SO2 with H2S and H2O. This newly discovered feature is relatively weak, having a core only 5% below the continuum at this resolving power. Our survey from 1.98 to 2.46 micrometers (5050-4065 cm-1) at this same resolving power revealed no other feature greater than 1% of the continuum level shortward of 2.35 micrometers, and 3% elsewhere. The feature does not correspond to any gas- or solid-phase absorption that might be expected from previously identified constituents of Ios surface. No temporal or longitudinal variation has been detected in the course of 18 nights of observation over the past year and no significant variation in the strength of the feature was seen during an emergence from eclipse. These observations indicate that the source material of the feature is reasonably stable, and is more uniformly distributed in longitude than Ios hot spots. These characteristics all indicate that the feature belongs to a class different from those characterizing other known absorption features in Ios spectrum. Consequently, it should reveal important new information about Ios atmosphere-surface composition and interaction. A series of laboratory experiments of plausible surface ices indicates that (i) the band does not arise from overtones or combinations of any of the molecular vibrations associated with species already identified on Io (SO2, H2S, H2O) or from chemical complexes of these molecules, (ii) the band does not arise from H2 trapped in SO2, and (iii) the band may arise from the 2 nu3 mode of CO2. If the band arises from CO2, it is clear from its detailed shape and position that the molecules are not embedded in an SO2 matrix, as are H2S and H2O, but may be present as multimers or clusters.