Chemical sources of haze formation in Titan's atmosphere

A prominent feature of Titan's atmosphere is a thick haze region that acts as the end product of hydrocarbon and nitrile chemistry. Using a one-dimensional photochemical model, an investigation into the chemical mechanisms responsible for the formation of this haze region is conducted. The model derives profiles for Titan's atmospheric constituents that are consistent with observations. Included is an updated benzene profile that matches more closely with—recent ISO observations (Icarus 161 (2003) 383), replacing the profile given in the benzene study of Wilson et al. (J. Geophys. Res. 108 (2003) 5014). Using these profiles, pathways from polyynes, aromatics, and nitriles are considered, as well as possible copolymerization among the pathways. The model demonstrates that the growth of polycyclic aromatic hydrocarbons throughout the lower stratosphere plays an important role in furnishing the main haze layer, with nitriles playing a secondary role. The peak chemical production of haze layer ranges from 140 to 300 km peaking at an altitude of 220 km, with a production rate of 3.2×10⁻¹⁴ gcm⁻² s⁻¹. Possible mechanisms for polymerization and copolymerization and suggestions for further kinetic study are discussed, along with the implications for the distribution of haze in Titan's atmosphere.     

The shape of the sunspot cycle

The temporal behavior of a sunspot cycle, as described by the International sunspot numbers, can be represented by a simple function with four parameters: starting time, amplitude, rise time, and asymmetry. Of these, the parameter that governs the asymmetry between the rise to maximum and the fall to minimum is found to vary little from cycle to cycle and can be fixed at a single value for all cycles. A close relationship is found between rise time and amplitude which allows for a representation of each cycle by a function containing only two parameters: the starting time and the amplitude. These parameters are determined for the previous 22 sunspot cycles and examined for any predictable behavior. A weak correlation is found between the amplitude of a cycle and the length of the previous cycle. This allows for an estimate of the amplitude accurate to within about 30% right at the start of the cycle. As the cycle progresses, the amplitude can be better determined to within 20% at 30 months and to within 10% at 42 months into the cycle, thereby providing a good prediction both for the timing and size of sunspot maximum and for the behavior of the remaining 7–12 years of the cycle. The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Long-Term Evolution of FU Ori-Type Stars at Infrared Wavelengths

We investigated the brightness evolution of 5 FU Ori systems in the 1–200 μm wavelength range using observations from the Infrared Space Observatory, 2MASS and MSX data. The SEDs were compared with earlier ones derived from the IRAS photometry and ground-based observations around the epoch 1983. In three cases no difference was seen, in one object the near- and mid-infrared part of the spectrum became fainter by a factor of 2, and in the fifth case a marginal fading was observed. We study in detail the case of V1057 Cyg.     

The Solar Cycle Workshop

The contributions to the third meeting of the Solar Cycle Workshop are briefly summarized. The topics discussed at the meeting included (i) predictions and precursors, (ii) large and small-scale magnetic fields, (iii) photospheric velocity fields, (iv) coronal phenomena, (v) the Sun as a star, (vi) limb temperature measurements and helioseismic data, (vii) theoretical modelling of the cycle, (viii) cyclic activity in stars, and (ix) the interpretation of the Elatina Sandstone Layers.This paper was presented at the third meeting of the Solar Cycle Workshop, held in Sydney, Australia, January 9–13, 1989.     

The Evolution of Trailing Plumes from Active Regions

We have studied the evolution of several high-latitude flux `plumes', i.e., unipolar regions, trailing from active regions which emerged near sunspot maximum in cycle 23. The observed patterns are compared with simulations using a simple flux transport equation based on the observed flux for an earlier Carrington rotation. In addition to the long recognized poleward migration and diffusion of flux from active regions, it is found that the evolution of the trailing plumes may be influenced by flux which emerges above latitude 35° over areas of all scales. We describe two cases in which the emerging flux appears in the form of bipolar flux patterns which are not obviously related to sunspots. Further, we find instances in which the observed surface flux decreases or spreads at rates which cannot be explained solely in terms of diffusion using the normally accepted rates. Thus in several cases the poleward migration of flux cannot be described in terms of passive transport by advection and diffusion as considered here, and further investigation of the processes that contribute to the evolution of the polar fields is required.     

Near-infrared spectra of the leading and trailing hemispheres of Enceladus

We present individual spectra 0.8-2.5 μm of the leading and trailing hemispheres of Enceladus obtained with the CorMASS spectrograph on the 1.8 m Vatican Advanced Technology Telescope (VATT) at the Mount Graham International Observatory. While the absorption bands of water ice dominate the spectrum of both hemispheres, most of these bands are stronger on the leading hemisphere than the trailing hemisphere. In addition, longward of 1 μm, the continuum slope is greater on the leading hemisphere than the trailing hemisphere. These differences could be produced by the presence of particles on the trailing side that are smaller and/or microstructurally more complex than those on the leading side, consistent with the preferential erosion or structural degradation of regolith particle grains on the trailing side by magnetospheric sweeping. We also explore compositional differences between the two hemispheres by applying Hapke spectrophotometric mixture models to the spectra whose components include water ice and ammonia hydrate (1% NH₃⋅H₂O). We find that spectral models which include as much as 25% by weight ammonia hydrate intimately mixed with water ice and covering 80% of the illuminated area of the satellite fit the observed spectrum of both the leading and trailing hemispheres. Areal (checkerboard) mixing models of ammonia hydrate and water ice fit the leading hemisphere with 15% of the surface comprised of ammonia hydrate and the trailing hemisphere with 10% ammonia hydrate. Therefore, while these spectral data do not contain an unambiguous detection of ammonia hydrate on Enceladus, our spectral models do not preclude the presence of a modest amount of 1% NH₃⋅H₂O on both hemispheres. We examine spectral differences and similarities between both hemispheres and the tenuous E ring within which Enceladus orbits. The spectral resolution (R=λ/Δλ) of these CorMASS data (R∼300) is comparable to but nevertheless higher than that of the Visual-Infrared Mapping Spectrometer (VIMS) (R=225) onboard the Cassini spacecraft.