Tobias C. Owen

Evidence for Methane Segregation at the Surface of Pluto

Abstract In May 1995, a set of spectrophotometric curves of the system Pluto–Charon were recorded with the UKIRT telescope equipped with the spectrometer CGS4. The spectra cover the near-infrared range between 1.4 and 2.55 μm with a resolution of approximately 700. The existence of solid methane is confirmed by numerous absorption bands, and carbon monoxide and nitrogen ices are identified by their respective signatures at 2.35 and 2.15 μm. We have modeled the spectrum of May 15 that corresponds to the maximum of Plutos visible lightcurve using a radiative transfer algorithm dealing with compact and stratified media. A geographical mixture of three distinct units is required to explain all the significant structures of the analyzed spectrum. The first unit is a thin, fine-grained layer of pure CH4 covering a compact polycrystalline substratum of N2–CH4–CO, which are in a molecular mixture (concentrations of CH4 and CO of the order of 0.5 and 0.1–0.2% respectively). It covers about 70% of the observed area and corresponds to volatile deposits that are sublimating under solar illumination. The second unit is either (a) a single thick layer of pure large-grained methane or (b) a unit with large-grained CH4 forming a substratum and the N2–CH4–CO mixture a superficial layer of fine grains covering 20% of the surface. Finally, the third unit is bright and spectrally neutral and is first modeled as a layer of very fine grains of nearly pure N2. Tholin, suggested to explain the red slope in the visible, is also found to be spectrally compatible with this unit. It covers the remainder of the surface (about 10–15%). All these results allow a better understanding of the processes of deposition, metamorphism, sublimation, and transport affecting the different ices detected on Pluto during its climatic cycles.

First observations of CO and HCN on Neptune and Uranus at millimeter wavelengths and the implications for atmospheric chemistry

Observations are presented which show that CO is present in both the troposphere and stratosphere of Neptune, whereas is confined to the Neptune stratosphere with a mean mole fraction in the 0.003-30 mbar pressure level range of 1.0 x 10 exp -9. CO is present in both the stratosphere and in the troposphere with a uniformly mixed model fraction of 1.2 x 10 exp -6. Upper limits of 1.0 x 10 exp -10 and 3.0 x 10 exp -8 mole fractions are derived for HCN and CO respectively on Uranus. The origin of these species in the atmosphere of Neptune and their nondetection in that of Uranus are discussed in detail. It is concluded that the most plausible scenario involves upward convection of CO and N2 from Neptunes deep interior and a failure of chemical equilibrium at deep atmospheric levels, allowing excess CO and presumably N2 to reach the upper atmosphere. Nondetection in Uranus may be explained by the lack of a significant internal heat source in the planet and consequent suppression of vertical convection.

Monodeuterated methane in the outer solar system. II. Its detection on Uranus at 1. 6 microns

Deuterium in the atmosphere of Uranus has been studied only via measurements of the exceedingly weak dipole lines of hydrogen-deuteride (HD) seen in the visible region of the spectrum. The other sensitive indicator of deuterium in the outer solar system is monodeuterated methane (CH3D) but the two bands normally used ot study this molecule, NU sub 2 near 2200 1/cm and NU sub 6 near 1161 1/cm, have not been detected in Uranus.

Monodeuterated methane in the outer solar system. III - Its abundance of Titan

The 3nv2 band of CH3D has been detected in spectra of Titan recorded at 1.6 microns with the Fourier transform spectrometer at the 4 m telescope of the Kitt Peak National Observatory. A value of the CH3D/Ch4 mixing ratio is obtained from a comparison between the observed Titan spectra and synthetic spectra. This value is about 2 times higher than the value measured on Uranus (de Bergh et al. 1981, 1986) and about 6 times higher than on Jupiter and on Saturn (Courtin et al. 1984; de Bergh et al.). The value found on Titan for D/H in methane is comparable to the D/H ratio measured in terrestrial H2O. 23 references.

Monodeuterated methane in the outer solar system. IV - Its detection and abundance on Neptune

We have detected the 3 nu 2 band of CH3D in the spectrum of Neptune near 1.6 micrometers recorded at a spectral resolution of 4 cm-1 with the Cassegrain Fourier Transform Spectrometer at the 3.6 m Canada-France-Hawaii Telescope (CFHT) on Mauna Kea. Our analysis of this spectrum, using spectral synthesis techniques, yielded a CH3D/CH4 ratio of 6(+6)(-4) x 10(-4), which corresponds to a global D/H ratio for Neptune of 1.2(+1.2)(-0.8) x 10(-4), if CH3D is in isotopic fractionation equilibrium with HD. This value is about an order of magnitude larger than an earlier estimate by Orton et al. based on deconvolution measurements of unresolved molecular emission in the 8-10 micrometers region. Comparison of this new determination with previous studies of CH3D in the outer solar system shows that, as in the case of Uranus, the D/H on Neptune is strongly enhanced over that found on Jupiter and Saturn and is comparable to the D/H in methane on Titan and in terrestrial methane and water.

On the possible detection of CH3D on Titan and Uranus

The possible spectral detection of monodeuterated methane, CH3D, in the atmospheres of Titan and Uranus is reported. The detection was made on the basis of the identification of a new band centered near 6425 per cm in laboratory spectra of CH3D, which could possibly account for the strength of the methane absorption near 6400 per cm observed in the interferometric spectra of Titan and Uranus of Fink and Larson (1979). On the basis of measurements of the CH3D/CH4 natural abundance ratio and the estimated band strength of the 6425 per cm band, it is concluded that approximately 15% of the observed methane absorption is due to CH3D, rather than a pure temperature effect in CH4.

The Collision of the Comet Shoemaker-Levy 9 with Jupiter: Detection and Evolution of HCN in the Stratosphere of the Planet

We report submillimeter heterodyne observations of Jupiter taken with the JCMT during and after the infall of Comet Shoemaker-Levy 9 into the planet. We detected the J=4-3 and J=3-2 rotational transitions of HCN in emission at many of the impact sites. Measurements suggest for fragment G a mixing ratio of ~5x10 -s above the 0.5-mbar pressure level and a total HCN mass of 6 x 10•g. Subsequent observations, made in September and November 1994, reveal that HCN is still present but that the lines now appear in absorption. This results from a cooling of the stratospheric thermal profile between July and September. Chemical implications of the observed persistence of HCN in the Jovian stratosphere for over 6 months