Triton: do we see to the surface?

The quantity and physical state of methane and nitrogen in the atmosphere of Neptune's satellite Triton and on the surface are evaluated by means of new telescopic data and laboratory measurements of these volatiles. Methane ice is seen in some spectral regions, indicating that the atmosphere is sufficiently transparent to permit sunlight penetration to the surface. Some of the molecular nitrogen absorption occurs in the atmosphere, though some must occur in condensed nitrogen (liquid or solid) on Triton's surface, or in a thin cloud of condensed nitrogen. The Voyager spacecraft cameras should see the surface of Triton.

Production of organic compounds in plasmas: a comparison among electric sparks, laser-induced plasmas, and UV light.

The chemistry in planetary atmospheres that is induced by processes associated with high-temperature plasmas is of broad interest because such processes may explain many of the chemical species observed. There are at least two important phenomena that are known to generate plasmas (and shocks) in planetary atmospheres: lightning and meteor impacts. For both phenomena, rapid heating of atmospheric gases leads to formation of a high-temperature plasma which emits radiation and produces shock waves that propagate through the surrounding atmosphere. These processes initiate chemical reactions that can transform simple gases into more complex compounds. In order to study the production of organic compounds in plasmas (shocks), various mixtures of N2, CH4, and H2, modeling the atmosphere of Titan, were exposed to discrete sparks, laser-induced plasmas (LIP), an ultraviolet radiation. The yields of HCN and several simple hydrocarbons were measured by gas chromatography and compared to those calculated from a simple quenched thermodynamic equilibrium model. The agreement between experiment and theory was fair for HCN and C2H2. However, the agreement for C2H6 and the other hydrocarbons was poor, indicating that a more comprehensive theory is needed. Our experiments suggest that photolysis by ultraviolet light from the plasma is an important process in the synthesis. This was confirmed by the photolysis of gas samples exposed to the light but not to the shock waves emitted by the sparks. Hence, the results of these experiments demonstrate that the thermodynamic equilibrium theory does not adequately model lightning and meteor impacts and that photolysis must be included. Finally, the similarity in yields between the spark and the LIP experiments suggest that LIP provide valid and clean simulations of lightning and meteor impacts and that photolysis must be included. Finally, the similarity in yields between the spark and the LIP experiments suggests that LIP provide valid and clean simulations of lightning in planetary atmospheres.

Lightning production of hydrocarbons and HCN on Titan: laboratory measurements.

Many hydrocarbon species have been detected in the atmosphere of Titan. It is possible that lightning activity is occurring in the troposphere and that it contributes to the hydrocarbon inventory. Measurements of the chemical yields of hydrogen cyanide, acetylene, ethylene, ethane, and propane from simulated lightning discharges are reported. A comparison of the experimental results with those based on thermodynamic equilibrium assumptions shows significant disagreement and implies that theories based solely on thermodynamic equilibrium are inadequate. Although photochemistry and charged particle chemistry occurring in the stratosphere can account for many of the observed hydrocarbon species, the predicted abundance of ethylene is too low by a factor of 10 to 40. While some ethylene will be produced by charged-particle chemistry, the production of ethylene by lightning and its subsequent diffusion into the stratosphere appears to be an adequate source.

Radiative flux measurements in the troposphere.

The results of radiative flux-density measurements in the troposphere, made using an especially designed radiometer mounted on a Cessna 402B aircraft, are reported. The radiometer incorporates several well-known principles that result in highly accurate determinations of radiative fluxes in the atmosphere. Heating rates for gases and for aerosols are calculated, using measurements and radiosonde data. Instrument performance is verified by calculating the solar constant at the top of the atmosphere, using the radiative flux densities measured in the troposphere. Total heating rates of 0.175 and 0.377 K h(-1) are determined for hazy and foggy atmospheres, respectively. Aerosol heating rates of 0.065 and 0.235 K h(-1) are deduced from the total heating rates. Environmental noise measurements during data acquisition are presented. The solar constant value of 1387 +/- 21 W m(-2) derived from the experiments agrees within 4% of the standard value.

Measurement of weak ir absorption with a tunable laser: the hydrogen S(2)(1) line strength.

A tunable laser light source has been used to obtain the intensity of the S(2)(1) hydrogen quadrupole line at ~8604.2 cm(-1). The result (0.0140 +/- 0.0007 cm(-1)/km-amagat) is in excellent agreement with a previous determination as well as with ab initio calculations.