Laboratory Needs

Julianne I. Moses, April 2002

My thoughts on some specific laboratory needs related to giant-planet atmospheres (in response to some prompting
by Melissa McGrath).

1. Need photoabsorption, photodissociation, and photoionization cross
   sections (UV), photolysis branching ratios, and quantum yields for
   appropriate species AT LOW TEMPERATURES
   
   Examples:
   1. Characterize CH4 photolysis at H Lyman alpha (we still do
      not know the photolysis branching ratios and product quantum
      yields). What is the relative importance of branches that
      produce CH3, CH2, and CH?
   2. Measure low-temperature photoabsorption and photodissociation
      cross sections for hydrocarbons containing from 2 to 6 carbon
      atoms (e.g., stable forms of C6H6, C3H4, C3H6, C4H4, and C4H6)
      at appropriate temperatures (50-200 K). Need better understanding
      of the photolysis of hydrocarbons with long lifetimes against
      photodissociation (e.g., benzene, acetylene) under conditions
      relevant to the outer planets (including any pressure
      dependence).

 

2. Need rate coefficients for appropriate chemical reactions at low
   temperatures and pressures


Examples:
1. Low-pressure limiting rate constants for many important termolecular hydrocarbon addition reactions have not been obtained experimentally or theoretically (e.g. H + hydrocarbons; CH3 + CH3; CH3 + C2Hx, C3Hx, and C4Hx radicals; C2H3 or C2H5 + C2Hx, C3Hx, and C4Hx radicals). Again, appropriate temperature range is 50-200 K. Any information about falloff region behavior would be helpful.
2. LOW-TEMPERATURE rate constants for important bimolecular reactions that have not been obtained or are not well constrained (e.g., H2 + hydrocarbon radicals; (1)CH2 + H2 and hydrocarbons; reactions involving C3Hx, C4Hx, C5Hx, and C6Hx species). Information about reaction products is also critical.
3. Need rate-constant measurements for reactions related to tropospheric ammonia and phosphine chemistry (e.g., PH2 + H; PH2 + NH2; PH2 + PH2; NH2 + N2H4; all reactions involving N2H3 and P2H3).
4. Photochemistry of H2S under tropospheric conditions needs to be better characterized.

 

3. Need laboratory measurements that will help us develop a better understanding of the chemical pathways that lead to aerosol formation on the giant planets.


Examples:
1. Need rate coefficients for charge exchange and recombination
   reactions, branching ratios, and other details of ion
   chemistry relevant to the polar regions of the giant
   planets.
2. Need rate coefficients for the key steps in the reaction sequences leading to the formation of heavy organic molecules under jovian atmospheric conditions.

 

4. Need vapor pressure measurements and other physical and optical properties of potential condensed phases in giant-planet atmospheres.


Examples:
1. Need accurate low-temperature vapor pressure measurements for N2H4, P2H4, NH2PH2, CH3C2H, C3H8, C4Hx, C6H6, and other hydrocarbon molecules.
2. Need measurements of the optical properties of NH4SH, C4H2, C4H4, C4H10, C6H6, and other organic solids. Need measurements of the thermodynamic and physical properties (e.g., density, surface energy, shape, scattering properties, "wetability") of potential condensed phases.
3. Need laboratory studies of the effects of UV irradiation of NH4SH, NH3, H2S, C2H2, C4H2, and other ices relevant to the tropospheres and stratospheres of the giant planets.

 

5. Need laboratory measurements or theoretical calculations that will help in modeling the radiative properties of giant-planet atmospheres.


Examples:
1. Need more detailed near-infrared absorption spectrum of CH4 to help in calculating stratospheric heating rates.
2. Need low-temperature collisional deactivation rates of relevant hydrocarbons (C2H2, C2H4, C6H6, CH3) with H2 and He because a significant portion of the infrared emission comes from high-altitude (low-density) atmospheric regions in which non-LTE effects might be important. Need information on excited state quenching and energy transfer for CH4 and other hydrocarbons.

 

6. Need laboratory measurements or theoretical calculations that will help in interpreting spectra from giant-planet atmospheres.


Examples:
1. Assignment of and measurement of spectral features (UV, vis, IR, microwave) for hydrocarbons and other relevant species: line strengths, line widths, pressure broadening characteristics, absorption coefficients.
2. Temperature dependence of line wing absorption (sub-mm, microwave) of NH3, PH3, and other tropospheric species.

 

7. Need laboratory measurements or theoretical calculations to better characterize the equation of state of H2 and H2-He at high temperatures and pressures.