Feasibility of determining haze properties during high-speed Titan entry

Abstract

An international cooperative project between the National Aeronautics and Space Administration and the European Space Agency is planning to send a probe into the atmosphere of Titan (a moon of Saturn) as part of the Cassini Mission to Saturn. This article analyzes the feasibility of measuring the intensity of atomic carbon and hydrogen line emission in the shock layer during the high-velocity portion of the entry to determine the number density and composition of the organic haze particles in the Titan atmosphere. Analysis indicates that the line radiation signal-to-noise ratios are high enough so that determination of the haze particle number density and composition appears to be feasible. The analysis may be applicable to other future planetary Nomenclature A ul = Einstein coefficient for spontaneous emission, 1/s C p = particle specific heat, 1.80 kJ/(kg-K) c = speed of light D = particle diameter, m/x d {} = gas-molecule diameter, 6.0 x 10~1 () rn E u = energy of upper state, J Fi_ 2 = view-factor for radiometer g fl = degeneracy of upper state of transition H = enthalpy, kJ/kg h = Planck's constant, J/K / A = radiation emission, w/(cm 3 -stradian) K np = Knudsen number of haze particle k = Boltzmann's constant, J-s L -particle vaporization energy, J/kg L () = particle heat of evaporation, 360 kJ/kg N •= number density, I/cm 3 P = gas pressure, Pa P -particle flow rate, particles/(cm' 2 -s) Q = total heat transfer, J Q c = convective heat transfer rate, W Q r = radiative heat transfer rate, W q c = convective flux to haze particle, W/cm 2 4, = radiative flux to haze particle, W/cm 2 R = radius of view-cylinder, cm S = photons emitted S = photon emission rate, photons/s T = temperature, K 7 1 ,.= vibrational temperature, K va P = particle evaporation temperature, 300 K t = time, s /,. = particle evaporation time, s V = velocity, m/s V v fl = view-volume times solid angle, stradian-cm 3 V (} = initial entry velocity, 6220 m/s x = distance in front of window, cm Z = electronic partition function