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Titolo:
TITANS GEOMETRIC ALBEDO - ROLE OF THE FRACTAL STRUCTURE OF THE AEROSOLS
Autore:
RANNOU P; CABANE M; CHASSEFIERE E; BOTET R; MCKAY CP; COURTIN R;
Indirizzi:
UNIV PARIS 06,SERV AERON,TOUR 15,B102,PL JUSSIEU F-75005 PARIS FRANCE CNRS,SERV AERON F-91371 VERRIERES BUISSON FRANCE UNIV PARIS 11,LAB PHYS SOLIDES F-91405 ORSAY FRANCE NASA,AMES RES CTR,DIV SPACE SCI MOFFETT FIELD CA 94035 OBSERV PARIS,CNRS,LAB RECH SPATIALE F-92195 MEUDON FRANCE
Titolo Testata:
Icarus
fascicolo: 2, volume: 118, anno: 1995,
pagine: 355 - 372
SICI:
0019-1035(1995)118:2<355:TGA-RO>2.0.ZU;2-V
Fonte:
ISI
Lingua:
ENG
Soggetto:
OPTICAL-PROPERTIES; AGGREGATE PARTICLES; HEATING RATES; ATMOSPHERE; MODELS; PHOTOLYSIS; CLUSTERS; GROWTH;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Science Citation Index Expanded
Citazioni:
27
Recensione:
Indirizzi per estratti:
Citazione:
P. Rannou et al., "TITANS GEOMETRIC ALBEDO - ROLE OF THE FRACTAL STRUCTURE OF THE AEROSOLS", Icarus, 118(2), 1995, pp. 355-372

Abstract

The collisional mechanisms of Titan's aerosols may lead to a fractal structure in which the aerosols are built by the aggregation of spherical submicrometer particles (monomers). In this initial study of the problem, the optical behavior of these aggregates is modeled assuming that each monomer radiates a dipole field in response to the incident radiation including the radiated fields of all the other elements in the aggregate. This dipole approximation, valid if the monomer radius issmaller than the wavelength, is used to calculate the scattering and extinction efficiencies of such aerosol particles, which are assumed to be composed of tholins. By applying the two-stream approximation forradiative transfer to the vertical distribution of aerosols obtained by microphysical modeling, we compute the geometric albedo of Titan. Computed values and observational values of the albedo are compared forwavelengths from 0.22 to 1.0 mu m, and the effects of parameters, such as the fractal dimension of aerosols, their formation altitude or mass production rate, and, in addition, the methane abundance, are investigated. The hypothesized fractal structure of particles can explain both the visible and the UV albedos. In previous models these measurements could only be matched simultaneously under the assumption of a bimodal population. For a fractal dimension D-f approximate to 2 in the settling region, corresponding to a growth governed by cluster-cluster aggregation, the computed albedo in the near-UV range matches the observations. A good fit between measurement and calculated albedo is obtained, for a formation altitude z(0) = 535 km, over the whole wavelength range by adjusting the absorption coefficient of the particles within a factor of two from that of tholins and a production rate between 0.2 and 1.5 times 3.5 x 10(-13) kg m(-2) sec(-1). Lower formation altitudes,like our preferred case, z(0) = 385 km, cannot be investigated inthe UV range due to limitations of the dipolar approximation, but we expect this case to give the same behavior. (C) 1995 Academic Press, Inc.

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Documento generato il 27/09/20 alle ore 22:38:57