Physics
Scientific paper
Jun 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997icar..127..319c&link_type=abstract
Icarus, Volume 127, Issue 2, pp. 319-353.
Physics
82
Scientific paper
A new step of development of the 3-D circumnuclear coma (``CNC'') model described in Crifo et al. (1995) is presented: now the gas and dust production are computed from an explicit dusty-ice sublimation model and a realistic dust mass spectrum is used. For the first time, (1) a clear distinction is made between nucleus active area fraction g and active surface icy area fraction f; (2) the dependence of the dusty ice sublimation rate on f and that of f on the heliocentric distance r_h due to the evolution of the dust cover are explicitly taken into account; (3) the 3-D structure of the CNC is described thoroughly instead of only in one symmetry plane; (4) the 3-D CNC model is quantitatively fitted to observational data; (5) a detailed comparison is made between the CNC structures resulting from two alternative assumptions concerning the nucleus. The first assumed nucleus is a homogeneous sphere of dusty ice (g = 1). We show that, in this case: (1) The maximum ejectable mass of spherical dust grains has a ~ f(r_h) cos z/r^2_h dependence upon solar zenith angle z and upon r_h. (2) The terminal ejected dust velocities have an approximate ~ cos z dependence on z and a strong ~ f(r_h)/r_h dependence on r_h. Fitting-for definiteness-the model to the light curve of the weak Comet P/Wirtanen (P/W), target of the future Rosetta mission, we find, assuming an upper limit nucleus radius of 1.4 km, that: (1) f decreases from about 14% at perihelion to possibly 0.025% at r_h = 3 AU; (2) for r_h <= 2.5 AU all or most of the sunward circumnuclear CNC is in fluid regime; (3) for 2.5 <= r_h <= 3 AU, most of the sunward CNC is in the so-called ``transition regime'' (4) due to their dependence upon f, the dust velocities are in absolute value much smaller than expected from the usually accepted algorithms (which assume f = 1) and decrease strongly with increasing r_h. The physical significance of these results is discussed. The second assumed nucleus fitted to P/W is an inhomogeneous sphere of dusty ice, most of the gas and dust production being due to four identical active areas separated by a background area, and covering a fraction g = 0.43 of the surface. It is found that, up to at least 2 AU, the structure of the CNC is, in such a case complex. (1) Owing to the difference in solar zenith angle, identical active areas produce differing CNC gas and dust distributions: therefore, in an inhomogeneous rotating nucleus, an active region is not characterized by a fixed corotating CNC pattern. (2) As with more productive comets, the interaction between gas issuing from different active regions leads to the formation of three-dimensional quasi stationary shock structures. (3) In particular, the weaker active areas cannot expand freely in the sunward hemisphere, but are surrounded by a low-altitude concave sheath of shocked gas and deflected dust. (4) Near to the shock surfaces, the dust density distribution is characterized, as in more productive comets, by density patterns that mimic dust jets. (5) Over the background areas, and close to the surface, the gas flows transverse to the vertical, and exhibits steep increases in density outwards. (6) The results reveal a fast smoothing-out of the CNC near-surface patterns with increasing distance to the surface. The implications of the present results for (1) the general physical characterization of comet nuclei on the basis of their coma-averaged properties, and (2) the assessment of the environmental parameters of future cometary close encounter missions are discussed. In particular, we show that simple, unidimensional heuristic models based on observations of a comet from the Earth are unable to provide relevant predictions concerning the physical conditions near to its nucleus. This conclusion is further enforced by the companion paper-Crifo and Rodionov (1996)-in which an aspherical homogeneous nucleus is assumed.
Crifo Jean-François
Rodionov Alexander V.
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