Heliocentric Distance Rh Research Articles

Activity on the surface of the nucleus of comet 46P/Wirtanen

We present a thermal model of the nucleus of comet 46P/Wirtanen, constrained by the temporal variations of the water production rate, in order to understand the activity on its surface. We consider a spherical nucleus with a macroscopic mosaic of small and numerous active and inactive regions. At heliocentric distances rh > 1.5 AU, the active regions represent 5–15% of the surface. At ~1.5 AU, a rapid increase takes place and the active fraction reaches 70 to 100% in about 10 days, and then remains approximately constant up to perihelion where molecule s-1. Post-perihelion, this fraction returns to ~10%. The model is consistent with a geometric albedo ≤0.06. A refractory crust likely forms post-perihelion and can explain the variations of the activity over an orbit. Finally, we derived an erosion of ~0.5 m per revolution and a remaining lifetime for the nucleus of several hundred revolutions.

C/1995 O1 (Hale-Bopp): Gas Production Curves And Their Interpretation

The discovery of C/1995 O1 (Hale-Bopp) at 7 AU from the Sun provided the first opportunity to follow the activity of a bright comet over a large range of heliocentric distances rh. Production rates of a number of parent molecules and daughter species have been monitored both pre- and postperihelion. CO was found to be the major driver of the activity far from the Sun, surpassed by water within 3 AU whose production rate reached 1031 s−1 at perihelion. Gas production curves obtained for various species show several behaviours with rh. Gas production curves contain important information concerning the physical state of cometary ices, the structure of the nucleus and all the processes taking place inside the nucleus leading to outgassing. They are relevant to the study of several other phenomena such as the sublimation from icy grains, dust mantling or seasonal effects. For some species, such as H2CO or HNC, they permit to constrain their origin in the coma. We discuss models of subsurface gas production in distant comets and predictions of how such a source may vary as the comet moves along its orbit, approaching perihelion and receding again. Features in the observed gas production curves of comet Hale-Bopp are generally interpretable in terms of either subsurface production (typical example: CO at large rh) or free sublimation (typical example: H2O). Possible implications for the vertical stratification of the cometary ices are reviewed, and preference is found for a model with crystallization of amorphous ice close to the nuclear surface.