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 r{ h} (greater than) 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 Q H2O=1.3(plus-minus)0.3*10^28 molecule s^-1. Post-perihelion, this fraction returns to ~10%. The model is consistent with a geometric albedo (less than)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.