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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 > 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 thendoi:10.1051/0004-6361:20031496 fatcat:nulq6qsyjbhwvhqv6v3lzdc3dm