Variation of Nongravitational Parameters for Comet Encke as a Result of its Decay . ... To account for decrease of nongravitational parame-ters three variants of physical processes are proposed: 1) deposition of a substantial nonvolatile mass not impeding the sublimation; 2) generation of a little-massive mantle shrinking the effective area linearly; 3) generation of a little-massive mantle shrinking the area exponentially. ...