Energy balance and the gas flux from the surface of comet 46P/Wirtanen

Understanding the power balance at the surface of the nucleus is essential to study the chemical and physical evolution of a comet. Therefore, we pres ent a detailed energy budget analysis for the surface of a model comet in t he orbit of 46P/Wirtanen, target comet of the European space craft mission Rosetta, for a variety of parameters and assumptions. We will show that for a fast spinning Jupiter-family comet such as 46P/Wirtanen with a rotation period of about 6 h, a fast rotator approximation underestimates the effect ive energy input. This yields lower gas fluxes from the surface. For an 100 % active, non-dust covered surface we obtain a water gas flux on the order of about 1.5 x 10(28) molecules s(-1) at perihelion, assuming a radius of 6 00 m. The calculated gas flux of water is within the order of measured valu es for comet 46P/Wirtanen. But our calculated values are maximum gas fluxes at noon-not averaged over one cometary day or taking the lesser insolation at the polar areas into account. Therefore, we conclude that either the ra dius of comet 46P/Wirtanen may be much larger than the accepted value of 60 0 m. A radius in the order of 2 km seems more likely to explain the measure ments. Or, an other possibility could be that water-ice particles are blown off from the surface like dust particles. This may also increase the effec tive surface area of sublimation. (C) 1999 Elsevier Science Ltd. All rights reserved.