Extrapolations beyond 2 AU of 67P pre-perihelion water-loss rates. Triangle, diamond and squares: observations (Fig. 19). Symbol  × : post-perihelion CN-loss rates (Schleicher 2006 at sun distances lower than 2 AU, Schulz et al. 2004) beyond). Vertical error bars: upper limits of pre-perihelion CN-loss rate (Schulz, private communication) multiplied by 10³. Solid line: water-loss rate obtained by the dust-loss rate assuming a constant dust-to-gas mass ratio of 3. Dotted lines: water-loss rate predicted by Gortsas et al. (2010) assuming an active area fraction of 3% and heat conductivities of 0.1 (lower line) and 10^-3 W m^-1 K^-1 (upper line). Dashed line: water-loss rate predicted by De Sanctis et al. (2006) rescaled to 4% their assumed active area fraction of 50% (see text for discussion, assumed heat conductivity of 3 W m^-1 K^-1).