with ρ s being the particle density and ρ g being the gas density. However, this
equation for u th is invalid for grains smaller than about 100 μm because of cohesive
forces. Shao and Lu’s formulation is
Fig. 2.95 Observations of ripples in the Hapi region of the nucleus of 67P. Top: Image on
18 September 2014 pre-perihelion (N20140918T003342230ID10F24). Bottom: Image from
8 June 2016 post-perihelion (N20160608T152638721ID10F22). Although the observation geometry is different, it is quite apparent that significant changes have occurred
2.10 Surface Appearance and Cometary “Geology”
163
equation for u th is invalid for grains smaller than about 100 μm because of cohesive
forces. Shao and Lu’s formulation is
Fig. 2.95 Observations of ripples in the Hapi region of the nucleus of 67P. Top: Image on
18 September 2014 pre-perihelion (N20140918T003342230ID10F24). Bottom: Image from
8 June 2016 post-perihelion (N20160608T152638721ID10F22). Although the observation geometry is different, it is quite apparent that significant changes have occurred
2.10 Surface Appearance and Cometary “Geology”
163