Formation of smooth terrains on Comet Tempel 1

We suggest that the regions of smooth terrain which were observed on Comet 9P/Tempel 1 by the Deep Impact spacecraft were formed by blowing ice grains in an outburst of gas from the comet interior. When gas is released from 10 to 20 m deep layers which were heated to 135 K, it is released quiescently onto the surface by individual conduits. If large amounts of gas are released, the drainage system cannot release them fast enough and wider interconnected channels are formed, leading to sudden outburst of gas. Instability triggering a sudden shift of flow is well known in subglacial drainage of water. The ballistic trajectory of the ice particles reach a distance of 3 km in the atmosphereless comet, whose gravity is 0.034 cm s⁻¹, if ejected at an angle of 45° at a speed of 95 cm s⁻¹. This speed is close to the speeds measured in laboratory experiments: 167, 140×sini and 167 cm s⁻¹, for particles of 0.3, 1000 and 14-650 μm, respectively. Blowing of ice grains can overcome the 1650 m long horizontal section of smooth terrain i1 (Fig. 1), whereas simple flow of material downhill would stop close to the foot of the hill. The ice particles at the end of their trajectory have a horizontal velocity component and this low velocity ballistic sedimentation would lead to formation of lineaments on the smooth terrain, like in solid-particulate volcanic eruptions.