THE STICKING PROPERTIES OF WATER FROST PRODUCED UNDER VARIOUS AMBIENTCONDITIONS

To form planetary systems, small solid particles which condense out of the cooling gas of the primitive solar nebula must aggregate together to form larger bodies. Centimeter-sized particles can grow out of mic rometer-sized grains; planets can form from the coagulation of kilomet er-sized planetesimals. The formation of stable, long-lived kilometer- sized objects from centimeter-sized particles is, however, not so stra ightforward. Some sort of surface sticking force is needed to hold the se aggregates together against rotational forces as well as collisions in a turbulent solar nebula. We have performed experiments to determi ne the surface sticking force of water frosts under a variety of ambie nt conditions. Our primary results are listed below. 1. The structure of the frost is critical in determining its sticking properties; thin, porous frosts are more likely to adhere than are thick, dense frosts. 2. Sticking forces range up to 250 dyn/mm(2). 3. Temperature fluctuat ions can increase the sticking force by significant amounts. 4. The fr ost bond acts like a spring: it stretches before breaking, with the di splacement proportional to the applied force. Measured spring constant s for many different water frosts cluster between 10(5) and 10(6) dyn cm(-1) (over a total area of 78 mm(2)). Based on these findings, we su ggest that frosts of volatiles such as water could provide the necessa ry surface sticking mechanism in some low-temperature regions of the s olar nebula. (C) 1997 Academic Press.