Comet striae formation by the sublimation driven YORP effect or SYORP

A cartoon of the stria formation process as hypothesized with the SYORP effect. (1) A parent chunk is disturbed and leaves the surface of the comet, (2) sublimation causes anti-sunward drift and rotationally accelerates (SYORP effect) the parent chunk, (3) the parent chunk rotationally fissions, (4) this process repeats at an increasingly rapid rate creating a fragmentation cascade, and (5) the small daughter chunks transition from sublimation to radiation pressure dominated anti-sunward drift and appear as a stria.

A cartoon of the stria formation process as hypothesized with the SYORP effect. (1) A parent chunk is disturbed and leaves the surface of the comet, (2) sublimation causes anti-sunward drift and rotationally accelerates (SYORP effect) the parent chunk, (3) the parent chunk rotationally fissions, (4) this process repeats at an increasingly rapid rate creating a fragmentation cascade, and (5) the small daughter chunks transition from sublimation to radiation pressure dominated anti-sunward drift and appear as a stria.

Our paper describing how comet striae are formed from the repeated rotational fissioning of sublimating lofted ice-rich chunks was published online and will be in the January 15, 2016 issue of Icarus. Here is a link to the publication in Icarus and here is a link to the preprint if you don't have access to Icarus.

A one-paragraph summary:

Comet striae are beautiful sub-structures within the tails of Great Comets, which are comets from the Oort cloud that are very sublimatively active when they pass close to the Sun. The comet is composed by roughly 50% ices that sublimate creating a gaseous halo about the comet and lofting dust into orbit. Great Comets have incredibly large tails that contain synchrones and striae (see these examples from APOD: example 1, example 2, and example 3). The basics of stria creation were already known, it's a structure composed of small dust grains that due to size differences between the different grains are strung out by radiation pressure into a long filament. What wasn't understood is how this clump of dust traveled together in the tail of the comet until it reached the appropriate place to make the stria. We solved this problem by considering that sublimation pressure not radiation pressure is the fundamental force until stria formation begins. As explained in the cartoon caption, once a parent chunk, perhaps ten meters in size, is ejected from the surface of the comet, it is driven sublimatively into the tail of the comet. While this occuring, a sublimative torque acts on the chunk, much like the YORP effect acts on asteroids, spinning the chunk up until it rotationally bursts. This process continues until the daughter chunks are small enough that radiation pressure dominates over sublimation pressure.

More details:

Stria formation is just one possible application of the SYORP effect. The SYORP effect is very analogous to the YORP effect, but instead of photons transporting angular momentum, it is much more massive (hence effective) atomic and molecular species, typically water ice. Unlike the YORP effect, whose strength decreases gradually with distance, the SYORP effect turns on and off suddenly when the body heats up or cools down past a specific temperature. We show that the SYORP effect can be orders of magnitude more powerful than the YORP effect, so this effect is likely to be acting on the comet nucleus as well, although on much longer timescales since it is much larger. In the future, we will be studying the SYORP effect in much more detail.