Saturn’s G and D Rings Provide Nearly Complete Measured Scattering Phase Functions of Nearby Debris Disks

Abstract

The appearance of debris disks around distant stars depends upon the scattering/phase function (SPF) of the material in the disk. However, characterizing the SPFs of these extrasolar debris disks is challenging because only a limited range of scattering angles are visible to Earth-based observers. By contrast, Saturn's tenuous rings can be observed over a much broader range of geometries, so their SPFs can be much better constrained. Since these rings are composed of small particles released from the surfaces of larger bodies, they are reasonable analogs to debris disks and so their SPFs can provide insights into the plausible scattering properties of debris disks. This work examines two of Saturn's dusty rings: the G ring (at 167,500 km from Saturn's center) and the D68 ringlet (at 67,600 km). Using data from the cameras onboard the Cassini spacecraft, we are able to estimate the rings' brightnesses at scattering angles ranging from 170• to 0.5 • . We find that both of the rings exhibit extremely strong forwardscattering peaks, but for scattering angles above 60• their brightnesses are nearly constant. These SPFs can be well approximated by a linear combination of three Henyey-Greenstein functions, and are roughly consistent with the SPFs of irregular particles from laboratory measurements. Comparing these data to Fraunhofer and Mie models highlights several challenges involved in extracting information about particle compositions and size distributions from SPFs alone. The SPFs of these rings also indicate that the degree of forward scattering in debris disks may be greatly underestimated.

Discovery of an Inner Disk Component Around Hd 141569 A*

“…This is consistent with the result of Weinberger et al (1999) and Mouillet et al (2001). We note that our g value is a lower limit because the degree of forward scattering for debris disks is underestimated (Hedman & Stark 2015). This problem was addressed in Stark et al (2014).…”

Collisions and drag in debris discs with eccentric parent belts

“…Löhne et al: Collisions and drag in debris discs with eccentric parent belts Hedman & Stark (2015) for Saturn's G ring. The disc is viewed from an angle 45 degrees below its midplane, with the line of nodes aligned east-west and the pericentre due west.…”

“…In models of debris discs, these more symmetric phase functions are often associated with the presence of smaller, spherical Mie grains. This degeneracy between small grains and grains with small structures is discussed by Min et al (2010) and Hedman & Stark (2015) in their models for the Fomalhaut disc and Saturn's G and D68 rings, respectively.…”

Bright clumps in the D68 ringlet near the end of the Cassini Mission

“…This ringlet, designated D68, was first observed in a small number of images obtained by the Voyager spacecraft (Showalter 1996), and more recently has been imaged repeatedly by the cameras onboard the Cassini spacecraft, enabling several aspects of its structure and composition to be documented. These images show that the ringlet is very faint in back-scattered light, and that its brightness increases dramatically at higher phase angles (Hedman and Stark 2015). This implies that the visible material in this ringlet is very tenuous, and composed primarily of dust-sized particles in the 1-100 micron size range.…”