Hubble Space Telescope Pre-perihelion ACS/WFC Imaging Polarimetry of Comet Ison (c/2012 s1) at 3.81 AU

We present polarization images of Comet ISON (C/2012 S1) taken with the Hubble Space Telescope (HST) on UTC 2013 May 8 (r h = 3.81 AU, Δ = 4.34 AU), when the phase angle was α ≈ 1216. This phase angle is approximately centered in the negative polarization branch for cometary dust. The region beyond 1000 km (~0.32 arcsec ≈ 6 pixels) from the nucleus shows a negative polarization amplitude of p% ~ –1.6%. Within 1000 km of the nucleus, the polarization position angle rotates to be approximately perpendicular to the scattering plane, with an amplitude p% ~ +2.5%. Such positive polarization has been observed previously as a characteristic feature of cometary jets, and we show that Comet ISON does indeed harbor a jet-like feature. These HST observations of Comet ISON represent the first visible light, imaging polarimetry with subarcsecond spatial resolution of a Nearly Isotropic Comet beyond 3.8 AU from the Sun at a small phase angle. The observations provide an early glimpse of the properties of the cometary dust preserved in this Oort-Cloud comet.