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We present polarization images of Comet ISON (C/2012 S1) taken with theHubble Space Telescope (HST) on UTC 2013 May 8 (rh = 3.81 AU, Delta = 4.34 AU),when the phase angle was alpha = 12.16 degrees. This phase angle isapproximately centered in the negative polarization branch for cometary dust.The region beyond 1000 km from the nucleus shows a negative polarizationamplitude of p% -1.6%. Within 1000 km of the nucleus, the polarization positionangle rotates to be approximately perpendicular to the scattering plane, withan amplitude p% +2.5%. Such positive polarization has been observed previouslyas a characteristic feature of cometary jets, and we show that Comet ISON doesindeed harbor a jet-like feature. These HST observations of Comet ISONrepresent the first visible light, imaging polarimetry with sub-arcsecondspatial resolution of a Nearly Isotropic Comet (NIC) beyond 3.8 AU from the Sunat a small phase angle. The observations provide an early glimpse of theproperties of the cometary dust preserved in this Oort-cloud comet.

Hubble Space Telescope Pre-Perihelion ACS/WFC Imaging Polarimetry of Comet ISON (C/2012 S1) at 3.81 AU