PacMan returns: An electron-generated thermal anomaly on Tethys C.J.A. Howett a,⇑ , J.R. Spencer a , T. Hurford b , A. Verbiscer c , M. Segura b a Southwest Research Institute, 1050 Walnut Street, Boulder, CO 80403, USA b Goddard Space Flight Center, Greenbelt, MD 20771, USA c University of Virginia, Charlottesville, VA 22904, USA article info Article history: Received 13 August 2012 Revised 12 October 2012 Accepted 13 October 2012 Available online 29 October 2012 Keywords: Saturn, Satellites Satellites, Surfaces Satellites, Composition abstract New Cassini observations show that Saturn’s moon Tethys, like Mimas, has a region of anomalously high thermal inertia at low latitudes centered on its leading hemisphere. Derivation of the thermophysical properties of the surface across three representative regions indicate that the bolometric Bond albedo across Tethys’ leading hemisphere remains constant, within error, whilst the thermal inertia increases from 5 ± 1 J s 1 m 1 K 1 outside of the anomalous region to 25 ± 3 J s 1 m 1 K 1 inside. The thermally anomalous region is spatially correlated with a decrease in the IR/UV surface coloration. The discovery greatly strengthens the hypothesis that high-energy electrons, which preferentially bombard the leading hemispheres on both satellites, produce dramatic alterations in surface texture. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction The rich data sets returned by the Cassini spacecraft allow us to use Saturn’s moons as natural laboratories for understanding the evolution of icy surfaces. Recent infrared and color observations re- turned by Cassini’s Composite Infrared Spectrometer (CIRS) and Imaging Subsystem (ISS) showed a thermal and color anomaly at low latitudes on Mimas’ leading hemisphere. This anomaly, dubbed PacMan (because its shape mimics that of the 1980s video icon) displays warmer nighttime and cooler daytime temperatures than its surroundings and appears darker in IR/UV color ratio maps (Howett et al., 2011; Schenk et al., 2011). The color and thermal anomalies on Mimas are believed to be the result of surface alteration by high-energy electrons, which preferen- tially bombard this region, altering its surface and increasing its ther- mal inertia. These high-energy electrons, in Saturn’s magnetosphere, drift in a retrograde direction relative to corotation. Recent modeling efforts, supported by the Low Energy Magenotospheric Measure- ment System (LEMMS) on Cassini’s Magnetospheric Imaging Instru- ment (MIMI), have shown that these electrons preferentially bombard low latitudes on Mimas’ and Tethys’ leading hemisphere (Paranicas et al., 2012; Howett et al., 2011; Schenk et al., 2011). Tethys shows a similar color anomaly in this region to that seen on Mimas, but until now thermal data sufficient to search for a corre- sponding thermal anomaly on Tethys have been lacking. CIRS is a Fourier Transform Spectrometer; the instrument has two interferometers that share a telescope and scan mechanism. Two thermopile detectors, known as focal plane 1 (or FP1), cover 10–600 cm 1 (9.1–1000 lm) over a 3.9 mrad field of view. Mid- infrared emissions are detected by a Michelson interferometer and two linear focal plane arrays of 10 HgCdTe detectors, known as focal planes 3 and 4 (or FP3 and FP4). Both focal planes have a 0.273 by 0.273 mrad field of view per detector, with FP3 sensitive from 600 to 1100 cm 1 (9.1–16.7 lm) and FP4 from 1100 to 1400 cm 1 (7.1–9.1 lm) (Flasar et al., 2004). In September 2011 (during Cassini’s orbit 153) CIRS obtained a daytime medium-spatial resolution observation (the average reso- lution was 84 km/pixel) of Tethys’ leading hemisphere. FP3 was used for these observations since it provides the best compromise between sensitivity to thermal radiation and spatial resolution for daytime observations of icy satellites. These observations were at a significantly better spatial resolution than the previous best ther- mal map of this hemisphere, a nighttime map obtained in June 2007 (Cassini’s orbit 47) (150 km/pixel) using FP1. The lower- spatial resolution FP1 is the only CIRS detector sensitive to the cool nighttime surface emission of Saturn’s icy satellites; therefore, spa- tial resolution differences in the night-day-time coverage with CIRS are common. Combination of the orbit 47 and orbit 153 obser- vations provides sufficient local time coverage to constrain surface albedo and thermal inertia. Details of both orbit 47 and 153 obser- vations are given in Table 1. 2. Method and results The analysis method closely follows that of Howett et al. (2011). This includes using the same pointing refinement technique: small tweaks to the pointing so that the predicted limb location agrees with the observed position of the warm limb against the cold 0019-1035/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2012.10.013 ⇑ Corresponding author. Address: 1050 Walnut Street, Suite 300, Boulder, CO 80302, USA. Fax: +1 303 546 9687. E-mail address: howett@boulder.swri.edu (C.J.A. Howett). Icarus 221 (2012) 1084–1088 Contents lists available at SciVerse ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus