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Science 10 March 2006:
Vol. 311. no. 5766, pp. 1401 - 1405
DOI: 10.1126/science.1121661
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Research Articles

Cassini Encounters Enceladus: Background and the Discovery of a South Polar Hot Spot

J. R. Spencer1*, J. C. Pearl2, M. Segura2, F. M. Flasar2, A. Mamoutkine2, P. Romani2, B. J. Buratti3, A. R. Hendrix3, L. J. Spilker3 and R. M. C. Lopes3

1 Department of Space Studies, Southwest Research Institute, 1050 Walnut Street, Suite 400, Boulder, CO 80302, USA.
2 NASA Goddard Spaceflight Center, Code 693, Greenbelt, MD 20771, USA.
3 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA.


Figure 1 Fig. 1. Far-IR observations of Enceladus. (A) Far-IR brightness temperature images of the nighttime and daytime thermal emission from the anti-Saturn hemisphere of Enceladus (centered at longitude 180°W) from the three Cassini encounters. The apparent signal beyond the limb of Enceladus is an artifact of the low spatial resolution and the plotting technique. (B) Thermal model fits to the far-IR day and night brightness temperatures at longitude 180°W and two different latitudes, as measured on orbits 3 and 4, showing the large spatial variations in thermal inertia [TI, in m kg s (MKS units)]. Horizontal bars show the local time range of each observation. (C) Far-IR spectrum of the north pole, with best fit blackbody and graybody spectra, showing the lack of the high-temperature component seen at the south pole. The fine structure in this and all spectra shown is due to noise. [View Larger Version of this Image (63K GIF file)]
 

Figure 2 Fig. 2. (A) Mid-IR brightness temperature image of Enceladus from orbit 11, showing the prominent south polar hot spot. The dashed line is the terminator. (B) Brightness temperature contours derived from the observationin (A), superposed on an ISS base map (19), showing the spatial correlation of the hot material with the region containing the tiger stripe troughs. Spatial resolution (lower right) of the temperature map is about 50 km after projection and smoothing. The yellow dashed line shows the latitude boundary of the average spectrum shown in (C). (C) Thermal emission spectrum of the region south of latitude 65°S, compared with the best fit blackbody spectrum, which does not match the data, and the best fit graybody plus background model spectrum, which matches the data very well. (D to F). Probability distributions for the temperature (D), filling factor (E), and total radiated power (F) for the hot material in the south polar region, on the assumption of a single temperature for the hot material, derived by Monte Carlo techniques. The power distribution is shown for assumed hot material albedos of 0.81 and 0.00. [View Larger Version of this Image (105K GIF file)]
 

Figure 3 Fig. 3. (A) Color-coded south polar brightness temperatures at high spatial resolution, derived from the ISS ride-along CIRS observations, superposed on an ISS base map (19). This shows the correlation between high brightness temperatures and the individual tiger stripe troughs. Isolated colored rectangles represent observations taken during slews, and these have less reliable locations than other observations. The locations of the seven hot sources described in Table 2 and the rest of this figure are indicated. (B and C) Precise location of hot sources A and B relative to the topography as derived from simultaneous ISS images (19). Each box shows a single mid-IR field of view and its associated brightness temperature, with uncertainties. Field of view size is 17.5 km for source A and 6.0 km for source B. (D) The hottest individual CIRS spectrum (source D) and the best fit graybody spectrum. (E and F) Probability distribution of the temperature and filling factor for the spectrum of source D, assuming a single temperature. [View Larger Version of this Image (105K GIF file)]
 

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