Enceladus: Cosmic Graffiti Artist Caught in the Act

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Science  09 Feb 2007:
Vol. 315, Issue 5813, pp. 815
DOI: 10.1126/science.1134681


As one of the most geologically active bodies in the solar system, Saturn's moon Enceladus not only coats itself with water ice particles, it accounts for the unusually high albedos of the other satellites orbiting within Saturn's vast, tenuous E ring. This effect is evident in Hubble Space Telescope observations obtained at true opposition on 13 and 14 January 2005 that reveal that the mean geometric albedos of satellites embedded within the E ring approximate or exceed unity.

Saturn's geologically active moon Enceladus (1) is fittingly named for the mythological giant who produces Mount Etna's volcanic fires. The moon's south polar plume eruptions give rise to the vast, tenuous E ring that enshrouds at least 11 satellites. Despite its small size, Enceladus is even more deserving of its gigantic namesake. Here, we report Hubble Space Telescope (HST) observations of Saturn's satellites at true opposition that show how material originating from Enceladus alters the appearance of its E-ring neighbors.

On the night of 13 January 2005, a rare perfect alignment of the Sun, Earth, and Saturn enabled the measurement of the albedos of the saturnian satellites at the smallest solar phase (Earth-Sun-Saturn) angle (α ≅ 0.01°) attainable at Saturn's heliocentric distance. Previously, their geometric albedos were poorly determined and could be estimated only by extrapolating photometric models to zero phase. Geometric albedo, p, is the ratio of a satellite's reflectance at α = 0° to that of a perfectly diffusing disk viewed at the same position and apparent size. All albedos measured at true opposition are considerably higher than previous estimates (table S1). For example, p is 1.23 for Tethys versus 0.80 previously, and p is 1.00 for Dione versus 0.55 previously (2). With the exception of Janus (p = 0.71) and Epimetheus (p = 0.73), which orbit interior to the E ring, the albedos of all satellites measured here approximate or exceed unity (3). A strong correlation exists (Fig. 1) between p and the pole-on radial reflectance profile of the E ring as a function of orbital radius. Calypso and Helene, Lagrangian satellites of Tethys and Dione, respectively, are visible within HST's field of view. Both also have p > 1; however, these values are uncertain because of low signal-to-noise ratios and large corrections for charge transfer efficiency. In addition, Helene's shape is not well represented by a simple triaxial ellipsoid.

Fig. 1.

The mean visual geometric albedo, p (left vertical axis), of the embedded satellites Mimas (0.96), Enceladus (1.38), Tethys (1.23), Dione (1.00), and Rhea (0.95) versus radial distance from Saturn (RS) mimics the pole-on reflectance (I/F) profile (solid line and right vertical axis) of the E ring. HST's Wide Field and Planetary Camera 2 (WFPC2) observed both the 1995 ring plane crossings (10) and the satellites at true opposition with the same filter (F555W).

The extraordinarily high albedo of Enceladus is reasonable for a geologically active body undergoing continuous resurfacing. The fact that p ≳ 1 for all embedded satellites is, however, striking, because the other bodies have ancient, inactive surfaces. Most icy moons in the outer solar system have p values between 0.2 and 0.4 (2); exceptions include satellites with relatively young surfaces such as Jupiter's Europa [p = 0.9 (4)] and Neptune's Triton [p = 0.8 (5)], which also has active geysers.

We propose that interactions with E-ring particles, ultimately from Enceladus' plumes, produce the high albedos on Tethys, Mimas, Dione, and Rhea. After ejection from Enceladus' south pole, nongravitational forces excite the eccentricities of E-ring grains (6). These particles collide with the embedded satellite surfaces at high relative velocities (6). Most of the ejecta resulting from this sandblasting falls back onto the satellite surfaces, coating them with clean, icy microstructures suited to enhancing the reflectance at opposition (7).

Although a more complete understanding of the regolith physical properties is ultimately expected from the analysis of full solar phase curves (reflectance versus α for 0° ≤ α ≤ 180°), a preliminary analysis of the phase curves near opposition (α < 6.4°) using the Hapke (8) model suggests that differences in the opposition effect (OE) among the embedded satellites may indeed be related to the degree to which they encounter E-ring grains. OE, the dramatic, nonlinear increase in reflectance as α→ 0°, is characterized by its amplitude and angular width, both of which depend on the microphysical structure and density of regolith grains (8). The OE amplitudes and widths of the embedded satellites' phase curves correlate with position in the E-ring (fig. S1). Three groupings of satellites emerge with similar OE amplitudes and widths: Tethys and Enceladus; Mimas, Dione, and Rhea; and Janus and Epimetheus. In models that successfully reproduce the E-ring brightness profile, Tethys and Enceladus experience substantially higher impact rates from E-ring particles than do Mimas, Dione, and Rhea (9). E-ring particles do not impact Janus and Epimetheus.

At least four additional small satellites orbit within the E ring: two between Mimas and Enceladus and two more Lagrangian companions of Tethys and Dione. Although measurements must await the acquisition of low-α Cassini images, we predict that p ≳ 1 for these moons as well. Inevitably, material from Enceladus impacts all satellites orbiting within the E ring, enhancing their albedos at the hands of a diminutive giant.

Supporting Online Material

Fig. S1

Tables S1 and S2



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