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

Transcription

1 Cassini Encounters Enceladus: Background and the Discovery of a South Polar Hot Spot Spencer et al., 10 March 2006, Science, 311, 1401 Presented by Shannon Guiles Astronomy 671 April 10, 2006 Image Credit: NASA/JPL/GSFC

2 Tiger! Tiger! burning bright, In the forests of the night, What immortal hand or eye Could frame thy fearful symmetry?

3 Cassini Cassini entered orbit around Saturn on 1 July 2004, and it will orbit there for four years. Credit: NASA Cassini has 12 instruments while the Hyugens probe (which descended to Titan's surface in January 2005) had 6. CIRS Credit: NASA Cassini had three close flybys of Enceladus between Feb & July One more scheduled for Spring 2008.

4 Cassini Composite Infrared Spectrometer Reference: Flazar et al. (2004)

5 In what distant deeps or skies Burnt the fire of thine eyes? On what wings dare he aspire? What the hand dare seize the fire?

6 Enceladus Credit: NASA/JPL/Space Science Institute. Composite of UV (338 nm), green (568 nm), and IR (752 nm). discovered in 1789 by William Hershel 6th largest moon of Saturn orbits close to Saturn at r= 3.94 r_saturn eccentricity = , in 2:1 orbital resonance with Dione Voyager found (1981) r_enceladus = 252 km high visual geom. albedo implying fresh snow or ice vastly different surfaces resides in the middle of Saturn's E ring, and probably its source surface dominated by water ice with organics and CO2 coincident with tiger stripes

7 Why do we care? Scientists like a good puzzle. Before Cassini observations of Enceladus' South Pole, it was expected that Enceladus would be cold. Surprise! Enceladus is one of only 3 outer solar system bodies (along with Jupiter's Io & Neptune's Triton) where active geological eruptions have now been seen. What is the origin of Enceladus' internal heating? Possibility of life?

8 The Puzzle Only previous measurement of Enceladus' thermal radiation made by Voyager 2 in 1981 inferred temperatures of 75 +/ 3 K. Left: Predicted temperatures on Enceladus based on solar heating. Right: Temperature image from measurements of Enceladus' heat radiation from 9 to 16.5 microns. Image Credit: NASA/JPL/GSFC

9 And what shoulder and what art Could twist the sinews of thy heart? And, when thy heart began to beat, What dread hand and what dread feet?

10 Cassini CIRS Observations of Enceladus Far IR brightness temperature images of the thermal emission from the anti Saturn hemisphere of Enceladus Figure 1(A) of Spencer et al. (2006).

11 Thermal Model Fits to Determine Physical Properties Temperatures near 76 K consistent with Voyager estimates Note the large spatial variations in the thermal inertia (TI). TI is 100 x < TI of solid water ice, implying a very lose surface. The high albedo is consistent with Voyager determination of / Figure 1 (B) of Spencer et al. (2006). Temperatures measured during orbits 3 and 4. Thermal Inertia (T.I.) in MKS units of J m 2 s 1/2 K 1

12 What the hammer? what the chain? In what furnace was thy brain? What the anvil? what dread grasp Dare its deadly terrors clasp?

13 Mid-IR Data on Enceladus' South Pole (12.5 to 16 µm) Figure 2 of Spencer et al. (2006).

14 Mid-IR Data on Enceladus' South Pole The FIR detector has low spatial resolution, but MIR detector made a map with resolution of 25 km from 12.5 to 16 µm (Fig. 2 A and B). Equatorial regions had 650 cm 1 brightness temperatures ~77 K, in agreement with models of passive solar heating based on thermal inertias and albedos determined from FIR observations (Fig. 1). Surprise: near the south pole brightness temperatures reach 85 K (thermal models predicted a temperature of ~68 K, assuming zero thermal inertia, at the pole because of the oblique angle of Sun's rays there).

15 Mid-IR Data on Enceladus' South Pole A greybody fit to the south polar spectrum implies temperatures of 133 +/ 12 K from ~ % of the area (Fig. 2 C). They used Monte Carlo simulations to determine the uncertainties in the temperature and area found from the spectral fit (Fig. 2 D E). Temperatures below 110 K are very unlikely. The spectral fit was used to estimate the total radiated power from the southern hot spot of 5.8 +/ 1.9 GW (Fig. 2F).

16 What Causes the Hot Is it seasonal effects? No. The season is midway between the southern summer solstice & fall equinox, and a nonzero thermal inertia would cause the temperature to be even lower. How about a solid state greenhouse effect? Spot at the South Pole? Probably not. This is where solar radiation gets trapped beneath the surface, & might make warm gases that could escape along fractures and cause plumes. Proposed for Neptune's Triton. Good in theory, difficult in practice. A heat source from within? Quite possibly. The hot spot is located in a recently resurfaced region that has four big troughs (tiger stripes ~130 km long, ~500 m deep, ~2 km wide from ISS imaging).

17 Thermal Emission Associated with Individual Tiger Stripes Precise locations of hot sources A and B. MIR FOV is 17.5 km for source A and 6.0 km for source B. Figure 3 of Spencer et al. (2006).

18 Of Hot Spots and Tiger Stripes During the last 2.5 hours of the approach to Enceladus' South Pole, CIRS pointed where the optical cameras pointed. These high spatial resolution observations confirm that the hot spots are associated with individual tiger stripe troughs (Fig. 3). Properties of individual hot spots: Table 2 of Spencer et al. (2006).

19 When the stars threw down their spears, And watered heaven with their tears, Did He smile His work to see? Did He who made the Lamb, make thee?

20 Plume, Hot SpotS and Tiger Stripes Likely that warm tiger stripes are the source of the dust and vapor plume seen by other Cassini instruments Assuming the plume originates from thermal sublimation of warm water ice this warm ice is visible to CIRS Then using Cassini ISS image of Fountains of Enceladus. Credit: NASA/JPL/Space Science Institute escape rate = (5 10)x1027 molecules/second mean south polar radiance at 900 cm 1 of ~ 5x10 10 W cm 2str 1(cm 1) 1 Get plume source temperature > 180 K

21 Plume, Hot SpotS, & Tiger Stripes In order to produce the escape rate in the plume, a 180 K plume source must have an area of ~ 28 km2 consistent with a 50 m width along each of the four tiger stripes The 345 km2 area of the south polar hot spots is in agreement with all the hot material being concentrated along the tiger stripes derive a width of ~660 m (4 stripes x 139 km length x km width = 343 km2 ), consistent with that derived from high resolution spectra.

22 In what funace was thy brain? - Possible Heat Sources Radioactive decay. The max power from this, assuming a meteor (chondritic = silicate blob) composition for non ice material in Enceladus is ~ 0.1 GW << the 6 GW inferred from observations. Tidal heating may generate 1 10 GW Enceladus is in a 2:1 mean motion orbital resonance with Dione. Enceladus may have been in a 1:4 spin orbit resonance in the past (based on the analysis of Enceladus' shape, Porco et al, 2006). This would have caused strong tidal heating, from which Enceladus may still be cooling down.

23 Tiger! Tiger! burning bright, In the forests of the night, What immortal hand or eye Dare frame thy fearful symmetry?

24 Summary Cassini CIRS found Enceladus is generating ~ 6 GW from within at temperatures around 150 K Comparison with Cassini visual images most if not all of the heat is concentrated along the tiger stripe troughs Warm troughs probably source of vapor and dust plumes seen by other Cassini instruments

25 References Cassini Encounters Enceladus: Background and the Discovery of a South Polar Hot Spot, Spencer et al. (2006) Science, 311, 1401 Exploring the Saturn System in the Thermal Infrared: the Composite Infrared Spectrometer, Flazar et al. (2004) Space Science Reviews, 115, 169 Cassini Observes the Active South Pole of Enceladus, Porco et al. (2006) Science, 311, 1393 wikipedia article on Enceladus at: Cassini home page at:

26 The Tiger Tiger! Tiger! burning bright, In the forests of the night, What immortal hand or eye Could frame thy fearful symmetry? What the hammer? what the chain? In what furnace was thy brain? What the anvil? what dread grasp Dare its deadly terrors clasp? In what distant deeps or skies Burnt the fire of thine eyes? On what wings dare he aspire? What the hand dare seize the fire? When the stars threw down their spears, And watered heaven with their tears, Did He smile His work to see? Did He who made the Lamb, make thee? And what shoulder and what art Could twist the sinews of thy heart? And, when thy heart began to beat, What dread hand and what dread feet? Tiger! Tiger! burning bright, In the forests of the night, What immortal hand or eye Dare frame thy fearful symmetry? William Blake ( )

27 Artist's Conception of Enceladus Image Credit: NASA