LETTERdoi:10.1038/nature14368

Curtain eruptions from Enceladus’ south-polarterrainJoseph N. Spitale1, Terry A. Hurford2, Alyssa R. Rhoden3, Emily E. Berkson4 & Symeon S. Platts5

Observations of the south pole of the Saturnian moon Enceladusrevealed large rifts in the south-polar terrain, informally called‘tiger stripes’, named Alexandria, Baghdad, Cairo and DamascusSulci. These fractures have been shown to be the sources of theobserved jets of water vapour and icy particles1–4 and to exhibithigher temperatures than the surrounding terrain5,6. Subsequentobservations have focused on obtaining close-up imaging of thisregion to better characterize these emissions. Recent work7 exam-ined those newer data sets and used triangulation of discrete jets3 toproduce maps of jetting activity at various times. Here we showthat much of the eruptive activity can be explained by broad, cur-tain-like eruptions. Optical illusions in the curtain eruptionsresulting from a combination of viewing direction and local frac-ture geometry produce image features that were probably misinter-preted previously as discrete jets. We present maps of the totalemission along the fractures, rather than just the jet-like compon-ent, for five times during an approximately one-year period in 2009and 2010. An accurate picture of the style, timing and spatial dis-tribution of the south-polar eruptions is crucial to evaluating the-ories for the mechanism controlling the eruptions.

At high resolution, the material emitted by Enceladus’ south-polarfractures is not adequately represented by discrete ‘jets’, as was done inprevious studies3,7. Rather, much of the emission seen in the imagesappears as broad vertical curtains extending over many kilometres offracture, with scale heights of a few kilometres. Intermittent prominentjets with much larger scale heights, and thus greater visibility, appear tobe superimposed on the curtain structure. A recent study7 used tri-angulation of jet-like features among multiple Cassini images to maperuptive activity, but did not specify how, or whether, the ubiquitouscurtain eruptions were treated.

We examined the eruptions by simulating the appearance of cur-tains of material emanating from extended sources on the surface, asdemonstrated in Fig. 1. Figure 1a shows Cassini image N1671564077taken on 2010-354 (year-day; upper image), and a synthetic image inthe same viewing geometry with rays projected vertically from samplepoints on Baghdad and Damascus Sulci spaced 250 m apart (lowerimage). The rays have uniform ground-level intensities and a uniformscale height of 4 km. The camera scale is about 750 m per pixel. Theshadow of Enceladus is visible on the synthetic curtains and compareswell with shadows in the image, demonstrating a correspondencebetween the observed plume material and the mapped fractures. Noother visible surface features produce curtains that match the patternseen in the image.

Despite the uniform sampling of the curtains in Fig. 1a, which is wellbelow the projected camera scale, the simulated curtains display anarray of fine jet-like features similar in appearance to fine structureseen in the Cassini image. Moreover, some of the broader structuresline up between the real and synthetic images. These ‘phantom’ jetsarise from variations in curtain optical thickness caused by the mean-ders in the source fractures relative to the line of sight. The features in

the synthetic image are all phantoms; the correspondence with the realimage suggests that much of the discrete structure in that image is alsoillusory.

Figure 1b shows a more striking example of the phantom jet phe-nomenon (top, observation; bottom, synthetic image). The scale heightis 1 km and the image scale is 80 m per pixel. In this case, nearly everyprominent bright feature in the Cassini image corresponds to aphantom jet in the simulated curtains. Finer-scale image features areseen where finer-scale phantoms occur in the synthetic image, thoughaliasing is a possibility at this image scale.

The presence of phantoms raises additional concerns about repre-senting the observed activity in terms of discrete jets only and usingtriangulation to locate their sources7. First, as shown in Fig. 2, theapparent source location of a phantom depends on the viewing angle,so triangulation is inappropriate. Attempting to triangulate a phantom

ba 6141647361N7704651761N

Figure 1 | Curtain simulations illustrating the phantom jet phenomenon.a, Top, Cassini image N1671564077 stretched and cropped to focus on jetcurtains from Baghdad and Damascus Sulci. Bottom, synthetic image in thesame geometry with uniform jet sheets from Baghdad and Damascus Sulci. Thehigh-frequency brightness variations in the simulated image are phantom jets.Some larger image structures may also be phantoms. b, Top, Cassini imageN1637461416 stretched and cropped to make the erupted material visible.Bottom, simulated uniform curtains overlain on the unstretched image.Phantom jets in the synthesized curtains correspond well to regions ofenhanced brightness in the image.

1Planetary Science Institute, 1700 East Fort Lowell Road, Suite 106, Tucson, Arizona 85719, USA. 2NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA. 3Johns Hopkins University AppliedPhysics Laboratory, 11100 Johns Hopkins Road, Laurel, Maryland 20723, USA. 4Rochester Institute of Technology, One Lomb Memorial Drive, Rochester, New York 14623, USA. 5Film and TelevisionDepartment, University of Arizona, Tucson, Arizona 85721, USA.

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produces a spurious solution whose accuracy diminishes as the par-allax increases. Such solutions are most accurate at low divergenceangles, where triangulation is least precise. Second, there is no proofof enhanced emission at any of these locations, so the presence ofphantoms is likely to bias the outcome: activity may be preferentiallyreported where phantoms appear.

For our maps, we identified active fractures by simulating curtainsoriginating along obvious candidate fractures to compare with theemission seen in Cassini images. The best information came fromthe shadows cast on the erupted material rising into the sunlightbecause they provide a unique constraint on the location of the fractureresponsible for the emission, though there were many cases where itwas possible to identify the source fracture by looking at the emissionon the sky, or because the fracture itself was visible and emitting.Unique determination of relative eruptive intensities along a fractureis not usually feasible because phantom jets and local variations ineruptive intensity often look the same. This approach captures boththe jet-like and curtain-like eruptions because discrete jets are a specialcase of the curtain geometry.

On days 2010-138 and 2010-354, when the viewing direction waswithin ,20u of the fracture direction, simple collimated vertical cur-tains were not adequate to explain the observations. On BaghdadSulcus on both days, a curtain spreading with an opening angle ofabout 610u gave a better match to the observed shadow edge and tothe observed shape of the curtains (Fig. 3), than to the collimatedcurtain. On Damascus Sulcus on day 2010-138, a spreading of 614uwith a zenith angle of about 8u was required. On Cairo Sulcus on day2010-138, a spreading of 67u with a zenith angle of about 3u wasrequired. Other viewing geometries were not sensitive to these para-meters.

The observed geometry variations among the curtains may reflectdifferences in the vent/reservoir geometries. For example, fracturesmay not be perfectly vertical, or reservoir depths and geometriesmay vary. Assuming that the observed curtain spreading reflects thegas dispersion, a deeper reservoir would allow particles more time to

interact with, and be affected by, the gas before exiting into space; agreater liquid surface area relative to chamber volume would producehigher gas velocities for a given temperature (that is, dispersion velo-city).

Figure 4a–e shows maps of fracture activity at five times during aninterval of about one year (day given at top right of each panel). Wetested every plausible fracture in the base map and recorded whetherthe observed pattern of emission is consistent with only one candidatefracture (in which case the fracture is coloured green), or activity on afracture cannot be determined (blue), or activity from a given fracturewould produce a pattern of emission that is not detected in the image(red). The fracture mapped in cyan could not usually be distinguishedfrom the nearby parallel fracture. Because of changing image para-meters and geometry, and varying shadow heights, the quantitativedetection limits implied for locations mapped in red varies. Fracturesthat were never determined to be active are not included in the maps.Because our test locations were chosen from a finite set of candidatescorresponding to features in the base map, the uncertainties in thoselocations correspond to and are correlated with the uncertainties in thebase map, so we do not display those uncertainties over the base map.

We produced an averaged activity map (Fig. 4f) by dividing thenumber of times each location was determined to be active (green) bythe total number of times a unique determination could be made (green

Triangulated positions

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Figure 2 | Schematic illustrating the dependence of phantom jet locationson viewing angle. Assuming the fracture is erupting uniformly, imagesobtained from directions A, B and C will show phantoms at locations a, b and crespectively, where the sight lines are tangential to the fracture. Triangulationamong the various combinations of sight lines produces solutions at locationsab, ac and bc, where the respective ground tracks intersect. Because theapparent location of a phantom depends on the viewing angle, attempts totriangulate phantoms produce spurious solutions whose accuracies diminishwith increasing parallax. In contrast, the precision of triangulation of stationaryobjects increases with increasing parallax.

a Baghdad Sulcus

b Baghdad Sulcus

N1652854202Spread = 10°Zenith = 0°

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Figure 3 | Shadow anomalies and spreading curtains. Left-hand column,reference images are shown with no overlays. Middle column, simulatedvertical collimated curtains are overlain. Right-hand column, simulatedcurtains with the indicated spreading and zenith angles are overlain. OnBaghdad and Damascus Sulci, the shape of the shadows are more consistentwith a spreading curtain than a collimated curtain. In every case, the shape ofthe observed curtain is consistent with a spreading with altitude. On DamascusSulcus, the horizontal shadow right of centre is consistent with a fracture notseen in the base map, but which appears in a different Cassini image (ExtendedData Fig. 1).

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or red). The alternative fracture mapped in cyan was excluded from theaverage. As expected, we find that the bulk of the emission originates onthe four main tiger stripes and their major branches. The hottest regionsdetected by the Cassini Composite Infrared Spectrometer (CIRS; figure 3in ref. 6) are generally the areas in our averaged map that show thelargest number of activity counts, though a cooler segment on CairoSulcus on the trailing hemisphere shows high activity counts in ouraveraged map and some warm areas were not observed to be active inany of our maps. However, given the small number of samples, thosediscrepancies may not be statistically significant. We also saw activity onfractures where no thermal anomaly is apparent in ref. 6. In particular, a

lengthy network of fractures extending from Baghdad Sulcus towardsCairo Sulcus was active on day 2010-225.

The curtain representation for Enceladus’ south-polar eruptionsexplains the broad emission seen in the images that cannot be mod-elled using discrete sources alone. The phantom jet phenomenon thatemerges from that representation demonstrates that there are far fewerlocalized eruption sources along the tiger stripes than previouslythought. Although prominent discrete jets with significant non-nadirzenith angles are still required to explain the activity seen in someimages (for example, N1669800820), other images (for example,Fig. 1b) can be explained with almost no discrete jets.

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Figure 4 | Stereographic projections of the south-polar terrain showingactivity at five different times, and the average. a–e, Green areas are active,red areas are inactive, and blue areas are undetermined. One possiblealternative segment on Baghdad Sulcus was mapped using cyan. f, Averaged

activity map, where the plotted value at each point is proportional to the totalnumber of times activity was detected divided by the total number of times adetermination could be made. Lighter colours represent larger values. Sourcelocations from ref. 3 are indicated with white circles.

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Online Content Methods, along with any additional Extended Data display itemsandSourceData, are available in the online version of the paper; references uniqueto these sections appear only in the online paper.

Received 5 August 2014; accepted 27 February 2015.

1. Porco, C. C. et al. Cassini observes the active south pole of Enceladus. Science 311,1393–1401 (2006).

2. Hansen,C. J.etal.Enceladus’water vaporplume.Science311,1422–1425(2006).3. Spitale, J. N. & Porco, C. C. Association of the jets of Enceladus with the warmest

regions on its south-polar fractures. Nature 449, 695–697 (2007).4. Hansen, C. J. et al. Water vapour jets inside the plume of gas leaving Enceladus.

Nature 456, 477–479 (2008).5. Spencer, J. R. et al. Cassini encounters Enceladus: background and the discovery

of a south polar hot spot. Science 311, 1401–1405 (2006).6. Howett, C. J. A., Spencer, J. R., Pearl, J. & Segura, M. High heat flow from Enceladus’

south polar region measured using 10–600 cm21 Cassini/CIRS data. J. Geophys.Res. 116, E03003 (2011).

7. Porco, C. C., DiNino, D. & Nimmo, F. How the geysers, tidal stresses, and thermalemission across the south polar terrain of Enceladus are related. Astron. J. 148, 45(2014).

Acknowledgements This work was funded by grant number NNX13AG45G of theCassini Data Analysis and Participating Scientists Program. We thank M. Hedman,P. Thomas and C. Howett for conversations on this topic.

Author Contributions J.N.S. devised the approach and wrote the majority of the text.T.A.H. and A.R.R. contributed substantially to the interpretation and to the text. E.E.B.and S.S.P. digitized many of the fractures and contributed to early efforts that led to thecurrent approach.

Author Information Reprints and permissions information is available atwww.nature.com/reprints. The authors declare no competing financial interests.Readers are welcome to comment on the online version of the paper. Correspondenceand requests for materials should be addressed to J.N.S. (jnspitale@psi.edu).

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METHODSData reduction. For this investigation, we used images from five Enceladus fly-bys, on days 2009-325, 2010-138, 2010-225, 2010-334 and 2010-354. The imageswere all taken in the clear filters and had camera scales ranging from ,1,500 m perpixel to ,50 m per pixel and phase angles greater than 142u. Images were radio-metrically calibrated using version 3.6 of the Cassini CISSCAL (Cassini ImagingScience Subsystem Calibration) software8, and where possible images were co-added to improve the signal relative to the noise. Pointing corrections were carriedout using landmarks on the satellite; either the limb, or known features on thesurface. The satellite was modelled as an ellipsoid with radii 256.6, 251.4 and 248.3km (ref. 1).Identifying active fractures. The procedure for simulating the curtains was afollows. First, candidate fractures were digitized by selecting a set of surface pointson a base map (we used EN_120828_DLR_south_equator9) and re-sampling themto a 250-m spacing. Finer spacings were too CPU/memory intensive. For the fewimages with pixel scales finer than that, we were careful not to over-interpret fine-scale phantoms. Next, we produced a simulated image using the geometry of a realcomparison image. To produce the simulated image, we projected a ray from eachsample location on the surface of Enceladus. The intensity of the ray decreasedexponentially with altitude, and points that were shadowed by Enceladus wereassigned zero intensity.

The mapping from Enceladus’ shadow on a curtain to the surface trace of thesource fracture is effectively unique. The shadow is a cone formed by the projectionof Enceladus’ terminator into space directly away from the Sun. Each point on thesurface of the satellite maps to a unique point on the surface of the shadow cone byway of a ray perpendicular to the satellite surface at that point. Because none of theimages used in this study were taken during eclipse, rays projected from thecamera that do not hit the satellite intersect the shadow cone at two points,implying emission from two possible surface locations. However, there is nearlyalways only one reasonable choice for the true source; the other choice is usually somuch closer to the camera that the shadow height rules out any reasonable pos-sibility of detecting any emission. In this study, there were no cases where thatdegeneracy could lead to any confusion.Non-ideal curtain geometries. On days 2010-138 and 2010-354, the shadow caston the curtain above Baghdad Sulcus was a poor match to shadows on idealsimulated curtains. We obtained a better match to those shadows by assumingthe curtain spread with altitude with a 610u spreading angle (Fig. 3a and b).Spreading was modelled by projecting two collimated curtains from a fractureto represent the envelope of the spread curtain. Points internal to the envelope

were not included. As shown in the figure, the 10u spreading angle is also consist-ent with the observed shape of the curtains themselves in images that are sensitiveto that aspect of the curtains. Figure 3 also shows that the curtains above Damascusand Cairo Sulci are better matched when spreading and small non-nadir zenithangles are included. Even better matches to the shadows could be obtained byallowing the spreading angles and zenith angles to vary along the fractures insteadof assuming a simple model with uniform parameters.Unseen fractures. Another possible explanation for the anomalous shadows onthe curtain above Baghdad Sulcus would be emission arising from additionalfractures that are not apparent in the base map. Assuming vertical collimatedcurtains, the observed shadows can be used to trace out unique surface sourcesto arbitrary precision. We searched all Cassini images with surface pixel scalesfiner than 500 m and phase angles less than 135u for fractures that couldproduce the observed curtains near Baghdad Sulcus assuming vertical colli-mated emission and found no candidates to explain the entire observed pattern,though the base map does contain one fracture that could explain part of theanomalous shadow on day 2010-354. Moreover, while vertical collimated emis-sion from unseen fractures can match the curtain shadows, it does not match theobserved shape of the curtains themselves, and it requires a different fracture toexplain the anomalous Baghdad shadow on each day; the inclusion of a 10uspreading angle does a reasonable job of explaining both anomalous shadowssimultaneously.

An anomalous curtain shadow near Damascus Sulcus probably does imply thepresence of a fracture not visible in the base map. In Fig. 3c, the prominenthorizontal shadow near right of centre in the frame cannot be explained by anycandidate fracture visible in the base map. A search for candidate fractures pro-duced the result shown in Extended Data Fig. 1. Note that because the locations forpoints along this fracture were inferred from the shadows instead of being selectedfrom visible features in the base map, uncertainties for those points are independ-ent of the base map and are explicitly shown in the figure. Those uncertainties werederived from the half-width of the shadow in the image.Sample size. No statistical methods were used to predetermine sample size.Code availability. The analysis in this paper was performed using IDL-basedsoftware that is not currently available to the public.

8. Porco, C. C. et al. Cassini imaging science: instrument characteristics andanticipated scientific investigations at Saturn. Space Sci. Rev. 115, 363–497(2004).

9. Roatsch, T. J. et al. in Saturn from Cassini-Huygens (eds Dougherty, M. Esposito, L. &Krimigis, S.) 763–781 (Springer, 2009).

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Extended Data Figure 1 | Portion of image N1602274088 showing sourcelocations inferred from anomalous emission near Damascus Sulcus.Diamonds are sample points inferred from the observed curtains; uncertaintiesare shown as circles about each displayed sample point. The inferred sourcelocations correspond well with features that are not apparent in the base map.

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