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Bradák, B.; Kimura, J.; Asahina, D.; El Yazidi, M.; Orgel, C. The Wispy Terrain and Dione's Cryotectonic Features. Encyclopedia. Available online: https://encyclopedia.pub/entry/51731 (accessed on 30 June 2024).
Bradák B, Kimura J, Asahina D, El Yazidi M, Orgel C. The Wispy Terrain and Dione's Cryotectonic Features. Encyclopedia. Available at: https://encyclopedia.pub/entry/51731. Accessed June 30, 2024.
Bradák, Balázs, Jun Kimura, Daisuke Asahina, Mayssa El Yazidi, Csilla Orgel. "The Wispy Terrain and Dione's Cryotectonic Features" Encyclopedia, https://encyclopedia.pub/entry/51731 (accessed June 30, 2024).
Bradák, B., Kimura, J., Asahina, D., El Yazidi, M., & Orgel, C. (2023, November 17). The Wispy Terrain and Dione's Cryotectonic Features. In Encyclopedia. https://encyclopedia.pub/entry/51731
Bradák, Balázs, et al. "The Wispy Terrain and Dione's Cryotectonic Features." Encyclopedia. Web. 17 November, 2023.
The Wispy Terrain and Dione's Cryotectonic Features
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This entry is adapted from the peer-reviewed paper 10.3390/rs15215177

The so-called Wispy Terrain, named after observing frequently appearing wispy streaks, markings, and lineaments in the images of the Voyager spacecraft, is one of the enigmatic features of the icy moon Dione (a satellite of Saturn). Its characteristics and formation have been the target of a long-lasting scientific debate and ongoing research, along with other cryotectonic features of the icy satellite. 

Wispy Terrain Dione Saturn icy satellite cryotectonic features

1. The Wispy Terrain

The higher resolution images of the Cassini spacecraft revealed that the markings of Wispy Terrain and the so-called chasmata or chasm system consist of quasi-parallel faults, troughs, and more complex “horst and graben” structures indicating extensional and shear stresses in Dione’s icy crust. Despite the popularity of Wispy Terrain as a research target, there are still ongoing debates about the evolution of the highly tectonized region of the icy satellite. Neither the age of the surface nor the formation of various features observed in the area are fully understood.
The early planetary geologic mapping of Dione provided a preliminary view of the features of the Wispy Terrain. Still, beyond a chronostratigraphic proposal, no precise age determination could be made about its formation time [1][2]. Following the analysis of the more detailed Cassini images, the impact crater dating from the Eurotas Chasmata indicated that the surface of Wispy Terrain is relatively young, and the cryotectonic processes in the region might have been active until ~3 Ga or even until ~1 Ga ago, keeping the uncertainty in the cratering models in mind [3][4]. Almost three decades passed after the first geological mapping, and the formation of the main terrains of Dione was re-evaluated in light of the new results [5]. The chasmata systems, including the Eurotas and Palatine Chasmata (Wispy Terrain), were defined as fractured cratered plains and subdivided into three facies types, regarding the timing of their formation by tectonic episodes, which date back to 3.7 Ga (with 100 Ma uncertainties) or between 2.7 Ga and 260 Ma [5]. Later calculations showed 4.5 (+0.2/−2.7) and 2.5 (+2.0/−1.9) Ga computed impact crater ages for the Faulted Terrain, including the Wispy Terrain. The results suggested that those ages do not reflect the timing of cryotectonic activity but the period while the larger craters and their ejecta blankets erased and covered the smaller craters in the region [6]. Among the newer studies, Fergusson et al. [7] suggest that surface renewal is not only limited to certain areas but appears in many regions on Dione.
Along with the continuously updated impact crater ages, there has been some development in theories related to the formation of the region as well. Based on the analog geological characteristics between the fault system of the chasmata and Earth’s provinces with divergent tectonic plates, Dione’s Faulted Terrain (along with the Rhea and Tethys chasmata) was defined as hemisphere-scale rift zones [8][9].
In addition, the possibility of a subsurface ocean under the satellite’s ice shell brought a promise of a still active surface and a potential for life under the icy cover [10]. The study of the stratigraphic relationship between the craters and faults on the Wispy Terrain suggests that the faulting is a geologically very recent event, dating back to 0.3–0.79 Ga [11]. Some of the newest studies go even further and suggest that the upper limit for the age of the studied fault on Dione’s Wispy Terrain is only 152 Ma, which supports the hypothesis that the cryotectonism might be still active or was active a very short time ago (ca. 100 Ma) on the satellite [12].
One of the primary research topics about icy satellites with potential subsurface oceans (e.g., Europa) is the understanding of downward processes from the frozen surface and the description of the material exchange between surface and subsurface regions, which may markedly contribute to the oxygenation of the subsurface oceans and provide potential habitat for living organisms. Ice or cryotectonic processes may support such downward material transport [13].

2. A Brief Review of Dione’s Cryotectonic Features

During the more and more detailed study of the satellite, various cryotectonic features have been observed in the icy crust of Dione, which are summarized in Table 1. Out of the different basic fault types (normal, reverse, and strike-slip faults), normal faults seem to be the most common, indicating an extensional stress field and resulting in the formation of various simple and more complex tectonic features, such as troughs, scarps, and the so-called horst and graben structure [2][4][14][15] (Table 1).
Table 1. The summary of putative cryotectonic features observed on the surface of Dione.
Putative Cryotectonic Features Short Description Stress Field References
Bright wispy markings (a) Surficial deposit of high-albedo material associated with eruptive events along fractures
(b) Trough walls coated by bright material
(c) Lineaments with slopes facing toward the Sun
(d) Fault scarps		 Extension (divergence) [1][2][14]
Fossae|Chasmata Initially, such features were called “wispy material” or “wispy markings” (former lineae) Extension (divergence) [16]
Fractures Non-Wispy Terrain features (from polygonal impact craters); fractures consistent with the global deformation from a combination of satellite despinning and volume expansion   [15]
Lineaments In general, various linear features; characteristic global lineament trend (NE and NW—middle latitudes and equatorial region; E–W—polar region) → origin; stresses by (i) loss of angular momentum associated with despinning, and (ii) effect of the orbital recession, superimposed on (iii) tensional stress (global surface extension) Extension (divergence) [2]
(Bright) lineaments Widely abundant; single or densely spaced, sub-parallel lineaments; may reach lengths of several hundred kilometers; some of them may be scarps with slight vertical displacement Ext. (diverg.) or compr. (converg.) [4][14]
(Radial) lineaments or ray crater Cassandra: bright ray crater/system of radial lineaments and scarps (set of radial scarps radiating away from a point source); bright exposure of ice on the slopes of the scarps Extension due to diapir formation [4][14]
Ridges (Janiculum Dorsa) Flexural deformation: 500 km long, north–south trending ridge; flanked by parallel flexural depressions; leading hemispheres; 4 Ga old Compression (convergence) [17]
Ridges Extending 50–300 km long, <0.5 km high, broad, “convex in cross-section” features; merge into lineaments or escarpments; parallel and subparallel ridge systems → origin ((volcanic or) tectonic): (a) parallel normal faults; (b) fault scarps; (c) graben; (d) high-angle reverse faulting
Ridge complex: prominent ridge associated with minor sub-parallel ridges and troughs (Janiculum Dorsa)		 Extension (divergence; a, b, and c), or compression (convergence; d) [2][4]
Rift zones Large-scale extensional deformation (extends ~1300 km, subtend 133° of arc, and varies 40–130 km in width); concentrated within or at the borders of the trailing hemisphere; shows a preference for ~N–S-oriented strikes (Palatine, Eurotas, and Padua Chasmata); complex fault structure Extension (divergence) [8][9]
Scarps Extending ≤100 km long, <1 km high, linear, uncommon features; extension of large polygonal crater rims → origin: (a) faulting; (b) mass-movements   [2]
Broad bands Formed by densely spaced (graben) horsts and scarps; bright albedo due to exposure to clean water ice Extension (divergence) [4]
Shallow normal fault slopes Normal faults with steep dips → viscous relaxation (due to lithospheric heating events related to radionuclide decay), which affects fault slopes (Padua and Palatine Chasmata) Extension (divergence) + viscous rel. [15]
Troughs|chasma|chasmata Long linear, narrow, or wider (shallower) troughs; branching (Tibur Chasma and Larissa and Latium Chasmata); parallel ridges may appear in their bottom (Tibur Chasma); ~30 to 100 km (>500 km), 0.3 ± 0.1 km deep, irregular, or scalloped walls, rims may appear; rectilinear troughs → grabens; parallel rectilinear troughs → horst and graben structure (Palatine Chasmata) Extension (divergence) [1][2]
Troughs Several kilometers or a few tens of kilometers wide; several hundred kilometers long; linear, arcuate, or curved; single or in parallel sets Extension (divergence) [4]
Linear features have been named in various ways since the first images from Dione arrived, such as “bright wispy markings” [1][2][14], “bright lineaments” [4][14], in general, “lineaments” [2], fossae, and chasmata (chasma) [2][16]. The terms fossae and chasmata have been applied to linear depressions with various depths. Still, such features have many characteristic marks, which may indicate different ways of formation and maturity in their development, e.g., on a scale from “simple” troughs to complex ones, even with parallel ridges in their bottom [1][2][16]. They often appear as high albedo features, most likely due to (i) the exposure of ice on the slopes of the scarps in the case of lineaments with a minimal vertical displacement [4][14], (ii) some bright material coating on the wall of some troughs (if the linear feature is identified as a trough), (iii) the lineaments had some Sun-facing slopes when the image was taken [1][2], and (iv) some subsurface material, which might resurface during various (cryotectonic–cryovolcanic) processes [1][2]. In the case of Eurotas, Palatine, and Padua Chasmata, the structures reached a specific size and complexity, which led to further theories describing the system of scarps, troughs, and normal faults as rift zones, similar to the ones on other satellites, such as Rhea and Tethys [8][9].
Most of the studies involved in the study of various tectonic features agree about the formation process of the morphological features described above, including the overall extension- (and shear stress-) dominated stress fields in the icy crust of the satellite (the triggering processes will be discussed later in this section). Such agreement does not apply to ridges, which are interpreted as (i) parallel normal faults, (ii) fault scarps, (iii) graben, (iv) high-angle reverse faults [2], and (v) flexural deformations [17]. Suppose one of the former three applies to ridges. In that case, the stress field during its formation agrees with the stress field that appears during the formation of troughs, chasmata, and other features suggested above. Suppose the ridges are defined as reverse faults and flexural deformations (the latter two). In that case, it indicates compression (or shear stress), which would be unusual regarding the studies about cryotectonic features on the surface of Dione (Table 1).
In general, in his early synthesis of the geological evolution of Dione, Moore [2] suggested sequences of heat, expansion, melting, and contraction as a trigger of stresses in its icy crust. Regarding the processes that may induce those stresses and result in the formation of various cryotectonic deformation-related features, various mechanisms, such as changes in volume due to phase changes within a satellite [18], solid-state convection [19], polar wander [20], diurnal tidal flexing [21][22], forced eccentricity [22][23], and obliquity [22][24][25][26], the nonsynchronous rotation of the ice shell to the tidal torques [27][28] can be mentioned. Along such long-lasting (at a geological scale), quasi-permanent, and periodic processes, catastrophic events, namely large impacts, may be considered the triggers of deep-seated lines of weakness in the crust that later determined the location of cryotectonic features [2][29].
Using craters as stress-field markers, partly introduced in Bradák et al. [30], revealed the frequent appearance of fragmentary or incomplete craters in the region located between Palatine Chasmata and Eurotas Chasmata, which may indicate compression in specific locations instead of the highly expected dilatation. 

References

  1. Plescia, J.B. The geology of Dione. Icarus 1983, 56, 255–277.
  2. Moore, J.M. The tectonic and volcanic history of Dione. Icarus 1984, 59, 205–220.
  3. Zahnle, K.; Schenk, P.; Levison, H.; Dones, L. Cratering rates in the outer Solar System. Icarus 2003, 163, 263–289.
  4. Wagner, R.J.; Neukum, G.; Stephan, K.; Roatsch, T.; Wolf, U.; Porco, C.C. Stratigraphy of tectonic features on Saturnś satellite Dione derived from Cassini ISS camera data. In Proceedings of the 40th Lunar and Planetary Science Conference, The Woodlands, TX, USA, 23–27 March 2009; 2142.pdf. Available online: https://www.lpi.usra.edu/meetings/lpsc2009/pdf/2142.pdf (accessed on 11 December 2022).
  5. Stephan, K.; Jaumann, R.; Wagner, R.; Clark, R.N.; Cruikshank, D.P.; Hibbitts, C.A.; Roatsch, T.; Hoffmann, H.; Brown, R.H.; Filiacchione, G.; et al. Dione’s spectral and geological properties. Icarus 2010, 206, 631–652.
  6. Kirchoff, M.R.; Schenk, P. Dione’s resurfacing history as determined from a global impact crater database. Icarus 2015, 256, 78–89.
  7. Ferguson, S.N.; Rhoden, A.R.; Kirchoff, M.R. Regional impact crater mapping and analysis on Saturn’s moon Dione and the relation to source impactors. J. Geophys. Res. Planets 2022, 127, e2022JE007204.
  8. Byrne, P.K.; Schenk, P.M.; McGovern, P.J. Tectonic mapping of rift zones on Rhea, Tethys and Dione. In Proceedings of the 46th Lunar and Planetary Science Conference, The Woodlands, TX, USA, 16–20 March 2015; 2251.pdf. Available online: https://www.hou.usra.edu/meetings/lpsc2015/pdf/2251.pdf (accessed on 11 December 2022).
  9. Byrne, P.K.; Schenk, P.M.; McGovern, P.J.; Collins, G.C. Hemispheric-scale rift zones on Rhea, Tethys, and Dione. In Proceedings of the Enceladus and the Icy Moons of Saturn, Boulder, CO, USA, 26–29 July 2016; 3020.pdf. Available online: https://www.hou.usra.edu/meetings/enceladus2016/pdf/3020.pdf (accessed on 11 December 2022).
  10. Beuthe, M.; Rivoldini, A.; Trinh, A. Enceladus’ and Dione’s floating ice shells supported by minimum stress isostasy. Geophys. Res. Lett. 2016, 43, 10088–10096.
  11. Hirata, N. Timing of the faulting on the Wispy Terrain of Dione based on stratigraphic relationships with impact craters. J. Geophys. Res. Planets 2016, 121, 2325–2334.
  12. Dalle Ore, C.M.; Long, C.J.; Nichols-Fleming, F.; Scipioni, F.; Rivera Valentín, E.G.; Lopez Oquendo, A.J.; Cruikshank, D.P. Dione’s Wispy Terrain: A cryovolcanic story? Planet. Sci. J. 2021, 2, 83.
  13. Howell, S.M.; Pappalardo, R.T. NASA’s Europa Clipper—A mission to a potentially habitable ocean world. Nat. Commun. 2020, 11, 1311.
  14. Wagner, R.; Neukum, G.; Giese, B.; Roatsch, T.; Wolf, U.; Denk, T.; the Cassini ISS Team. Geology, ages, and topography of Saturn`s satellite Dione observed by the Cassini ISS camera. In Proceedings of the 37th Lunar and Planetary Science Conference, The Woodlands, TX, USA, 13–17 March 2006; 1805.pdf. Available online: https://www.lpi.usra.edu/meetings/lpsc2006/pdf/1805.pdf (accessed on 11 December 2022).
  15. Beddingfield, C.B.; Burr, D.M.; Dunne, W.M. Shallow normal fault slopes on Saturnian icy satellites. J. Geophys. Res. Planets 2015, 120, 2053–2083.
  16. Roatsch, T.; Wählisch, M.; Hoffmeister, A.; Matz, K.-D.; Scholten, F.; Kersten, E.; Wagner, R.; Denk, T.; Neukum, G.; Porco, C. High-resolution Dione atlas derived from Cassini-ISS images. Planet. Space Sci. 2008, 56, 1499–1505.
  17. Hammond, N.P.; Phillips, C.B.; Nimmo, F.; Kattenhorn, S.A. Flexure on Dione: Investigating subsurface structure and thermal history. Icarus 2013, 223, 418–422.
  18. Hillier, J.; Squyres, S.W. Thermal stress tectonics on the satellites of Saturn and Uranus. J. Geophys. Res. 1991, 96, 15665–15674.
  19. Czechowski, L.; Leliwa-Kopystyńskia, J. Solid state convection in the icy satellites: Discussion of its possibility. Adv. Space Res. 2002, 29, 751–756.
  20. Schenk, P.; Matsuyama, I.; Nimmo, F. A very young age for true polar wander on Europa from related fracturing. Geophys. Res. Lett. 2020, 47, e2020GL088364.
  21. Marshall, S.T.; Kattenhorn, S.A. A revised model for cycloid growth mechanics on Europa: Evidence from surface morphologies and geometries. Icarus 2005, 177, 341–366.
  22. Rhoden, A.R.; Militzer, B.; Huff, E.M.; Hurford, T.A.; Manga, M.; Richards, M. Constraints on Europa’s rotational dynamics from modeling of tidally-driven fractures. Icarus 2010, 210, 770–784.
  23. Hoppa, G.; Greenberg, R.; Tufts, B.R.; Geissler, P.; Phillips, C.; Milazzo, M. Distribution of strike-slip faults on Europa. J. Geophys. Res. 2000, 105, 22617–22627.
  24. Bills, B.G. Free and forced obliquities of the Galilean satellites of Jupiter. Icarus 2005, 175, 233–247.
  25. Hurford, T.A.; Sarid, A.R.; Greenberg, R.; Bills, B.G. The influence of obliquity on Europan cycloid formation. Icarus 2009, 202, 197–215.
  26. Rhoden, A.R.; Hurford, T.A. Lineament azimuths on Europa: Implications for obliquity and non-synchronous rotation. Icarus 2013, 226, 841–859.
  27. Helfenstein, P.; Parmentier, E.M. Patterns of fracture and tidal stresses due to nonsynchronous rotation: Implications for fracturing on Europa. Icarus 1985, 61, 175–184.
  28. Kattenhorn, S.A. Nonsynchronous rotation evidence and fracture history in the bright plains region, Europa. Icarus 2002, 157, 490–506.
  29. Smith, B.A.; Soderblom, L.; Beebe, R.; Boyce, J.; Briggs, G.; Bunker, A.; Collins, S.A.; Hansen, C.J.; Johnson, T.V.; the Voyager Imaging Team; et al. Encounter with Saturn: Voyager 1 imaging science results. Science 1981, 212, 163–191.
  30. Bradák, B.; Nishikawa, M.; Gomez, C. A Theory about a hidden Evander-size impact and the renewal of the Intermediate Cratered Terrain on Dione. Universe 2023, 9, 247.
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Balázs Bradák
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Jun Kimura
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Daisuke Asahina
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Mayssa El Yazidi
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Csilla Orgel
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