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Analysis of the geology of Jovian moon

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Abstract: Europa, a lunar sized ice-covered body has been fascinating geologist with its extreme surface features. From the past fly-by missions has provided us with high resolution SSI images revealing various surface features and units. This paper briefly reviews on the understanding of the surface composition of the Jovian moon: Europa. Images and spectroscopic data have disclosed various surface features. On the basis of the initial mapping from the Galileo data, it has been suggested that Europa’s lithosphere is composed of five primary materials: plains, chaos, band, ridges and craters.

Keywords: Europa, Galileo, planetary geology, geologic mapping

1.Introduction:

Europa, the fourth largest Galilean satellite of Jupiter, is ice covered rocky object with a diameter of 3122km [slightly smaller than earth’s moon (3474km). It’s extremely young surface age, indications of potentially habitable environment and extreme geological features have fascinated and become the area of interest of various astrobiologist and planetary geologist around the globe. It has been stated that it possesses an outer water-ice shell of approximately 100km thick. On the basis of the major geological data analysis from Voyager and Galileo spacecraft, it allows us to differentiate various geological units, structural features and predicting the model of formation of various geological features.

Ridges are most extensive surface materials on Europa. Ridged plain material vary widely in length and width over a vast area. Smooth plains material typically overprints various terrains and material units. Chaos material are dark albedo fine textured polygonal blocks from pre-existing lineated plains. Whereas, bands are linear, curvilinear, cuspate or wedge-shaped zones which exhibit parallel to subparallel ridges and troughs. Ridge material are narrow chain of crest in continuous manner. Ridges are sub divided as single ridge, doublet ridges more complex ridge consisting of more than two ridges. Crater material are deep circular depressions on any planetary surface that can either form due to asteroidal impact of volcanic explosions.

2. Material Units:

2.1 Plain Materials:

Most of the Europan surface is occupied by plain material unit. Plain units are further divided on the basis of albedos. The equatorial region is more likely to have a low albedo. Probably because of the frequent magnetospheric ion bombardment of high energy particles onto the surface, driving out the ice and leaving back the net deposits of silicates and non-ice materials on the surface. The ultra violet spectral features show that the polar plains indicate high albedo which could be the result of the poleward migration of ice grains. [5]

2.1.1 Ridged plain material

-Ridged plain (Figure 1) materials are multiple cross cutting ridges and troughs on a planar surface which consists of parallel and multiple orientations.

  • Individual ridges generally have a spacing of 100 m.
  • Ridges lying on a belt can lie up to >4km wide with individually spacing up to 200-400m apart.
  • The surrounding terrain are subdued and hence has a high albedo and are sharp with surrounding terrain owing to high topographic or albedo difference.
  • Several models have been proposed for the formation of ridges which are stated in section.

2.2.2 Smooth plain material

  • These plains (Figure 2) are characterized by the smooth surface with almost negligible texture and has a low albedo as compared to other plains.
  • Generally, smooth plain material has a limited area of coverage and tends to subdue the pre-existing feature.
  • They are found in circular to irregular shaped areas;
  • They are interpreted as a result of cryovolcanic deposition or melting and re-crystallization of surface material.

2.2 Chaos material

Chaos material are disrupted fine texture polygonal blocks of pre-existing plains. They form dark albedo features making contacts with brighter, smoother plain units. They are characterized by various terrain sets in hummocky matrix which have been disrupted into place of icy crus of different volumes. Chaos are irregularly shaped stratigraphically young plates with 10-100km in diameter which are rotated, tilted and translated.

Two sub units of chaos material are observed at regional and high resolution:

2.2.1 Platy chaos material:

Platy chaos material is characterized by slabs and plates which are pre-existing materials. They are composed of individual hillocks which form rugged hummocky terrains which range from few kilometres to thousands of kilometres.

2.2.2 Knobby chaos material:

Knobby chaos material is characterized by irregularly shaped knobs that raise above he surrounding matrix area. Each knob can stand up to hundreds of meters above the matrix and ~3km approx. e.g. The “Mitton” knob rises above 150m of surrounding matrix.

2.3 Bands Material

Bands are linear, curvilinear, cuspate or wedge-shaped zones which exhibit sharp parallel-to-parallel ridges and troughs. It was observed that in region Galileo images in which an internal structure of ridges and troughs move sub parallel to each other along the boundaries of the band. [7]. they are characterized by the zones of separation and total replacement of background plains which can occur due to lateral movement strike slip movement and non-orthogonal separation. [2]. Band material comprises triple bands gray bands, bright bands and wedged spaced bands. [5]. the features of wedge shaped and gray bands allow them to restore and reconstruct their original structure which was displaced when bands opened due to fractures [7,5] along with the lithosphere was separated.

Reconstruction of bands showed that some have opened along-existing ridges and demonstrated that surrounding ridge plain which can results into tectonic volcanic resurfacing of Europa. [5] The triple bands are doublet ridges which situated along dark material.

These materials can be well distinguished by the variance in albedo or surface texture when compared with surrounding areas. The albedos vary widely in bands. Lower Albedos are observed at a younger band than older bands. Agenor Linea is an important exception which is a rare bright band on Europa’s surface which is analysed probably if compressional origin.

2.4 Ridges

Ridges are one of the most dominant geological material unit making distinct contact with other units. A ridge can be characterized by a narrow chain of crests in a continuous manner. Long narrow ridges can range from ~200m to >4km which can extend up to less than 1000km in length and can be 200 – 350m high.

Ridges cover features that are straight, curvilinear or cycloidal. They can be subdivided into single ridge, doublet ridges (two ridges separated by a central trough) or Ridge complexes which consist more than two ridges. Ridges consistently cross younger plains and exhibit as tapering, flank slopes and mass wasting. From the high-resolution Galileo images, it was observed that most of the ridges are found as doublet ridges which can vary from ~500m – `2km in width and are minutely convex to trapezoidal with a central trough. Flank by dark diffuse material which lags a sharp outer margin. In low resolution albedo appearance in low incidence angle, triple bands as complex, multi component which lineaments consist of bright strips flanked on each side by dark bands.

3.1 Lenticulae

Lenticulae are circular and elliptical structural features on Europan surface. They are further classified as Domes, Pits, Smooth and Dark spots which are 7-15km in diameter and 5-20km apart. [2,6] They generally alter the pre-existing rigid plains and consist of subparallel ridges and grooves that overlaps in successive generation. [6] e.g.: A dome looks like pieces of older plains around them and looks like domes were formed when the plains were pushed up. [9] Lenticulae are characterized by widespread mottled terrain and make up most of the texture. [2]

3.2 Troughs

Troughs are laterally extending linear structure which are formed on a narrow basin or a geological rift. [10] They are generally characterized by V- and U- shape in cross-section and have minutely raised rim and are mostly found on tectonic rim. They can intersect all terrain types. As they are narrow, troughs are generally considered as lineaments on global mapping. They are analysed as a narrow line along trough axis with inward pointing arrowheads. [5,10]

3.3 Strike-slip faults

Strike slip is a fracture and discontinuity in a plain surface due to the relative movement of geological feature (i.e. ridges, troughs, lineaments, scraps). [5,12] A strike slip fault is mapped with a line in a faulting region, with a half arrow on one side indicating direction of relative motion. [5]

3.4 Scraps

Scraps are sharp fractures in a surface where on one sides of faults are displaced with respect to other. [13] As they are linear to curvilinear in platform, they can be distinguished from closed crater, rims, domes and depressions. [5]A scarp is mapped with a line in the base region of the slope with the hachures pointing in the downhill direction.[5]

3.5 Depressions

Depressions can be enclosed circular, elliptical or irregularly shaped negative relief features which are depressed below the surrounding area. Depressions can be easily distinguished under near terminator lightening conditions. They are mapped by a line that follow the outline of its upper slope and marked with downstairs directed contour.[5]

3.5 Pits, domes and spots

Dome is a structural feature of geology which are positive relief. [4] They are characterised by youngest layer at outside and growing inwards and can raise up to 10- 100 meter high. They can be easily observed under near terminator lightning conditions. [5] A dome is map with a line along its base and marked with outward pointing contour.

3.6 Crater rims

Crater rim is the part that raise above the height of surface, usually in a circular or elliptical edge. [15] A large crater (that is more than 15 km) have complex origin and can be distinguished by central uplifts within impact zone. [5,15] A crater is mapped with a close circular curve along the rim crest and sets of paired in word pointing contour.

4. Formation of geological features of Europa

4.1 Ridges

Analysing various geological and morphological aspects from high resolution Galileo images various models were proposed for formation of ridges

Each of these have been revived in short:

a. Volcanism model

It was proposed by kadel et al (1998) that double ridges are linear volcanic structure which were found during the gas driven fisher eruptions. Evolving gases like CO2 or SO2 with water vapour erupted onto the surface dumping the debris to form doublet ridges with central trough. [6,7]this model has a difficulty in explaining for the ridges extending up to few km.[6]

b. Tidal squeezing model

Greenberg et al (1998) proposed that ridges are piles if linear debris that gets up deposited along the fractures with each cycle. [6]This can be due to diurnal stresses, which pumps ice debris above the surface which leads to ridge construct. Here the author states a hypothesis of a thin icy shell above the ocean and diurnal stresses transfers in the ice debris to the surface forming ridges and the surrounding terrains adjust the pumping compression. [6,7]

Figure 6. Types of formation of various geological features

a) volcanism b) tidal squeezing c) Diaprism d) compression e) wedging

There were difficulties with the crack penetrating ice shell. It was found that crack pumping the ice debris upward cannot be held for a long time. [7] Other problem states that incomplete pumping can lead to freeze the ice and block the crack which may remain open for many tidal cycles. Further modification is required to develop systematic pumping of ice to the surface within a specific opening and closing time scale. [6,7]

d. Diaprism model

Head et al (1999) stated that cracking and linear diapiric assent of warm ice push the surface to form doublets ridges with the central trough. [7] e.g. A slab or wall of diapiric material would raise upward due to the bouncy and apparently breaking the thin ice upwards. In this model, it suggests that rather than ice shell the crack penetrates to the sub surface and the warm sub surface ice moves buoyantly into the fracture due to tidal heating. This warm low viscosity ice raise buoyantly to create a ridge, building a few 100-meter-high ridges in a timeline of 10 years.[6]

e. compression model

Sullivan et al (1997) proposed the model for formation of ridges by compression of 2 plates due to stress. Due to deforming warm subsurface more than cold near surface ice by compressive stress, the deformed icy material icy compressed and forced upwards in fractures, pushing cold near surface plates leading to formation of ridges. [5]Mass wasting affects ridge. Flank morphology, complicating efforts to determine how much ridge relief takes place due to unwrapping of pre-existing terrain, extrusion, tidal flexing or other process. [6,7]

f. Wedging

It was proposed by Turtle et al which states that ridges are formed due to the melt addition inside a vertically shallow crack at the surface. This will cause an outward and upward push of the near surface to build a ridge. Though, it is not sure that do water filled crack would necessarily perform the function of the model. [6,7]

5. Age and Stratigraphy of Europa

Geological mapping and analysis of the Europan surface has enabled to suggest a stratigraphically sequence for the evolution and stability of the physical feature. [5] Ridges plain material are considered as the oldest while the most chaotic terrains and dark material, are among the youngest material unit. The two sub divisions of chaos-knobby and platy reflect the difference in geographical formations and various properties of major material units.[5]All explanations for the formation of chaos terrain includes various processes like endogenic processes like diapiric intrusion, heat driven mantel, melting ice crust and extrusion of subsurface material i.e. cryovolcanic. Even there are several models proposed for the ridged plain formation like volcanism, tidal squeezing model, diaprism model, compression and wedging modes. [5,7]

According to the recent studies, if the assumed interpretations are correct, Europa would appear to change its geology and topography over a timeline from plain formation to mottled terrains formation [7]

If the average surface age is assumed as ~60Myr old, then it seems unlikely to change its fundamental features only during ~1% of satellite’s total age. [7]

There have evolutionary scenarios explored:

  1. Europa is in steady state and resurfaces in a patchy style.
  2. Europa is now at a very special time in history.
  3. The global resurfacing is episodic and sporadic.
  4. The Europan surface is in steady state old.

6. Conclusion

In this paper, 5 geological units, 6 surface features and their formations were studied on the basis of the high-resolution images and spectral data from Galileo spacecraft. The defined units revealed the morphological features from the images at both global and regional resolutions. Definitive analysis and structural identification from the current data and further exploration in Europa will bring a clear idea on it’s potential habitability and its evolution of extremely young surface and features in a given timeline. From the current data of Galileo its not possible to predict the current interior composition of the Europa. Hence, a further analysis is required for answering the major questions like:

  1. What is the composition of the ice layer on the Europa?
  2. What is interior composition of the Europa?
  3. How young is the Europan surface and its impact on the morphological processes?
  4. What is the current possibility of the habitable environment potential?

Improved research and exploration of the topography, geology and compositional analysis would provide a better understanding of the Europan surface.

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