S

Fig. 6.21. This Galileo spacecraft view of chaotic terrain on Europa shows an area where the icy surface has been broken into many separate plates that have moved laterally and rotated. The plates are surrounded by a topographically lower matrix that may have been emplaced by water, slush, or warm flowing ice, which rose up from below the surface. One of the plates is seen as a flat, lineated area in the upper portion of the image. (Credit: NASA/JPL/Caltech)

Fig. 6.21. This Galileo spacecraft view of chaotic terrain on Europa shows an area where the icy surface has been broken into many separate plates that have moved laterally and rotated. The plates are surrounded by a topographically lower matrix that may have been emplaced by water, slush, or warm flowing ice, which rose up from below the surface. One of the plates is seen as a flat, lineated area in the upper portion of the image. (Credit: NASA/JPL/Caltech)

chaos in recognizable form. Chaos may also have formed concurrently with ridge formation, as a long-term process [379].

Cryovolcanism is the eruption of liquid or vapor phases of water or other volatiles that would be frozen solid at the normal temperatures of an icy satellite's surface. Cryovolcanism has apparently resurfaced broad areas of Europa by disruption, displacement, and reworking of crustal ice [380] (Fig. 6.22).

There are craters on Europa's surface, but very few of them. The rarity of large impact craters suggests a geologically young surface, perhaps due to widespread resurfacing processes already alluded to [381]. The craters seem to fall into two categories. One type, referred to as 'palimpsests' are 100 km across with concentric fractures and low surface relief, as though the impact was later filled in. This 'filling in' leaves a very shallow crater or one that is almost not there. The second class consists of about a dozen craters with diameters of ~25 km (Fig. 6.23). The ice was £ probably thin at the time of impact. One crater in particular, Pwyll Crater, displays

£ fantastically bright ejecta rays. These bright rays are fresh fine water-ice particles c that were ejected and painted streaks for thousands of kilometers.

O ^ The bright rays indicate a very young crater. The lack of craters overall suggests

•> V that Europa's surface is relatively young [382]. According to Moore et al., Europa's

,0 craters appear to be anomalously shallow compared to similarly sized craters on q © other solid-surface bodies, which may be due to post-impact isostatic adjustment

O [383]. Thus, if there is a subsurface liquid water layer on Europa, the overlying ice must be thick enough that ~3-6 km deep craters do not penetrate completely

[384]. Ruiz believes that analysis of size and depth of the largest impact structures suggests that these features were formed in an icy shell at least ~19-25 km thick

[385]. Even the largest impact feature, Tyre, transported material to the surface from

Fig. 6.22. A wide-field image of a portion of Europa's icy surface, revealing ridges, plateaus, and patches of smooth, low-lying darker materials. Note the absence of craters, indicating this region is composed of a young surface material, suggesting that cryovolcanism has resurfaced the region. (Credit: NASA/JPL/Arizona State University)

Fig. 6.22. A wide-field image of a portion of Europa's icy surface, revealing ridges, plateaus, and patches of smooth, low-lying darker materials. Note the absence of craters, indicating this region is composed of a young surface material, suggesting that cryovolcanism has resurfaced the region. (Credit: NASA/JPL/Arizona State University)

Fig. 6.23. This enhanced color image from the Galileo spacecraft is of a young impact crater named Pwyll, a feature that is about 26 km in diameter. Pwyll is thought to be one of the youngest features on Europa. The bright white ejecta rays extend in all directions for over a thousand kilometers from the impact site. (Credit: NASA/JPL/University of Arizona/PIRL)

Fig. 6.23. This enhanced color image from the Galileo spacecraft is of a young impact crater named Pwyll, a feature that is about 26 km in diameter. Pwyll is thought to be one of the youngest features on Europa. The bright white ejecta rays extend in all directions for over a thousand kilometers from the impact site. (Credit: NASA/JPL/University of Arizona/PIRL)

a depth not greater than ~4 km deep. And, Pwyll and Manannan transported material from a depth not greater than ~2 km [386]. Many crater floors lie at the same general elevation as the surrounding terrain beyond the continuous ejecta blanket, as opposed to being lower than surrounding terrain due to the excavation of the impact. This is true for craters Cilix and Manannan and may be a result of fluid fill in or isostatic adjustment following the impact [387]. Europa's larger craters, Cilix, Maeve, and Pwyll appear to exhibit central peaks, or central peak complexes. Cilix displays an elongated central peak complex surrounded by a flat crater floor, terrace walls, a circular rim, and reddish brown continuous ejecta blanket. The central peak complex is composed of two prominent massifs. Both massifs exhibit ~300 m relief, and is located in the center of the crater floor. The crater floor reveals only a few tens of meters relief and is mottled by a number of sub-kilometer reddish-brown patches [388]. Unlike the Moon or Mars, the central peaks of Europa's largest craters rise well above the surrounding crater rim. In the case of Pwyll, the central peak rises ~800 m above the crater floor, or ~300 m above the average rim height. By contrast, central peaks rising above rims are rare for the Moon and Mars [389].

Shoemaker estimated Europa's crater retention age as 30 Myr for craters less than 10 km, and found that the cratering record is consistent with a 10-km-thick ice crust overlying a liquid ocean [390]. There are ~150 impact craters less than 1 km across and these craters show no evidence of degradation by tectonic activity, again suggesting a decline in geologic processes on Europa, and that impact cratering was one of the last geologic processes to have occurred [391] (Fig. 6.24).

Impact into ice does not produce exactly the same effect as an impact into a silicate solid. Consequently, most craters on Europa are dissimilar to craters seen on Earth's Moon. Likewise, impact into solid ice produces an effect different from an impact into a low-viscosity material. Impact simulations suggest that the Europan features Callinish and Tyre would not be produced by impact into a solid ice target, but might be explained by impact into an ice layer of order of 10 km thick overlying a low-viscosity material [392]. It is believed that Tyre is a relatively £ recent cratering event [393].

£ It is important to understand the order in which resurfacing events have c occurred on Europa. It is believed that tectonic resurfacing dominates the early

O ^ formation of background plains by the intricate superposition of lineaments, the

•> V opening of wide bands with infilling of inter-plate gaps, and the buildup of ridges

,0 and ridge complexes along prominent fractures in the ice. Due to the lack of craters q © being overprinted by lineaments, it is thought that tectonic resurfacing decreased

O rapidly after ridged plains formation (Fig. 6.25). Later, the degree of cryogenic resurfacing increased with time. The transition from tectonic to cryogenic dominated resurfacing may be attributed to the gradual thickening of Europa's cryosphere. The evolution from a brittle ice shell to a thicker one would cause a decrease in fracturing or the melt through of sub-surface material due to tidal and endogenic processes; fracturing and plate displacements decreasing with time as the shell thickened [394].

Fig. 6.24. This Galileo spacecraft image shows the Conamara Chaos region, revealing craters that range in size from 30 m to over 450 m. The large number of craters seen here is unusual for Europa. This section of Conamara Chaos lies inside a bright ray of material that was ejected from the large impact crater Pwyll. The presence of craters within the bright ray suggests that many are secondaries which formed from the impact of chunks of material that were thrown out by the enormous energy of the impact which formed Pwyll. (Credit: NASA/JPL/Caltech)

Fig. 6.24. This Galileo spacecraft image shows the Conamara Chaos region, revealing craters that range in size from 30 m to over 450 m. The large number of craters seen here is unusual for Europa. This section of Conamara Chaos lies inside a bright ray of material that was ejected from the large impact crater Pwyll. The presence of craters within the bright ray suggests that many are secondaries which formed from the impact of chunks of material that were thrown out by the enormous energy of the impact which formed Pwyll. (Credit: NASA/JPL/Caltech)

eye"pattern appears to be a 140-km wide impact scar that formed as the surface fractured minutes after a mountain-sized asteroid or comet slammed into the moon. This color composite represents a series of events. The earliest event was the impact that formed the Tyre structure. The impact was followed by the formation of the reddish lines superposed on Trye, the red color indicating areas that are probably a dirty water ice mixture. The fine blue-green lines are ridges that formed after the crater from west to east. (Credit: NASA/JPL/University of Arizona/PIRL)

eye"pattern appears to be a 140-km wide impact scar that formed as the surface fractured minutes after a mountain-sized asteroid or comet slammed into the moon. This color composite represents a series of events. The earliest event was the impact that formed the Tyre structure. The impact was followed by the formation of the reddish lines superposed on Trye, the red color indicating areas that are probably a dirty water ice mixture. The fine blue-green lines are ridges that formed after the crater from west to east. (Credit: NASA/JPL/University of Arizona/PIRL)

The resurfacing of Europa did not occur as discreet events, but rather more likely as a continuous process, or stages. The first stage involved the sequence of formation and deformation of the background plains, a complex process. Tectonic processes were predominant at this stage, far outweighing cryovolcanism or impact cratering. The early second stage was probably entirely dominated by tectonic resurfacing. Tectonic processes also dominated the late second stage, when several sets of lineaments with very consistent orientation developed. During the third stage, resurfacing was primarily cryovolcanic and involved the formation of chaos and subdued pitted plains. The fourth resurfacing stage on the trailing hemisphere of Europa involved the development of the last sets of lineaments and the cryovol-canic formation of some chaos at middle southern latitudes. Lineaments include ridges and ridge complexes. On the trailing hemisphere the final events include the opening of linear and slightly cuspate regional fractures, and the formation of impact craters and associated ejecta deposits. On the leading hemisphere, the final stage involves the development of cuspate smooth bands and double ridges at high latitudes [395].

Even though the resurfacing process changed from tectonic-dominated processes to cryovolcanic-dominated processes, the transition appears to have been gradual, with both processes coexisting at most times but in varying degrees [396]. Kargel et al., believe that all of these features, their relationships, and their ages indicate that the crust of Europa has been completely resurfaced in the recent geological history [397].

Previous modeling supports the hypothesis that Europa has a massive metallic core in which a magnetic field may be generated [398]. Europa's core radii is estimated to be 426-510 km for an iron (Fe) core and 610-706 km if composed of iron and iron sulfide (Fe - FeS) [399] (Fig. 6.26). Thus, core size is between 10 and 45% of Europa's radius and contains up to 15% of the moon's mass [400].

Based upon Galileo data, modeling suggests that Europa is differentiated with a metallic core rich in iron (Fe), underlying a dehydrated silicate rock mantle and an ice - liquid shell ~120-140 km thick. The thickness of the water ice shell is uncertain and may range from less than 100-200 km. This water ice shell contains ~10% of the moon's total mass. Europa's weak magnetic field may be explained by either the existence of a metallic core or a liquid water ocean (beneath a thin solid shell) containing some electrolyte, or a combination of both [401]. There is indirect geological and geophysical evidence that Europa may posses a subsurface salty £ liquid water ocean [402]. Voyager and Galileo imaging and infrared spectroscopy

£ show that Europa's surface is covered with water ice [403]. There is geological c evidence that the water ice shell is decoupled from Europa's deep interior due to

O ^ the existence of a subsurface liquid water ocean or at least a soft ice layer. The

•> V most convincing argument for a subsurface ocean results form an interpretation

J5 IjS of Galileo magnetometer data that requires an electrically conducting layer at q © a shallow depth in which a magnetic field is induced as Europa moves through

.C O the magnetosphere of Jupiter [404]. The observed magnetic field perturbations

Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment