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           Jim Klemaszewski - Water on Europa


The planet Jupiter is over half a billion kilometers (384,000 miles) from Earth. This large planet, which is eleven earth-diameters across, is orbited by 16 moons- a miniature "solar system" in its own right. Four of Jupiter's natural satellites, large enough to be seen with telescopes or binoculars from Earth, were first observed by Galileo in 1610. The Galilean satellites, as they are now called, each have a unique geologic history.

[NOTE: The link beneath each image will take you to the Planetary Photojournal page where you can download the image, and which also contains the original caption.] 

Io, the rocky moon closest to Jupiter, is the most volcanically active body in the solar system

Europa, about the same size as Io and the Earth's Moon, has an outer shell composed of water ice that has been modified by geologic activity for most if not all of its history

The faulted and fractured surface of Ganymede, the icy Galilean satellite larger than the
planet Mercury, records an early period of tectonic activity

Although Callisto is nearly the size of Ganymede, its surface does not reveal evidence of
significant tectonic or icy volcanic activity indicating that these two large moons evolved
down separate paths.

The surfaces of the icy satellites Callisto, Ganymede and Europa display amounts of geologic
activity that decrease with distance from Jupiter. This decrease in activity reflects the amount of internal heat within each body. Although the icy shell of Callisto was thought to be completely frozen based on its heavily cratered surface, a thin salty subsurface ocean is now thought to lie within 150 kilometers (90 miles) of the surface. A subsurface ocean may also be present beneath Ganymede's fractured surface, although evidence of its presence is masked by the strong magnetic field generated by Ganymede's still-molten core. Geologic
features seen in images of Europa's youthful surface obtained by the Voyager spacecraft twenty years ago were speculated to have formed in an ice shell that covered a global ocean. One of the primary objectives of the Galileo mission was to determine if an ocean was present in Europa's past, and if so, whether or not it is still there today. 

The surface of Europa can be divided into two basic types of terrain: mottled terrain and icy plains. Europa's icy plains are criss-crossed by dark linear features, some of which exceed 1000 kilometers (620 miles) in length, and dark curvilinear bands. These bands and ridges are disrupted and occur less frequently in mottled terrain. The observation that very few impact craters are seen on Europa, coupled with the relatively high average brightness of Europa indicates that the surface is relatively young (geologically speaking). These
features can be seen in the first Galileo images of Europa, obtained in June of 1996.

These dark bands are younger than the surrounding icy plains, and formed when the icy surface fractured with subsequent rotation of the icy plates over a subsurface that was able to flow. However, it cannot be determined from these images whether that movement occurred rapidly over a subsurface that was liquid water or slowly over warm ice that deformed and flowed over hundreds to thousands (or millions) of years. One of these wedge-shaped bands was imaged at higher resolution on the third orbit. 

The trajectory of Galileo's second orbit of Jupiter provided global-resolution of Europa's trailing hemisphere (centered at longitude 270) which was previously imaged by Voyager at very low resolution.

This global view of Europa (left = true color; right = false color) shows that mottled terrain has a geographic distribution that is broadly equatorial. The equatorial presence of mottled terrain, and its absence near the poles probably indicates that the ice at Europa's poles is relatively thicker and therefore more resistant to mottled terrain formation. This global image was also used to refine imaging targets on the fourth and sixth orbits. On the fourth orbit the dark circular spot on the lower left side of Europa was imaged, as well as
features in the dark, mottled terrain. The irregularly shaped dark spot below the prominent dark "X" formed by two intersecting bands (northeast of center), the bright impact crater near the bottom of the disk, and some bright icy plains were imaged on the sixth orbit. [A question/exercise for students: What feature(s) would you take a picture of, and why (i.e., what question(s) do you want to try to answer)? 

Imaging on the fourth orbit targeted mottled terrain at moderate and high resolutions. The moderate resolution image shows features that resemble volcanic ice flows. If that is indeed what they are, this would be good evidence for "geothermal" heat within Europa. However, the interpretation of these features remains is ambiguous.

The surface of Europa at high resolution is seen to consist of numerous criss-crossing ridges that range in nature from simple and double ridges to complex parallel ridges. In some locations these ridges are disrupted by subsequent geologic activity

One feature that was met with enthusiasm is a smooth circular area about 10 km (6 miles)

Although this feature may represent the extrusion (eruption) of water or slush onto the surface, it may also have originated from localized heating of near-surface materials. In either event, this feature is a surficial expression of internal, near-surface heat. An additional observation on the fourth orbit included a dark circular feature. This feature is an impact crater that is interpreted to have penetrated Europa's icy shell.

The sixth orbit provided the strongest evidence for a subsurface ocean. The dark area
beneath the intersecting bands is a region where Europa's icy crust was disrupted from

The disruptive activity broke the crust into blocks with a range of sizes, some up to ~6
miles across, which then "floated" around until the surface refroze. 

This terrain on Europa produced by subsurface heating and disruption is referred to as "chaos terrain", because of its resemblance to chaos terrain on Mars where subsurface water appears to have erupted onto the surface at the head of many outflow channels. Chaos terrain has now been identified in many areas on Europa, some of which are nearly 100,000 square kilometers in extent, indicating widespread heating and disruption of Europa's crust. 
Bright plains were also imaged to determine the types of geologic features of which they are composed. 

This image shows that the plains, which appeared "smooth" at lower resolutions, consist of numerous criss-crossing ridges. Scientists (and students) can use the principle of cross-cutting relationships to determine the relative ages of the ridges: The youngest ridges are on top and cross-cut the older ridges below. This principle can be used to determine if the orientation of ridges on Europa has changed with time. (Question: how has the size of ridges changed with time in this region?) 

The impact crater Pwyll

was imaged on the 6th orbit, and then again on the 12th orbit. The data from these orbits
were combined by our colleagues at the German Aerospace Center to produce a digital
elevation model of Pwyll's topography.

The principle behind this is based on how we (people) see depth -- we have two eyes that
view objects from slightly different angles. Our brain is able to combine the two images
that it receives from our eyes into one 3-D image. Similarly, this 3-D image of Pwyll

or any 3-D image, is produced by taking two pictures of the same feature from two different
angles. So what does this 3-D image of Pwyll tell us? It tells us that Pwyll is very flat
for an impact crater. Impact craters are circular pits in the ground that form when a
fast-moving object slams into the surface. Because Pwyll does not have the typical "bowl"
shape, we believe that the ice which makes up the crust has been warmed from below, allowing
the floor of the crater to relax and rebound upwardly. 

Although we cannot see beneath Europa's ice at present, we can look at features on the
surface to help us understand what may present below the surface. The features observed on
Europa during Galileo's 24 orbits of Jupiter: chaos, ridges, relaxed impact craters, and

all point to a consistent story that Europa most likely had a subsurface ocean at the time these features formed. The big question is whether or not that ocean is still present today.

If it is, there could be up to twice as much liquid water on Europa compared to Earth's oceans. That's a lot of water. And with all that water, scientists are wondering if life might be present in Europa's ocean. How are we going to find out? NASA is planning on sending another spacecraft to go into orbit around Europa. This spacecraft will take pictures of the entire surface, but will also use radar to try to penetrate the ice shell to determine its thickness and whether or not an ocean is there today. If that mission is successful, then a lander will be sent to Europa that will have the ability to melt through the ice and launch a small robotic submersible into this alien ocean. And some of today's students -- not just those who excel in math and science, but also artists, writers, and business majors -- will be the explorers of tomorrow.

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