Introduction

Europa is a world that is both familiar and utterly alien. Caught in a gravitational tug-of-war millions of years old between Jupiter and its neighboring satellites, Europa's interior is constantly being churned and heated. This inner turmoil has manifested itself in a surface which is a jumbled mosaic of ice - an activity which may well continue to this day.. The pictures released today show this amazing world in unprecedented detail: plate tectonics, ice flows, and icy volcanoes are clearly visible.

As is often the case with a sudden barrage of information from the outer solar system, the pictures pose far more questions than they answer. In reminiscing about the last batch of data from Europa via the Voyager probes in the late 70's Terrence Johnson, from JPL summarized it this way: "After waiting 17 years we're still saying 'What the heck is that' ". James Head, from Brown University, put a more hip spin on it: "Europa is really weird. It makes the 'X-Files' look like a predictable soap opera".

Four main thrusts were presented as being the drivers behind the investigation of Europa: its interior configuration, whether or not Europa is currently active, the nature and origin of Europa's surface features, and the potential for Exobiology (life) on Europa.

The Pictures

Some of the pictures released have a resolution of 35 meters per pixel and were taken at distances as close as 692 kilometers above Europa on 19 December 1996. Many of these pictures show greater surface detail than has ever been received for any jovian satellite. On the highest resolution images, the smallest structure visible was on the order of a football stadium. And more is still to come. The next encounter with Europa will occur in February when Galileo will get even closer to Europa.

Much of the imagery shown at this press conference had only been "on the ground" for a few days leaving little time for scientists to do detailed analysis. Only one third of the images from this encounter have been received - and none are currently being transmitted. This is due to the fact that Jupiter and Earth are currently exactly opposite one another - with the sun in between - thus blocking all radio transmissions. Transmissions are set to resume on 27 January when Jupiter emerges from behind the sun.

A Complex Surface

The source for all of these surface features is presumed to come from heat generated by the gravitational deformation of Europa itself. This has yet to be proven conclusively. Indeed, in describing the surface of Europa, the participants were cautious about making absolute statements as to what they thought was going on.

Three major types of surface deformation seem to have occurred on Europa: lateral plate spreading, volcanism, and localized obliteration. Three types of surface activity were also described: icy volcanism, thermal disruption, and crustal spreading.

While there are some similarities between Europa and its neighbor Ganymede there are also some significant differences. The crust on Ganymede has been stretched and has broken up. It is also littered with rather old features. Europa, on the other hand, exhibits plates which seem to float on a liquid substrate, perhaps water. The surface is also rather lacking in older features.

Sublimation Erosion:

A process described as 'sublimation erosion' seems to be at work wherein the surface is lifted from below, possibly by thermal disruption. As the surface is lifted, cracks begin to form. This exposes different layers of the surface to space and a differential erosion of water, Methane, and Ammonia-containing materials occurs. The result is a jumbled chaotic terrain somewhat reminiscent of that found on Mars.

Plate Tectonics

Even a cursory examination of these images shows features that clearly resemble plate tectonics on Earth. The operative word here though is "resemble". In analyzing the surface patters on Europa, scientists used an approach similar to that used in understanding plate tectonics on earth- the fit of South America and Africa being the most vivid example. When pieced together, various fragments on Europa could clearly be assembled into larger structures. The plates seem to retain their shape as they move about and seem to exhibit the same rift-like features that appear on earth.

The presenters also readily admit that the similarities may be just superficial and that the plates on Europa may well float and that their movement may be driven by processes other than mantel convection which is the case for terrestrial plate tectonics.

Ridges and Ice Flows

All of the photos show a surface covered with intersecting and parallel ridges, in patterns similar to those seen on Ganymede. These pictures also revealed the first ice flows yet discovered on any of Jupiter's icy moons. These flows can be 100 meters or more in thickness and can be hundreds of kilometers long. A number of instances were shown where these flows seem to have spread over and obliterated some of the ubiquitous ridges. Similarities were noted between these flows and the contiental scale glaciers on Earth.

Maculae

In targeting the cameras for this flyby, a large dark feature or "macula", first seen by Voyager, was chosen. Upon close study it can be seen that this structure has obliterated whatever surface features existed before its formation. There is no clear consensus as to what his feature is. One group of scientists thinks that it is the remnants of large impact feature. Others think that it may be volcanic in origin and bear some similarity to features seen on Venus.

Is there an Ocean on Europa?

"Nothing yet received from Europa tells us Yes or No" was the answer. There is some evidence in the subsurface activity, such as plate tectonics, that leads to the possibility that there is liquid water present. Europa's density is pretty much that of a world made of solid rock. As such, the thickness of ice on its surface can range from several hundred kilometers to perhaps only a few kilometers. There may be some regional melting occurring that would allow bodies of water to exist. This is all speculation and will remain as such until instruments such as gravitometers are in orbit about Europa which would allow us to peak though the ice.

Is Europa Active Today?

Further studies will need to be done of the images before this assessment can be made. Part of this process will include crater counts to get a measure of the age of Europa's surface. If a number of ice flows were found to be devoid of craters this would argue towards a young surface. Also to be performed is a comparison between Voyager imagery in the late 70's and that obtained from Galileo to see if there have been any changes in the interim - much as has been done to calibrate changes on Io.

Possibility of Life on Europa

Europa was described as being a "high priority target" in the search for life. Reference was made several times to the ability for life to thrive in terrestrial environments such as deep ocean trenches and hot springs where none had ever been expected.

Three factors were presented as being required in order for life to be possible on Europa . All of these factors have a probability of existing on Europa. As such, interest in Europa as a possible abode of life continues to grow.

1. The presence of water, preferably liquid. It has been known for quite some time that there is abundant water ice on Europa mixed perhaps with Ammonia and Methane with some Sulfur likely to have originated from nearby Io. While no conclusive evidence for bodies of liquid water has been obtained, it is clear at some point that the ice on Europa has melted such that the surfaces features we now see could form.

2. The presence of organic chemicals. Given the bombardment history of the other jovian satellites, as well as that in other portions of the solar system, it is likely that cometary bodies have deposited considerable organic material upon Europa during its history.

3. An available heat source to support any possible biomass. It is obvious from surface structures on Europa that some heat source has been available to allow the surface ice to become mobile enough to move. Internal heat sources are likely to exist in Europa - as they are in Io as the result of gravitational interactions derived from various orbital resonances between Jupiter and the other bodies in the jovian system.

The presenters were all cautious about jumping to conclusions when a feature seemed to have an analog on another planet. In examining all of the worlds in our solar system, James Head put it this way: "this is comparative planetology. This is not a herd but a zoo. The worlds are so different". None the less, during the course of this briefing, similarities were noted between Europa and Venus (maculae), Ganymede (ridges), Earth (Plate tectonics and glaciers) Mars (chaotic terrain) and Io (interior structure).

Future Exploration of Europa

When asked about possible uses of the Galileo spacecraft after its baseline mission has been completed, Torrence Johnson (JPL) stated that the spacecraft remains healthy and that ample fuel reserves will remain. He said that studies have been drafted and received favorable reviews both by advisory committees and by NASA Headquarters. Such a mission might well focus on Europa with some additional studies of Io.