It's Time for Europa!

After more than a year in the Jovian system, after an historic probe mission, and after close flybys of three major moons--Io, Ganymede, and Callisto--it is time for Europa.

December 19, 1996. The Galileo spacecraft prepares for E4, its first close flyby of the mysterious, ice-encrusted Europa. Images from earlier flybys, notably G1 and C3, have yielded increasingly tantalizing views of the frozen surface [see The Galileo Messenger, issues 40 (Oct. 1996) and 41 (Jan. 1997) and the Europa web sites at http: //www.jpl.nasa.gov/galileo/europa/ and http://www.jpl. nasa.gov/galileo/countdown/e4.html]. Now, at E4, the Orbiter will swoop to within 692 km (430 mi) of the ice, some 300 times closer than Voyager 2 and 50 times closer than Galileo at C3.

The Orbiter nears Europa from the inside of its orbit and from behind. It will overtake the moon and pass in front of its leading face before swinging toward the outer portions of a new orbit. Approach is over the morning side of the Europan globe, heading toward the terminator over long, west-pointing shadows. All the high-resolution pictures will be taken within 15 degrees of this dawn line (about 340 degrees west). This is important, because Europa is so flat that any enhancement of its minimal relief will help the science investigators back on Earth. The point of closest approach will be over the night side, just after both the Earth and Sun drop behind the limb in occultation.

At a range of over 60,000 km (39,000 mi) and more than 3 hours before closest approach, the Orbiter's solid-state imaging (SSI) camera captures an area centered about 5 degrees north, some 250-by-390 km (160-by-240 mi), showing features as small as 1.6-km (1.0-mi) across. The Sun, shining from low in the east, or right, reveals a mix of ridges, plateaus, and patches of smooth, low-lying, darker materials. The lack of prominent impact craters indicates a relatively young surface, geologically speaking. Many of the ridges are interrupted by gaps, and some of them appear to be partly buried or subdued by flows of viscous, glacier-like masses of ice.

In the news briefing 4 weeks later and 900,000,000 km away at NASA headquarters in Washington, DC, Ron Greeley of Arizona State (ASU) would explain, "This is the first time we've seen actual ice flows on any of the moons of Jupiter...These flows, as well as dark scarring on some of Europa's cracks and ridges, appear to be remnants of ice volcanoes or geysers." Rob Sullivan, also of ASU, would compare the slot-like trough in the upper center of the same field to a volcanic vent, and the flow pattern to the Snake River Plain in Idaho.

A Closer Look

At a range of about 12,000 km (7,400 mi) and 39 minutes before closest approach, the orbiter images an area about 50-by-150 km (30-by-90 mi), centered about 15 degrees south, showing features as small as 240-m (800-ft) across. The macula, or large circular feature in the upper left of the image, may be the 100-km-wide scar of a large meteorite impact, modified into a central zone of rugged topography surrounded by circular fractures that reflect adjustments to stress in the surrounding icy crust.

An alternate model suggests upwelling of warmer material from below, a bulging dome topped by random cracks, and, finally, a chaotic patch of rubble remains after ices--water, methane, and ammonia--are lost to space, much like dry ice evaporating on Earth, through sublimation erosion.

Ridges, clearly double, cross the plains in the right part of the image. Younger ridges overlap older ones in mind-numbing, "ball-of-string" complexity, revealing the sequence of formation. Gaps in ridges where new surface material has erased older features are obvious.

NIMS Imagery

About 27 minutes before closest approach, the near-infrared mapping spectrometer (NIMS) scans the trailing hemisphere to generate a high-resolution spectral map, showing minerals in the water-ice crust. A spectrum of several hundred wavelengths identifies the chemical composition at each pixel. The resolution (about 10 km per pixel) of the 0.7-µm NIMS image isolates the spectral signatures of different types of terrain, which consist of water frost and hydrated minerals in various proportions.

The Closest Yet

This composite of two images spans 17-by-26 km (11-by-17 mi) and, at some 3400 km (2100 mi) and 13 minutes before closest approach, shows features as small as 70-m (230-ft) across! This single mosaic really showcases the structural complexity of the Europan surface. The low morning sun, shining from the right, reveals complex overlapping ridges and fractures in the upper and lower portions of the mosaic. Rugged, more chaotic terrain spreads across the center in stark contrast to patches of smooth, craterless terrain, relatively unscarred by ridges. Offsets along the ridges (upper left) show lateral faulting. More missing ridge segments (center of the mosaic) indicate obliteration of pre-existing materials and emplacement of new terrain. Population densities of impact craters vary from one location to the next, suggesting surfaces of very different ages.

This image is a blowup of the top portion of the composite above and is centered near 6 degrees north. This is the highest-resolution image ever taken of Europa. The area shown is about 10-by-16 km (6-by-10 mi), and the smallest visible feature is about the size of a football field. In this view, the ice-rich surface has been broken into a complex pattern by crosscutting ridges and grooves, resulting from an icy tectonism. Sinuous, rille-like features and knobby terrain suggest surface modifications of unknown origins. Small craters, ranging in size from less than 100-m (330-ft) to about 400-m (1300-ft) across, are clearly visible.

Closest Approach

Answers

Has Galileo solved any of the big mysteries of Europa? Not quite, though we do know much more now about its surface than ever before. Our closer images give much better crater counts and a real appreciation of the geological youth of the surface (but no clear sense of just how young). We have a better appreciation for the thinness of a crust that may still overlie an active, plastic subsurface. Evidence is everywhere: doublet ridges show crustal spreading; glacier-like tongues of ice flow over and through the ridges; fault lines divide the surface into what resemble floes of floating ice; and possible vents for icy eruptions and rubbly fields of sublimated ice dot the landscape. The Europan surface is unlike any in the Jovian system--unlike any in the solar system. It's clearly different from the ancient, corrugated crust of Ganymede, clearly different from any Earthly terrain--sea ice, glacial ice, volcanic rock, or continental crust.

And life? All the pieces are probably there: abundant water; organics, which are prevalent in the solar system; and a source of subsurface heat. The E4 encounter is just the first of several close, planned flybys: E6 in February (closer still), E11 in November, and then, several more in an extended mission that will continue to probe Europa's icy enigmas. It's time for Europa!

--Larry Palkovic

[During the E6 encounter, Galileo sped past Europa at an altitude of 586 km (364 mi). We await the arrival of images from that event.] --Ed.

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