Space exploration has been filled with its share of frustrations, the most obvious being the lack of follow-up with travel to the Moon following Apollo 17. That’s been a 50-year gap and counting, but a gap of half that size is also unsettling. It was in late 1995 that the Galileo probe began orbital operations at Jupiter, and since then we’ve had to rely on its imagery of Europa when we needed close up views of the ocean-filled moon. While we await Europa Clipper, scheduled for 2024 launch, and Jupiter Icy Moon Explorer (JUICE), slated by ESA for a departure in 2022, we’re still refining Galileo images in preparation for future flybys.
One thing the newly touched up images should remind us of is that Europa Clipper is going to give us views of much larger parts of Europa’s surface at high resolution, complementing but considerably extending what Galileo was able to do. The latter was a mission with its own set of frustrations, of course, as a recollection of its unusable high gain antenna makes clear, but the superb work by ground controllers in recovering these Europa images is an object lesson in getting the most out of the equipment you’ve got left. Let’s hope Europa Clipper runs into no comparable difficulties.
Image: This image shows a transitional location between blocky chaos terrain, on the left, and ridged plains on the right. A few chaos blocks are visible on the left as individually broken and rotated pieces of preexisting surface material; their shadows indicate that some of these blocks have tilted as well. A ridge passes through the center of this image. These ridges, which contain arc-shaped segments joined together by a series of cusps, may be related to how the icy surface crust of Europa fractures when subjected to stresses from Jupiter’s strong gravity. The right side of this image shows a few lenticulae, which are small rounded surface features, commonly domed in appearance. The image resolution is 247 yards (226 meters per pixel, and this image depicts an area about 180 miles (300 kilometers) across. Credit: Mario Valenti / SETI Institute / NASA/JPL-Caltech.
The three images here were captured on the eighth of Galileo’s targeted flybys, showing features as small as 460 meters in size. They were taken through a clear filter in grayscale, with lower-resolution images from the same region on a different flyby used to map color onto the grayscale, producing, at a cost of considerable time and processing power, what we see here. These are enhanced-color images, thus exaggerating color variations to bring out the chemical composition of the surface. The light blue or white areas are predominantly water ice, while reddish areas are laden with other materials, like salts.
Image: This image shows a region of Europa’s surface covered with ridges and bands, with a few small disrupted chaos regions. Ridges, a common surface feature type, may form when a crack in the surface opens and closes repeatedly, building up a feature that’s typically a few hundred yards tall, a few miles wide and that can stretch horizontally for thousands of miles. In contrast, bands are locations where a crack appears to have continued pulling apart horizontally, producing large, wide, relatively flat features. This image shows both ridges and bands, which interact with each other in complex ways that are somewhat similar to tectonic activity on the Earth. Credit: NASA/JPL-Caltech/SETI Institute.
All three images were captured in the spacecraft’s 17th orbit of Jupiter (E17), with the lower-resolution color images used for mapping taken in orbit E14. The striking thing about the Europan surface has always been its relative youth, some 40 to 90 million years old, meaning what we see is much younger than the moon itself, which would have formed 4.6 billion years ago. This is one of the youngest surfaces in the Solar System, a place of long linear ridges and bands that indicate crustal stretching under the influence of Jupiter’s strong gravity.
Image: A region of blocky chaos terrain, where the surface has broken apart into many smaller chaos blocks that are surrounded by featureless matrix material. Many of the chaos blocks have moved sideways, rotated, or tilted before being refrozen into their new locations, and some larger blocks preserve features of the pre-existing terrain before it was broken up. Using these features as clues, scientists have been able to reconstruct some chaos regions like jjgsaw puzzles to track the motion of blocks. Cutting through the chaos terrain near the bottom, from left to right, is a broad flat band. Called Agenor Linea, it is one of the longest bands on Europa and is distinctive for its two-color appearance, with a bright region at the top and a darker region below. Another rare bright band, Katreus Linea, cuts across the top portion of this image. The image resolution is 243 yards (222 meters) per pixel, and this image depicts an area about 170 miles (280 kilometers) across. Credit: NASA/JPL-Caltech/SETI Institute.
Europa Clipper will conduct numerous flybys of Europa as we try to learn more about the ocean now believed to exist beneath the icy crust, and its interactions with the surface. The disrupted areas known as ‘chaos terrain’ are particularly interesting, as they show blocks of surface material that have been moved through gravitational stresses and then refrozen into a new location. The jigsaw puzzle analogy JPL uses to describe them is exactly on target.
Image: The locations on Europa depicted in the newly processed images, with Chaos Transition at top. This image is centered approximately at 6.4 degrees north latitude, and 135.3 degrees east positive longitude. Credit: NASA/JPL-Caltech/SETI Institute.
Original plans for a Europa orbiter were scrapped at least partly due to concerns over the radiation levels produced by Jupiter’s magnetosphere. Instead, we’ll get over 40 close flybys as Europa Clipper makes its way around the giant planet in an elliptical orbit. The JUICE mission will conduct two Europa flybys of its own, while adding multiple flybys of Callisto before settling into orbit around Ganymede. By the end of this decade, then, we should start updating these spectacular images with high-resolution views of entirely new terrain, and the long wait for a return to Europa will finally end.