Jupiter has four moons that could qualify as planets if they orbited the Sun. All four have unique characteristics that set them apart from each other. One moon appeared quite different from the rest when Voyagers 1&2 flew past in 1979 because it looked as bland and smooth as a cue ball and lacked craters. Smooth Europa appeared very bright and was covered with cracks making it appear like a cracked egg. Back then scientists were suspicious that Europa was something different from the rest of the moons seen so far as it even lacked mountains.
Io is the neighboring moon, closer to Jupiter, and found to have huge, powerfully active volcanoes spewing hot sulfurous lava and several plumes of sulfur dioxide ice towering hundreds of miles into space. Powerful tidal action between Jupiter and the other large moons, Europa, Ganymede, and Callisto, flex Io like putty heating its interior to an explosive inferno. Io is by far the most active world in the Solar System. It was suspected that Jupiter could also raise tides on Europa strong enough to heat its interior and create geysers of water ice. It could be warm enough for Europa to have liquid water not too far beneath its icy crust. In spite of intense analysis of the Voyager images no plumes could be seen erupting from Europa’s cracks. Neither Voyager came close enough to Europa to detect any evidence of geysers. A dedicated orbiter around Jupiter was needed that could make several close flybys of Europa; the Galileo mission was born. Galileo made its dramatic arrival at Jupiter on December 7, 1995 going into orbit and deploying a probe into its atmosphere. Galileo performed about a dozen flybys of Europa during its nearly 8-year orbit of Jupiter before its fiery plunge into Jupiter on September 21, 2003. It detected an induced magnetic field at Europa, which means that a conducting fluid exists at shallow depths within Europa’s interior. Analysis of the surface composition and internal structure of Europa make convincing evidence that a global ocean of liquid water exists under Europa’s icy crust. There may be twice as much water within Europa as there is here on Earth. Data from Galileo indicates that the crust may be thin enough where tidal stresses create the cracks and water may ooze out. Without an atmosphere the water cannot remain a liquid and would rapidly freeze in the -200ºF cold. Minerals and salts may also be dredged up from deep within Europa and deposited on the surface along the cracks staining it reddish.
Europa has one of the smoothest surfaces known anywhere with hills no more than 300 feet high. Even more exciting is that the Galileo data reveals that Europa’s rotation rate is complex because the ice shell is being tugged by Jupiter, adding to the evidence that there is a global ocean beneath the crust and that it may be thin, maybe no more than a few miles thick. There are patchy, chaotic surface areas that are disrupted very much like Antarctica, which indicates the possibility of subsurface lakes. These are the reddish regions that could be havens for life. Galileo was hindered by a faulty main antenna that failed to fully deploy, so the amount of data it could gather was limited. Global imagery is not at sufficient resolution to map Europa in fine detail and topographic data is sparse and crude. This limits what is known about the composition of the surface and evidence for an ocean is indirect. The presence of organic molecules is suspected but cannot be confirmed.
Europa was no longer unique as a possible water world besides Earth as of January 2005 when Cassini discovered active geysers at the south polar region of Saturn’s moon Enceladus. Tidal stresses also crack Enceladus and open and close fissures called Tiger Stripes where the geysers erupt. The geysers pump enough water ice into space that it creates a tenuous ring around Saturn. The amount of data being collected at Enceladus by Cassini has proved how little data there is of Europa in spite of Galileo’s efforts. Technology is better with the more modern Cassini, which is a healthy, fully-functioning spacecraft. Europa is creating a strong desire to return but missions to the outer Solar System are expensive. Scientists are working hard on designing a mission to Europa in an era of budget cuts. There have been many plans including placing an orbiter around Europa. Orbiters can take high resolution images of the surface at varying wavelengths, which is vital for future landers. The problem is the intense radiation at Europa is so powerful it can quickly render unshielded electronics useless. Spacecraft operating constantly at Io and Europa would be constantly bathed in Jupiter’s radiation. Hefty shielding is required, which makes spacecraft heavy and expensive. This has scrapped several plans for a Europa orbiter keeping any dreams of directly exploring the subsurface ocean expressly forbidden. The best method for exploring Europa has already been proven at Saturn with Cassini flying repeatedly past Enceladus, thus dodging most of the worst radiation there yet obtaining excellent science. A spacecraft in orbit around Jupiter could repeatedly fly close to Europa and only briefly be exposed to the intense radiation. A new, relatively low-cost mission is being designed to explore Europa; the Europa Multiple-Flyby Mission is born.
The Europa Multiple-Flyby Mission, formerly known as Europa Clipper, is a sophisticated spacecraft that will orbit Jupiter at least 45 times and each time will fly past Europa, sometimes getting as close as 16 miles from the surface. The spacecraft will image strips of Europa’s icy surface with a stereo camera that should give a 3-D view better with than 150 feet resolution. High resolution imagery as good as a few feet should be sufficient to see large boulders, and aid in planning future lander missions. Spectra of the surface in infrared light will look for traces of salts and organic compounds on the surface erupted from the ocean below. The spacecraft will be able to detect molecules being knocked off the surface by radiation, but better yet, it will be possible for it to fly close to or through the newly discovered geysers. The risk of dangerous collisions with geyser particles is low as Cassini has already proven it could safely fly through the geysers of Enceladus. It will be able to gather information on the composition of the geyser plumes and any tenuous atmosphere. Ice-penetrating radar is planned to be the most important instrument aboard the spacecraft. Ice is transparent to certain radar waves but liquid water is not. It would ping the icy surface with radar and be able to determine how thick the ice is and the nature and extent of the ocean beneath. The spacecraft will also be able to measure the gravity and tides of Europa along with the magnetic field, which would further determine the size and depth of the ocean, and its salinity.
The Europa Multiple-Flyby Mission is estimated to cost about $2 billion, much less than the approximately $5 billion for an orbiter. It has been given formal approval by Congress, known as a “new start”, meaning development can finally begin. It may be launched as soon as 2022 or 2023 and is expected to use the new Space Launch System, a powerful new rocket that should cut the travel time to Jupiter from 6 to 3 years than if launched using a conventional Atlas V rocket. Europa is among the most geologically dynamic worlds in the Solar System. The possibility of a global ocean completely wrapping Europa and being warmed by the heat generated by tides and the decay of radioactive material makes it one of the best places to look for life. Life thrives in unexpected places on Earth. Europa is truly an astrobiologist’s dream.