There is a fascinating world nearly 500 million miles away unlike any other. It was once just a point of light as seen through even the largest and best telescopes. The Pioneer 10 & 11 spacecrafts revealed it as a fuzzball with hints of unusual surface features when it flew past it during 1973-74. The big reveal had to wait until March 1979 when Voyager 1 flew past the fascinating world of Europa.

Europa is a moon that orbits Jupiter, one of a group of four large moons: Io, Europa, Ganymede, and Callisto, collectively known as the Galilean Moons, after their discoverer Galileo Galilei. These four moons went from mere points of light to amazing worlds rich in geology. Voyager 1 quickly revealed Io to be a world of huge active volcanoes with towering plumes nearly 200 miles above the surface. Ganymede was a fractured world of active geology with faulting. Callisto was a dead moon, but there were so many craters on its surface that new ones could only form by destroying old ones. Europa would soon become famous in a way that the volcanic and radiation-ravaged Io could not claim.

Europa is slightly smaller than Earth’s Moon at 1940 miles in diameter and the smallest of the Galileans. Neither of the Voyagers passed close to Europa, but the images were good enough to reveal a very unusual surface. Europa turned out to be the smoothest world in the Solar System with no craters and the highest hill barely rising 300 feet, as smooth as a cue ball and cracked like an egg. After volcanoes were discovered on Io, it was speculated that Europa was warm enough beneath the icy crust to possibly have a deep global ocean. This could neatly explain why Europa was so smooth and cracked with no craters; fresh ice explained why Europa was so bright. The crust was thought to be thin enough that the gravitational pull from Jupiter could cause it to crack allowing water from below to flood the cracks which would quickly freeze in the -250ºF cold. Scientists studied the images carefully looking for active geysers but found none. Where there is water, there may be life, and scientists could not wait until the Galileo spacecraft arrived at Jupiter where it would orbit for several years making several close flybys of Europa.

The wait was longer than anticipated due to the Space Shuttle Challenger disaster of January 28, 1986, which possibly contributed to a critical problem that nearly doomed the mission before it ever arrived at Jupiter. The launch of Galileo from the Space Shuttle in May 1986 with arrival at Jupiter in December 1988 had to be cancelled; Galileo was placed into storage until the Space Shuttle returned to flight. Galileo was finally launched on October 18, 1989, with a less-powerful solid fuel booster rocket as the liquid-fueled Centaur booster was now considered too dangerous to fly aboard the Space Shuttle under the new, post-Challenger accident guidelines. The boost from a less less-powerful rocket meant that Galileo would not arrive at Jupiter until December 7, 1995. The high-gain antenna dish was folded up like an umbrella when Galileo was prepped for launch in 1986 ending up stowed until after the 1989 launch when it was commanded to open in April 1991. It failed to do so and remained partially jammed, useless. It is possible that the lubricant dried out during the longer than expected storage. Galileo would have to relay information through its low-gain antenna, which meant slower data relay rates and fewer images.

Against all odds and hardship Galileo carried out its mission for nearly eight years from 1995-2003 flying as close as 120 miles above Europa’s surface from 1997-99 during an extended mission that was mainly devoted to this watery world, which proved exciting. Close-up images confirmed that Europa was resurfaced by water ice that left very few craters and a very flat surface. The largest crater, Pwyll, had fresh ice radiating hundreds of miles and gave that side of Europa an eerie resemblance to Earth’s Moon. Pwyll is nearly level with the surface indicating that it must have softened the crust and filled in with something fluid from underneath. The central peak is much higher than the rim of the crater indicating that water may have oozed out and built it skyward even after the crater collapsed and froze. There is evidence of upheaval and collapse in the crust possibly from the upwelling of water below. There are dark stains along several of the cracks, which may be where water spewed out from the ocean and vaporized in the vacuum leaving minerals and salts behind. Europa may have tides up to 100 feet, which could heat the moon enough to keep the ocean from freezing. One theory is that Europa has a rocky core that is overlain with an ocean sixty miles deep and an icy crust a few miles thick. The discovery of hydrated minerals and salts added proof that Europa has plenty of water.

Juno has been orbiting Jupiter since July 4, 2016. It is primarily a mission to study Jupiter, its weather, interior, and the environment surrounding it. An extended mission from 2021-2025 made it possible for Juno to visit the Galilean Moons except for Callisto which remained too far away for Juno to study. Juno made three visits to Europa with the closest occurring on September 29, 2022, when it flew 219 miles above the surface. It did not find any geysers or transient plumes but did resolve surface features reminiscent of cryovolcanic plume deposits. It also supported the idea that Europa’s icy shell may give way in locations where pockets of briny water from the subsurface ocean are present beneath the surface. Complex surface features stood out including intricate networks of cross-cutting ridges and dark stains from potential plumes of water vapor. The flybys also support the theory that a huge ocean resides beneath the icy crust and that the outer ice shell is essentially free-floating and moves. This would explain the cracks and chaotic terrain. Juno will not last much longer so a bigger, fully dedicated mission to Europa was needed to answer many questions including a nagging one.

Could there be life on Europa? Three ingredients are necessary for life as we know it: an energy source, organic compounds, and liquid water. All three conditions appear to exist on Europa. Galileo discovered an energy source on the surface, sulfuric acid. This may seem corrosive, but it is an oxidant, and an energy source for life on Earth. It is perpetually dark under the icy crust of Europa; therefore, sulfur could provide the energy that living organisms need. The sulfur originates from deep within Europa and most likely would spew out on the ocean floor like the black smokers on the bottom of the Earth’s oceans. Life is abundant near these vents despite the darkness as they spew out heat and vital minerals. Europa Clipper is a sophisticated spacecraft costing $5.2 billion that was launched on October 14, 2024, and will arrive on April 11, 2030. It will orbit Jupiter at least 49 times, each time flying 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 with resolutions better than 150 feet. 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 that may have 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 any geysers it discovers. 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 of any tenuous atmosphere. 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, depth, and salinity of the ocean. The European Space Agency (ESA) launched the Jupiter Icy Moons Explorer (JUICE) on April 14, 2023, with arrival at Jupiter on July 14, 2031. JUICE will go into orbit around Jupiter in such a way that it will initially make several close flybys of Europa and Callisto before it settles into orbit around Ganymede, which will be its primary mission through 2035.

Europa Clipper is a bold mission, and combined with JUICE, will provide important information about Europa that will pave the way for an eventual Europa lander, rover, and even a hydrobot which would land on a thin spot on the crust and melt its way through the ice to the ocean below to look for life. This is an exciting era of Jupiter exploration. The next decade will be one to remember!