Jupiter has an amazing family of moons with volcanic Io, groovy Ganymede, and cratered Callisto, but Europa is one of the most fascinating in the Solar System. It is nothing more than a point of light as seen through a telescope from Earth. The Pioneer spacecraft revealed it as a fuzzball with hints of unusual surface features when it flew past during 1973-74. It is a very bright moon, but any clue to what Europa was really like had to wait until Voyager 1 flew by during March 1979.
Europa is a world slightly smaller than Earth’s Moon at 1950 miles in diameter. Although neither Voyager 1 nor 2 flew close to Europa, Voyager 1 did reveal Europa to be as smooth as a cue ball and cracked like an egg. No craters were seen anywhere and 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 nicely explained 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 and water from below could flood the cracks and quickly freeze in the -250ºF cold. Scientists studied the Voyager images carefully to see if there were any 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 and make several close flybys of Europa.
After several delays and a busted main antenna, Galileo arrived at Jupiter on December 7, 1995 for nearly eight years of exploration. Galileo flew as close as 120 miles to Europa from 1997-99 during an extended mission that was mainly devoted to the watery moon and every flyby was exciting. Close-up images revealed that Europa was definitely resurfaced by water ice that left very few craters and was very flat with the highest spot only a hill about 300 feet high. 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 warmer 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 to leave 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 60 miles deep and an icy crust a few miles thick, possibly thicker or thinner. The discovery of hydrated minerals and salts added proof that Europa has plenty of water.
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 may 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 Earth’s oceans. Life is abundant near the vents in spite of the darkness as they spew out heat and vital minerals. All of this has fueled scientists with a desire to return to the Jovian system with a dedicated orbiter for Europa so it could be studied for months. Placing a spacecraft around any moon is a challenge, but Jupiter’s intense radiation belt only adds to it because its strength is powerful enough to be lethal to a spacecraft’s components if too much radiation is accumulated. The only way to put a spacecraft in orbit around Europa is for it to fly through the dangerous radiation belts several times. Many different mission scenarios have been drafted and discarded, but there is finally one mission that will fly someday.
The Europa Jupiter System Mission (EJSM) was approved for development in February 2009 and is a flagship mission, which means it will cost around $4.5 billion and will consist of two orbiters: a Europa orbiter to be built by the US and a Ganymede orbiter to be built by the European Space Agency (ESA). The Jupiter Europa Orbiter (JEO) will be launched on February 29, 2020 and arrive at Jupiter on December 21, 2025. It will be heavily shielded from the radiation and its electronics will also be radiation hardened. It will go into orbit around Jupiter for 30 months and make several flybys of Io, Europa, Ganymede, and Callisto. JEO will then go into orbit around Europa for at least nine months and will completely map it at a resolution of a few feet. It will also have a powerful radar or sounder and it is hoped that it will be able to measure the thickness of the crust and determine any thin spots, which will be crucial for any future lander missions as it will need easy access to the ocean below. JEO will also measure the heat flow and map the mineral composition and distribution of the surface to look for any clues of upwellings from the ocean beneath. The warm upwellings would keep the crust above them thinner. If there is life in the ocean, it is possible that it could be belched out onto the surface through one of the cracks. The water would quickly freeze, but any life would be quickly freeze-dried and lying on the surface. This is a critical issue that JEO will address since any future mission would have to land in a spot where life is most likely to be found. It will also look for geysers, magnetic fields, and any trace of an atmosphere. If the spacecraft remains healthy, the mission could be extended beyond 2029.
ESA plans to launch the Jupiter Ganymede Orbiter (JGO) on March 23, 2020 with arrival at Jupiter in February 13, 2026. JGO will go into orbit around Jupiter in such a way where it will initially be in resonance with Callisto, allowing several close flybys before it would settle into orbit around Ganymede. It is expected to orbit Ganymede for the rest of the mission until at least 2029 and possibly longer if the spacecraft remains healthy. Ganymede’s ocean is much deeper and forever inaccessible. It is an unusual moon as it has chaotic terrain, polar ice caps, a thin atmosphere of oxygen, and a magnetic field. Low-lying valleys appear to have been flooded with water or slush around a billion years ago and froze over leaving bright terrain that dramatically abuts older, darker terrain giving the moon a marbled appearance. Ganymede is the only moon known to have a magnetic field, a clue that there must be an ocean beneath the crust as something liquid is necessary to create it. The polar icecaps are thought to be fine deposits of water ice that are deposited courtesy of the magnetic field. Energetic particles get trapped in the magnetic field and strike the surface releasing a fine spray of fresh bright ice. The origin of the polar icecaps, thin atmosphere, and magnetic field will definitely come under scrutiny as Ganymede is mapped in fine resolution to equal Europa.
This dual mission is bold, but 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 to the ocean below to look for life. Russia is planning a Europa lander for the 2020’s, but is not yet committed to development. There are also other missions being considered by the US for exploring Io at lower cost, which will compliment the missions that will fly. Juno is the next mission that will fly to Jupiter with launch during August 2011. It will arrive at Jupiter on August 3, 2016, however it will be in a low polar orbit around Jupiter and focus primarily on Jupiter. It will come nowhere near any of the moons; therefore everything rests on the eventual launch of EJSM. Europa is one moon that may hold more surprises than anyone has imagined. There is certainly no end to the ways Jupiter and its moons can be explored and it will be an exciting century of exploration.