Europa Clipper, sailing to new discoveries on habitability

The early days of planetary exploration must have been a little disappointing for those looking for biological activity, as one after another the terrestrial planets turned out to be cratered, old and with little potential nooks for life.  Venus, of course, was the exception but with its extreme atmosphere of carbon dioxide, high temperatures, and large surface pressures, touched with spiciness of acid - meant that not even the plucky Venera landers could last more than two hours on its surface, let alone anything more squidgy.  

As the gas giants were approached, perhaps little was expected - new and exotic physics, yes - but the prospect of anything able to live where there is no solid ground seemed remote.  An image from Voyager 2 in July 1979 changed all that.

Voyager 2 had entered the Jovian system and was the first to get a close look at the four moons first discovered by Galileo in 1610.  The Galilean moons were named Io, Europa, Ganymede and Callisto in jest originally by Johannes Kepler after the lovers of Zeus (the Greek equivalent of Jupiter). The discovery of the Galilean moons had revolutionised science at the time of their discovery, showing that every celestial body does not revolve around the Earth, a paradigm shift from the hundreds of years of historical thinking.

Centuries later, our first close glimpses of these moons showed hints of the dynamic worlds that planetary scientists had been searching for, with the active volcanoes of Io and the enigmatic cracked icy surface of Europa.  Suddenly planetary science moved from the study of old landscapes to dynamic young surfaces, more like our own Earth.  

But what were the surfaces made from?  Voyager 2 (and 1) didn’t have instruments that could discern the chemistry in detail but could infer the temperature.  That information coupled with the images captured by these early spacecraft led to the prevailing thoughts that the surface was mainly water ice, and with few impact craters seen this meant that the surface had to be geologically young.  More had to be found out. 

The Jovian system had proved itself sufficiently interesting, hence 10 years after Voyager 2 had given a tantalising peak of the system a dedicated Jupiter-system mission was launched.  The Galileo spacecraft overcame several challenges to become the first spacecraft to orbit another planet.  During its 8-year tour of the planet and its moons, it conducted 11 flybys of Europa, passing at its closest, 200 km from the moon’s surface.

Despite the fact that Galieo’s high gain antenna never opened up (meaning that much of NASA’s Deep Space Network - including the node in Australia - had to be upgraded to receive any signal from the craft’s low gain antenna), mounds of data made it back to Earth.  Galileo offered a picture of the chemistry of the surface with its spectrometers, which coupled with the visitable light images meant that we could start speculating about the history of the moon’s patchwork surface.  Data from the limping magnetometer (it was severely tested by the enormity of Jupiter’s own magnetic field) support the idea that Europa had a worldwide ocean under the ice on the surface.  Since Voyager 2 had revealed an icy surface, many had speculated that there would be an ocean under the ice crust, due to the rising temperature and pressure at depth.  Galileo’s magnetometer found that Europa caused a change in the wider magnetic field of Jupiter, and the conclusion was drawn that it was salt in the ocean that was doing this.  

The 11 Galileo flybys meant that we have strips of high-resolution data across the surface, but not whole coverage.  The instruments that took these data were state-of-the-art at the time that they were constructed in the 1980s, but unfortunately, they were not able to tell us definitively what the materials on the surface were.  Analysing this light has pointed to several materials that could be on the surface, mainly hydrates - materials made of water ice and salts.  These are notoriously complicated materials, with novel ones still being discovered (including by the author of this piece!).  

By the time Galieo undertook its swansong, diving into the depths of Jupiter itself, Europa’s sister moons Ganymede and Callisto had also seen to have evidence of under-ice oceans, joined later by Saturn’s moons Titan and Enceladus in the wake of the Cassini mission.  The case for a close examination of one of these ocean worlds grew substantially.   

Europa Clipper was already well into being planned when a campaign of observations with the Hubble Space Telescope showed that the moon was actively spouting possible water plumes into space.  Sifting through older, archival Hubble data also showed that plumes had also been observed in 2012.  Suddenly, not only was there more evidence of liquid water below the surface, but also a way for it to reach space and get detected.  Europa became the best place to investigate the habitability of another world. 

Information from Voyager 2, Galileo and Hubble helped form three science questions that Europa Clipper has been designed to investigate. These are: how thick is the ice that overlays the ocean, and does it interact with the surface?  What is the moon made out of? What is the history of its surface and interior?  Assessed together the overall hope is that these will point to whether there are conditions on Europa that can support life, as we know it.

Harking to naval links, the Clipper spacecraft is the largest interplanetary mission to date, swelled by its solar panels to harvest energy from fainter sunlight.  The tea clippers of the 19th century zipped around the Atlantic, and Europa Clipper’s voyage will also be swift - it will overtake the European Space Agency’s JUICE mission that launched for the Jovian system last year - and arrive in April 2030.  Once it arrives in an orbit designed to minimise time in the ferocious radiation of Jupter’s magnetic field, the nine instruments will run simultaneously over each of the 49 planned flybys.  The images will be spectacular, and hopefully the data crisp enough to show that there is a place where life can bloom away from Earth.