In the coming decade, NASA and the ESA will be sending two dedicated missions that will explore Jupiter’s moon Europa. These missions are known as the Europa Clipper and the JUpiter ICy moons Explorer (JUICE) missions, which will fulfill a dream that has been decades in the making – searching for possible evidence of life inside Europa. Since the 1970s, astronomers have theorized that this satellite contains a warm-water ocean that could support life.
The case for life in Europa has only been bolstered thanks to multiple flybys and observation campaigns that have been mounted since. According to new research led by the University of Hawaii at Manoa, the best way to look for potential signs of life (aka. biosignatures) would be to analyze small impact craters on Europa’s surface. These patches of exposed subsurface ice could point the way towards life that might exist deeper in the moon’s interior.
Speculation about the possible existence of an interior ocean in Europa began in 1979 after the Voyager 1 and 2 missions flew past Jupiter and its moons on their way to the outer Solar System. With data obtained by the Galileo and New Horizons spacecraft and the Hubble Space Telescope have provided additional indications, which included how it interacted with Jupiter’s magnetic field, tidal models, surface features, and plume activity.
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Between resurfacing events and surface plumes that originate from the interior, scientists have speculated that biosignatures – chemicals produced by living organisms – that are the result of life in Europa’s interior ocean may have made it to the surface as well. However, since Europa orbits within Jupiter’s powerful magnetic field, its surface is subject to intense amounts of radiation that would destroy any traces of biological material.
This means that any biomolecules that are periodically ejected by plume activity or resurfacing events would only be likely to survive beneath the surface. Luckily, Europa’s surface is covered with small impacts that have taken place over the course of millions of years, which measure about 30 cm (12 inches) deep. These impacts would have also resulted in what is known as “impact gardening,” where impacts cause material from above and below the surface to be mixed.
Led by Emily S. Costello, a postdoctoral researcher at the Hawaii Institute of Geophysics and Planetology (HIGP), part of the UH Manoa’s School of Ocean and Earth Science and Technology (SOEST), the researchers sought to create the first comprehensive estimate of the effects of impact gardening on Europa. Their results are described in a study that recently appeared on July 12th in the scientific journal Nature Astronomy.
As Costello indicated in a recent SOEST press release, searching for potential signs of life on airless bodies like Europa presents a significant challenge. “If we hope to find pristine, chemical biosignatures, we will have to look below the zone where impacts have been gardening,” she said. “Chemical biosignatures in areas shallower than that zone may have been exposed to destructive radiation.”
Costello was joined by Professor of planetary science Paul G. Lucey, who is also a researcher with the HIGP; Cynthia B. Phillips, a Europa scientist at NASA’s Jet Propulsion Laboratory (JPL); and Rebecca Ghent, a senior scientist with the Planetary Science Institute (PSI). Thanks to a program grant from the NASA Solar System Workings (SSW) program, Ghent and Costello developed the original impact gardening model for this study. As Costello explained in a PSI press release:
“Radiation at Europa’s surface is so intense that it can break delicate biomolecules. Impact gardening cycles possible biomolecules into the zone of radiation. This work therefore provides some valuable new constraints on where to look if we hope to find the evidence of life.
“If we want to find evidence of pristine biomolecules unaltered by radiation in Europa’s ice, we have to either dig down beyond about a 30 centimeter depth – deeper in some regions – or find places where fresh material has recently been brought to the surface by recent impact craters.”
For some time now, astronomers have believed that impact gardening was a likely process on Europa and other airless bodies in the Solar System, but this new model provides the most comprehensive picture of that process to date. What’s more, it is the first to take into account secondary impacts that are caused by debris kicked up from the initial impact as it falls back to Europa’s surface.
“This is new because this is the first time the effects of impact gardening have been considered when predicting where on Europa biomolecules might be found and the first time impact gardening has been modeled to consider Europa’s unique icy surface and the impactor population in the Outer Solar System,” said Costello.
The research also indicates that Europa’s surface would be less affected by double impact gardening and radiation around the moon’s mid- and high-latitudes. In the near future, research like this could assist NASA and ESA planners develop mission profiles for the Europa Clipper and JUICE. Given that both of these missions will be examining Jovian moons for possible signs of life, knowing where it is most likely to be found is crucial.
In addition, this research could guide the design of instruments and future missions that are also dedicated to finding biosignatures in the “Ocean Worlds” of the Solar System. Beyond Europa and Ganymede, this includes Saturn’s moons Titan and Enceladus, Uranus’ moons Titania and Oberon, Neptune’s largest moon Triton, Pluto, and other icy satellites that are thought to have interior oceans. As Ghent added:
“[I]t also provides a framework for future investigation using higher-resolution images from upcoming missions, which would help to generate more precise estimates on the depth of gardening in various specific regions. The key parameters in this study are the impact flux and cratering rates. With better estimates of these parameters, and higher-resolution imaging resulting from upcoming missions, it will be possible to better predict the depths to which gardening has affected the shallow ice in specific regions.”
“This work broadens our understanding of the fundamental processes on surfaces across the Solar System,” said Phillips. “If we want to understand the physical characteristics and how planets in general evolve, we need to understand the role impact gardening has in reshaping them.” This research is part of wider NASA-funded efforts to study the cumulative effects of small impacts on Europa’s surface in preparation for the Europa Clipper mission.
This mission, which is expected to launch sometime in 2024, will orbit Jupiter as it conducts a series of close flybys of Europa. Its suite of scientific instruments will include optical and thermal imagers, spectrometers, magnetometers, and radar sounding devices. These will allow the spacecraft to survey Europa’s surface, measure its magnetic moment, and determine the chemical composition of its ice.
It will also carry a mass spectrometer and dust analyzer to study Europa’s tenuous atmosphere, plume activity, and sample the dust and gases that are kicked up above the surface. The data obtained from these missions could also inform future missions to the surface – like the Europa Lander concept – that could search for biosignatures directly and might even perform a sample return.