Nearly 600 million kilometers away, an icy moon holds a vast salty ocean that is one of scientists’ prime candidates for finding life outside of Earth. Last week, a team of scientists released a 264-page report that spells out how, someday, we could land there.
The moon is Europa, a satellite of Jupiter, and one of the few ocean worlds in the solar system. In response to a congressional directive at the end of 2015, NASA recruited an interdisciplinary team of scientists to put together a plan for a hypothetical landing mission on Europa. The team undertook the study to determine the scientific goals and the feasibility of such a mission.
Why a Lander?
Remote measurements can tell scientists a lot about a celestial object: its size, general composition, and information about any existing atmosphere. A spacecraft can reveal even more details about a planet’s geology and composition—even its interior, in the case of Europa. In the 1990s, the passing Galileo spacecraft revealed that Europa very likely housed a salty ocean underneath its icy shell. Since this revelation, scientists have viewed Europa as an ideal candidate on which to find life.
“Every mission is a stepping stone for the future,” said Sarah Hörst, an atmospheric chemist at Johns Hopkins University in Baltimore, Md., and member of the Science Definition Team (SDT) that created the report. At NASA, the mantra is “first you fly by, then you orbit, then you land, then you rove,” she said.
Currently, plans are in the works for a Europa flyby mission that will launch in the 2020s and orbit Jupiter, providing scientists with more than 40 flybys of the icy moon. But even a flyby mission can’t tell you everything, Hörst said. The “history of exploring the solar system has told us that every place we go is more interesting than we thought it was when we decided to go there,” she continued. A lander would be able to reveal small-scale details about Europa’s surface composition—and whether it’s habitable or already supporting life.
Europa Mission Goals
Mission planners have come up with a simple directive for the envisioned lander: find life on Europa. However, executing this mission will be complicated.
Specifically, scientists are itching to know whether the moon’s ocean has hydrothermal vents on its rocky bottom, like those on Earth. Our vents occur where water percolates down into the ocean floor at spreading centers or subduction zones, where it heats up and gets shot back into the sea. The interaction between the cold seawater and hot geothermal fluids causes chemical reactions that spew fine-grained particles that form columns. Since the 1970s, scientists have found rich ecosystems around these vents, and they’re even thought by some scientists to have been vital to life’s origins, Hörst said.
Unfortunately, however, life doesn’t come with a neat and tidy definition, she continued. Developing the report opened questions within the SDT about what kinds of evidence of life the team would find convincing. Therefore, the group chose a multipronged approach to guarantee conclusive, rather than simply compelling, evidence of life.
Take amino acids, for instance. They’re the building blocks of proteins and essential to life. But that doesn’t mean their presence alone signifies life. Scientists find amino acids on meteorites—Hörst can even make them in her lab. But in living things, amino acids have certain characteristics and ratios, which a hypothetical lander could search for.
Instruments suited to look for life would need to include imagers for examining the surface, assorted spectrometers to detect organic molecules, a specialized microscope system to investigate possible cellular life, and other devices that characterize the geology and chemistry of the moon’s surface. The second and third directives of such a mission would be to assess Europa’s habitability in the absence of life and to characterize its surface and composition to prepare for any future mission.
Earth as Example
Without a rover on every celestial body, scientists turn to Earth to find environments that might exist in the solar system. For Europa, the SDT turned to ocean scientists.
The team specifically targeted studies of subglacial environments, like Lake Vostok, which lies 4 kilometers under the ice in East Antarctica. In the 1990s, a research team discovered microbes living in the lake. Researchers suspect that geothermal activity keeps the lake liquid. Without sunlight, microbes would gain energy instead by taking advantage of chemical reactions that result from the geothermal activity in the lake. This process is called chemosynthesis, said Chris German, a marine geochemist at Woods Hole Oceanographic Institution in Massachusetts and a member of the SDT.
The SDT also considered deep, cold brines, such as Lake Vida—another Antarctic lake—which is permanently covered by 27 meters of ice. This lake is 6 times saltier than normal seawater, yet scientists have found life in abundance.
“While any dark ocean beneath thick ice far from the Sun presents a pretty dim prospect for photosynthesis,” German said (pun intended, he noted), “there is no reason why one couldn’t imagine water-rock interactions driven by geothermal processes from a planet’s interior working just as well anywhere in space, given the right conditions.”
The Next Steps
According to the report, a lander would require 5 years to journey to Europa, with multiple gravity assists from Earth and Jupiter. The lander would descend to the surface via sky crane—like Curiosity’s landing on Mars in 2012—and spend a minimum of 20 days analyzing Europa’s surface.
But at this point, a lander is merely hypothetical. The report could be “the first step to the launch pad,” Hörst said, although she was quick to point out there are no guarantees. The purpose of a report like this is to tell NASA that the mission is possible, she continued. Thanks to the report, we know that we can launch a spacecraft, send it to Europa, and put a lander on its surface.
But even if no lander comes to fruition, Hörst hopes that the report will continue to generate discussion about life and its possible scope throughout the solar system.
Read the full report here.
—JoAnna Wendel (@JoAnnaScience), Staff Writer
Wendel, J. (2017), New report details how NASA could land on Europa, Eos, 98, https://doi.org/10.1029/2017EO067885. Published on 15 February 2017.
Text © 2017. The authors. CC BY-NC-ND 3.0
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