Recent imagery from the Hubble Space Telescope adds to the evidence that Jupiter’s largest moon, Ganymede, harbors a salty ocean beneath its icy surface comprised of more water than found on Earth. With this discovery, Ganymede joins the growing number of tantalizing worlds in our solar system in which liquid water is prevalent. On Earth, where there is liquid water, there is life. As such, understanding our solar system’s water worlds has potentially profound ramifications for the emergence of life on Earth, and the existence of life elsewhere in our solar system and the universe (helping us identify these same characteristics in Earth-like worlds around other stars).
In addition to Earth, our present list of water worlds includes Jupiter’s moons Europa, Ganymede and Callisto, Saturn’s moons Enceladus and Titan, and Neptune’s moon Triton. Including the vast hydrocarbon seas on Titan’s surface, a compelling scientific quest could be defined focused on the diversity and distribution of liquid resources across the solar system. Enceladus and Europa may be the two worlds in our solar system best suited to search for life as we know it; Titan is likely the best place to search for life as we don’t know it.
What we know of these worlds today is based largely on spacecraft and instruments designed in the 1970s and 1980s (Voyager 2, Galileo and Cassini), and observations generally completed more than a decade ago. Europa, for example, was last explored in 2000 by the Galileo spacecraft, which found that Europa’s icy surface likely encases a global ocean of liquid water heated by Jupiter’s tidal forces. A single Voyager 2 flyby of Triton in 1989 is the basis for its estimated water ice content and potential existence of a subterranean ocean. Plumes emanating from Enceladus’ south polar region were first observed by the Cassini spacecraft in 2005. More recently, Cassini has identified organics within these plumes, determined that they were produced by salty liquid water, and returned evidence of a regional sea beneath the Enceladus south pole.
Today, we have the tools and technology to move beyond the foundational investigations carried out by the Galileo, Cassini and Voyager spacecraft, and create robotic explorers to answer fundamental questions of habitability and life beyond Earth. To do so, we have to return to the outer planets with regularity and consistency of purpose. Unfortunately, there are no missions in NASA’s planned planetary science portfolio to access the water at the destinations in which we know it to exist.
Perhaps now is the time to initiate an Outer Planet Exploration Program focused on the fundamental questions of evolution, habitability and life across our solar system’s water worlds. Informed by the lessons and success of NASA’s Mars Exploration Program, an Outer Planet Exploration Program focused on the compelling science that results from direct access to these watery abodes may be both technically achievable and fiscally viable.
It has not been any one mission or science measurement that has singularly changed our view of Mars. Rather, it has been the synthesis of evidence, gathered through an interconnected set of measurements, obtained by a carefully engineered sequence of missions. Advancing Mars science required a prioritization of investigations, opportunities for relatively frequent launch, and a building-block approach in which technology advancement was utilized to improve science return over time. Built upon these same principles and the scientific foundation obtained from past missions, a program approach to exploring our solar system’s water worlds is possible today as a result of critical technology investments and new capabilities that bring the outer planets within our reach.
Technology advancement being pursued today in power (both solar and nuclear), radiation protection, sensing, landing, navigation and communications should greatly reduce the cost and increase the readiness of our flight systems for the exploration of Enceladus, Europa, Titan, Ganymede, Callisto and Triton as well as the gas giants Jupiter, Saturn, Uranus and Neptune. With the advent of NASA’s Space Launch System and/or SpaceX’s Falcon Heavy, a steady cadence of outer planet missions is possible in the decade of the 2020s. Because the transit times, distances, radiation environment and surface environments of these worlds differ so significantly from vistas that we have visited often (Earth, Mars and the moon), these emerging capabilities are critical for sustained exploration of the outer planets.
At present, NASA is formally initiating the Europa Clipper flight project in accordance with the objectives of the planetary science decadal survey. Viewed through a program lens, it is evident that adding a small, astrobiology-focused lander to directly access the surface of this water world should be considered for potential launch with the Europa Clipper. Proving our ability to safely and precisely access the fundamentally different surface environment of these water worlds should be the primary objective of this first U.S. outer planets lander. Providing imagery and chemical analysis of the icy moon terrain, such a mission would be a pathfinder for a suite of future surface and subsurface astrobiology missions to access the water in these ocean worlds. Compiled as a sequence of interconnected missions, this is a journey sure to inspire the world.
By any objective measure, planetary science is one of America’s crown jewels. This endeavor has consistently demonstrated that the United States is a bold and curious nation interested in understanding our world and discovering and exploring the richness of worlds beyond our own. In addition to informing our worldview, these missions are inspirational beacons, pulling young people into educational and career paths aligned with science, technology, engineering and mathematics, the foundation of continued U.S. economic competitiveness. Accessing the water, in destinations where we know it exists, is the next great quest towards understanding the possibilities for life across our solar system and the universe.
It’s time to accelerate the search for life in our solar system. It’s time to explore our solar system’s water worlds and the mysteries of the oceans beneath their ice encrusted surfaces. It’s time to initiate a NASA Outer Planet Exploration Program.
Robert D. Braun is the David and Andrew Lewis Professor of Space Technology at the Georgia Institute of Technology and served as the NASA chief technologist in 2010 and 2011.