2.0 Science Goals and Objectives of the Europa-Jupiter System Mission
2.1 The Relevance of Jupiter Exploration
The Joint Jupiter Science Definition Team (JJSDT) identified The Emergence of Habitable Worlds Around Gas Giants as the overarching goal for the mission. Jupiter is the largest of the gas giants in our solar system and serves as the archetype for extrasolar planets, some 300 of which have been discovered thus far. Since their initial discovery in the late 1980s, the rate of discovery of extrasolar planets has increased tremendously and it has been suggested that 10% of all sun-like stars have planets (Marcy et al., 2005). Consequently, the Europa-Jupiter System Mission (EJSM) affords the opportunity for studying a gas giant and its array of satellites and is important not only for understanding our solar system, but also for the insights it could provide into planetary systems as a whole.
The broad objectives to meet the EJSM goal include assessment of the formation of the Jupiter system, building on previous missions and anticipated measurements from JUNO. While including the Jupiter system as a whole, the primary focus for EJSM will be the study of the ice-rich worlds Europa and Ganymede through in-depth comparative science. This will include characterizing their current and past environments, determining their surface and near-surface compositions, and deriving their geological histories. Europa is essentially a rocky world with an outer ~150-km-thick shell of water ice composition, while Ganymede is composed mostly of water ice and represents volatile-rich satellites; thus, these objects provide a natural laboratory for comparative analysis of potential habitability.
Figure 2.1-1 A Europa-Jupiter System mission can be carried out by multiple spacecraft with specific scientific targets and overlapping science objectives.
JSM has been crafted around the NASA Jupiter Europa Orbiter (JEO) and the ESA Jupiter Ganymede Obiter (JGO); a Jupiter Magnetospheric Orbiter (JMO) might also be flown by JAXA. As shown in Figure 2.1-1, each spacecraft element has its own mission targets, as well as overlapping science objectives. While the primary focus of JEO is to orbit Europa, flybys of Io, Ganymede, Callisto and observations of Jupiter and the system expand its science return. Similarly, JGO would ultimately orbit Ganymede, but focused observations of Callisto and Jupiter and the system will complement those of JEO, while JAXA's Magnetospheric Orbiter has the potential to focus on particles and fields observations of the Jovian magnetosphere. It should be noted that while JEO and JGO are complementary and synergistic, both are designed as "stand-alone" missions, should that be necessary.
2.2 The Relevance and Prominence of Europa Exploration
Nearly 400 years after Galileo Galilei’s discovery of Jupiter’s moons advanced the Copernican Revolution, one of these moons, Europa, has the potential for discoveries just as profound.
Europa’s icy surface is believed to hide a global subsurface ocean with a volume more than twice that of Earth’s oceans (Figure 2.2-1). The moon’s surface is young, with an estimated age of 60 million years [Schenk et al, 2004], implying that it is most likely geologically active today. The molecular constituents of life have rained onto Europa throughout solar system history, are potentially created by radiation chemistry at its surface, and may pour from vents at the ocean’s deep bottom. On Earth, microbial extremophiles take advantage of environmental niches arguably as harsh as within Europa’s subsurface ocean. If the subsurface waters of this Galilean moon are found to contain life, the discovery would spawn another revolution, this time in our understanding of life in the universe.
Although it is now recognized that water may exist within several of the solar system’s icy satellites, Europa’s relatively thin ice shell and potentially active surface-ocean exchange elevate its priority for exploration [SSER 2006]. A Europa mission is the first step in understanding the potential of icy satellites as abodes for life.
Europa’s high astrobiological potential and its complex interrelated processes have been recognized by the National Research Council (NRC) and by NASA, making Europa an extremely high priority for future exploration. The NRC’s Committee on Planetary and Lunar Exploration [COMPLEX 1999] recognized that Europa “offers the potential for major new discoveries in planetary geology and geophysics, planetary atmospheres, and, possibly, studies of extraterrestrial life. In light of these possibilities … , COMPLEX feels justified in assigning the future exploration of Europa a priority equal to that for the future exploration of Mars.”
The Solar System Exploration Survey (hereafter referred to as the “Decadal Survey”) convened by the National Research Council of the National Academy of Sciences [SSES 2003] identifies a Europa Geophysical Explorer as the top priority Flagship mission for the decade 2003–2013. This is principally because such a mission addresses the fundamental science question: “Where are the habitable zones for life in the solar system, and what are the planetary processes responsible for producing and sustaining habitable worlds?”
NASA’s scientific community-based Outer Planets Assessment Group (OPAG) “affirms the findings of the Decadal Survey, COMPLEX, and SSES, that Europa is the top-priority science destination in the outer solar system” [OPAG 2006].
These recommendations are reflected in the NASA Science Mission Directorate’s 2006 Solar System Exploration Roadmap [SSER 2006], which states that “Europa should be the next target for a Flagship mission.” The Roadmap calls out five high-level “Science Questions” (traced from the Decadal Survey’s “Scientific Goals”), four of which are directly addressed by an orbital mission to Europa:
How did the Sun’s family of planets and minor bodies originate?
How did the Solar System evolve to its current diverse state?
What are the characteristics of the Solar System that led to the origin of life?
How did life begin and evolve on Earth and has it evolved elsewhere in the Solar System?
Noting that Europa’s neighbors Ganymede and Callisto are also believed to have internal oceans, the Roadmap further states: “It is critical to determine how the components of the Jovian system operate and interact, leading to potentially habitable environments within icy moons. By studying the Jupiter system as a whole, we can better understand the type example for habitable planetary systems within and beyond our Solar System.”
NASA’s 2007 Science Plan [NSP 2007] echoes the many previous recommendations, calling out Europa as “an extremely high-priority target for a future mission.” Acknowledging that several icy satellites are now believed to have subsurface oceans, it states: “Although oceans may exist within many of the solar system’s large icy satellites, Europa’s is extremely compelling for astrobiological exploration. This is because Europa’s geology provides evidence for recent communication between the icy surface and ocean, and the ocean might be supplied from above and/or below with the chemical energy necessary to support microbial life.” The Science Plan affirms the priority of Europa exploration in addressing fundamental themes of Solar System origin, evolution, processes, habitability, and life.
The NASA Astrobiology Roadmap [Des Marais et al., 2003] includes as a goal: “Explore for past or present habitable environments, prebiotic chemistry, and signs of life elsewhere in our Solar System.” A subsidiary objective is to “provide scientific guidance for outer Solar System missions. Such missions should explore the Galilean moons Europa, Ganymede, and Callisto for habitable environments where liquid water could have supported prebiotic chemical evolution or life.” A 2007 letter from the NASA Astrobiology Institute’s Executive Council to the previous Europa Explorer (EE) SDT reaffirms that a Europa orbiter mission as represented by EE “is in its highest priority mission category for advancing the astrobiological goals of Solar system exploration.”
The exploration of the Jovian System and Europa is a high priority of the European Space Agency’s Cosmic Vision strategic document [ESA 2005]. Key questions about the Jupiter System and its habitability strongly resonate with two of the four major scientific questions identified by the Cosmic Vision. Specifically, these being: (1) “What are the conditions for planet formation and the emergence of life?” This question includes the sub-topic “Life and habitability in the Solar System,” and the goal “Explore in situ the surface and subsurface of solid bodies in the Solar System most likely to host – or have hosted – life.” (2) How does the Solar System work? This includes the sub-topic “The giant planets and their environments,” and the goal “Study Jupiter in situ, its atmosphere and internal structure.”
If a Europa orbital mission finds that Europa is a habitable environment today, with active communication between subsurface water and the near surface, then a Europa Astrobiology Lander has been recommended as an important next step beyond the orbiter in the satellite’s exploration [SSES 2003; SSER 2006]. Present exploration of Europa would feed forward to a future landed mission.
All of the above recommendations are consistent with the Vision for Space Exploration document [VSE 2004], which places high priority on robotic exploration across the solar system, “In particular, to explore Jupiter’s moons … to search for evidence of life, [and] to understand the history of the solar system. …”
The scientific foundation of a mission to Europa has been clearly laid out. Next are summarized key aspects of the state of knowledge regarding Europa, providing the framework for the Europa Explorer mission.