Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032 (2022)

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3 Priority Science Questions The research strategy described in the prior decadal survey in planetary science, Vision and Voyages, was structured largely by destination or destination class. Panels constituted to address the Inner Planets, Mars, Giant Planets, Satellites, and Primitive Bodies (a.k.a. small bodies) each contributed a chapter that identified science goals, future directions for investigations and measurements, and related missions, technology, and other activities to advance knowledge within their topical scope. In its guidelines to this committee, NASA requested that this decadal survey report instead “be organized according to the significant, overarching questions in planetary science, astrobiology, and planetary defense” (see Appendix A). Implementing a report structure that would, for the first time, be based on overarching science questions required re-assessment of committee organizational structure, as well as new processes to define these science questions and to construct the corresponding report. Creating panels based on science questions was determined to be impractical because it required the priority questions to be defined before the panels could be formed, and defining the questions themselves was to be an important initial deliberative task for the committee. Destination-based panels would, as they had previously, provide target-specific recommendations for missions and other activities throughout the decadal process. Destination-specific expertise also allowed the panels to efficiently identify additional mission studies, beyond those already available, early in the process as required for study completion prior to committee prioritizations. The committee structure was comprised of a steering group and six destination-oriented panels that addressed Mercury and Moon, Venus, Mars, Small Bodies, Giant Planet Systems, and ocean worlds and dwarf planets. Initial deliberations, led by the steering group, defined the priority science questions. Writing groups—one for each question topic—were created to each contribute a corresponding chapter in the report. The writing groups were informal, fluid structures, comprised of committee (panel and steering group) members with appropriate expertise and interests, with committee members typically participating in multiple writing groups. 1 This organization effectively drove interaction and discussion across target- focused sub-communities, providing an integrated view of how a question related to, e.g., planetary formation or habitability, might be addressed at destinations across the solar system. A primary initial task for the steering group was to define the priority science questions around which the writing groups and the report would be structured. These questions would ultimately provide the intellectual framework for the evaluation of potential research activities by the committee. The steering group sought priority question topics that were sufficiently broad and high-level so as to (1) clearly convey, even to a non-specialist, a topic of fundamental importance, and (2) emphasize cross-cutting connections rather than specific singular objects, in keeping with NASA’s request. However, it was clear that the question topics needed to be defined narrowly enough to allow one to distinguish between different activities as being more or less broadly impactful, and to keep question-oriented chapters to a manageable scope. An analogous writing group structure was also adopted to address key topical areas, including State of Profession, 1 Research and Analysis, Planetary Defense, Human Exploration, Infrastructure, and Technology. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 3-1 

Steering group members began by each submitting a few draft questions that they considered to be significant, overarching questions for planetary science, astrobiology, and planetary defense. Steering group members considered as inputs to their deliberations white papers submitted to the committee, prior crosscutting themes and priority questions identified in Table 3.1 of Vision and Voyages, and ∼50 “big questions” that had been previously identified by the various Assessment Groups (AGs) in response to a request from PSD Director Dr. Lori Glaze. The ~70 initial steering group questions were compiled and organized by general topic. Figure 3.1 shows the distribution of questions by topic for this initial group of steering group questions and for those previously identified by the AGs. The two distributions were remarkably similar, reflecting an average of 33 percent of the questions relating to planetary processes, 27 percent to habitability and life, 25 percent to solar system origins, 8 percent to bombardment and impact hazard, 5 percent to exoplanets, and 3 percent to human exploration. The main differences between the two distributions were a somewhat greater emphasis on astrobiology-related topics by the steering group, whose membership included multiple astrobiologists in keeping with the decadal scope, and a somewhat greater emphasis on exoplanets by the AGs. FIGURE 3.1 Comparison of topical distribution of cross-cutting questions initially identified by the steering group (SG; left) with those previously identified by Assessment Groups (AGs, including LEAG, MAPSIT, MEPAG, OPAG, SBAG, MEXAG, and VEXAG; right). Priority questions were progressively developed through discussion (including feedback from the panels) and polls that assessed, e.g., whether draft questions were “appropriate in scope”, “too broad”, or “too narrow”, as well as the sub-topics to be included in each. The result was the 12 priority science questions shown in Table 3.1. Each of what the committee henceforth defines as the 12 priority science questions is comprised of a single overarching topic, as well as a one-to-two sentence description of the question’s scope. The 12 priority questions provide the organizational structure around which the scientific portions of the report are organized. The scope of scientific inquiry encompassed by each of the 12 priority questions is of high and comparable priority for optimizing advances in planetary science and astrobiology over the next decade. The first eleven priority science questions are grouped into three scientific themes: Origin of the solar system (Theme A), Solar System Worlds and Processes (Theme B), and Life and Habitability (Theme C). The number of priority questions in each theme approximately reflects the topical PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 3-2 

distribution shown in the Figure 1 plots, with bombardment included in Theme B. The twelfth priority question, which addresses Exoplanets, is a cross-cutting question that relates to all three themes. Writing groups focused on each priority question subsequently identified key open issues and sub- questions. A central element of this process was the identification of the strategic research areas that would provide substantial progress in addressing the questions over the next decade. These strategic research areas provided important inputs for assessing potential missions and for identifying key technology development needs for the coming decade. Additionally, they are intended to provide useful guidance and context in support of activities beyond those specifically prioritized in this report, e.g., involving Discovery-class or smaller missions, basic research, and instrumentation and technology development. Initial drafts of each science chapter were made available to the full committee for their review and comment. The resulting 12 science chapters identify the most compelling science questions, goals, and challenges that should motivate future strategy in planetary science and astrobiology, and provide a comprehensive research strategy to advance the frontiers of planetary science, thus addressing items (3) and (5) of the statement of task. When considering activities, such as future missions, the committee evaluated their potential for addressing the priority science questions and sub-questions identified in these chapters, providing direct traceability between recommended activities and science goals (see Table 22.4). Structuring the report around priority science questions led to greatly increased interactions among committee members across the panels and steering group. It is hoped that this material will provide broadly useful background for the sponsors and the community, with content complementary to that produced by destination-focused AG science goal documents. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 3-3 

TABLE 3.1 The Twelve Priority Science Questions Scientific Themes Priority Science Question Topics and Descriptions A) Origins Q1. Evolution of the protoplanetary disk: What were the initial conditions in the solar system? What processes led to the production of planetary building blocks, and what was the nature and evolution of these materials? Q2. Accretion in the outer solar system: How and when did the giant planets and their satellite systems originate, and did their orbits migrate early in their history? How and when did dwarf planets and cometary bodies orbiting beyond the giant planets form, and how were they affected by the early evolution of the solar system? Q3. Origin of Earth and inner solar system bodies: How and when did the terrestrial planets, their moons, and the asteroids accrete, and what processes determined their initial properties? To what extent were outer solar system materials incorporated? B) Worlds and Q4. Impacts and dynamics: How has the population of solar system bodies changed Processes through time, and how has bombardment varied across the solar system? How have collisions affected the evolution of planetary bodies? Q5. Solid body interiors and surfaces: How do the interiors of solid bodies evolve, and how is this evolution recorded in a body’s physical and chemical properties? How are solid surfaces shaped by subsurface, surface, and external processes? Q6. Solid body atmospheres, exospheres, magnetospheres, and climate evolution: What establishes the properties and dynamics of solid body atmospheres and exospheres, and what governs material loss to space and exchange between the atmosphere and the surface and interior? Why did planetary climates evolve to their current varied states? Q7. Giant planet structure and evolution: What processes influence the structure, evolution, and dynamics of giant planet interiors, atmospheres, and magnetospheres? Q8. Circumplanetary systems: What processes and interactions establish the diverse properties of satellite and ring systems, and how do these systems interact with the host planet and the external environment? C) Life and Q9. Insights from Terrestrial Life: What conditions and processes led to the emergence Habitability and evolution of life on Earth, what is the range of possible metabolisms in the surface, subsurface and/or atmosphere, and how can this inform our understanding of the likelihood of life elsewhere? Q10. Dynamic Habitability: Where in the solar system do potentially habitable environments exist, what processes led to their formation, and how do planetary environments and habitable conditions co-evolve over time? Q11. Search for life elsewhere: Is there evidence of past or present life in the solar system beyond Earth and how do we detect it? Cross-cutting A-C Q12. Exoplanets: What does our planetary system and its circumplanetary systems of linkage satellites and rings reveal about exoplanetary systems, and what can circumstellar disks and exoplanetary systems teach us about the solar system? PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 3-4 

Q1 PLATE: An Atacama Large Millimeter/submillimeter Array (ALMA) image of the protoplanetary disk around the star HL Tau in 2014. The disk is approximately 200 astronomical units in diameter. SOURCE: ESO/NOAJ/NRAO. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION