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Suggested Citation:"2 Advances Since the Publication of Vision and Voyages." National Academies of Sciences, Engineering, and Medicine. 2020. Report Series: Committee on Astrobiology and Planetary Science: Options for the Fifth New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/25868.
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Suggested Citation:"2 Advances Since the Publication of Vision and Voyages." National Academies of Sciences, Engineering, and Medicine. 2020. Report Series: Committee on Astrobiology and Planetary Science: Options for the Fifth New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/25868.
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Suggested Citation:"2 Advances Since the Publication of Vision and Voyages." National Academies of Sciences, Engineering, and Medicine. 2020. Report Series: Committee on Astrobiology and Planetary Science: Options for the Fifth New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/25868.
×
Page 8
Suggested Citation:"2 Advances Since the Publication of Vision and Voyages." National Academies of Sciences, Engineering, and Medicine. 2020. Report Series: Committee on Astrobiology and Planetary Science: Options for the Fifth New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/25868.
×
Page 9
Suggested Citation:"2 Advances Since the Publication of Vision and Voyages." National Academies of Sciences, Engineering, and Medicine. 2020. Report Series: Committee on Astrobiology and Planetary Science: Options for the Fifth New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/25868.
×
Page 10
Suggested Citation:"2 Advances Since the Publication of Vision and Voyages." National Academies of Sciences, Engineering, and Medicine. 2020. Report Series: Committee on Astrobiology and Planetary Science: Options for the Fifth New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/25868.
×
Page 11
Suggested Citation:"2 Advances Since the Publication of Vision and Voyages." National Academies of Sciences, Engineering, and Medicine. 2020. Report Series: Committee on Astrobiology and Planetary Science: Options for the Fifth New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/25868.
×
Page 12
Suggested Citation:"2 Advances Since the Publication of Vision and Voyages." National Academies of Sciences, Engineering, and Medicine. 2020. Report Series: Committee on Astrobiology and Planetary Science: Options for the Fifth New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/25868.
×
Page 13
Suggested Citation:"2 Advances Since the Publication of Vision and Voyages." National Academies of Sciences, Engineering, and Medicine. 2020. Report Series: Committee on Astrobiology and Planetary Science: Options for the Fifth New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/25868.
×
Page 14

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

2 Advances Since the Publication of Vision and Voyages The first question in the statement of task asked if there have been significant changes in scientific understanding, programmatic developments, or technological advances relevant to Ocean Worlds, Trojan Tour and Rendezvous, Io Observer, and the LGN mission themes since the release of the 2011 planetary science decadal survey1 or its midterm review.2 Subsequent sections address each of these mission themes in turn. OCEAN WORLDS Since the 2011 decadal survey, our understanding of the icy worlds in the solar system has greatly advanced. Missions like Cassini, which orbited the Saturn system; Dawn, which orbited the two largest asteroids; and New Horizons, which flew past Pluto and into the Kuiper belt, have revolutionized our view of the role of liquid water in the outer solar system. Plumes were discovered to be erupting from a water ocean inside Enceladus. A subsurface ocean was identified inside Titan and possibly Pluto. Even asteroid Ceres shows evidence for briny eruptions—a tantalizing continuum of ocean worlds that may be or may once have been habitable.3 In the coming decade, NASA is investing in the further exploration of Ocean Worlds through missions (Europa Clipper and its contribution to the European Space Agency’s JUICE,4 to Europa and Ganymede, respectively) and major technology investments and studies of cryogenic sample return and curation (through, for example, the Scientific Exploration Subsurface Access Mechanism for Europa (SESAME), Instrument Concepts for Europa Exploration (ICEE), and Concepts for Ocean Worlds Life Detection Technology (COLDTech) programs). As stated above, NASA added the Ocean Worlds theme to the NF4 call, soliciting missions to explore Titan and/or Enceladus. Such missions were not specifically called out as New Frontiers mission themes in V&V, but as noted in its midterm review,5 the scientific importance of both Titan and Enceladus is discussed in detail in V&V. Specifically, Enceladus was identified as a potential target for a large mission if additional financial resources were made available to NASA by Congress. Similarly, Titan was identified as the destination for a high-priority, but deferred, large mission. The committee used the overall scientific priorities for Titan and Enceladus discussed in V&V as the basis for evaluating the Ocean Worlds New Frontiers missions below in terms of their potential decadal-level science. 1 National Research Council, Vision and Voyages for Planetary Science in the Decade 2013-2022, The National Academies Press, Washington, D.C., 2011, http://www.nap.edu/catalog/13117.html; hereafter V&V. 2 National Academies of Sciences, Engineering, and Medicine (NASEM), Visions and Voyages for Planetary Science in the Decade 2013-2022: A Midterm Review, The National Academies Press, Washington, D.C., 2018. 3 J.I. Lunine, “Ocean World Exploration,” Acta Astronautica 131, 123-130, 2017. 4 For more about the scientific context for the exploration of Ocean Worlds see, for example, A.R. Hendrix, T.A. Hurford, L.M. Barge, M.T. Bland, J.S. Bowman, et al., “The NASA Roadmap to Ocean Worlds,” Astrobiology 19, pp. 1-27, 2019. 5 National Academies of Sciences, Engineering, and Medicine (NASEM), Visions and Voyages for Planetary Science in the Decade 2013-2022: A Midterm Review, The National Academies Press, Washington, D.C., 2018. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 6

The committee notes, after clarifying discussions with PSD director Glaze, that it was asked to address whether scientific or programmatic developments warrant reconsidering the Titan and Enceladus missions themes in NF5. The committee was not asked to consider broadly all possible Ocean Worlds targets, as this would require a decadal-level assessment. The recently initiated planetary science and astrobiology decadal survey will comprehensively evaluate the merits of Ocean Worlds exploration endeavors at different bodies and for a variety of mission classes. Nevertheless, the Ocean Worlds mission theme presents a certain level of ambiguity in regard to programmatic decisions within the New Frontiers program. As noted above, once a New Frontiers target has been addressed by a mission, it is to be removed from the list of future-mission options. With the selection of Dragonfly, should the Ocean Worlds mission theme be considered satisfied? Or given the breadth of scientific objectives in this mission theme, including two distinct target locales, should an Enceladus mission be retained under the Ocean Worlds mission theme? Moreover, even given that Dragonfly addresses the preponderance of scientific objectives for Titan (remembering that “preponderance” in the NF4 AO is not necessarily defined by the number but by importance or influence of the scientific objectives addressed), are the scientific objectives so broad that a Titan mission might be retained under the Ocean Worlds mission theme? These questions are addressed in this report. Titan Titan is unique among the Ocean Worlds in that carbon, water, and energy (e.g., chemical potential energy) are abundant and intermittently interact on its surface. Moreover, Titan’s dense methane-rich atmosphere and Earth-like surface processes make it an ideal laboratory for studying the prebiotic chemistry that led to the development of life on Earth.6 Since the release of V&V, the primary programmatic development relevant to an Ocean Worlds theme for NF5 is the selection of the Dragonfly mission in NF4. Scheduled to launch in 2026, Dragonfly is a relocatable lander that will explore the region surrounding Selk Crater in Titan’s equatorial terrain. Using a suite of remote sensing and in situ instrumentation, Dragonfly will characterize the habitability of Titan’s environment, investigate how far prebiotic chemistry has progressed, and search for chemical evidence of water and/or hydrocarbon-based life.7 Dragonfly’s measurements have the potential to revolutionize our understanding of Titan’s organic and methanogenic cycle, including its relation to prebiotic chemistry and the processes occurring during the emergence of life on Earth. By collecting and analyzing samples directly from the surface, Dragonfly will characterize the complex, diverse, and unknown chemical composition of Titan’s organic reservoir. As a relocatable lander with an expected mission range of hundreds of kilometers, however, Dragonfly will not investigate global-scale processes, such as large-scale sediment transport pathways and precipitation patterns. Nor can it visit Titan’s hydrocarbon lakes and seas, which are exclusively found in polar terrains that are too distant to be visited by a rotorcraft inserted near the equator. As a result, Dragonfly will not fully investigate Titan’s subsurface water ocean nor its surface hydrocarbon liquid reservoirs (the second scientific objective for Titan in the NF4 AO), although Dragonfly may constrain the depth to Titan’s subsurface ocean through its seismometer or via electric field measurements of Schumann resonances in its atmosphere. These comments do not reduce in any way the value of Dragonfly as a New Frontiers class mission, but simply serve to illustrate that Titan is a complex and spatially heterogeneous world8 whose 6 NASA, The NASA Astrobiology Strategy 2015, L. Hays (ed.), NASA, Washington, D.C., 2015, https://nai.nasa.gov/media/medialibrary/2015/10/NASA_Astrobiology_Strategy_2015_151008.pdf. 7 E.P. Turtle, M. G. Trainer, J.W. Barnes, R.D. Lorenz, K.E. Hibbard, et al., “Dragonfly: In Situ Exploration of Titan's Organic Chemistry and Habitability,” 51st Lunar and Planetary Science Conference (2020), abstract 2228, https://www.hou.usra.edu/meetings/lpsc2020/pdf/2288.pdf. 8 I. Müller-Wodarg, C.A. Griffith, E. Lellouch, T.E. Cravens (eds.), Titan: Interior, Surface, Atmosphere, and Space Environment, Cambridge University Press, New York, 2014. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7

exploration will likely require multiple in situ and orbital investigations. The committee anticipates that the substantial scientific progress in understanding Titan’s interior,9 solid surface,10 hydrocarbon lakes and seas,11 atmosphere,12 climate,13 and space environment14 since V&V will inform assessment of the next steps in Titan exploration in the forthcoming planetary science and astrobiology decadal survey. Enceladus Enceladus is the only world with confirmed cryovolcanic plumes.15 Relative to other ocean worlds of the solar system, Enceladus is special—not only do data clearly show the existence of a present- day subsurface ocean, but that ocean and its chemistry are presently accessible to exploration without having to penetrate the ice shell.16 Consequently, Enceladus is uniquely compelling, or nearly so,17 for understanding the processes that generate ocean worlds, as well as for assessing habitability and searching for extant life. At the time of V&V, it was known that water vapor and ice particles emanated from the “tiger stripe” fracture systems at Enceladus’ southern pole, although it was unknown whether or not the plume material came from a global subsurface ocean of liquid water. Initial analyses of the chemistry of these particles and vapor revealed the presence of organics and ammonia,18 as well as the presence of sodium and potassium salts and bicarbonates, demonstrating that the plume material came from a liquid 9 D. Durante, D.J. Hemingway, P. Racioppa, L. Iess, and D.J. Stevenson, “Titan’s Gravity Field and Interior Structure after Cassini,” Icarus 326, 123-132, 2019. 10 R.M. C. Lopes, M.J. Malaska, A.M. Schoenfeld, A. Solomonidou, S.P.D. Birch, M. Florence, A.G. Hayes, D. A. William, J. Radebaugh, T. Verlander, E.P. Turtle, A. LeGall and S.D. Wall, A Global Geomorphologic Map of Saturn’s Moon Titan (2020), Nature Astronomy, 4, 228-223, 2020. 11 A.G. Hayes, The Lakes and Seas of Titan, Annual Review of Earth and Planetary Sciences, 44, 57-83, 2016. 12 S.M. Hörst, “Titan’s Atmosphere and Climate,” Journal of Geophysical Research: Planets, 122, 432-482, 2017. 13 J.M. Mitchell and J.M. Lora, “The Climate of Titan,” Annual Reviews of Earth and Planetary Sciences, 44, 353-380, 2016; A. G. Hayes, A Post-Cassini view of Titan’s Methane-based Hydrologic Cycle, Nature Geoscience, 11, 306-313, 2018. 14 See, for example, K. Szego, Z. Bebesi, G. Erdos, L. Foldy, F. Crary, D. McComas, et al., The Global Plasma Environment of Titan as Observed by Cassini Plasma Spectrometer During the First Two Close Encounters with Titan, Geophysical Research Letters, 32, 20, 2005; R.T. Desai, A.J. Coates; A. Wellbrock, V. Vuitton, D. González- Caniulef, et al., “Carbon Chain Anions and the Growth of Complex Organic Molecules in Titan's Ionosphere,” Astrophysical Journal Letters 844, L18, 2017. 15 Plumes on Triton have long been considered to be insolation-driven (e.g., R.L. Kirk, L.A. Soderblom, R.H. Brown, S.W. Kieffer, and J.S. Kargel, “Triton's Plumes: Discovery, Characteristics, and Models,” pp. 949-989 in Neptune and Triton (D.P. Cruikshank, ed.), University of Arizona Press, Tucson, 1995), but given the low resolution of relevant Voyager 2 imagery a cryovolcanic origin is possible. 16 Observations of Saturn’s E ring, sourced from the plumes, go back to the 1960s (S. Kempf, M. Horányi, H.- W. Hsu, T.W. Hill, A, Juhász, and H.T. Smith, “Saturn’s Diffuse E Ring and its Connection with Enceladus,” pp. 195-210 in Enceladus and the Icy Moons of Saturn. (P.M. Schenk et al., eds.), University of Arizona Press, Tucson, 2018). 17 The existence of present-day plumes on Europa is less secure. See, for example, X. Jia, M.G. Kivelson, K.K. Khurana, and W.S. Kurth, “Evidence of a Plume on Europa from Galileo Magnetic and Plasma Wave Signatures,” Nature Astronomy 2, 459-464, 2018, and references therein. 18 J.H. Waite Jr, W.S. Lewis, B.A. Magee, J.I. Lunine, W.B. McKinnon, et al, “Liquid Water on Enceladus from Observations of Ammonia and 40Ar in the Plume,” Nature 460, pp. 487-490, 2009. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 8

source.19,20 Gravity and libration measurements confirmed that this liquid source came from a global subsurface ocean.21 That the waters streaming from the Enceladus ocean are potentially habitable has since been established based on several lines of evidence: • Silica and molecular hydrogen were detected in Saturn stream particles and Enceladus plume vapor, respectively, and are inferred to be generated from hydrothermal reactions.22,23 • Carbon dioxide concentration and carbonate chemistry in the plume were analyzed and found to support a compositionally heterogeneous rocky core with deep surface serpentinization and shallower carbonate (similar to the terrestrial oceans).24 Further, the inferred concentrations of H2 and CO2 within the ocean could support microbial metabolism (methanogenesis). • Complex, locally generated organics were identified and characterized via mass spectrometry while flying through the plume.25,26 Thus, analysis of the ocean waters at Enceladus is poised to not only assess habitability, but move directly to life detection in the next mission. Concurrently, fundamental questions about the processes that allow the generation of this ocean have matured. A key question in light of Enceladus’ small size is how the heating required to sustain the subsurface ocean can be maintained over geologic time. Recent studies of the rings of Saturn have led to the proposal that the inner midsize moons may be geologically young, and tidal flexure of a porous core has been proposed as a means to sustain long-term heating that supports an ocean.27 Modern processes are also of interest, as solar occultation studies and maps of plume fallout on the Enceladus surface elucidate the means of their emplacement.28,29 Ewald and Ingersoll have found a long-term periodicity to plume water ice flux so far unexplained by models of tidal flexure.30 Key 19 F. Postberg, S. Kempf, J. Schmidt, N. Brilliantov, A. Beinsen, B. Abel, U. Buck, and R. Srama, “Sodium Salts in E-ring Ice Grains from an Ocean Below the Surface of Enceladus,” Nature 459, pp. 1098-1101, 2009. 20 J.R. Spencer, F. Nimmo, A.P. Ingersoll, T.A. Hurford, E.S. Kite, A.R. Rhoden, J. Schmidt, and C.J.A. Howett, “Plume Origins and Plumbing: From Ocean to Surface,” pp. 163-174 in Enceladus and the Icy Moons of Saturn. (P. M. Schenk et al., eds.), University of Arizona Press, Tucson, 2018. 21 See, for example, D. Hemingway et al. “The Interior of Enceladus,” pp. 57-77 in Enceladus and the Icy Moons of Saturn. (P. M. Schenk et al., eds.), University of Arizona Press, Tucson, 2018. 22 H.-W. Hsu, F. Postberg, Y. Sekine, T. Shibuya, S. Kempf, et al., “Ongoing Hydrothermal Activities within Enceladus,” Nature 519, pp. 207-210, 2015. 23 J.H Waite, Jr., C.R. Glein, R.S. Perryman1, B.D. Teolis, B.A. Magee, et al., Cassini Finds Molecular Hydrogen in the Enceladus Plume: Evidence for Hydrothermal Processes,” Science 356, 155-159, 2017. 24 C.R. Glein and J.H Waite, Jr., “The Carbonate Geochemistry of Enceladus' Ocean,” Geophysical Research Letters 47, e85885, 2020. 25 F. Postberg, N. Khawaja, B. Abel, G. Choblet, C.R. Glein, et al., “Macromolecular Organic Compounds from the Depths of Enceladus,” Nature 558, 564-568, 2018. 26 N. Khawaja, F. Postberg, J. Hillier, F. Klenner, S. Kempf, et al., “Low-mass Nitrogen-, Oxygen-bearing, and Aromatic Compounds in Enceladean Ice Grains, Monthly Notices of the Royal Astronomical Society 489, pp. 5231- 5243, 2019 27 L. Iess, B. Militzer, Y. Kaspi, P. Nicholson, D. Durante, et al.,” Measurement and Implications of Saturn’s Gravity Field and Ring Mass,” Science 2019, 10.1126/science.aat2965; and G. Choblet, G. Tobie, C. Sotin, M. Běhounková, O. Čadek, et al., “Powering Prolonged Hydrothermal Activity Inside Enceladus,” Nature Astronomy 1, 841-847, 2017. 28 M.M. Hedman, D. Dhingraa, P.D. Nicholson, C.J. Hansen, G. Portyankina, et al., “Spatial Variations in the Dust-to-Gas Ratio of Enceladus’ Plume,” Icarus, 305, 123-138, 2018. 29 B.S. Southworth, S. Kempf, and J. Spitale, “Surface Deposition of the Enceladus Plume and the Zenith Angle of Emissions,” Icarus, 319, 33-42, 2019 30 A. Ingersoll and S. Ewald, “Decadal Timescale Variability of the Enceladus Plumes Inferred from Cassini Images,” Icarus, 282, 260-275, 2017. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 9

outstanding questions include the deep structure of Enceladus and the interaction with tidal processes to enable this unique exposure of the waters from an interior ocean. The potential exploration of Enceladus has also benefited from substantial technology investment, both from the Enceladus-focused missions proposed (albeit unsuccessfully) to NF4, and from technology investments applicable to Ocean Worlds generally. As part of the NF4 selections, the Enceladus Life Signatures and Habitability mission was selected for a technology development effort to enable maturation of cost-effective techniques that limit spacecraft contamination during plume sampling. Biosignature and life detection instrumentation has been a focus of ICEE-2, COLDTech, and Maturation of Instruments for Solar System Exploration programs. Future Enceladus exploration can also leverage technologies under development for a possible Europa lander.31 TROJAN TOUR AND RENDEZVOUS Knowledge of the Trojan asteroids has increased considerably since the completion of V&V, in terms of the quality and completeness of their astronomical characterization; new kinds of telescopic observations, including in the thermal-infrared and mid-ultraviolet; and the discoveries of additional binary systems, for which we can determine masses and bulk densities, and dynamical families indicative of disruptions. We have a more complete view of their great diversity in size, shape, and composition.32 Additionally, significant advancements in solar system dynamics have provided powerful constraints on the migration of Jupiter and other giant planets after their formation, and the possible origin of the Trojan swarm. Indeed, one of the key motivations for a Trojan Tour in V&V was its unique ability to resolve some of the primary debates concerning early planet formation, with great relevance to debates concerning the formation of Earth and other planets. The traditional theory was that Trojans are remnants of Jupiter’s feedstock planetesimals, a collection of primitive bodies that were trapped in the co-orbital gravitational potential wells (Sun-Jupiter Lagrange points L4 and L5) that lead and trail Jupiter by 60° in its orbit around the Sun. Modern ideas of giant planet migration make the survival of such an original swarm unlikely and have led to a new paradigm: Trojans are proto- KBOs that were scattered inwards during giant planet migration, with a fraction of them being captured by Jupiter to become the L4 and L5 swarms. If true, this paradigm places important quantitative constraints on planet formation and migration,33 with predictions for the growth of terrestrial planets and the delivery of volatiles to Earth. While modern dynamical models make a strong case for captured Trojans, there are substantial, unexplained differences between current spectroscopic properties (and thus, taxonomies) of Trojans and those of KBOs. They look like different populations. One idea is that KBOs, once trapped in Jupiter’s orbit at 5 AU, evolved thermally, chemically, and dynamically (e.g., through collisions) in ways that led to the diversity of the modern population. This idea connects deeply with our understanding of primitive asteroids in general, and with the still poorly understood connections between asteroids and comets, and the compositions of bodies that were accreted by the terrestrial planets, including Earth. As noted in V&V,34 “In-depth study of these objects will provide the opportunity to understand the degree of mixing in the solar system and to determine the composition and physical characteristics of bodies that are among the most primitive in the solar system.” 31 K.P. Hand, A.E. Murray, W.B. Brinkerhoff, B.C. Christner, K.S. Edgett, et al., Report of the Europa Lander Science Definition Team, JPL D-97667, Jet Propulsion Laboratory, Pasadena, Calif., 2016, https://europa.nasa.gov/resources/58/europa-lander-study-2016-report/. 32 J.P. Emery, F. Marzari, A. Morbidelli, L.M. French, and T. Grav, 2015, The complex history of Trojan asteroids. In Asteroids IV (P. Michel et al., eds.), pp. 203-220. University of Arizona Press, Tucson. 33 See, for example, D. Nesvorný, D. Vokrouhlický, W.F. Bottke, et al., 2018, Evidence for very early migration of the solar system planets from the Patroclus–Menoetius binary Jupiter Trojan. Nature Astronomy 2, 878-882. 34 See NASEM, Vision and Voyages for Planetary Science in the Decade 2013-2022, The National Academies Press, Washington, D.C., 2011, p. 102. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 10

In sum, the scientific understanding of the Trojan asteroids has advanced considerably since V&V due to Earth-based observations and theoretical modelling efforts. Nevertheless, the major scientific questions posed in V&V regarding these bodies remain unanswered and, if anything, have been accentuated and made more relevant to planetary science. To address these questions requires in-depth investigation of one or more of these worlds, and these questions form the scientific basis for the selection of the Lucy multiple-Trojan-flyby Discovery mission, scheduled for launch in 2021. This mission, discussed further in Chapter 3, is the major programmatic development since V&V regarding the Trojan asteroids. Lucy takes advantage of the significant technological advancement in space solar power systems, which enables operations at Jupiter’s distance from the Sun.35 It also leverages technological advancement in instrument design. As described below, a key scientific difference between Lucy and the Trojan Tour and Rendezvous New Frontiers mission envisioned in V&V is the lack of a final rendezvous encounter and the associated instrumentation that would measure elemental composition of near-surface materials. IO OBSERVER Driven by its orbital resonance with Europa and Ganymede, Io exhibits extraordinary characteristics that are unprecedented in the solar system—inferred extreme flexure and tidal heating, high heat flow, extensive ongoing volcanism, striking tectonic mountains, an unusual and dynamic atmosphere dominated by sulfur dioxide gas, and an outsized influence on the jovian magnetosphere.36 Io’s heat flux, as directly measured in the infrared from Earth, is at least 30 times that of Earth’s average value today (per unit area). As an indirect result of the extreme volcanism, Io is a source of roughly 1 ton of material per second injected into the jovian magnetosphere, primarily sulfur and oxygen, but also important, less abundant species such as sodium, potassium, and chlorine. This material rapidly fills and inflates Jupiter’s magnetosphere, by far the largest in the solar system. Material from Io also reaches and influences the surfaces and environments of other jovian satellites. The extremes of high heat flow and extensive volcanism currently exhibited by Io are thought to have been much more common very early in solar system history—that is, when bodies such as Earth and the Moon may have possessed magma oceans—making studies of Io particularly important for informing our understanding of the earliest evolution of the solid planets of the solar system. Io is also an important analog for extremely heated exoplanets, including both tidally heated worlds (e.g., the resonant super-Ios in TRAPPIST-1)37 and worlds heated by extreme solar insolation (e.g., close-in “lava worlds” such as 55 Cancri e).38 Since V&V, data sets from the Voyager, Galileo, New Horizons missions, along with limited new measurements from Juno, continue to be analyzed. Analysis of Galileo magnetic induction measurements on two passes by Io, which suggested a global magma ocean, have been both supported and challenged.39 35 Other examples of solar-powered spacecraft designed to operate in the asteroid belt and beyond include Juno, JUICE, and Europa Clipper. 36 J. Perry, R.M.C. Lopes, J.R. Spencer, and C. Alexander, “A summary of the Galileo mission and its observations of Io,” pp. 35-60 in Io After Galileo (R.M.C. Lopes and J.R. Spencer, eds.), Springer-Praxis, Chichester, U.K., 2006. 37 A.C. Barr, V. Dobos, and L.L. Kiss, “Interior Structures and Tidal Heating in the TRAPPIST-1 Planets,” Astronomy and Astrophysics, 613, A37, 2018. 38 B.-O. Demory, M. Gillon, J. de Wit, N. Madhusudhan, E. Bolmont, et al., “A Map of the Large Day–Night Temperature Gradient of a Super-Earth Exoplanet,” Nature 532, 207-209, 2016. 39 O. Šebek, P.M. Trávníček, R.J. Walker, and P. Hellinger, “Dynamic Plasma Interaction at Io: Multispecies Hybrid Simulations,” Journal of Geophysical Research: Space Physics 124, 313-341, 2019; L. Roth, J. Saur, K.D. Retherford, A. Blöcker, D.F. Strobel, and P.D. Feldman, “Constraints on Io’s Interior from Auroral Spot Oscillations,” Journal of Geophysical Research: Space Physics 122, 1903-1927, 2017; and A. Blöcker, J. Saur, L. Roth, D.F. Strobel, “MHD Modeling of the Plasma Interaction With Io's Asymmetric Atmosphere,” Journal of Geophysical Research: Space Physics 123, 9286-9311, 2018. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 11

Alternative interpretations involving both an asymmetric ionosphere and a highly conducting metallic core have been offered to explain both the induction signature and auroral observations of Io from the Hubble Space Telescope. On the theoretical side, there are multiple alternative hypotheses for the nature of the Io interior that might enable its extraordinary volcanism. Earth-based observations of Io have improved dramatically. High-resolution, high-cadence, adaptive-optics near-infrared imaging has enabled observation of non-symmetric patterns in the volcanism of specific hot spots, as well as the overall time evolution of volcanic eruptions across the surface of Io.40 Earth-based observations have provided the first evidence of a sharp decrease in Io’s atmospheric pressure as Io is eclipsed by Jupiter.41 Observations from the Atacama Large Millimeter/Submillimeter Array can now measure chemical species and isotopologues (so far, those of sulfur) in Io’s atmosphere.42 Despite the considerable scientific progress summarized above, major knowledge gaps remain with respect to geophysical and geochemical measurements (e.g., gravity/magnetic fields, atmospheric chemistry, etc.), the extent and variability of the atmosphere, and the link between Io’s active volcanism, its interior structure, and the copious amount of material injected into the jovian magnetosphere. Improved geophysical data, for example, gravity and magnetic field data, are required to differentiate between competing hypotheses. Technological advancements relevant to missions to Io since V&V include improved solar power at 5 AU as a result of the Juno, JUICE, Europa Clipper, and Lucy missions, as noted above, and radiation hardening and mitigation pioneered on Juno and Europa Clipper. The latter include new radiation models, improvements in shielding, and development of radiation-hardened instruments. LUNAR GEOPHYSICAL NETWORK Since V&V and the midterm review, substantial advancements have been made in our scientific understanding of the lunar interior through the results of the Gravity Recovery and Interior Laboratory (GRAIL) and Lunar Reconnaissance Orbiter (LRO) missions, re-analysis of Apollo seismic data, modeling efforts to explain the different lunar surface terranes, and petrological, isotopic, and paleomagnetic studies of lunar samples. GRAIL data have revealed features of the lunar crust in unprecedented detail. These include deep crustal porosity, fractures and other tectonic structures, mascons, lava tubes and other volcanic landforms, impact basin rings, and the shape and size of complex to peak-ring lunar craters.43 For example, GRAIL gravity gradient data over the Procellarum KREEP (potassium, rare earth elements, and phosphorus) Terrane revealed a surrounding dyke system, suggesting it may be a magmatic-tectonic feature overlying the nearside “magma plumbing system” that supplied the mare with their basaltic infills.44 LRO data revealed subtle tectonic features across the lunar surface, 40 See, for example, K. de Kleer, M. Skrutskie, J. Leisenring, A.G. Davies, A. Conrad, et al., “Multi-phase Volcanic Resurfacing at Loki Patera on Io,” Nature 545, 199-202, 2017; and K. de Kleer, I. de Pater, E.M. Molter, E. Banks, A.G. Davies, et al., “Io’s Volcanic Activity from Time Domain Adaptive Optics Observations,” Astronomical Journal 158, 29, 2019. 41 C.C.C. Tsang, J.R. Spencer, E. Lellouch, M.A. Lopez-Valverde, and M.J. Richter, “The Collapse of Io’s Primary Atmosphere in Jupiter Eclipse,” Journal of Geophysical Research: Planets 121, 1400-1410, 2016. 42 A. Moullet, E. Lellouch, R. Moreno, M. Gurwell, J.H Black, and B. Butler, “Exploring Io’s Atmospheric Composition With APEX: First Measurement of 34SO2 and Tentative Detection of KCl,” Astrophysical Journal 776, 32, 2013. 43 See, for example, M.A. Wieczorek1, G.A. Neumann, F. Nimmo, W.S. Kiefer, G.J. Taylor, et al., “The Crust of the Moon as Seen by GRAIL,” Science 339, 671-675, 2013. 44 J.C. Andrews-Hanna, J. Besserer, J.W. Head III, C.J. A. Howett, W.S. Kiefer, et al., “Structure and Evolution of the Lunar Procellarum Region as Revealed by GRAIL Gravity Data,” Nature 514, 68-71, 2014. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 12

including thrust faults that have been proposed to be the cause of the high-magnitude shallow moonquakes, although a definitive linkage cannot be made without a seismic network.45 Analyses of GRAIL data have also produced a family of core models consistent with geodetic parameters (including constraints from ongoing lunar laser ranging experiments),46 but gravity data alone have not yet definitively identified the presence of an inner core. Laser ranging data suggest a liquid core, although combined gravity, topography, and laser ranging data predict a solid inner core, along with a total core size similar to the core modeled using Apollo seismic data.47 Paleomagnetic studies of Apollo samples have demonstrated that the Moon had surface magnetic fields of ~30–100 μT lasting from at least 4.2 Ga until 3.56 Ga, interpreted as evidence for the existence of an ancient core dynamo.48 When the dynamo initiated and ended is uncertain, however, as is the mechanism or mechanisms that allowed it to operate. Substantial efforts have been made to mature potential lunar geophysical instruments over the last decade, including investments by NASA, and in our understanding of the scattering properties of the lunar regolith through the application of terrestrial seismic data-analysis techniques to Apollo data. Such studies have fed instrument design and deployment needs. The seismometer currently in operation on the Interior Exploration using Interior, Seismic Investigations, Geodesy and Heat Transport (InSIGHT) spacecraft on Mars is a notable example and represents a very broadband seismometer with a sensitivity and frequency range far better than Apollo. In addition, other seismometer developments have been funded by NASA through the Development and Advancement of Lunar Instrumentation program and other technology development activities. Heat flow probes for the Moon have also been improved. Because the lunar regolith has experienced millions of years of seismic shaking and porosity reduction, a mechanical mole-type heat probe, which pushes regolith out of the way as it is deployed underground, is not tenable on the Moon. Rather, compressed gas-drive removal technologies to remove regolith from boreholes have been tested in vacuum and instruments ready for flight opportunities have matured.49 Magnetotelluric methods for gathering electromagnetic data combine analyses of electrical and magnetic fields at a single point on a planetary surface without knowledge of the source field; relevant instrumentation has also advanced and matured under NASA funding. A new passive laser retroreflector—the Next Generation Laser Retroreflector (NGLR)—developed through NASA and international funding can support up to 100-times better range accuracy.50 This will produce lower science parameter uncertainty, such as in the selenocentric reflector coordinates, fractional moment-of-inertia differences, a lunar Love number, and rotational dissipation. Adding these to and expanding the existing lunar laser ranging network would yield greatly improved estimates of shape and dissipation of the liquid-core/solid-mantle boundary and, in conjunction with GRAIL data, would improve the quantification of principal moments of inertia of the Moon. Programmatically, NASA is currently investing heavily in several activities centered on the Moon within SMD, STMD, and HEOMD. Within SMD, a key component of the Lunar Discovery and 45 P.S Kumar, U. Sruthi, N. Krishna, K.J.P. Lakshmi, R. Menon, et al., “Recent Shallow Moonquake and Impact‐Triggered Boulder Falls on the Moon: New Insights from the Schrödinger Basin,” Journal of Geophysical Research: Planets 121, 147-179, 2016. 46 See, for example, M. Laneuville, M.A. Wieczorek, D. Breuer, J. Aubert, G. Morard, and T. Rückriemen, “A Long-Lived Lunar Dynamo Powered by Core Crystallization,” Earth and Planetary Science Letters 401, 251-260, 2014. 47 R.C. Weber, P.-Y. Lin, E.J. Garnero, Q. Williams, and P. Lognonné, “Seismic Detection of the Lunar Core,” Science 331, 309-312, 2011. 48 See, for example, B.P. Weiss and S.M. Tikoo, “The Lunar Dynamo,” Science 346, 1198, 2014. 49 S. Nagihara, P. Ngo, L. Sanasarian, V. Sanigepalli, and K. Zacny, “Heat Flow Probe for Short-Duration Lunar Missions on Small Landers,” 50th Lunar and Planetary Science Conference, abstract1557, 2019. 50 D.G. Currie, S. Dell-Agnello, G.O. Delle Monache, B. Behr, and J.G. Williams, “A Lunar Laser Ranging Retroreflector Array for the 21st Century,” Nuclear Physics B 243-244, 218-228, 2013. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 13

Exploration Program (LDEP) is the Commercial Lunar Payload Services (CLPS) initiative.51 CLPS aims to contract with private companies to send small lunar landers to the lunar surface and to deliver scientific and other payloads. At present, CLPS landers are solar powered and not designed to last through the lunar night. The first three launch providers have been selected, with two payloads to the Moon in 2021 and one in 2022. SMD has also announced, as part of LDEP, a future rover mission to lunar South Pole—Volatiles Investigating Polar Exploration Rover (VIPER)—which will focus on resource prospecting, particularly the search for water ice in the lunar regolith, and is unrelated to the objectives of the LGN. Within STMD, the Lunar Surface Innovation Initiative (LSII) has just been started (end of February 2020) and aims to develop a number of advanced technologies to enable future robotic and human exploration of the Moon. The human exploration component, Artemis, is in early stages of formulation.52 51 NASEM, Review of the Planetary Science Aspects of NASA SMD’s Lunar Science and Exploration Initiative, and NASEM, Review of the Commercial Aspects of NASA SMD’s Lunar Science and Exploration Initiative, both published by The National Academies Press, Washington, D.C., 2019. 52 NASA, NASA’s Plan for Sustained Lunar Exploration and Development, Washington, D.C., 2020, https://www.nasa.gov/sites/default/files/atoms/files/a_sustained_lunar_presence_nspc_report4220final.pdf. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 14

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Report Series: Committee on Astrobiology and Planetary Science: Options for the Fifth New Frontiers Announcement of Opportunity Get This Book
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The Committee on Astrobiology and Planetary Sciences of the National Academies of Sciences, Engineering, and Medicine is tasked with monitoring the progress in implementation of the recommendations of the most recent planetary science decadal survey, Vision and Voyages for Planetary Science in the Decade 2013-2022. Planetary science decadal surveys evaluate the state of the field, identify the most important scientific questions and themes, and prioritize missions and activities for the decade in question based on scientific merit, technical feasibility, and anticipated cost. The need for careful monitoring is underscored by the fact that some of the decadal survey's recommendations are triggered at specific programmatic decision points. Options for the Fifth New Frontiers Announcement of Opportunity addresses one such decision point.

For each of the following four New Frontiers targets: Ocean Worlds, Trojan Tour and Rendezvous, Io Observer and Lunar Geophysical, this report summarizes changes in scientific understanding or external factors since the release of Vision and Voyages or its midterm review and considers whether those changes have been sufficiently substantial to warrant reconsideration of the four targets for inclusion in the New Frontiers 5 announcement of opportunity, scheduled for release in early 2022.

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