Origins, Worlds, and Life A Decadal Strategy for Planetary Science and Astrobiology 2023-2032 (2023) / Chapter Skim
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28 ORIGINS, WORLDS, AND LIFE REFERENCES Hicks, D., M Zullo, A
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2 Tour of the Solar System: A Transformative Decade of Exploration The past decade has witnessed an explosive growth in the state of knowledge of planetary science and astrobiology through the invaluable combination of new missions and data, supporting theoretical and modeling research, telescopic observations, and laboratory and experimental advances. In this chapter, the committee discusses some of the most exciting advances from the past decade, organized by destination or destination class as represented by the committee panels.
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30 ORIGINS, WORLDS, AND LIFE Some of the most notable discoveries of the past decade include measurements of Mercury's crustal geochemistry: elevated abundances of sulfur and carbon, low abundances of iron and oxygen, and a nearly chondritic chlorine/ potassium ratio. These characteristics have important implications for the thermochemical evolution of Mercury and point to a planet that is surprisingly rich in volatiles and that formed under highly reducing conditions.
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TOUR OF THE SOLAR SYSTEM 31 FIGURE 2-1 Giant tectonic landforms like Carnegie Rupes, which is more than 2 km high in places, provide evidence that Mercury shrunk as it cooled, creating a lobate scarp (arrowed)
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32 ORIGINS, WORLDS, AND LIFE FIGURE 2-2 Volatile-rich rocks on Mercury are thought to sublime when exposed by impact events, leaving behind features known as hollows, such as those that cover floor Kertész crater (31 km in diameter)
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TOUR OF THE SOLAR SYSTEM 33 FIGURE 2-3 Despite their hot surroundings, permanently shadowed regions near Mercury's poles stay cold enough to host abundant deposits of water ice. SOURCE: Courtesy of N.L.
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34 ORIGINS, WORLDS, AND LIFE magnetoplasma circulation. Mercury's neutral exosphere contains nine known species: hydrogen, helium, sodium, potassium, calcium, magnesium, aluminum, iron, and manganese.
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TOUR OF THE SOLAR SYSTEM 35 THE MOON A wealth of data from new missions, reinterpretation of old data, and new analyses of Apollo samples and meteorites, have, over the past decade, provided new insight into the evolution of the Moon, Earth, and the early solar system. The Lunar Reconnaissance Orbiter (LRO)
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36 ORIGINS, WORLDS, AND LIFE fractionated relative to terrestrial values further suggests that the Moon lost a substantial fraction of its volatile elements both during and after it formed. Volatile loss from the lunar interior was likely important for generating transient atmospheres and for distributing some of the volatile species presently on the lunar surface.
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TOUR OF THE SOLAR SYSTEM 37 Such large-scale crustal asymmetry is further found in nearside/farside differences in porosity, heat-producing elements, and extent of volcanism; the cause of the Moon's crustal asymmetry remains one of the greatest outstanding mysteries regarding lunar early evolution. The crustal thickness estimates also provide constraints on the bulk composition of the silicate portion of the Moon.
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38 ORIGINS, WORLDS, AND LIFE FIGURE 2-5 The Moon's volcanic activity may have extended into geologically recent times to produce features like Ina, thought to be a volcanic caldera some 2 km across. North is to the right in this image from NASA's Lunar Reconnaissance Orbiter.
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TOUR OF THE SOLAR SYSTEM 39 impact bombardment of the lunar surface would be mirrored across the inner solar system, including on the early Earth. Only the most recent impact events are preserved on Earth, and LRO data combined with the terrestrial evidence suggest that the flux of impactors to the Earth-Moon system may have increased at about 290 Ma, and the breakup of specific asteroid families may be recorded in the Moon's crater population.
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40 ORIGINS, WORLDS, AND LIFE FIGURE 2-7 A map of the lunar south pole (poleward of 87.5°S)
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TOUR OF THE SOLAR SYSTEM 41 hydrogen, energetic protons, or chemically altered in the regolith and released as new H-bearing species, including H2 and methane. The LADEE mission did not find a substantial and sustained water exosphere.
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42 ORIGINS, WORLDS, AND LIFE VENUS A Tale of Two Planets Viewed from afar -- say, a few tens of light-years -- the solar system contains two remarkably similar large, rocky worlds. They are close in size: one is only slightly less massive, and slightly smaller, than the other.
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TOUR OF THE SOLAR SYSTEM 43 currently operating at Venus. The last dedicated U.S.
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44 ORIGINS, WORLDS, AND LIFE FIGURE 2-9 Examples of volcanic, tectonic, and impact features on Venus. Magellan radar data revealed a remarkably diverse planetary surface at Venus.
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TOUR OF THE SOLAR SYSTEM 45 FIGURE 2-10 Venera 13 on the Venus surface. Artist's impression of the Soviet Venera 13 lander sitting in Navka Planitia in March 1982.
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46 ORIGINS, WORLDS, AND LIFE have come into operation in the past decade, allowing for the study of these weathering processes. Limited by the few constraints we actually have for rocks on the Venus surface, and by even less information about the types of surface-atmospheric interactions that weather rocks there, such studies are nonetheless valuable.
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TOUR OF THE SOLAR SYSTEM 47 FIGURE 2-11 The Vega-1 balloon in flight. Artist's impression of the Soviet Vega-1 balloon drifting in the middle Venus atmosphere in June 1985.
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48 ORIGINS, WORLDS, AND LIFE have been much more efficient at stripping away atmosphere than at present, this question is especially important to our efforts to piece together the ancient history of the Venus climate. Excitingly, it is this climate history that we have recently begun to rethink.
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TOUR OF THE SOLAR SYSTEM 49 The Case for Continued Exploration of Venus Venus is once again back in the spotlight. In the coming years and decades, we will continue to develop the recently emerging view of Venus as a complex, active world, augmented by new spacecraft data, ever more sophisticated climate modeling, and the finding of an increasing number of Venus-size rocky worlds in close proximity to their host stars.
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50 ORIGINS, WORLDS, AND LIFE MARS Significant advances in understanding Mars as a system have been made during the previous decade since Vision and Voyages (NRC 2011)
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TOUR OF THE SOLAR SYSTEM 51 Preserved Organic Compounds A groundbreaking discovery of the decade relating to the possibility of past life on Mars was the detection of organic matter in the lake sediments of Gale Crater by the Curiosity rover's SAM instrument (Figure 2-12)
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52 ORIGINS, WORLDS, AND LIFE of infall of exogenic carbonaceous materials and abiotic synthesis on Mars during water–rock reactions and magmatic processes. Although meteorite results so far have not found any organic biosignatures, they support the widespread occurrence and preservation of prebiotic compounds on Mars.
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TOUR OF THE SOLAR SYSTEM 53 Seasonal Behavior of the Atmosphere The seasonal and annual behavior of weather in today's lower atmosphere is dominated by the seasonal cycles of dust, water vapor, and CO2. Although we have known about dust storms for over a century, improved observations and continuous monitoring over the past decade have resulted in greater understanding of how they begin and evolve, and of annual patterns in when and where they occur.
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54 ORIGINS, WORLDS, AND LIFE FIGURE 2-13 A layer of underground ice exposed along a steep slope; the ice appears bright blue in this enhanced-color view from the HiRISE camera on NASA's Mars Reconnaissance Orbiter. The scene is about 500 m wide.
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TOUR OF THE SOLAR SYSTEM 55 ice could deposit at higher altitudes and provide a source for liquid water. Second, recent modeling and laboratory experiments have shown that greenhouse warming from a combination of CO2 and reducing gases such as hydrogen or methane could have warmed early Mars.
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56 ORIGINS, WORLDS, AND LIFE Evolution of the Martian Surface and Interior Geologic findings in the past decade have significantly altered our understanding of the first billion years of martian history, as well as revealed active surface and subsurface processes on modern-day Mars. New Discoveries from Very Old Rocks The ancient igneous crust of Mars is now recognized to be much more compositionally complex than previously realized.
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TOUR OF THE SOLAR SYSTEM 57 FIGURE 2-15 The NWA 7034 martian meteorite, a breccia containing fragments of 4.4-billion-year-old volcanic rocks, has provided unique insights into the earliest period of Mars history. SOURCE: Institute of Meteoritics UNM © The University of New Mexico.
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58 ORIGINS, WORLDS, AND LIFE FIGURE 2-16 Ground acceleration as a function of time over a period of 40 minutes recorded by the very broad band seismometer on InSight for a marsquake located near Cerberus Fossae, the site of geologically recent eruptions of magma and discharge of water. The three traces show accelerations in the east, north, and vertical directions, and dashed lines show the arrival of P and S waves and began at about 2:23 (local mean solar time)
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TOUR OF THE SOLAR SYSTEM 59 Key Discoveries from the Past Decade • Detection of organic matter in the lake sediments of Gale Crater. Curiosity rover data acquired from sedimentary rocks show similar evidence as data from igneous Mars meteorites (exogenic infall of carbonaceous organic matter and chemical reactions on Mars that synthesize or alter organic matter)
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60 ORIGINS, WORLDS, AND LIFE SMALL SOLAR SYSTEM BODIES Small bodies are rocky and icy worlds that span the solar system. The most commonly known ones are asteroids and comets, and their major reservoirs include the asteroid belt, Trojan populations, trans-neptunian region, and Oort cloud.
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TOUR OF THE SOLAR SYSTEM 61 FIGURE 2-17 Major populations of solar system small bodies. Historically, the reservoir of small bodies between ~30 and ~50 AU has been referred to as the Kuiper belt.
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62 ORIGINS, WORLDS, AND LIFE FIGURE 2-18 Analyses of extraterrestrial samples occur in laboratories across the country. Upper left: Meteorite analysis being conducted at Carnegie Institution.
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TOUR OF THE SOLAR SYSTEM 63 FIGURE 2-19 The orbits of the most distant trans-neptunian objects in the solar system are shown. The magenta orbit is one possible orbit for a distant, as-yet undiscovered, giant planet that dynamical simulations suggest could be responsible for the clustering of trans-neptunian objects.
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64 ORIGINS, WORLDS, AND LIFE FIGURE 2-20 New Horizons composite view of Arrokoth from images obtained on January 1, 2019. Arrokoth measures approximately 36 km along its longest axis.
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TOUR OF THE SOLAR SYSTEM 65 Main Asteroid Belt Planetesimals and the Moons of Mars A highlight of the past decade was the first spacecraft exploration of the asteroid Vesta, the second-largest body in the main belt (Figure 2-21)
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66 ORIGINS, WORLDS, AND LIFE planetary mantles, would be exposed. The missing olivine is likely sequestered in the deep mantle, whereas the upper mantle is dominated by orthopyroxene-rich diogenite.
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TOUR OF THE SOLAR SYSTEM 67 planned for launch in 2024 with NASA as a contributing partner agency, is positioned to test these theories for the origin of the martian moons, through spacecraft measurements of the two moons as well as bringing samples of Phobos to Earth in 2029. Exploration of Near-Earth Asteroids Near-Earth asteroids are fragments of main belt asteroids that can approach or possibly strike Earth.
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68 ORIGINS, WORLDS, AND LIFE FIGURE 2-22 OSIRIS-REx sampling the asteroid Bennu on October 20, 2020. The white circular sampling head is about 30 cm across.
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TOUR OF THE SOLAR SYSTEM 69 mission is, at the time of writing, scheduled to launch with Psyche and send twin smallsat to explore two binary asteroid systems in 2026. Knowledge gained about near-Earth asteroids also has implications beyond the scientific exploration of the solar system.
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70 ORIGINS, WORLDS, AND LIFE Critical connections between life and impacts can also be indirect. For example, large impacts appear to enhance the output of existing volcanic plumes via seismic shaking.
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TOUR OF THE SOLAR SYSTEM 71 in the past decade and the first spacecraft exploration of a primitive trans-neptunian object suggest distinctive accretion and evolutionary processes, with still much to discover about this population we have only begun to explore. • Inner solar system planetesimals provide new insights into planetary evolutionary processes.
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72 ORIGINS, WORLDS, AND LIFE GIANT PLANET SYSTEMS Overall Architecture The giant planets -- Jupiter, Saturn, Uranus, and Neptune -- dominate the solar system, containing more than 99 percent of the mass and angular momentum outside of the Sun. Their formation, and likely orbital migration, dictated the structure of the rest of the solar system, controlling terrestrial planet growth rates, locations, and much of their volatile inventories, as well as the ultimate distribution of all small body populations in the solar system.
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TOUR OF THE SOLAR SYSTEM 73 and their host planets provide provides a third energy source in addition to energy of accretion and radiogenic heating, helping to maintain subsurface oceans which may harbor life. To understand whether the solar system architecture is typical or unique requires comparing a deep understanding of the formation and evolution of the giant planet systems, while surveying the size, number, and composition of the more than 2,000 giant exoplanets discovered to date.
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74 ORIGINS, WORLDS, AND LIFE Giant planet atmospheres are mostly hydrogen and helium, with other common elements like carbon included as hydrogenated molecules in small amounts, and a general trend of more enriched elemental abundances in the ice giants than the gas giants. The noble gases (helium, neon, argon, krypton, and xenon)
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TOUR OF THE SOLAR SYSTEM 75 FIGURE 2-24 Cassini highlight montage. Top row, left to right: Red streaks on Tethys; south pole of Enceladus; Saturn's rings seen from high elevation.
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76 ORIGINS, WORLDS, AND LIFE 3,000 km and 9,000 km for Jupiter and Saturn, respectively. The flows decay where the electrical conductivity is sufficiently high to drag the flow into nearly uniform rotation at greater depth.
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TOUR OF THE SOLAR SYSTEM 77 RP) and use the distance from the planet's center to the subsolar boundary (basically the smallest scale)
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78 ORIGINS, WORLDS, AND LIFE The magnetospheres of Jupiter and Saturn are dominated by plasma sources from their geologically active moons -- Io and Enceladus, respectively. The interaction of the surrounding plasma with Io's atmosphere causes tons of atmospheric gases (mostly SO2 and dissociation products O, S)
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TOUR OF THE SOLAR SYSTEM 79 Astrobiological Potentials of Giant Planet Moons Three key ingredients required to support life on Earth are liquid water, source(s) of energy (oxidants and reductants)
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80 ORIGINS, WORLDS, AND LIFE that records the oscillations and asymmetries in the planet's gravitational field, thereby providing new insights into Saturn's internal structure and rotation. More dramatically, a variety of measurements of both the mass of Saturn's main rings and the mass fluxes between the rings and planet made around the end of the Cassini mission suggest that the rings may be only about 100 million years old.
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TOUR OF THE SOLAR SYSTEM 81 Key Discoveries from the Past Decade • Jupiter and Saturn have dilute cores, not the small well-defined cores assumed by models. Gravity data from Cassini's final orbits at Saturn, and Juno's high inclination orbits at Jupiter have revealed that the deep interior structure of both planets is not sharply defined as most models had assumed.
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82 ORIGINS, WORLDS, AND LIFE OCEAN WORLDS AND DWARF PLANETS In addition to Earth, we have identified more than 20 worlds throughout the solar system that may once have had or currently support large liquid water oceans. These so-called ocean worlds include several icy moons of Jupiter and Saturn, which harbor confirmed modern oceans, as well as icy moons of Saturn, Uranus, and Neptune and several dwarf planets, including Pluto and Ceres, in which candidate oceans may exist (Figure 2-27)
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TOUR OF THE SOLAR SYSTEM 83 vents and continental subsurface settings on Earth informs investigations of hydrothermal activity with similar characteristics within Saturn's moon Enceladus (see below for details)
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84 ORIGINS, WORLDS, AND LIFE Europa Prior to the previous decadal survey, Visions and Voyages (NRC 2011) , the Voyager and Galileo missions revealed that Europa has a global liquid water ocean that is sandwiched between a dynamic ice shell and a silicate core (Figure 2-28a)
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TOUR OF THE SOLAR SYSTEM 85 In the coming decade, the Europa Clipper mission will begin its exploration of the Jupiter system, conducting dozens of close flybys of Europa (closest approach of each encounter is 35–100 km) , with the goal of assessing the moon's habitability and addressing questions such as: How does physical and chemical oceanography affect Europa's past and present state (e.g., ocean thickness and geochemical exchange between the ocean, ice shell, and seafloor)
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86 ORIGINS, WORLDS, AND LIFE The plume's source within Enceladus is a global subsurface ocean, as established by Cassini in 2014 from analyses of Enceladus's gravity, topography, and wobble as it rotates. The ocean is deduced to be about 40 km (25 mi)
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TOUR OF THE SOLAR SYSTEM 87 Surface A Reflection from Hydrocarbon seas sea floor. Reflection from Distance from De-coupled outer shell surface reflection (water-ice/clathrate)
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88 ORIGINS, WORLDS, AND LIFE Other Bodies The identification of ocean worlds as a new class of planetary bodies brings a new perspective on the diversity of worlds across the solar system. Reexamination of pre-Cassini mission era data from the Galileo and Voyager 2 spacecraft has led to new discoveries at Jupiter's large moons Ganymede and Callisto, the uranian moons, and Neptune's largest moon Triton.
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TOUR OF THE SOLAR SYSTEM 89 Triton Triton is a Kuiper belt object that was captured into Neptune's orbit. Voyager images showed a young surface with relatively few craters and discovered active plumes that stand out among the icy satellites of the outer solar system and put Triton in a class with Io, Europa, Enceladus, and Titan -- moons with geological processes active today.
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90 ORIGINS, WORLDS, AND LIFE FIGURE 2-31 NASA's Dawn mission revealed Ceres as a carbon-rich world shaped by processes involving liquid water. Against Ceres's relatively uniform surface, which is blanketed by minerals formed through the action of liquid water early in its history, a few landmarks stand out.
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TOUR OF THE SOLAR SYSTEM 91 FIGURE 2-32 Pluto and Charon through the eyes of New Horizons. From top left, counterclockwise: (a)
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92 ORIGINS, WORLDS, AND LIFE Other Bodies Many Pluto-scale worlds orbit the Sun beyond Neptune, including all known dwarf planets except Ceres (see Figure 2-32)
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TOUR OF THE SOLAR SYSTEM 93 Key Discoveries from the Past Decade • Intermittent plumes on Europa. Although Europa has a young surface, space telescope observations suggesting intermittent plumes of water vapor jetting into space were a complete surprise.
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94 ORIGINS, WORLDS, AND LIFE Hayne, P.O., P.G. Lucey, T.R.
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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, icy satellites, and primitive bodies (a.k.a.
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96 ORIGINS, WORLDS, AND LIFE FIGURE 3-1 Comparison of topical distribution of crosscutting questions initially identified by the steering group (left) with those previously identified by the analysis and assessment groups, including LEAG, MAPSIT, MEPAG, OPAG, SBAG, MEXAG, and VEXAG (right)
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