Origins, Worlds, and Life A Decadal Strategy for Planetary Science and Astrobiology 2023-2032 (2023) / Chapter Skim
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2 Tour of the Solar System: A Transformative Decade of Exploration
2 Tour of the Solar System: A Transformative Decade of Exploration
Pages 29-94
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From page 29... ...
The fluid outer core presently supports an active magnetic field that is about 100 times weaker at Mercury's surface than Earth's. Mercury's magnetic field is predominantly axisymmetric and dipolar, closely aligned with Mercury's spin axis, although the magnetic equator is offset along the spin axis to the north by ~20 percent of the planet's radius, for reasons that are not fully understood.
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From page 30... ...
In the case of graphite, its presence at the surface could also reconcile the seeming contradiction of the planet's low average surface reflectance with its bulk composition that would otherwise yield bright surface materials. Most of the regions where Mercury's reflectance is lowest, and thus may have the highest abundance of graphite, are found either in areas with the highest density of large craters, or in the ejecta of some impact craters and basins.
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From page 31... ...
Because of its orbit close to the Sun, Mercury has experienced more intense impact bombardment than anywhere else in the solar system, and impactors strike the surface at an average velocity at least three times that of the Earth–Moon system. However, in contrast to the expectation of a heavily cratered surface, MESSENGER revealed that Mercury has a lower total population of large impact craters (>20 km diameter)
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From page 32... ...
One environment on Mercury is, however, well suited to volatile preservation: regions of permanent shadows in impact craters and topographic lows near Mercury's poles. Deposits of ice were initially suggested to exist near the poles based on Earth-based radar observations, and MESSENGER provided multiple lines of evidence that these deposits are mostly water ice, including hydrogen abundances inferred from neutron spectroscopy, reflectance measurements, and models of the thermal environment (Figure 2-3)
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From page 33... ...
MESSENGER observations demonstrated that plasma contained within the magnetosphere is predominantly solar wind protons that enter the magnetosphere via dayside magnetic field reconnection. The entering plasma subsequently flows to a high latitude "mantle" region and then into the central anti-sunward magnetic tail.
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From page 34... ...
Interaction between Mercury's magnetic field and the solar wind results in the generation of currents that induce external magnetic fields. With induced external fields that can be as large as the planetary field, the dynamic magnetosphere at Mercury is a unique natural laboratory for exploring magnetospheric physics and exospheres.
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Determining the formation time of the Moon is crucial because the impact event defined the initial condition of Earth. Moreover, the Moon-forming impact was likely the last catastrophic event in the inner solar system, and therefore it also defined the end of the accretion phase.
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(A) Lunar Reconnaissance Orbiter (LRO)
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Over the past decade, remanent crustal magnetism studies and laboratory analyses of returned lunar samples have revealed that the ancient Moon had an internally generated magnetic field. Between 4.25 and 3.56 Ga, surface field intensities reached values up to 40–110 T.
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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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From page 39... ...
, as neutron absorption measurements provide increased detail regarding the distribution of hydrogen, FIGURE 2-6 A new crater on the Moon that formed in 2012 or 2013. Hundreds of new impact craters on the Moon have been detected by NASA's Lunar Reconnaissance Orbiter by comparing images acquired at different times and looking for changes.
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. Locations consistent with the presence of water ice, based on anomalous ultraviolet reflectance, are indicated in cyan (Hayne et al.
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From page 41... ...
Furthermore, a particulate exosphere, produced by micrometeoroid impacts, was found by LADEE to extend many hundreds of kilometers above the surface, although no evidence was found for electrostatic transport of dust at altitudes >1 km. Over the past decade, numerous lunar space missions, analyses of lunar samples, and modeling approaches have deepened our understanding of the distribution of volatiles, impact history, magnetic field, and geological features not only for the Moon, but for early Earth and planetary objects in general.
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From page 42... ...
Since NASA's Mariner 2 flyby of Venus in 1962 -- the first successful encounter with any planet, by any nation -- the striking differences in surface conditions between Venus and Earth have motivated a major question in planetary science: Why is Earth's closest sibling not its twin? Enigmatic Venus Before the heady days of sustained Mars exploration, Venus attracted substantial attention from the major spacefaring nations of the time, the United States and former Soviet Union.
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From page 43... ...
Under this view, a perhaps-once-temperate planet lost its oceans to the sustained onslaught of radiation from a star that Venus unfortunately happened to orbit just a little too closely. Later missions helped flesh out that picture: a planet with major geological activity within the past billion years that all but erased any earlier record of surface conditions; a planet that, unlike Earth, lacks an internal magnetic field; a planet with a super-rotating atmosphere that at the cloud tops spins sixty times faster than the surface.
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From page 44... ...
Together, these observations strongly point to major horizontal motion of the Venus surface at some point since major volcanic burial of the crust began. Both the VERITAS and EnVision orbiter missions will search for evidence of ongoing volcanic and tectonic activity from space and, with an atmospheric entry probe, the DAVINCI mission will look for chemical signatures of volcanism in the atmosphere itself.
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From page 45... ...
Modern advances in instrumentation technology hold the promise of incredible breakthroughs for Venus and planetary science by carrying out in situ sampling and analysis of Venus surface materials, both in the planet's vast volcanic plains and in the tesserae. Just as in situ measurements of rocks by the Spirit, Opportunity, Curiosity, and Perseverance rovers have dramatically expanded our understanding of Mars's geological character and history, performing such analyses at the Venus surface would rewrite the textbooks about the second planet.
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. Unlike Earth, however, Venus has no strong magnetosphere; Pioneer Venus found that any internally generated magnetic field is at least one hundred thousand times weaker than that of Earth today.
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. Another discovery was the magnetization of the Venus upper atmosphere by electrical currents induced by the solar wind -- a finding that some magnetic fields are present at the planet, just not any intrinsic to the planet's interior.
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The James Webb Space Telescope (JWST) will soon observe high-priority exoplanet targets, but the vast distances involved mean that even a telescope as powerful as JWST will still acquire sparse and noisy exoplanet data.
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From page 49... ...
2018. "Venus Interior Structure and Dynamics." Space Science Reviews 214:88.
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From page 50... ...
Through Mars's preservation of a record of its entire history, we can examine the interplay between processes from the deep interior to the upper atmosphere to understand what controls the fate of habitable worlds. Our understanding of Mars feeds into an understanding of terrestrial planets in the solar system and of the possible evolutionary paths of terrestrial planets around other stars (many of which are thought to have undergone extensive loss of their atmospheres and water to space over time analogous to Mars)
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Laboratory analytical results from examined meteorites (all igneous rocks) conclude that organics trapped in martian meteorites represent a combination FIGURE 2-12 Views in Gale Crater from the Curiosity rover.
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From page 52... ...
Long-Term Habitable Environments at Gale Crater and Elsewhere on Mars Exploration of Gale Crater by the Curiosity rover has provided definitive evidence that water flowed and ponded on the surface of Mars billions of years ago. Sedimentary processes that took place over thousands to millions of years indicate that liquid water, although perhaps not necessarily present continuously, was stable and available for considerable periods of time on the planet's surface.
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From page 53... ...
Absent a detailed understanding of today's dust storms and the complex coupling to water and CO2 behavior, extrapolation to other epochs and determination of their long-term impact remains difficult. Dust behavior in the lower atmosphere affects the dynamics and composition of the upper atmosphere via lower-atmosphere heating, expansion, and the modification of vertically propagating waves.
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From page 54... ...
. When these ice deposits were emplaced, whether they represent a one-time deposition or a cyclical deposition and removal associated perhaps with obliquity cycles, and what is the total integrated amount of water they contain are uncertain.
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From page 55... ...
FIGURE 2-14 False-color ultraviolet images showing the extended coronae of carbon, oxygen, and hydrogen atoms that surround Mars and that are contributing to loss of atmospheric CO2 and H2O to space. Red circles in each image show location of the surface of the planet, showing that all three coronae extend well above the surface and lower atmosphere.
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From page 56... ...
The Curiosity rover found volcanic rocks of unusual compositions shed from the rim of Gale Crater, and laboratory analyses of the Northwest Africa (NWA) 7034 meteorite, first described in 2013, extended the occurrence of similar volcanic rocks back into the earliest period of Mars history.
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From page 57... ...
InSight's seismic data allows a determination of the size of the martian core and, combined with Mars's moment of inertia, the density and composition of the core were constrained, finding a significant proportion of light elements like sulfur or oxygen in addition to iron. InSight's magnetometer has provided ground truth for orbital measurements by measuring the magnetic field on the planet's surface for the first time.
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From page 58... ...
, frost, gullies, recurring slope lineae, new impact craters and degradation of older craters, sublimation of ice, new insights into dust storms and dust devils, seasonal ice, and growth of unusual geomorphic features within polar ice. These presently active processes have been occurring for millennia, and the long-term observations help constrain their rates.
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From page 59... ...
2016. "The Sustainability of Habitability on Terrestrial Planets: Insights, Questions, and Needed Measurements from Mars for Understanding the Evolution of Earth-Like Worlds." Journal of Geophysical Research: Planets 121(10)
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From page 60... ...
For this section, we do not consider objects that have formed equilibrium spherical shapes, roughly >800 km in diameter objects, which are covered in the Ocean Worlds and Dwarf Planets section, or small moons in the outer solar system, which are discussed in the Giant Planet Systems section. The diversity of small bodies in their sizes, orbits, compositions, and physical natures provides unique scientific opportunities unavailable for larger bodies.
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From page 61... ...
When combined with the continued analysis of returned samples from the Stardust and Hayabusa missions, they collectively provide us with a set of diverse planetesimal samples that formed from different compositional reservoirs in a range of solar system regions and eras, and insight into what might have been the prebiotic chemistry of Earth -- a key question for astrobiology. Characterization of newly discovered ungrouped meteorites and some carbonaceous chondrite groups have provided one of our first windows into planetesimals that formed in the giant planet and trans-neptunian regions.
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From page 62... ...
Gravitational interactions with Neptune cause some objects to evolve onto crossing orbits with the giant planets, producing the Centaur population, transitional objects between Jupiter and Neptune on their path to becoming Jupiter-family comets. The orbits of the most distant trans-neptunian objects (TNOs)
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From page 63... ...
In 2014, the Hubble Space Telescope discovered Arrokoth, an optimal target for the second flyby of the New Horizons mission in the dynamically cold classical TNO population. Cold classical TNOs are thought to be undisturbed by giant planet migration and formed in situ, and so they should represent pristine planetesimals whose nature can be used to probe the earliest epochs in the primordial trans-neptunian region.
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From page 64... ...
Remote sensing has advanced to routinely measure the volatile distribution in fainter cometary comae with facilities such as the Atacama Large Millimeter Array, where asymmetric outgassing, distributed and extended source species, and variability have been found. Observational studies have expanded the molecular inventory of complex organics detected in comets, providing new understanding into the materials available during the formation of the terrestrial planets.
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The presence of two large overlapping impact basins in the southern hemisphere of Vesta (Veneneia and Rheasilvia) , and the global trough systems (fossae)
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From page 66... ...
Now, instead of dates for primary aqueous alteration within parent objects spanning 10 million years, they are constrained to an interval of 1 million years to 4 million years following solar system formation. Meanwhile, advances in the sensitivity and spatial resolution of magnetometers have enabled the first magnetic measurements of many meteorite groups and identification of an asteroid dynamo and the nebular magnetic field.
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From page 67... ...
It will also provide key constraints into the entire history of both asteroids, from their preserved presolar grains components through to pre- and post-accretion environments and geologic activity, to surface processes and the overall dynamical evolution of each asteroid -- advances to be made in the coming decade. Hayabusa2 successfully returned to Earth with approximately 5.4 grams of sample on December 6, 2020.
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From page 68... ...
Rotational spin-up from YORP is on par with collisions in influencing the geologic evolution of small gravitational aggregates like Bennu or Ryugu. Additionally, the active asteroids motivated theoretical studies focused on the mechanical properties and behavior of small rubble pile asteroids.
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From page 69... ...
Impact craters can also be used to tell us about tremendous changes in the history of Earth. For example, most craters on Earth are found on stable regions called cratons and are younger than 650 million years old.
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From page 70... ...
As analytical techniques continue to advance, new measurements of interstellar materials contained in meteorites and other extraterrestrial samples will continue to provide constraints into the evolution of the rocky components of other solar systems. Key Discoveries from the Past Decade • Distinct chemical and physical reservoirs were produced during the evolution of the early solar system and the formation of the giant planet.These momentous events distributed small bodies across the solar system; chemical signatures measured in meteorites and remote observations of volatiles point to the extent of mixing between the reservoirs and the compositions of the building blocks available to the terrestrial planets.
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From page 71... ...
The first orbital expedition to the main asteroid belt documented Vesta's very early formation, battering by two giant impacts, and preservation of hydrated materials delivered to its surface by impactors. Meteoritic and telescopic studies investigated processes ranging from early solar system magnetic dynamos to current-day outburst events from active asteroids, and new models for the origin of the martian moons by a giant impact were developed, emphasizing the wide array of processes that affect the evolution of small bodies.
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From page 72... ...
. The tidal interactions between many icy satellites FIGURE 2-23 Schematic of the outer giant planet systems, showing a rich set of moons, rings, and varying interior structure indicated by color, and shown in more detail later in Figure 10-1.
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the "ice giants," Uranus and Neptune, which possess H-He atmospheres containing ~10–20 percent of their total mass and a larger proportion of ices. There are many similarities among the giant planets: all appear to have interiors dominated by a large, dense core; deep, dynamic hydrogen/helium atmospheres; multiple satellites, some of which suffer additional heating by tidal dissipation to have subsurface oceans; multiple rings that interact with small moons; and substantial magnetospheres.
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From page 74... ...
Additionally, various physical and chemical processes and their interplay govern the interiors; dynamics and magnetic fields, dynamo generation and composition, dynamics and rotational contribution to the density distribution, as well as processes like convection, core erosion, and immiscibility. In addition, the deep interiors of the giant planets serve as natural laboratories for materials at high pressures and temperatures, as a result, constraining the interiors of the outer planets is also of interest to the high-pressure physics community.
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From page 75... ...
SOURCES: Top left: Courtesy of NASA/JPL-Caltech/Space Science Institute/Lunar & Planetary Institute. Other images: NASA/JPL-Caltech/Space Science Institute.
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From page 76... ...
Understanding the magnetic fields of the giant planets is not only crucial for putting constraints on the composition, but also on the heat transport mechanisms within the planets and the interplay between rotation, interior, and dynamics. Magnetospheres Jupiter's magnetosphere -- the sphere of influence of its magnetic field -- is the largest planetary structure in the solar system, 10 times the volume of the Sun and stretching out past the orbit of Saturn.
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From page 77... ...
As the solar wind spreads out with distance, the pressure at farther planets correspondingly decreases, but the major cause of these ranges in size between these magnetospheres comes from the different strengths of the magnetic fields generated by their internal magnetic dynamos. In the case of Jupiter, a major additional factor is the internal pressure of hot ionized gases -- plasma -- trapped in the magnetic field that further inflate the magnetosphere by about a factor of two.
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From page 78... ...
The satellite systems of the giant planets reveal four very different outcomes of planet formation processes and the subsequent dynamical evolution that can modify or disrupt them, as recorded in their surface features and crater populations. As a result, the thermal and geologic processes that have shaped and sculpted the satellites of these four systems are also distinct.
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From page 79... ...
Rings Each of the giant planets is surrounded by a distinctive ring system composed of many small particles orbiting the planet. Jupiter has the most tenuous ring system, which is composed primarily of fine dust grains that were probably knocked off the planet's small inner moons.
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Atmospheric processes (cloud formation and spatial variability) in giant planets allow us to understand clouds in exoplanets and the sources of variable light curves.
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From page 81... ...
Rather, they likely have extended envelopes enriched with heavy elements. New models of giant planet formation and evolution are needed to explain this, along with similar data for Uranus and Neptune to understand if this is common.
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From page 82... ...
Starting with the initial discovery of hydrothermal vent ecosystems near the Galapagos Islands coinciding with Voyager 2's first images of Jupiter's moon Europa, exploration of ocean worlds has complemented advances in terrestrial ocean science that are redefining our understanding of how terrestrial life may have emerged. For example, definitive evidence for de novo abiotic synthesis of organic compounds at deep-sea hydrothermal FIGURE 2-27 Ocean worlds and dwarf planets of the solar system, shown to scale.
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From page 83... ...
In the following subsections, the committee describes major discoveries made in the past decade and list outstanding questions in planetary sciences and astrobiology associated with ocean moons of the giant planets and dwarf planets found in both the inner and outer solar system. This section ends with a discussion of the future of ocean world and dwarf planet exploration, including major discoveries that could occur over the course of the next decade.
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From page 84... ...
confirmed by modeling the local perturbation of the jovian magnetic field observed by the Galileo spacecraft.
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From page 85... ...
If Europa does not harbor life, but a habitable ocean exists, what are the limits on the emergence of life itself? Enceladus Thanks largely to the Cassini mission, the past decade has revealed Saturn's geologically active moon Enceladus to be a habitable world that contains significant liquid water, energy to sustain metabolism, and conditions favorable for the assembly of complex organic molecules.
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From page 86... ...
. The first, like its ocean world siblings, is a liquid water ocean that lies beneath a water-ice shell.
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From page 87... ...
New isotopic measurements of noble gases and methane would resolve key questions concerning the ocean composition, the evolution of the interior and atmosphere, and the formation of Titan, including the age of Titan's atmosphere and how it mysteriously remains methane-rich. Determining if Titan's ocean is interacting with its rocky core would provide a key constraint on the habitability of large ocean worlds both within and beyond the solar system.
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From page 88... ...
Results from Galileo demonstrated the likelihood of oceans beneath the surfaces of Ganymede and Callisto, but it was not until the discovery of a habitable ocean at Enceladus during the Cassini mission that moons of the uranian system and at Neptune became widely recognized as potential ocean worlds. Ganymede and Callisto Initial studies of Ganymede's ocean suggested that a layer of high-pressure ice, likely sandwiched between the rocky core and subsurface ocean, would significantly limit water–rock interaction, and hence limit the ocean's habitability potential.
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As a result, these two dwarf planets have been studied intensively, and both are now also regarded as candidate ocean worlds (see below)
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What processes have driven recent geological activity? Did Ceres form at its current location, further out among the giant planets, or did it migrate inward from the outer reaches of the solar system?
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Theoretical arguments showed that Pluto could retain a subsurface ocean beneath its ice shell. More subtly, Sputnik Planitia's location opposite to Charon could be explained if a subsurface ocean was present.
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From page 92... ...
Future of Ocean World and Dwarf Planet Exploration The initial ocean world discoveries made by the Voyager, Galileo, and Cassini missions motivated Vision and Voyages to recommend a Europa mission to confirm the presence of a subsurface ocean and take the first steps in understanding the potential of the outer solar system as an abode for life. This recommendation is being implemented as the Europa Clipper mission, which will enter orbit around Jupiter in the early 2030s.
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From page 93... ...
2016. "Ocean Worlds in the Outer Solar System." Journal of Geophysical Research: Planets 121:1378–1399.
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From page 94... ...
2020. "NASA's Europa Clipper -- A Mission to a Potentially Habitable Ocean World." Nature Communications 11:1311.
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