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Recent Scientific Discoveries

Since the publication of the 2011 planetary science decadal survey, Vision and Voyages for Planetary Science in the Decade 2013-2022 (NRC, 2011), planetary science has made many significant advances, including acquiring results from several highly successful missions. This chapter outlines some of the highlights, ordered according to the panels of the decadal survey: Inner Planets (Mercury, Venus, Moon); Mars; Giant Planets (Jupiter, Saturn, Uranus, Neptune); Satellites (Europa, Titan, Enceladus); and Primitive Bodies (asteroids, comets, Kuiper Belt Objects). This chapter is not intended to be comprehensive, and there are many other worthy scientific discoveries of the past few years.

MERCURY

After 4 years on orbit at Mercury, the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) mission ended in April 2015. Key results of the MESSENGER mission include the following:

• Mercury’s core, expected to fill a large fraction of its interior, is even larger than expected, which implies that instead of pure Fe and Ni, it must contain light elements. There is a solid inner core, a liquid outer core, and possibly an outer layer of solid FeS (Hauck et al., 2013). Proposed explanations of the large size of the core include impact stripping of the outer silicate layer, ablation of the silicate layer, or segregation of metal from silicate in the solar nebula, none of which is strongly supported by geologic and chemical evidence (Smith et al., 2012; Hauck et al., 2013). Instead, Mercury’s accretion in a highly reduced environment may have led to an enrichment in iron and a depletion in silica that was removed in a vapor phase (Ebel and Alexander, 2011).
• Comparison of observed crater populations to impactor flux models suggests that large volcanic deposits were emplaced by 3.5 Ga (Byrne et al., 2016). The composition of the volcanic materials varies across the surface from iron-poor komatiite to basaltic andesite (Vander Kaaden and McCubbin, 2015; Izenberg et al., 2014).
• Compressional structures (lobate scarps and wrinkle ridges) formed by horizontal shortening in response to cooling and contraction of the planet’s interior dominate Mercury’s tectonics. Mercury contracted by as much as 7 km in radius, substantially more than previously expected (Byrne et al., 2014). Surprisingly,

• extensional structures also occur, inside large impact basins and over buried craters where intra-basin processes and compaction dominated the stress field (Fassett et al., 2012; Klimczak et al., 2012).
• Mercury’s magnetic field is offset northward along the planetary spin axis by 20 percent of the planet’s radius. (See Figure 2.1.) The internal field is 100 times weaker than Earth’s and barely holds off the solar wind at the subsolar point. The interaction of the planetary field with the solar wind induces a response in the core that generates an external magnetic field of magnitude similar to or larger than the planetary field (Anderson et al., 2012, 2014; Johnson et al., 2012; Winslow et al., 2014) and modifies the global scale of the magnetosphere (Jia et al., 2015). The inductive response remotely sounds the core-mantle boundary (Johnson et al., 2016). Detection of a crustal remanent magnetic field indicates that a dynamo field at ~3.7-3.9 Ga must have been comparable to, or up to 100 times larger than, Mercury’s present field (Johnson et al., 2015).

Mercury’s magnetosphere is highly dynamic because of the planet’s small magnetic field and the properties of the solar wind at its radial distance from the Sun. The inner conducting boundary required to form a magnetosphere is not an ionosphere but is provided by the large iron core. Reconnection of the interplanetary and planetary magnetic field produces small structures called “flux transfer events” whose frequency of formation is so large that circulation of magnetic flux in the magnetosphere occurs 100 times faster than at Earth (DiBraccio et al., 2013; Gershman et al., 2013). These dynamics promote intense particle precipitation to the surface in regions of both open and closed magnetic field, so space weathering should be more intense on Mercury than on the Moon (Korth et al., 2015; Raines et al., 2014; Slavin et al., 2014).

Mercury is a volatile-rich planet with high contents of Na, K, Cl, S, and C in the crust. The formation environment of the planet was so oxygen-poor that normally volatile elements occur in refractory forms—for example, S occurs as Mg and Ca sulfides (Nittler et al., 2011; Peplowski et al., 2011; Evans et al., 2012). Carbon occurs as graphite that accounts for the planet’s low albedo, and is highest in concentration in the remnants of a graphite flotation crust that was stirred by impacts into overlying volcanics. Remnants of the primordial crust are exposed by large basins, and locally in the oldest terrains (Murchie et al., 2016; Peplowski et al., 2016).

Radar-bright deposits in permanently shadowed polar terrain are rich in H (Lawrence et al., 2013). They consist mostly of water ice, but ice is exposed only in the coldest areas close to the poles. Slightly equatorward they are covered by an organic-rich lag (Neumann et al., 2013), implicating a cometary source of volatiles (Delitsky et al.,

2017). Hollows, which may form by removal of volatile materials, are small-scale, shallow, irregular depressions that produce a surface that resembles the Swiss cheese terrain in the CO₂ veneer on Mars’s southern polar cap.

VENUS

Exciting Venus work has continued since the publication of Vision and Voyages; two areas highlighted here are continent-like plateaus and atmospheric dynamics.

Continent-Like Plateaus

Tesserae are intensely deformed terrains that have been proposed to be analogs to Earth’s continents based on their planform, shallow isostatic compensation and stratigraphically old age (Campbell and Taylor, 1983). On Earth, subduction and remelting of the original basaltic crust in the presence of water formed the huge volume of felsic (silica-rich, iron-poor) crust that comprises the continents. Continents are of lower density than basaltic seafloor and they do not readily subduct. Although originally designed for observing a comet (not Venus), the VIRTIS instrument on Venus Express was able to observe the surface near 1 micron through a transmission window in Venus’s CO₂ atmosphere. The VIRTIS data cover about half of the southern hemisphere, including one of Venus’s six tessera plateaus, with reasonable precision. The derived surface emissivity data show that Alpha Regio is felsic (iron-poor) compared to the basaltic plains (see Figure 2.2), consistent with the interpretation of tesserae as continental analogs (Gilmore et al., 2015). Although water is not stable on the surface today, Venus Express data showed that Venus continues to lose significant amounts of water via erosion from the upper atmosphere by solar wind stripping (Curry et al., 2015). Pioneer Venus showed that Venus has lost a shallow ocean’s worth of water (Kumar et al., 1984), but the timing of this water loss is not clear. The evidence for felsic crust at Alpha Regio suggests that the other tesserae terrains, comprising 7 percent of the surface, may also be felsic, and provide evidence of the role of interior or past surface water in shaping the evolution of Venus.

Venus as an Analog for Early Earth

Venus is of great interest as an analog for early Earth and for understanding the conditions needed for plate tectonics, which may be a key to understanding planetary habitability. Numerous potential subduction locations on Venus have clear morphologic, mechanical, and gravity field similarities to terrestrial subduction zones (e.g., Schubert and Sandwell, 1995) and there is much ongoing research seeking to identify the likely initiation process for these features, possibly involving plumes. The high lithospheric temperature on Venus today due to its greenhouse make it a good analog for Earth billions of years ago, when terrestrial plate tectonics began. Davaille et al. (2017) show that three-dimensional (3D) laboratory simulations of plume-induced subduction predict previously unexplained features of venusian subduction zones, such as partial arcs of subduction, and that current conditions on Venus are ideal for the initiation of subduction on rocky planets.

Atmospheric Dynamics

The Japanese mission Akatsuki observed for the first time dramatic planetary-scale standing gravity (buoyancy) waves at the cloud tops (see Figure 2.3) that are tied to specific topographic features and local time (Fukuhara

et al., 2017; Kouyama et al., 2017). These bow-shaped, 10,000-km-long standing waves form each evening, in locations tied to Venus’s largest topographic highlands. Large waves were observed in the atmosphere previously, but they were not standing waves and instead rotated along with the super-rotating atmosphere. Venus’s atmosphere rotates at an astounding 100 m/s at the cloud-top altitude of ~70 km. This super-rotation is a fundamental characteristic of Venus’s atmospheric dynamics that remains challenging to fully understand and model. The discovery of the gravity waves indicates much more coupling of the atmosphere with surface topography than previously known. The waves may transfer momentum from the lower atmosphere to the cloud-top levels, and help fuel super-rotation. Further modeling is required to explain how the cloud layer, time of day, and temperature affect these waves. It is likely that the atmospheric dynamics cause variations in the spin rate of Venus on the time scale of years to decades, but the Venus Express data do not clearly demonstrate a change (Mueller et al., 2012). A more complete understanding of atmospheric dynamics from modeling the gravity waves may provide constraints on the coupling of atmospheric and interior dynamics.

THE MOON

The relative accessibility of the Moon has led to its selection as the objective of a number of both National Aeronautics and Space Administration (NASA) and non-NASA missions. Additional motivation has been provided by the growing awareness that the Moon is not as dry as once thought, which has increased interest in its formation and evolution. The Moon is also a likely target for future human exploration.

The Lunar Reconnaissance Orbiter (LRO) was launched in 2009 and continues to collect important data. (See Figure 2.4.) Evidence for widespread OH or water on the surface has been inferred from spectral data (Bandfield et al., 2018), building on earlier results obtained from the Indian spacecraft Chandrayaan-1 (Pieters et al., 2009). The different forms of water now identified from the lunar interior, across the lunar surface, and sequestered at the poles have become areas of intense active research and are now known to represent different fundamental processes active on the Moon and other silicate bodies of the inner solar system (e.g., Hurley and Benna, 2017; Pernet-Fisher et al., 2017). The distribution of polar hydrogen deposits has led to the recognition of true polar wander arising from changes in the internal mass distribution of the Moon, most likely associated with lunar mare volcanism (Siegler et al., 2016).

NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE) orbited the Moon to gather detailed information about the structure and composition of the thin lunar atmosphere, and confirmed that there is a dust cloud surrounding the Moon over time, which is sustained by the continual bombardment of interplanetary dust particles (Horányi et al., 2015).

Gravity Recovery and Interior Laboratory (GRAIL; a pair of spacecraft that provided very accurate gravity and topography) provided many new results for lunar structure (Zuber et al., 2013), including confirmation of the existence of a small core, global mapping of the crustal thickness (less than thought) and crustal density (lower than thought) (Wieczorek et al., 2013). (See Figure 2.5.) Ancient igneous intrusions were identified, indicating an early phase of expansion of the Moon by a few kilometers (Andrews-Hanna et al., 2013). Mass anomalies on a new and increasingly precise level of spatial resolution were identified, and the circum-Procellarum fracture network was identified in the gravity gradient map (Andrews-Hanna et al., 2014). The mass anomalies are important for lunar geology, and for models of interior dynamics and volcanic processes. Topography and gravity suggest frozen-in bulges from both early faster rotation and tidal heating, together with possible true polar wander (a large reorientation of the Moon’s polar axis; Garrick-Bethell et al., 2014).

The Moon was included in the Inner Planets theme of Vision and Voyages. Priorities for spacecraft missions to the Moon, Mars, and other solar system bodies were treated in a unified manner. Lunar science addresses numerous cross-cutting investigation themes identified in Vision and Voyages, particularly the accretion, accretion timing, water supply, chemistry, and differentiation of the inner planets, the role of early bombardment, and current volatile composition and distribution. Significant progress has been made in fundamental lunar science since 2011. LRO continues to make fundamental discoveries about the Moon. Many GRAIL results were published in 2013-2016 time frame. The LADEE mission (2013-2014) characterized the Moon’s environment and surface-bound exosphere. And sample research continues on Apollo samples and meteorites.

Lunar Interior

Although the GRAIL mission was selected prior to Vision and Voyages, its flight operations and science results occurred after Vision and Voyages was published. The existence of a small core, crustal thickness (less than expected) and density (lower than expected) (Wieczorek et al., 2013), and mass anomalies on a new and increasingly precise level of spatial resolution, including the identification in the gravity gradient map of the circum-Procellarum fracture network (Andrews-Hanna et al., 2014), are important for lunar geology, including models of

interior dynamics and volcanic processes. GRAIL and LRO results yield definitive definition of the lunar inventory of impact basins (Neumann et al., 2015). Detailed laboratory measurements now possible with carefully prepared samples allow the character of the ancient lunar dynamo to be identified and constrained to be active from 4.25 to 3.56 billion years ago (Ga) (Garrick-Bethell et al., 2016; Tikoo et al., 2017), and the age of crustal components to be determined, including a Ferroan Anorthosite (FAN) age of 4360 +/– 3 million years, which requires either that the Moon solidified significantly later than most previous estimates or that the long-held assumption that FANs are flotation cumulates of a primordial magma ocean is incorrect (Gaffney and Borg, 2014) and the formation of the urKREEP at ¹⁷⁶Lu-¹⁷⁶Hf and Sm-Nd urKREEP ages, 4368 +/– 29 Ma, representing the time at which the magma ocean crystallized (Boyet et al., 2015; Carlson et al., 2014).

Lunar Surface

The Moon has a stunningly diverse array of volcanic landforms: large shield volcanoes, spatter cones, pyroclastic deposits, mare basalt provinces, and silicic volcanics—some of which occurred within the last 1 billion years of lunar history (Lawrence et al., 2013; Boyce et al., 2017; Braden et al., 2014; Whitten and Head, 2015). Global inventory of structures shows that the rate of tectonics due to cooling and solidification of core and seismic activity is associated with Earth tidal forces (Waters et al., 2016). Reexamination of both samples and lunar geologic relationships from high-resolution imaging have shown that the big basin chronology of the Moon is not as well understood as was once thought (Spudis et al., 2011; Fassett et al., 2012; Robbins, 2014; Norman et al., 2016), which has enormous implications for important issues such as the late heavy bombardment that drives dynamical models of the early solar system (e.g., Morbidelli et al., 2012). Discoveries continue to be made about the diversity of the lunar crust using the high spatial and spectral resolution of M³ and the Kaguya spectral suite data, including new detections of ilmenite, silica-rich mineralogies, spinels, pyroxene varieties, and nearly pure anorthosite (e.g., Yamamoto et al., 2013; other references). Exciting new science has been enabled by identification of surface mineralogy, with the first global UV bands (LROC) and thermal data (Diviner).

Lunar Volatiles

Vision and Voyages barely had time to incorporate the emerging knowledge of lunar volatiles. White papers were due just 3 months into the LRO mission, before the Lunar Crater Observation and Sensing Satellite (LCROSS) impact, and months after publication of M³ 3µm hydration observations. Results from LADEE; LRO; LCROSS; M³/Deep Impact/Cassini; Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun; Chang’e-3; and Kaguya have significantly contributed to the understanding of lunar volatiles (see reviews by Anand et al., 2014, and Denevi, 2017). The different forms of water now identified from the lunar interior, across the lunar surface, and sequestered at the poles have become areas of intense active research and are now known to represent different fundamental processes active on the Moon and other silicate bodies of the inner solar system (e.g., Hurley and Benna, 2017; Pernet-Fisher et al., 2017). LRO characterized the polar environment, including surface and subsurface temperatures (Diviner) and illumination conditions (LROC), and the possibility of surface frosts (Lyman-Alpha Mapping Project, LOLA) restricted to locations that never get warmer than 110 K, the cold-trap temperature above which water-ice sublimes (Gladstone et al., 2012; Lucey et al., 2014; Pernet-Fisher et al., 2017). Analyses of volatiles in lunar basalts and crustal rocks have confirmed the presence of small but significant amounts of water in the lunar magma ocean. The approximate constancy of volatile depletion in the Moon relative to Earth is explained by assuming that both acquired volatiles from a similar source or by a similar mechanism, but Earth was more efficient in acquiring the volatiles. The H₂O, F, and S concentrations in the primitive lunar mantle source are similar to or slightly lower than those in terrestrial MORB mantle (Hui et al., 2013; Chen et al., 2015).

Lunar Environment

Extensive further analysis of lunar orbital data sets from LADEE and LRO, theoretical modeling, and laboratory simulation of the lunar environment has led to tighter constraints on environmental (charged particles, fields, regolith, dust, exosphere, surface volatiles) dynamics, including the volatile sources, sinks, and transport. The LADEE neutral mass spectrometer (NMS) detected argon, mapping out how argon moves over the course of a lunar day. The NMS findings indicate that a very thin layer of argon sticks to the surface on the cold night side of the Moon (much like frost is deposited during the night on Earth) and is released as the Sun heats the surface. After release, these atoms do not immediately escape from the Moon, as gravity keeps them within the orbit and they bounce off the warmer daytime surface, where they can be detected by the NMS. These data provide the basis for higher fidelity models of the interaction of argon and other gases with the lunar surface, and by extension to other bodies in the solar system that have very thin atmospheres. The Lunar Dust Experiment (LDEX) on the LADEE spacecraft recorded over 11,000 unambiguous dust impacts during its mission at the Moon that lasted from October 2013 until April 2014. These findings confirm that there is a dust cloud surrounding the Moon, which is sustained by the continual bombardment of interplanetary dust particles. Carbonaceous matter on the surfaces of black pyroclastic beads, collected from the Shorty crater during the Apollo 17 mission, represents the first identification of complex organic material associated with any lunar sample. It formed through the accretion of exogenous meteoritic kerogen from micrometeorite impacts into the lunar regolith (Thomas-Keprta et al., 2014). LRO temporal pairs are creating critical new determinations of the lunar impact rate and revealing exciting new details of ejecta distribution and the structure of the regolith (Robinson et al., 2015; Speyerer et al., 2016). Diviner thermophysical property results, including global maps of the Diviner Rock Abundance (DRA), are excellent tools for regolith property investigations and landing site selection (Bandfield et al., 2011). Chang’e-3 provided ground-truth measurements on a relatively young, intermediate- to high-Ti basalt flow in the Imbrium basin. The LRO mission is providing data necessary to certify sites for safe landing, including high-resolution images and stereo Digital Terrain Models for many locations on the Moon for U.S. and international missions.

Current Implementation

The current lunar science program is driven by both sample studies on returned lunar samples and meteorites and by missions. The Lunar Quest (LQ) program included flight missions (LRO after the Human Exploration Office mission concluded, LADEE, International Lunar Network), instruments for lunar missions of opportunity, and research and analysis efforts for crosscutting lunar and exploration research (Lunar Advanced Science and Exploration Research; Lunar Mapping and Modeling Project). The LQ program was cancelled in 2014, at the conclusion of the LADEE mission, and the LRO extended mission was transferred to Discovery and research objectives to the Solar System Exploration Research Virtual Institute.

MARS

The most important new Mars results since publication of Vision and Voyages derive from the Mars Exploration Rover (MER) Opportunity, Mars Reconnaissance Orbiter (MRO), Mars Atmosphere and Volatile Evolution Mission (MAVEN), and Mars Science Laboratory (MSL) Curiosity measurements. Some expand on preliminary findings known at the time of Vision and Voyages; others reveal entirely new characteristics of Mars, and have implications for climate in the past few billion years and Mars’s astrobiologic potential. The findings fall into four broad categories.

Ancient Wet and Habitable Environments

MSL/Curiosity’s in situ analysis of a ~3.7 Ga mudstone in Gale crater reveals that it definitely formed in a habitable environment. The mudstone’s stratigraphy and lithology are consistent with deposition in a stream-fed lake. Depletion of the sediments in olivine and the presence of secondary gypsum veins and smectite indicate prolonged aqueous alteration and habitable pH and salinity conditions. Co-occurrence of minerals with Fe and S in mixed oxidation states would have provided a source of chemical energy (Grotzinger et al., 2014; Léveillé et al., 2014). By analogy with other similar lacustrine deposits identified by remote sensing, such habitable environments probably occurred in many locations on ~4 Ga Mars (Goudge et al., 2012, 2015).

Continued orbital reconnaissance has revealed outcrops that record at least a dozen distinct environments in which liquid water interacted with rock for a sufficiently long period to leave a mineral record, extending in age from the pre-4 Ga period to as recently as ~3 Ga. Of these, about half record surface water or hydrothermal environments whose mineral assemblages are consistent with pH, salinity, and water activity that could have been habitable.¹ The rest are not consistent with habitable conditions due to one or more factors (Carter et al., 2013; Ehlmann and Edwards, 2014; Wray et al., 2016). (See Figure 2.6.)

Hydration of the rock crust occurred mostly in the subsurface at ~4 Ga (Ehlmann et al., 2011a; Carter et al., 2013). This alteration is interpreted to have arisen by contact with saline hydrothermal fluids (Arvidson et al., 2014). There is evidence for both endogenic and impact heating having driven alteration (Tornabene et al., 2013; Viviano-Beck et al., 2017).

A later period, perhaps around 3.7 Ga, had “peak” abundance of surface waters as recorded by valley networks, detrital and chemical sediments deposited in lake environments, and compositionally stratified clay deposits (Ehlmann et al., 2011b) containing 30 wt% or more clays possibly formed by pedogenesis—that is, chemical soil formation by chemical weathering (Le Deit et al., 2012; Poulet et al., 2014).

Surface water continued to be present sporadically at least until ~3 Ga in Arabia and Valles Marineris, forming fluvial valley, stream-fed paleoloakes, and locally secondary mineral deposits (Weitz et al., 2013; Wilson et al., 2016). Rock-water interactions probably due to subglacial volcanism and melting occurred at high southern latitudes (Ackiss et al., 2016).

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¹ On Mars, “hydrothermal” can be much cooler than on Earth due to Mars’s geophysical conditions.

Ancient Climate and Loss of the Atmosphere

Since Vision and Voyages, two distinct episodes of carbonate formation on Mars have been recognized. Newly discovered Fe/Ca-carbonates are greater than 4 Ga in age and have been exhumed by impacts (Wray et al., 2016). Their co-occurrence with smectite clays would be consistent with formation under several hundred millibars of CO₂, which is comparable to current Earth total surface pressure (Zolotov and Mironenko, 2016). Mg carbonates formed during the subsequent few hundred million years are restricted to an annulus around the Isidis impact basin, and sequester no more than 12 mb of CO₂, limiting crustal sequestration of carbon since ~4 Ga (Edwards and Ehlmann, 2015).

MAVEN measurements support stripping of enough martian atmosphere by solar wind to remove ~500 millibars of CO₂ in 4 byr, mostly during the 300-500 Ma after the magnetic field shut down (Jakosky et al., 2015,

2017). These results are consistent with inferences above based on older carbonate-bearing deposits; with the finding from MSL Curiosity that D/H of water extracted from clays in ~4 Ga rocks was already increased by 3 times, indicating early, substantial loss of water (Mahaffy et al., 2015); and with Ar isotopic measurements indicating loss of ≥2/3 of atmospheric neutrals (Jakosky and Maven Science Team, 2017).

At the time of Vision and Voyages, later surface water was generally thought to have originated as rainfall. Newer modeling indicates that precipitation as highland snowfall (the “Icy Highlands” model) may better explain the distribution of morphologic and mineralogic indicators of these later wet environments (Fastook and Head, 2015; Wordsworth et al., 2015).

More Recent (<3 Ga Age) Climate Change

MRO radar sounding of the polar caps has revealed a massive amount of CO₂ ice buried in the south polar cap, which if released would double the present mass of atmosphere and greatly expand the temporal and spatial range at which surface temperature and pressure exceed the triple point of water (Phillips et al., 2011). It has also revealed nonuniform “packets” of reflectors, which may be related to obliquity changes. Modeling of the topmost layer suggests the last ice age was ~370,000 years ago (Smith et al., 2016).

Shallow (≥95 wt%), subsurface water ice has been observed directly at widespread midlatitude locations where the ice is exposed in fresh impact craters (Dundas et al., 2014). Where exposed locally in scarps, the water ice forms continuous sheets tens of meters in thickness (Dundas et al., 2018). Its presence is inferred over wide areas even where not directly observed, based on radar (Putzig et al., 2014; Stuurman et al., 2016) and occurrence of expanded secondary craters (Viola et al., 2015) and terraced craters (Bramson et al., 2015). There is not wide agreement on the process that could concentrate water ice in excess of available pore space (~35 percent): late Amazonian snowfall (Head et al., 2003; Bramson et al., 2017) or concentration by intergranular flow of vapor or brines (Sizemore et al., 2015) have been proposed.

Dynamic Modern Mars

At the time of Vision and Voyages, martian sand bedforms were thought to be inactive. From orbital and landed imaging, it is now known that modern Mars has active sand sedimentation, migrating dunes, and Earth-like transport rates of sand (Bridges et al., 2012, 2017). Sand originates from mineralogically diverse local sources (Chojnacki et al., 2014a, 2014b). Eolian sorting processes create further diversity in sand composition (Lapotre et al., 2017).

Intensive gully imaging by MRO has shown that gullies actively grow during late winter due to early springtime ablation of seasonal CO₂ frost, when temperature does not permit flow even of brines. Rather, dry granular flow initiated by CO₂ sublimation appears to be the dominant processes in forming gullies (Pilorget and Forget, 2016).

Recurring slope lineae (RSL) form in summertime on sunward-facing slopes where temperatures exceed 253K, grow and extend downslope over the summer, and fade during autumn (McEwen et al., 2011, 2014). At least locally, they form where perchlorates are present and could depress the freezing point of brines by up to 70° below that of pure water (Ojha et al., 2015). These properties suggest briny flow, deliquescence, or dry granular flow on warm slopes initiated by vapor evaporated from brine or deliquescence.

Occurrence of martian methane has been confirmed by MSL/SAM. Its abundance increases above a negligible background abruptly, and decreases more quickly than models of atmospheric chemistry predict (Webster et al., 2015). Although the origin could be biotic, alternatively it could form from serpentinization of olivine on modern Mars, or on ancient Mars but trapped, and released episodically (Etiope and Sherwood Lollar, 2013; Kite et al., 2017). (See Figure 2.7.)

JUPITER

Juno arrived in Jupiter orbit on July 5, 2016, and results from the mission are already leading to a new understanding of the planet’s workings and interior. Among the new discoveries are a deeply penetrating ammonia-based weather system and a stronger and more irregular magnetic field than previously thought (Bolton et al., 2017).

The discovery by Juno that Jupiter’s gravity field (Folkner et al., 2017) is north-south asymmetric and the determination of its nonzero odd gravitational moments J3, J5, J7, and J9 demonstrates that the observed east-west winds persist to a depth of about 3000 km (Kaspi et al., 2018). The initial results from the gravity experiment suggest an enrichment of heavier elements toward the center of the planet, perhaps of order 10 or so Earth masses, but this core may extend out as far as half the planet, meaning that the heavier elements are partially dissolved in hydrogen. (It could be water ice, for example. Gravity indicates nothing about the composition, only that the material is more dense than a solar composition mix.)

Juno discovered clusters of cyclones encircling Jupiter’s poles. (See Figure 2.8.) This is very different than anticipated, and the structure differs greatly from that at Saturn’s poles, where the north pole’s cyclones are organized in a hexagonal pattern (Adriani et al., 2018). Furthermore, Juno’s microwave radiometer discovered an equatorial belt of ammonia that appears to extend at least to a pressure of 250 bars, but away from this belt there is considerable variability throughout the deep atmosphere (Bolton et al., 2017). This discovery demonstrates that Jupiter’s atmosphere, at least with respect to ammonia (and presumably water), is not well mixed below the 5-10 bar region as previously assumed. Jupiter’s atmosphere also appears to be depleted in ammonia to at least 100 bars (Li et al., 2017).

The Jovian magnetic field contains patches of low and high magnetic field flux that are not yet fully characterized because of their limited spatial extent. Jupiter’s dynamo extends out to as much as 90 percent of the radius (Bolton et al., 2017).

Juno found similarities but also strong differences between Jupiter’s and Earth’s auroras (Mauk et al., 2017; Connerney et al., 2017). Juno discovered strong magnetic field-aligned electric potentials over Jupiter’s main aurora. Electric potentials are indicated (up to 400 kilovolts) that are an order of magnitude larger than the very largest observed at Earth. Despite the magnitude of these potentials, they are not associated with the most intense auroras at Jupiter as they are at Earth.

SATURN SYSTEM

The spectacular Cassini mission ended its 13-year voyage in the Saturn system in September 2017, going out in a blaze of glory. Some of the highlights of Cassini since the publication of Vision and Voyages are described here.

Cassini found that Enceladus is jetting water vapor to great distances beneath its south pole (Dougherty et. al., 2006). (See Figure 2.9.) The source of the water vapor was found to lie beneath distinct dark-colored cracks on the surface, requiring a lake or an ocean beneath the ice (Porco et al., 2006). Images of Enceladus, acquired over seven years, were used to determine the satellite’s precise rotation state and to thereby derive the physical libration of the body. The results were found to require a global subsurface ocean (Thomas et al., 2016), rather than a localized subsurface sea.

Data acquired by Cassini’s Cosmic Dust Analyzer (CDA) instrument on flybys through the Enceladus plumes indicate that the plumes originated in a salt-water reservoir that is, or has been, in contact with rock. Hsu et al.

(2015) found that the composition and nanometer size of silica particles in the plume imply high-temperature (>90ºC) hydrothermal reactions. Waite et al. (2017) used Ion and Neutral Mass Spectrometer (INMS) data to identify H₂ in the Enceladus plume (at levels of 0.4-1.4 percent by volume), also pointing to water-rock interactions. Together, these results suggest that hydrothermal activity is able to transport material from the ocean floor at least 40 km up to the surface through the Enceladus plumes. The hydrothermal material may mix in the ocean, cool to ice temperature, and then later vent. This is the first evidence of ongoing hydrothermal activity beyond Earth.

Data from the Visual and Infrared Mapping Spectrometer (VIMS) revealed that the brightness of the plumes is several times greater when Enceladus is near the point in its orbit farthest from Saturn (apokrone) than when near the point of closest approach to the planet (perikrone). Hedman and Nicholson (2014) interpreted this to mean that material escapes from beneath Enceladus’s surface most readily at times when the fissures from which the plumes emanate are predicted to be in tension, implying a direct correlation between plume activity and tidal stresses.

Cassini established that Titan is one of the most Earth-like bodies yet encountered, with weather, climate, and geology that provide new insights into the evolution of planet Earth. Cassini’s extended missions revealed seasonal variations at Titan, including evidence of spring methane rain showers (Turtle et al., 2011), changes in atmospheric circulation (Teanby et al., 2012), and appearance of low-latitude hydrocarbon lakes (Griffith et al., 2012). Precise radio science measurements (Iess et al., 2012) at different locations around Titan’s eccentric orbit demonstrated deformations in Titan’s interior on the time scale of its orbital period large enough to require a water-rich ocean under an ice shell. Further analysis of gravity data indicated that Titan’s ice shell is variable in thickness, with an average thickness of ~70 km, and that the ocean contains a high concentration of dissolved salts (Mitri et al., 2014). Titan is a new type of ocean world with stable surface liquids other than water.

Puzzling variations at periods slightly longer than Saturn’s rotation period but varying over months to years are observed in radiofrequency emissions (Gurnett et al., 2010), auroral activity, and many other magnetospheric phenomena. The effects are thought to be driven by rotating asymmetric flows in the high-latitude ionosphere arising from asymmetries in the upper atmosphere (Jia and Kivelson, 2012).

Cassini revealed the complexity of Saturn’s rings and the dramatic processes operating within them. Details of ring structure have provided clues to the locations of new moons and the presence of both internal oscillations and static mass anomalies in Saturn’s interior (Hedman and Nicholson, 2013, 2014). The mission also illuminated the critical role of charged dust in magnetospheric dynamics (Blanc et al., 2015), and the diversity of plasma interactions with moons in a single system (Simon et al., 2015).

In the grand finale, Cassini obtained some remarkable results for the rings and interior (gravity and magnetic fields), most of which are not yet published. The rings were found to be at the low mass end of previous estimates, suggesting that they are young. The rotation of Saturn’s interior is still undetermined because the internal part of the magnetic field exhibits no longitudinal structure. The gravity field shows clear evidence of strong differential rotation in the outer 10-15 percent of Saturn’s radius.

Europa

At Europa, enhanced hydrogen and oxygen ultraviolet (UV) emissions were observed above the southern hemisphere using the Hubble Space Telescope (HST) (Roth et al., 2014). These were interpreted as the dissociation products of water, hence evidence of water vapor plumes. (See Figure 2.10.) Although such UV emissions have not been observed again at any location on Europa so far, subsequent HST observations of Europa in transit against the disk of Jupiter have found indications of UV absorption by possible plume material at the southern hemisphere and at a low-latitude trailing hemisphere location, strengthening evidence for modern-day venting activity at Europa (Sparks et al., 2016).

Additional evidence of localized jets of vapor-forming plumes just above Europa’s surface (possibly intermittently) was found in archived data from the Galileo spacecraft two decades after the measurements were made (Jia et al., 2018). Galileo’s closest pass by Europa on December 16, 1997, took it within 200 km of Europa’s surface. Near closest approach, rapid changes of the magnetic field orientation and magnitude had been noted by the magnetometer team but had not been interpreted. Hubble images (e.g., Figure 2.10) suggesting the presence of plumes provided information on their height and width and the density of the vapor. Numerical simulations of the effect of a plume were used to determine how a plume would perturb the magnetic field along Galileo’s orbit. The analysis was carried out both with and without a vapor plume (mainly water) located near Galileo’s closest approach to Europa. The computation incorporated the effects of ionization of the neutral molecules by energetic electrons, revealing how such a structure would affect the magnetic field and the electron density near the plume. The results not only accounted for the previously puzzling changes of the measured magnetic field but also explained an extremely brief emission in the plasma wave spectrum that implied a significant increase of electron density coincident with the magnetic field anomaly. The highly localized increase of electron density was also consistent with the results of the numerical simulation. These results provide supporting evidence for strong hydrothermal activity at Europa and suggest the possibility of characterizing matter from Europa’s ocean from a spacecraft passing near Europa such as the Europa Clipper mission.

Analysis of Galileo images of Europa shows several lines of evidence for subduction, and hence plate tectonics, resolving a long-standing issue of how Europa’s ice shell accommodates its ubiquitous extensional surface features. A tectonic reconstruction of a portion of Europa’s surface reveals evidence of spreading, transform motions, and partial removal of a 99-km wide slab along a discrete tabular zone interpreted to be a subduction-like convergent boundary (Kattenhorn and Prockter, 2014).

Ceres and Vesta

The Dawn mission confirmed that Vesta, the second largest asteroid, is the parent body of the HED meteorites, based on data from three instruments: the gamma ray and neutron detector (GRaND), which gives elemental composition; the visible and infrared spectrometer (VIR), which provides mineralogy; and the framing camera (color bands). Dawn found that Vesta’s gravity field is consistent with an iron core of the size predicated by HED-based differentiation models, and thus also confirmed that this body underwent differentiation early in the history of the solar system (Russell et al., 2012). The heavily cratered topography found by Dawn is transitional between planets and smaller asteroids. Dawn measurements provided ground-truth for HED meteorites and geological context for the Vesta asteroid family, as well as revealing significant surface contamination by carbonaceous material (McCord et al., 2012). (See Figure 2.11.)

At Ceres, the largest asteroid, the Dawn spacecraft has demonstrated that a global ocean existed in its early history. Dawn measured sodium carbonate and ammonium salts on the bright deposits in Ceres’s Occator crater; these salts were dissolved in water and crystallized from subsurface brines, the chemistry of which has implications for models of Ceres’s formation and evolution and likely requires an early subsurface reservoir rich in ammonium or chloride (Nathues et al., 2017). Surprisingly, some of this activity may have occurred only tens of millions of years ago. Gravity and topography data support a partially differentiated structure with the outer region being a mixture of ice and silicates (Park et al., 2016). Morphological evidence (pits and furrows) suggests small amounts of water ice in the subsurface (Schmidt et al., 2017) and spectroscopic evidence shows small amounts of water ice on the surface (Combe et al., 2016). Cryovolcanism has been suggested for Ceres based on landforms suggestive of viscous dome formation (Ruesch et al., 2016). (See Figure 2.12.) Together, the discoveries at Ceres indicate a remarkably active, volatile-rich dwarf planet.

COMETS AND ‘OUMUAMUA

The Rosetta mission at comet 67P/Churyumov-Gerasimenko ended in September 2016, after releasing the first-ever lander onto a comet surface in November 2014. The comet’s D/H ratio was measured to be three times that of water on Earth, placing constraints on the type of object expected to have brought water to Earth early in

its history (Altwegg et al., 2015). Although it is unlikely that comets delivered most of Earth’s water, xenon isotope measurements from Rosetta show that comets did contribute to Earth’s atmosphere (Marty et al., 2017) and comets have a full range of D/H values including several (e.g., Hartley 2) that are terrestrial. Rosetta measured the composition of volatiles escaping from comet 67P/C-G and offered new insight into the interior structure of a comet; molecular oxygen was observed for the first time at the comet (Bieler et al., 2015), with implications for the comet’s formation. Rosetta included two U.S.-supplied instruments. Remarkably detailed morphological and mechanical details of the cometary surface were obtained, useful for both future cometary missions and for constraining ideas for how comets form and evolve. (See Figure 2.13.)

An interstellar interloper was discovered on October 19, 2017, with the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) 1 telescope at the University of Hawaii (Meech et al., 2017). This is a morphologically unusually prolate body. The interstellar nature of this object, known as A/2017 U1 (‘Oumuamua), was confirmed by the Jet Propulsion Laboratory (JPL) Center for Near-Earth Object Studies (CNEOS). The substantially hyperbolic orbit, with heliocentric encounter velocity of ~26 km/sec, far greater than could have arisen from solar planetary perturbations, requires that it originated outside the solar system (collision cannot lead to the observed eccentricity of the object). Although it passed perihelion only 0.25 AU from the Sun, no sign of coma or volatile activity was observed at any time, so this was not labeled as a comet, although it is possibly a body that once had cometary volatiles or has encased these volatiles beneath an involatile surface crust (Fitzsimmons et al., 2018). The existence of small bodies traveling between the stars is no surprise; the Oort Cloud of our own solar system provides ample evidence that planet formation is a messy process that results in many small bodies escaping into interstellar space.

PLUTO AND CHARON

The New Horizons spacecraft made its closest approach to Pluto in July 2015. (See Figure 2.14.) The mission spectacularly demonstrated that Pluto is a far more dynamic world than anticipated. The Pluto system is complex

in the variety of its landscapes, including 3.5 km high mountains, the diversity of its manifestation of activity, and its range of surface ages (Stern et al., 2015; Weaver et al., 2016; Moore et al., 2016; Grundy et al., 2016; Gladstone et al., 2016; McKinnon et al., 2016). The orientation of the region named Sputnik Planitia, a nitrogen ice sheet on the anti-Charon point of Pluto, may indicate reorientation and tidal wander of the shell facilitated by a subsurface ocean (Nimmo et al., 2016). Polygonal patterns on the nitrogen surface are thought to arise from convection in a layer of solid nitrogen (Trowbridge et al., 2016). The “bedrock” of Pluto is thought to be water ice, which has been detected on the surface (Protopapa et al., 2017). Charon’s dark red polar cap may be the result of atmospheric gases that escaped Pluto and settled on Charon’s surface (Grundy et al., 2016). Pluto’s atmosphere behaves differently than believed, pre-New Horizons; for instance, the atmospheric nitrogen was shown to escape at a much lower-than-predicted rate and it is now thought that the atmosphere does not freeze out when Pluto is near aphelion (Olkin et al., 2017). (See Figures 2.15, 2.16, 2.17, and 2.18)

EXOPLANETS

No summary of recent developments in planetary science would be complete without a mention of the remarkable explosion in discoveries of exoplanets since the last decadal survey. It is of particular significance that many of these bodies are intermediate in nature between Earth-like bodies and Uranus or Neptune-like bodies. They are variously described as super-Earths and sub-Neptunes, different from anything in our solar system, yet they can tell us much about how planets form and evolve (Fulton et al., 2017). Exoplanets form planetary systems that differ markedly from our own, with many planets, even ones far larger than Jupiter, at close distance from the central star. This discovery has given impetus to new concepts of planetary system formation, and has given new incentives to improve our understanding of how our own solar system formed.

CONCLUSION

This chapter has provided only a brief overview of the tremendous scientific discoveries made since the Vision and Voyages report was released in March 2011. New discoveries have been made from the core to the magnetospheres of all classes of objects in our solar system. A rich collection of missions is continuing to deliver profound advances in the understanding of solar system formation and evolution, which will inform science priorities for the remaining years of the Vision and Voyages decade and beyond.

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