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This article consists of three sections. The first discusses how we determine satellite internal structures and what we know about them. The primary probes of internal structure are measurements of magnetic induction, gravity, and topography, as well as rotation state and orientation. Enceladus, Europa, Ganymede, Callisto, Titan, and (perhaps) Pluto all have subsurface oceans; Callisto and Titan may be only incompletely differentiated. The second section describes dynamical processes that affect satellite interiors and surfaces: tidal and radioactive heating, flexure and relaxation, convection, cryovolcanism, true polar wander, non-synchronous rotation, orbital evolution, and impacts. The final section discusses how the satellites formed and evolved. Ancient tidal heating episodes and subsequent refreezing of a subsurface ocean are the likeliest explanation for the deformation observed at Ganymede, Tethys, Dione, Rhea, Miranda, Ariel, and Titania. The high heat output of Enceladus is a consequence of Saturn’s highly dissipative interior, but the dissipation rate is strongly frequency-dependent and does not necessarily imply that Saturn’s moons are young. Major remaining questions include the origins of Titan’s atmosphere and high eccentricity, the regular density progression in the Galilean satellites, and the orbital evolution of the Saturnian and Uranian moons.

Article

Duane W. Roller

The Atlantic Ocean (literally “the Ocean of Atlas”) was known to Greeks since the time of Homer, but the term did not come into use until the 5th century bce, because of mythological associations of the giant Atlas with the far western Mediterranean. Phoenicians were the first to sail on the ocean, perhaps as early as the beginning of the first millennium bce, and Greeks first went beyond the Pillars of Heracles into the Atlantic in the latter 7th century bce. Much of the early Greek exploration of the Atlantic was due to Massalians, who by 500 bce had gone south of the Pillars into the tropics, and north perhaps to the British Isles, primarily seeking trade connections. The Carthaginians also went beyond the Pillars, even farther than the Massalians, but their explorations were only vaguely known to the Graeco-Roman world until 146 bce. The greatest Greek explorer of the Atlantic was Pytheas of Massalia, who in the latter 4th century bce explored the British Isles and headed north into the Arctic, discovering Thule (probably Iceland), and reaching the Norwegian coast. After the fall of Carthage, the South Atlantic was open to Greeks (and eventually Romans). Polybius of Megalopolis went to the equatorial regions, and Eudoxus of Cyzicus attempted to perfect a route to India around the continent of Africa. The Atlantic islands were also explored, in part. There is evidence for contact with the Madeiras and Canaries, and less certain information about the Cape Verdes and Azores. There is, however, no reliable evidence that anyone from Graeco-Roman antiquity crossed the Atlantic and returned to report on it: casual finds of antiquities in the New World are generally dismissed. Yet exploration of the Atlantic led to the development of tidal theories—tides in the Mediterranean are minimal—first by Pytheas, and then later by Poseidonius and others. The Romans added little to ancient knowledge of the Atlantic, although they explored the region between the British Isles and Scandinavia, which they named the North Sea. But a series of maritime disasters in the early 1st century ce led the Romans to abandon travel on the ocean, and nothing more was discovered until medieval times.

Article

The 1992 Salvadoran peace accords ended a 12-year civil war and forced modest democratic reforms on a state long dominated by a ruthless oligarchy and military. However, the new system represented a shallow version of democracy that remained largely unresponsive to the population. For two decades the far-right Alianza Republicana Nacionalista (Nationalist Republican Alliance [ARENA]) party held the presidency and used it to enact pro-business economic policies of austerity, privatization, and deregulation. In 2009, the left-wing opposition party, the Farabundo Martí National Liberation Front (FMLN), won the presidential elections for the first time. Yet despite winning some notable progressive reforms, the FMLN did not seek, much less achieve, a radical break from the neoliberal policies of previous administrations. FMLN leaders opted to continue a number of pro-capitalist policies while pursuing reforms to ameliorate the worst symptoms of the system, not overthrow it. The FMLN’s shift away from revolutionary socialism is attributable to several factors: a political and media terrain that still heavily favors the right, the continued influence of the United States government, and private investors’ control over the economy. These constraints were vitally important during the tenures of FMLN presidents Mauricio Funes (2009–2014) and Salvador Sánchez Cerén (2014–2019). El Salvador’s political trajectory since 1992, and especially during the FMLN’s decade in the presidency, offers insights into the constraints facing various left-of-center governments elected across Latin America in the early 21st century.

Article

Washington Consensus policies evolved over time, both in Washington and among Latin American policymakers. These policies, involving trade liberalization and privatization (among other measures), were widely adopted in the region by the early 1990s. A generation of scholarly work sought to explain how and why Latin American countries embarked on economic reforms that governments had strongly resisted in the past. While many researchers focused on the top-down nature of the market-liberalization process, others called attention to its pluralist character and argued that the process had considerable public support. When the original Consensus ideas proved ineffective in promoting growth and improved living standards, technocratic Washington added new policies. By the early 2000s, Washington’s goal became that of reducing poverty while ensuring the completion of the original Washington Consensus reforms. In Latin America, however, there was a growing disillusionment with the original reform agenda and a strong challenge to key reforms. With the rise of social mobilization critical of neoliberal reforms and the election of left regimes challenging their main precepts, scholarship turned to a discussion of the nature of the new regimes and the extent to which their policies deviated from the Washington Consensus (both its original formulation and the later expanded version). While most scholars identify the left leaders of Ecuador, Bolivia, and particularly Venezuela, as offering the greatest challenges to neoliberalism, there is no consensus on the extent of the challenge to neoliberalism presented by Latin America’s left regimes. Research has also given attention to the rising demand of China for Latin American commodities as a key ingredient in the region’s left turn away from neoliberalism. The fall in commodity prices, however, set the stage for a resurgence of the political right, its business supporters, and the re-introduction of some key aspects of the original Washington Consensus.

Article

Pascal Rosenblatt, Ryuki Hyodo, Francesco Pignatale, Antony Trinh, Sebastien Charnoz, Kevin Dunseath, Mariko Dunseath-Terao, and Hidenori Genda

The origin of the natural satellites or moons of the solar system is as challenging to unravel as the formation of the planets. Before the start of the space probe exploration era, this topic of planetary science was restricted to telescopic observations, which limited the possibility of testing different formation scenarios. This era has considerably boosted this topic of research, particularly after the Apollo missions returned samples from the Moon’s surface to Earth. Observations from subsequent deep space missions such as Viking 1 and 2 Orbiters, Voyager 1 and 2, Phobos-2, Galileo, Cassini-Huygens, and the most recent Mars orbiters such as Mars Express, as well as from the Hubble space telescope, have served to intensify research in this area. Each moon system has its own specificities, with different origins and histories. It is widely accepted that the Earth’s Moon formed after a giant collision between the proto-Earth and a body similar in size to Mars. The Galilean moons of Jupiter, on the other hand, appear to have formed by accretion in a circum-Jovian disk, while smaller, irregularly shaped satellites were probably captured by the giant planet. The small and medium-sized Saturnian moons may have formed from the rings encircling the planet. Among the terrestrial planets, Mercury and Venus have no moons, the Earth has a single large moon, and Mars has two very small satellites. This raises some challenging questions: What processes can lead to moon formation around terrestrial planets and what parameters determine the possible outcomes, such as the number and size of moons? The answer to such fundamental questions necessarily entails a thorough understanding of the formation of the Martian system and may have relevance to the possible existence of (exo)moons orbiting exoplanets. The formation of such exomoons is of great importance as they could influence conditions for habitability or for maintaining life over long periods of time on the surface of Earth-like exoplanets, for example by limiting the variations of the orientation of the planet’s rotation axis and thus preventing frequent changes of its climate. Our current knowledge concerning the origin of Phobos and Deimos has been acquired from observational data as well as theoretical work. Early observations led to the idea that the two satellites were captured asteroids but this created difficulties in reconciling the current orbits of Phobos and Deimos with those of captured bodies, hence suggesting the need for an alternative theory. A giant-impact scenario provides a description of how moons similar to Phobos and Deimos can be formed in orbits similar to those observed today. This scenario also restricts the range of possible composition of the two moons, providing a motivation for future missions that aim for the first time to bring material from the Martian system back to Earth.

Article

The relationship between social movements and left governments in Latin America since the postwar period has evolved from top-down relationships of populism and vanguardism to more contemporary attempts to blend new social movement practices of horizontalism and direct democracy with the hierarchical structures of the capitalist state and the party system. This evolution represents a long and unfinished transformation in the character of popular struggles, which today stands at the crossroads between referring back to more traditional structures of resistance, and pushing forward to the creation of a new left that can feature radical democratic participation from below as its centerpiece.

Article

Hundreds of planets are already known to have orbits only a few times wider than the stars that host them. The tidal interaction between a planet and its host star is one of the main agents shaping the observed distributions of properties of these systems. Tidal dissipation in the planet tends make the orbit circular, as well as synchronizing and aligning the planet’s spin with the orbit, and can significantly heat the planet, potentially affecting its size and structure. Dissipation in the star typically leads to inward orbital migration of the planet, accelerating the star’s rotation, and in some cases destroying the planet. Some essential features of tidal evolution can be understood from the basic principles that angular momentum and energy are exchanged between spin and orbit by means of a gravitational field and that energy is dissipated. For example, most short-period exoplanetary systems have too little angular momentum to reach a tidal equilibrium state. Theoretical studies aim to explain tidal dissipation quantitatively by solving the equations of fluid and solid mechanics in stars and planets undergoing periodic tidal forcing. The equilibrium tide is a nearly hydrostatic bulge that is carried around the body by a large-scale flow, which can be damped by convection or hydrodynamic instability, or by viscoelastic dissipation in solid regions of planets. The dynamical tide is an additional component that generally takes the form of internal waves restored by Coriolis and buoyancy forces in a rotating and stratified fluid body. It can lead to significant dissipation if the waves are amplified by resonance, are efficiently damped when they attain a very short wavelength, or break because they exceed a critical amplitude. Thermal tides are excited in a planetary atmosphere by the variable heating by the star’s radiation. They can oppose gravitational tides and prevent tidal locking, with consequences for the climate and habitability of the planet. Ongoing observations of transiting exoplanets provide information on the orbital periods and eccentricities as well as the obliquity (spin–orbit misalignment) of the star and the size of the planet. These data reveal several tidal processes at work and provide constraints on the efficiency of tidal dissipation in a variety of stars and planets.

Article

In Latin America, urban popular movements emerged in the late 1940s as thousands of low-income migrants and city residents banded together to claim land, build self-help housing, and forge neighborhood organizations that fomented community participation and mobilized to demand land titles and city services. These neighborhoods were characterized by informal housing; inadequate provision of electricity, water, sanitation, transportation, and social services; and informal employment and underemployment. During the authoritarianism of the 1960s and 1970s, some urban popular movements resisted military dictatorship while others forged clientelist ties. Democratic and authoritarian leaders alike were forced to deal with the steady influx of rural migrants to cities, and regimes of all types often came to view informal neighborhoods founded by urban popular movements as an acceptable solution to some of the challenges of urbanization. In the 1980s and 1990s, neoliberal privatization of public utilities and cuts to social safety nets harmed urban popular movements, but national and local democratization expanded some avenues of participation, and the regional trend of urban popular movements expanded in numbers and extended its geographic reach. In the 2000s, socialist “Pink Tide” governments delivered benefits to low-income sectors, and many popular sectors supported these leftist regimes. Material gains proved modest, however, and state-movement alliances were rocky, leaving urban popular movements in the awkward position of being dissatisfied with national leadership, yet preferring the Pink Tide incumbents to most alternatives. And in the 2010s, a new “right turn” emerged, as conservative leaders replaced many Pink Tide presidents, threatening to reintroduce the repressive over-policing of popular sectors. Throughout these periods, the core conceptual identity of some urban popular movements shifted from the poblador (the “founder” seeking to meet his or her family’s needs) to the vecino (the “neighbor” collaborating with other movement participants through collective efforts), to the ciudadano (the empowered “citizen” who recognizes his or her needs as rights to be secured through political engagement).