Regular moon

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Titan (larger crescent) and Rhea (smaller crescent), two regular moons of Saturn Titan and Rhea - November 19 2009.png
Titan (larger crescent) and Rhea (smaller crescent), two regular moons of Saturn

In astronomy, a regular moon or a regular satellite is a natural satellite following a relatively close, stable, and circular orbit which is generally aligned to its primary's equator. They form within discs of debris and gas that once surrounded their primary, usually the aftermath of a large collision or leftover material accumulated from the protoplanetary disc. Young regular moons then begin to accumulate material within the circumplanetary disc in a process similar to planetary accretion, as opposed to irregular moons, which formed independently before being captured into orbit around the primary.

Contents

Regular moons are extremely diverse in their physical characteristics. The largest regular moons are massive enough to be gravitationally rounded, with two regular moons—Ganymede and Titan—being larger than the planet Mercury. Large regular moons also support varied and complex geology. Several are known to have atmospheres, although only one regular moon—Titan—hosts a significant atmosphere capable of supporting weather and climate. As a result of their complexity, the rounded regular moons are often considered planetary objects in their own right by planetary scientists. [1] In contrast, the smallest regular moons lack active geology. Most are heavily cratered and irregular in shape, often resembling small asteroids and other minor bodies in appearance.

Six of the eight planets of the Solar System host 60 regular satellites [lower-alpha 1] combined, with the four giant planets—Jupiter, Saturn, Uranus, and Neptune—hosting the most extensive and complex regular satellite systems. At least four of the nine likeliest dwarf planets also host regular moon systems: Pluto, Eris, Haumea, and Orcus.

Origin and orbital characteristics

Formation

Regular moons have several different formation mechanisms. The regular moons of the giant planets are generally believed to have formed from accreting material within circumplanetary discs, growing progressively from smaller moonlets in a manner similar to the formation of planets. Multiple generations of regular satellite systems may have formed around the giant planets before interactions with the circumplanetary disc and with each other resulted in inward spiralling into the parent planet. As gas inflow into the parent planet begins to end, the effects of gas-induced migration decrease, allowing for a final generation of moons to survive. [2]

In contrast, Earth's Moon and Pluto's five satellites are thought to have originated from giant impacts between two protoplanets early in the Solar System's history. These impacts ejected a dense disc of debris into orbit whence satellites can accrete. [3] [4] The giant impact model has also been applied to explain the origin of other dwarf planet satellite systems, including Eris's moon Dysnomia, Orcus's moon Vanth, and Haumea's ring and two moons. [5] In contrast to regular moon systems of the giant planets, giant impacts can give rise to unusually massive satellites; Charon's mass ratio to Pluto is roughly 0.12. [5]

Regular moons may also originate from secondary disruption events, being fragments of other regular moons following collisions or due to tidal disruption. The regular moons of Neptune are likely examples of this, as the capture of Neptune's largest moon—Triton—would have severely disrupted the existing primordial moon system. Once Triton was tidally dampened into a lower-eccentricity orbit, the debris resulting from the disruption of the primordial moons re-accreted into the current regular moons of Neptune. [6] [7] [8]

Martian moons

Despite the extensive exploration of Mars, the origin of Mars's two moons remains the subject of ongoing debate. Phobos and Deimos were originally proposed to be captured asteroids originating from the neighboring asteroid belt, and thus would not be classified as regular satellites. Their similarities to C-type asteroids with respect to spectra, density, and albedo further supported this model. [9]

However, the capture model may be inconsistent with the small, low-eccentricity, low-inclination orbits of the two moons, which are more typical of regular satellites. The rubble pile nature of Phobos has further pointed against a captured origin, and infrared observations of Deimos by the Hope orbiter have revealed that the moon's surface is basaltic in composition, more consistent with an origin around Mars. [10] [11] As a result, various models for the in situ formation of Phobos and Deimos have been proposed to better explain their origins and current configuration, including a giant impact scenario similar to the one which formed the Moon and a 'recycling' model for Phobos. [10]

Orbital characteristics

Orbits of Jupiter's Galilean moons, demonstrating the organized, low-eccentricity orbits typical of regular satellites Galilean moons around Jupiter.gif
Orbits of Jupiter's Galilean moons, demonstrating the organized, low-eccentricity orbits typical of regular satellites

Regular moons are characterized by prograde orbits, usually with little orbital inclination or eccentricity relative to their parent body. These traits are largely constrained by their origins and subsequent tidal interactions with the parent body. In the case of the giant planet satellite systems, much like protoplanetary discs, infalling material surrounding a forming planet flattens out into a disc aligned with the planet's equator due to conservation of angular momentum. [12] As a consequence, any moons formed from the circumplanetary disc will orbit roughly coplanar with the planet's equator; even if future perturbations increase a moon's inclination, tidal effects work to eventually decrease it back to a coplanar state. Likewise, tidal circularization acts to decrease the eccentricity of the regular moons by dissipating energy towards a circular orbit, which is a minimum-energy state. Several regular moons do depart from these orbital traits, such as Hyperion's unusually eccentric orbit and Miranda's unusually inclined orbit, but in these cases, orbital eccentricity and inclination are often increased and subsequently maintained by resonant interactions with neighboring moons. [13] [14]

Orbital resonances are a common feature in regular moon systems and are a crucial aspect in their evolution and structure. Such resonances can excite the eccentricity and inclination of participating moons, leading to appreciable tidal heating which can sustain geological activity. A particularly apparent example of this is the 1:2:4 mean-motion resonance (MMR) chain Io, Europa, and Ganymede participate in, contributing to Io's volcanism and Europa's liquid subsurface ocean. [15] Orbital resonances and near-resonances can also act as a stabilizing and shepherding mechanism, allowing for moons to be closely packed whilst still remaining stable, as is thought to be the case with Pluto's small outer moons. [16] A small handful of regular moons have been discovered to participate in various co-orbital configurations, such as the four trojan moons of Tethys and Dione within the Saturnian system. [17]

Shepherd moons

Regular moons which orbit near or within a ring system can gravitationally interact with nearby material, either confining material into narrow ringlets or clearing out gaps within a ring in a process known as 'shepherding'. Shepherd moons may also act as a direct source of ring material ejected from impacts. The material may then be corralled by the moon in its orbital path, as is the case with the Janus-Epimetheus ring around Saturn. [18]

Physical characteristics

Geology

Active plumes on the south pole of Saturn's moon Enceladus, fed by a global subsurface ocean of liquid water Enceladus geysers June 2009.jpg
Active plumes on the south pole of Saturn's moon Enceladus, fed by a global subsurface ocean of liquid water

Of the nineteen regular moons large enough to be gravitationally rounded, several of them show geological activity, and many more exhibit signs of past activity. Several regular moons, such as Europa, Titan, and Enceladus are known to host global subsurface oceans of liquid water, maintained by tidal heating from their respective parent planets. [19] [20] [21] These subsurface oceans can drive a variety of geological processes, including widespread cryovolcanism, resurfacing, and tectonics, acting as reservoirs of 'cryomagma' which can be erupted onto a moon's surface. [22] [23]

Io is unusual as, in contrast to most other regular moons of the giant planets, Io is rocky in composition with extremely little water. Io's high levels of volcanism instead erupt large basaltic flows which continuously resurfaces the moon, whilst also ejecting large volumes of sulfur and sulfur dioxide into its tenuous atmosphere. Analogous to the subsurface oceans of liquid water on icy moons such as Europa, Io may have a subsurface ocean of silicate magma beneath its crust, fuelling Io's volcanic activity. [24] [25]

Atmospheres

Significant atmospheres on regular moons are rare, likely due to the comparatively small sizes of most regular moons leading to high rates of atmospheric escape. Thinner atmospheres have been detected on several regular moons; the Galilean moons all have known atmospheres. The sparse atmospheres of Europa, Ganymede, and Callisto are composed largely of oxygen sputtered off from their icy surfaces due to space weathering. [26] [27] [28] The atmosphere of Io is endogenously produced by volcanic outgassing, creating a thin atmosphere composed primarily of sulfur dioxide (SO2). As Io's surface temperature is below the deposition point of sulfur dioxide, most of the outgassed material quickly freezes onto its surface, though it remains uncertain whether volcanic outgassing or sublimation is the dominant supporter of Io's atmosphere. [29] [30]

One regular moon, Titan, hosts a dense atmosphere dominated by nitrogen as well as stable hydrocarbon lakes on its surface. The complex interactions between Titan's thick, hazy atmosphere, its surface, and its 'hydrocarbon cycle' have led to the creation of many unusual features, including canyons and floodplains eroded by rivers, possible karst-like topography, and extensive equatorial dune fields. [31] [32]

Rotation

The majority of regular moons are tidally locked to their parent planet, though several exceptions are known. One such exception is Saturn's Hyperion, which exhibits chaotic rotation due to Titan's gravitational influence on its irregular shape; Hyperion's chaotic rotation may be further facilitated by its 3:4 orbital resonance with Titan. [13] The four small circumbinary moons of Pluto, which are similarly elongated, also rotate chaotically under the influence of Charon and generally have very high axial tilts. [33] Hi'iaka, the larger outer moon of Haumea, was revealed to have a very rapid rotational period of approximately 9.8 hours via lightcurve data, approximately 120 times faster than its orbital period. Results for Namaka were less clear, potentially pointing towards a slower rotational period or a pole-on configuration, with a significant axial tilt relative to its orbital plane. [34]

Uniquely, Charon is large enough to have also tidally locked Pluto, creating a mutual tidally locked state where Charon is only visible from one hemisphere of Pluto and vice versa. Similarly, Eris has been observed to be tidally locked to its satellite Dysnomia, which may indicate an unusually high density for the moon. [35]

Parent-satellite interactions

Bright auroral spots within Jupiter's northern aurorae, contributed by the Galilean moons Jupiter.Aurora.HST.mod.svg
Bright auroral spots within Jupiter's northern aurorae, contributed by the Galilean moons

Due to their close nature and long, shared histories, regular moons can have a significant influence on their primary. A familiar example of this are the ocean tides raised by the Moon on the Earth. Just as Earth raises tidal bulges on the Moon which results in tidal locking, the Moon raises tidal bulges on the Earth which manifest most noticeably as the rising and falling of the local sea level roughly diurnally (though local coastal topography can result in semidiurnal or complex patterns). [36]

Io's volcanic activity results in extreme interactions with Jupiter, constructing the Io plasma torus in a roughly toroidal region surrounding Io's orbit as well as a neutral cloud of sulfur, oxygen, sodium, and potassium atoms which immediately surround the moon. [37] Escaping ions from the plasma torus are responsible for Jupiter's unusually extensive magnetosphere, generating an internal pressure which inflates it from within. [38] Jupiter's intense magnetic field also couples an intense flux tube with Io's atmosphere and its associated neutral cloud to Jupiter's polar upper atmosphere, generating an intense region of auroral glow. [37] Similar, albeit much weaker flux tubes were also discovered to be associated with the other Galilean moons.

Exploration

Due to their ability to support large internal volumes of liquid water, regular moons are of particular interest to scientists as targets in the search for extraterrestrial life. Subsurface oceans are believed to be capable of hosting complex organic chemistry, an expectation which was supported after the potential indirect detection of various salts in Europa's ocean and the detection of organic compounds and hydrogen cyanide in Enceladus's plumes. [39] [40] [41] [42] As a result, dedicated missions to investigate the nature and potential habitability of several regular moons' internal oceans have been proposed and launched. [43] [44]

Active missions

Missions in development

See also

Notes

  1. ↑ Count derived by adding all inner moons and all rounded moons excluding Triton. For simplicity, Mars's two moons are included, while Saturn's spurious F ring moonlets are excluded.

Related Research Articles

<span class="mw-page-title-main">Galilean moons</span> Four largest moons of Jupiter

The Galilean moons, or Galilean satellites, are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. They are the most readily visible Solar System objects after the unaided visible Saturn, the dimmest of the classical planets, allowing observation with common binoculars, even under night sky conditions of high light pollution. The invention of the telescope enabled the discovery of the moons in 1610. Through this, they became the first Solar System objects discovered since humans have started tracking the classical planets, and the first objects to be found to orbit any planet beyond Earth.

<span class="mw-page-title-main">Orbital resonance</span> Regular and periodic mutual gravitational influence of orbiting bodies

In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationship is found between a pair of objects. The physical principle behind orbital resonance is similar in concept to pushing a child on a swing, whereby the orbit and the swing both have a natural frequency, and the body doing the "pushing" will act in periodic repetition to have a cumulative effect on the motion. Orbital resonances greatly enhance the mutual gravitational influence of the bodies. In most cases, this results in an unstable interaction, in which the bodies exchange momentum and shift orbits until the resonance no longer exists. Under some circumstances, a resonant system can be self-correcting and thus stable. Examples are the 1:2:4 resonance of Jupiter's moons Ganymede, Europa and Io, and the 2:3 resonance between Neptune and Pluto. Unstable resonances with Saturn's inner moons give rise to gaps in the rings of Saturn. The special case of 1:1 resonance between bodies with similar orbital radii causes large planetary system bodies to eject most other bodies sharing their orbits; this is part of the much more extensive process of clearing the neighbourhood, an effect that is used in the current definition of a planet.

<span class="mw-page-title-main">Jupiter</span> Fifth planet from the Sun

Jupiter is the fifth planet from the Sun and the largest in the Solar System. A gas giant, Jupiter's mass is more than two and a half times that of all the other planets in the Solar System combined and slightly less than one one-thousandth the mass of the Sun. Jupiter orbits the Sun at a distance of 5.20 AU (778.5 Gm) with an orbital period of 11.86 years. It is the third brightest natural object in the Earth's night sky after the Moon and Venus and has been observed since prehistoric times. Its name derives from Jupiter, the chief deity of ancient Roman religion.

<span class="mw-page-title-main">Callisto (moon)</span> Second-largest moon of Jupiter

Callisto, or Jupiter IV, is the second-largest moon of Jupiter, after Ganymede. In the Solar System it is the third-largest moon after Ganymede and Saturn's largest moon Titan, and as large as the smallest planet Mercury, though only about a third as massive. Callisto is, with a diameter of 4,821 km, roughly a third larger than Earth's Moon and orbits Jupiter on average at a distance of 1,883,000 km, which is about six times further out than the Moon orbiting Earth. It is the outermost of the four large Galilean moons of Jupiter, which were discovered in 1610 with one of the first telescopes, being visible from Earth with common binoculars.

<span class="mw-page-title-main">Europa (moon)</span> Smallest Galilean moon of Jupiter

Europa, or Jupiter II, is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet of all the 95 known moons of Jupiter. It is also the sixth-largest moon in the Solar System. Europa was discovered independently by Simon Marius and Galileo Galilei and was named after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus.

<span class="mw-page-title-main">Terrestrial planet</span> Planet that is composed primarily of silicate rocks or metals

A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate, rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, Venus, Earth and Mars. Among astronomers who use the geophysical definition of a planet, two or three planetary-mass satellites – Earth's Moon, Io, and sometimes Europa – may also be considered terrestrial planets. The large rocky asteroids Pallas and Vesta are sometimes included as well, albeit rarely. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth, as these planets are, in terms of structure, Earth-like. Terrestrial planets are generally studied by geologists, astronomers, and geophysicists.

<span class="mw-page-title-main">Natural satellite</span> Astronomical body that orbits a planet

A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body. Natural satellites are colloquially referred to as moons, a derivation from the Moon of Earth.

<span class="mw-page-title-main">Ganymede (moon)</span> Largest moon of Jupiter and in the Solar System

Ganymede, or Jupiter III, is the largest and most massive natural satellite of Jupiter and in the Solar System. It is the largest Solar System object without a substantial atmosphere, despite being the only moon in the Solar System with a substantial magnetic field. Like Titan, Saturn's largest moon, it is larger than the planet Mercury, but has somewhat less surface gravity than Mercury, Io, or the Moon due to its lower density compared to the three.

<span class="mw-page-title-main">Io (moon)</span> Innermost of the four Galilean moons of Jupiter

Io, or Jupiter I, is the innermost and second-smallest of the four Galilean moons of the planet Jupiter. Slightly larger than Earth's moon, Io is the fourth-largest moon in the Solar System, has the highest density of any moon, the strongest surface gravity of any moon, and the lowest amount of water by atomic ratio of any known astronomical object in the Solar System. It was discovered in 1610 by Galileo Galilei and was named after the mythological character Io, a priestess of Hera who became one of Zeus's lovers.

<span class="mw-page-title-main">Cryovolcano</span> Type of volcano that erupts volatiles such as water, ammonia or methane, instead of molten rock

A cryovolcano is a type of volcano that erupts gases and volatile material such as liquid water, ammonia, and hydrocarbons. The erupted material is collectively referred to as cryolava; it originates from a reservoir of subsurface cryomagma. Cryovolcanic eruptions can take many forms, such as fissure and curtain eruptions, effusive cryolava flows, and large-scale resurfacing, and can vary greatly in output volumes. Immediately after an eruption, cryolava quickly freezes, constructing geological features and altering the surface.

<span class="mw-page-title-main">Exploration of Jupiter</span> Overview of the exploration of Jupiter the planet and its moons

The exploration of Jupiter has been conducted via close observations by automated spacecraft. It began with the arrival of Pioneer 10 into the Jovian system in 1973, and, as of 2023, has continued with eight further spacecraft missions in the vicinity of Jupiter. All of these missions were undertaken by the National Aeronautics and Space Administration (NASA), and all but two were flybys taking detailed observations without landing or entering orbit. These probes make Jupiter the most visited of the Solar System's outer planets as all missions to the outer Solar System have used Jupiter flybys. On 5 July 2016, spacecraft Juno arrived and entered the planet's orbit—the second craft ever to do so. Sending a craft to Jupiter is difficult, mostly due to large fuel requirements and the effects of the planet's harsh radiation environment.

<span class="mw-page-title-main">Ocean world</span> Planet containing a significant amount of water or other liquid

An ocean world, ocean planet or water world is a type of planet that contains a substantial amount of water in the form of oceans, as part of its hydrosphere, either beneath the surface, as subsurface oceans, or on the surface, potentially submerging all dry land. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid or thalassogen, such as lava, ammonia or hydrocarbons. The study of extraterrestrial oceans is referred to as planetary oceanography.

Extraterrestrial liquid water is water in its liquid state that naturally occurs outside Earth. It is a subject of wide interest because it is recognized as one of the key prerequisites for life as we know it and is thus surmised to be essential for extraterrestrial life.

<span class="mw-page-title-main">Habitability of natural satellites</span> Measure of the potential of natural satellites to have environments hospitable to life

The habitability of natural satellites is the potential of moons to provide habitats for life, though it is not an indicator that they harbor it. Natural satellites are expected to outnumber planets by a large margin and the study of their habitability is therefore important to astrobiology and the search for extraterrestrial life. There are, nevertheless, significant environmental variables specific to moons.

<span class="mw-page-title-main">Exploration of Io</span> Overview of the exploration of Io, Jupiters innermost Galilean and third-largest moon

The exploration of Io, Jupiter's innermost Galilean and third-largest moon, began with its discovery in 1610 and continues today with Earth-based observations and visits by spacecraft to the Jupiter system. Italian astronomer Galileo Galilei was the first to record an observation of Io on January 8, 1610, though Simon Marius may have also observed Io at around the same time. During the 17th century, observations of Io and the other Galilean satellites helped with the measurement of longitude by map makers and surveyors, with validation of Kepler's Third Law of planetary motion, and with measurement of the speed of light. Based on ephemerides produced by astronomer Giovanni Cassini and others, Pierre-Simon Laplace created a mathematical theory to explain the resonant orbits of three of Jupiter's moons, Io, Europa, and Ganymede. This resonance was later found to have a profound effect on the geologies of these moons. Improved telescope technology in the late 19th and 20th centuries allowed astronomers to resolve large-scale surface features on Io as well as to estimate its diameter and mass.

<span class="mw-page-title-main">Planetary-mass moon</span> Planetary-mass bodies that are also natural satellites

A planetary-mass moon is a planetary-mass object that is also a natural satellite. They are large and ellipsoidal in shape. Moons may be in hydrostatic equilibrium due to tidal or radiogenic heating, in some cases forming a subsurface ocean. Two moons in the Solar System are larger than the planet Mercury : Ganymede and Titan, and seven are larger and more massive than the dwarf planets Pluto and Eris.

Planetary oceanography, also called astro-oceanography or exo-oceanography, is the study of oceans on planets and moons other than Earth. Unlike other planetary sciences like astrobiology, astrochemistry, and planetary geology, it only began after the discovery of underground oceans in Saturn's moon Titan and Jupiter's moon Europa. This field remains speculative until further missions reach the oceans beneath the rock or ice layer of the moons. There are many theories about oceans or even ocean worlds of celestial bodies in the Solar System, from oceans made of diamond in Neptune to a gigantic ocean of liquid hydrogen that may exist underneath Jupiter's surface.

<span class="mw-page-title-main">Satellite system (astronomy)</span> Set of gravitationally bound objects in orbit

A satellite system is a set of gravitationally bound objects in orbit around a planetary mass object or minor planet, or its barycenter. Generally speaking, it is a set of natural satellites (moons), although such systems may also consist of bodies such as circumplanetary disks, ring systems, moonlets, minor-planet moons and artificial satellites any of which may themselves have satellite systems of their own. Some bodies also possess quasi-satellites that have orbits gravitationally influenced by their primary, but are generally not considered to be part of a satellite system. Satellite systems can have complex interactions including magnetic, tidal, atmospheric and orbital interactions such as orbital resonances and libration. Individually major satellite objects are designated in Roman numerals. Satellite systems are referred to either by the possessive adjectives of their primary, or less commonly by the name of their primary. Where only one satellite is known, or it is a binary with a common centre of gravity, it may be referred to using the hyphenated names of the primary and major satellite.

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