Everything about Io Moon totally explained
Io (
EYE-oe, or as
Greek Ῑώ) is the innermost of the four
Galilean moons of
Jupiter and, with a
diameter of 3,642
kilometers, the
fourth-largest moon in the
Solar System. It was named after
Io, a priestess of
Hera that became one of the lovers of
Zeus.
With over 400 active volcanoes, Io is the most geologically active object in the Solar System. This extreme geologic activity is the result of tidal heating from friction generated within Io's interior by Jupiter's varying pull. Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km (310 mi). Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of the moon's silicate crust. Some of these peaks are taller than Earth's
Mount Everest. Unlike most satellites in the outer Solar System (which have a thick coating of ice), Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core. Most of Io's surface is characterized by extensive plains coated with sulfur and sulfur dioxide frost.
Io's volcanism is responsible for many of that satellite's unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various shades of red, yellow, white, black, and green, largely due to the sulfurous compounds. Numerous extensive lava flows, several longer than in length, also mark the surface. These volcanic processes have given rise to a comparison of the visual appearance of Io's surface to a pizza. The materials produced by this volcanism provide material for Io's thin, patchy atmosphere and Jupiter's extensive magnetosphere.
Io played a significant role in the development of astronomy in the 17th and 18th centuries. It was discovered in 1610 by
Galileo Galilei, along with the other Galilean satellites. This discovery furthered the adoption of the
Copernican model of the Solar System, the development of
Kepler's laws of motion, and the first measurement of the speed of light. From Earth, Io remained nothing more than a point of light until the late 19th and early 20th centuries, when it became possible to resolve its large-scale surface features, such as the dark red polar and bright equatorial regions. In 1979, the two
Voyager spacecraft revealed Io to be a geologically active world, with numerous volcanic features, large mountains, and a young surface with no obvious impact craters. The
Galileo spacecraft performed several close flybys in the 1990s and early 2000s, obtaining data about Io's interior structure and surface composition. These spacecraft also revealed the relationship between the satellite and Jupiter's magnetosphere and the existence of a belt of radiation centered on Io's orbit. The exploration of Io continued in the early months of 2007 with a distant flyby by Pluto-bound
New Horizons.
Name
While
Simon Marius isn't credited with the sole discovery of the Galilean satellites, his names for the moons have stuck. In his 1614 publication
Mundus Jovialis, he named the innermost large moon of Jupiter after the
Greek mythological figure
Io, one of the many lovers of
Zeus (who is also known as
Jupiter in
Roman mythology). Marius' names fell out of favor, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, Io is simply referred to by its
Roman numeral designation (a system introduced by Galileo) as "", or simply as "the first satellite of Jupiter". The most common adjectival form of the name is
Ionian.
Features on Io are named after characters and places from the Io myth, as well as deities of fire, volcanoes, the Sun, and thunder from various myths, and characters and places from
Dante's
Inferno, names appropriate to the volcanic nature of the surface. Since the surface was first seen up close by
Voyager 1 the
International Astronomical Union has approved 225 names for Io's volcanoes, mountains, plateaus, and large albedo features. The approved feature names used for Io include
patera (volcanic depression),
mons,
mensa,
planum, and
tholus (various types of mountain, with morphologic characteristics such as size, shape, and height determining the term used),
fluctus (lava flow),
vallis (lava channel),
regio (large-scale albedo feature), and
active eruptive center (location where plume activity was the first sign of volcanic activity at a particular volcano).
Observational history
The first reported observation of Io was made by
Galileo Galilei on
7 January,
1610. The discovery of Io and the other Galilean satellites of Jupiter was published in Galileo's
Sidereus Nuncius in March 1610. In his
Mundus Jovialis, published in 1614, Simon Marius claimed to have discovered Io and the other moons of Jupiter in 1609, one week before Galileo's discovery. Galileo doubted this claim and dismissed the work of Marius as plagiarism. Given that Galileo published his work before Marius, Galileo is credited with the discovery.
For the next two and a half centuries, Io remained an unresolved, 5th-magnitude point of light in astronomers' telescopes. During the 17th century, Io and the other Galilean satellites served a variety of purposes, such as helping mariners determine their
longitude, validating Kepler's
Third Law of planetary motion, and determining the time for light to travel between Jupiter and Earth. Later telescopic observations confirmed Io's distinct reddish-brown polar regions and yellow-white equatorial band.
Telescopic observations in the mid-20th century began to hint at Io's unusual nature. Spectroscopic observations suggested that Io's surface was devoid of water ice (a substance found to be plentiful on the other Galilean satellites). The same observations suggested a surface dominated by evaporates composed of
sodium salts and
sulfur. Radio telescopic observations revealed Io's influence on the Jovian
magnetosphere, as demonstrated by
decametric wavelength bursts tied to the orbital period of Io.
Pioneer
The first spacecraft to pass by Io were the twin
Pioneer 10 and
11 probes on
December 3,
1973 and
December 2,
1974 respectively. Radio tracking provided an improved estimate of Io's mass, which, along with the best available information of Io's size, suggested that Io had the highest density of the four Galilean satellites, and was composed primarily of silicate rock rather than water ice. The
Pioneers also revealed the presence of a thin atmosphere at Io and intense radiation belts near the orbit of Io. The camera on board
Pioneer 11 took the only good image of Io obtained by either spacecraft, showing its north polar region. Close-up images were planned during
Pioneer 10's encounter with Io, but those observations were lost due to the high-radiation environment. The images returned during the approach revealed a strange, multi-colored landscape devoid of impact craters. The highest-resolution images showed a relatively young surface punctuated by oddly shaped pits, mountains taller than Mount Everest, and features resembling volcanic lava flows.
Shortly after the encounter,
Voyager navigation engineer Linda A. Morabito noticed a "plume" emanating from the surface in one of the images. Analysis of other
Voyager 1 images showed nine such plumes scattered across the surface, proving that Io was volcanically active. This conclusion was predicted in a paper published shortly before the
Voyager 1 encounter by Stan J. Peale, Patrick Cassen, and R. T. Reynolds. The authors calculated that Io's interior must experience significant tidal heating caused by its orbital resonance with Europa and Ganymede (see the "
Tidal heating" section for a more detailed explanation of the process). Data from this flyby showed that the surface of Io is dominated by sulfur and
sulfur dioxide frosts. These compounds also dominate its thin
atmosphere and the
torus of plasma centered on Io's orbit (also discovered by
Voyager).
Voyager 2 passed Io on
July 9,
1979 at a distance of 1,130,000 km (702,150 mi). Though it didn't approach nearly as close as
Voyager 1, comparisons between images taken by the two spacecraft showed several surface changes that had occurred in the five months between the encounters. In addition, observations of Io as a crescent as
Voyager 2 departed the Jovian system revealed that eight of the nine plumes observed in March were still active in July 1979, with only the volcano
Pele shutting down between flybys.
Galileo
The
Galileo spacecraft arrived at Jupiter in 1995 after a six-year journey from Earth to follow up on the discoveries of the two
Voyager probes and ground-based observations taken in the intervening years. Io's location within one of Jupiter's most intense radiation belts precluded a prolonged close flyby, but
Galileo did pass close by shortly before entering orbit for its two-year, primary mission studying the Jovian system. While no images were taken during the close flyby on
December 7,
1995, the encounter did yield significant results, such as the discovery of a large iron core, similar to that found in the rocky planets of the inner solar system.
Despite the lack of close-up imaging and mechanical problems that greatly restricted the amount of data returned, several significant discoveries were made during
Galileo's primary mission.
Galileo observed the effects of a major eruption at Pillan Patera and confirmed that volcanic eruptions are composed of silicate magmas with magnesium-rich
mafic and
ultramafic compositions with sulfur and sulfur dioxide serving a similar role to water and
carbon dioxide on Earth. Distant imaging of Io was acquired for almost every orbit during the primary mission, revealing large numbers of active volcanoes (both thermal emission from cooling magma on the surface and volcanic plumes), numerous mountains with widely varying morphologies, and several surface changes that had taken place both between the
Voyager and
Galileo eras and between
Galileo orbits.
The
Galileo mission was twice extended, in 1997 and 2000. During these extended missions, the probe flew by Io three times in late 1999 and early 2000 and three times in late 2001 and early 2002. Observations during these encounters revealed the geologic processes occurring at Io's volcanoes and mountains, excluded the presence of a magnetic field, and demonstrated the extent of volcanic activity.
Subsequent observations
Following
Galileo's fiery demise in Jupiter's atmosphere in September 2003, new observations of Io's volcanism came from Earth-based telescopes. In particular,
adaptive optics imaging from the
Keck telescope in
Hawaii and imaging from the Hubble telescope have allowed astronomers to monitor Io's active volcanoes. This imaging has allowed scientists to monitor volcanic activity on Io, even without a spacecraft in the Jupiter system. The
New Horizons spacecraft, en route to
Pluto and the
Kuiper belt, flew by the Jupiter system and Io on
February 28,
2007. During the encounter, numerous distant observations of Io were obtained. Early results include images of a large plume at Tvashtar, providing the first detailed observations of the largest class of Ionian volcanic plume since observations of Pele's plume in 1979.
New Horizons also captured images of a volcano near
Girru Patera in the early stages of an eruption, and several volcanic eruptions that have occurred since
Galileo.
The only forthcoming mission planned for the Jupiter system,
Juno, doesn't have an imaging system powerful enough to perform Io surface science. The Europa/Jupiter System Mission, a joint NASA/ESA project currently in the concept study phase, would be able to study Io from a distance as well as during as many as four close flybys. If approved by the two space agencies, the two spacecraft would arrive in the 2021-2024 timeframe. Another possible mission, called the
Io Volcanic Observer, would launch in 2013 as a Discovery-class mission and involve multiple flybys of Io while in orbit around Jupiter, however at
present, this project is also in the concept study phase.
Orbit
Io orbits Jupiter at a distance of 421,700 km (262,000 mi) from the planet's center and 350,000 km (217,000 mi) from its cloudtops. It is the innermost of the Galilean satellites of Jupiter, its orbit lying between those of
Thebe and
Europa. Including Jupiter's inner satellites, Io is the fifth moon out from Jupiter. It takes 42.5 hours to revolve once (fast enough for its motion to be observed over a single night of observation). Io is in a 2:1 mean-motion
orbital resonance with Europa and a 4:1 mean-motion orbital resonance with
Ganymede, completing two orbits of Jupiter for every one orbit completed by Europa, and four orbits for every one completed by Ganymede. This resonance helps maintain Io's
orbital eccentricity (0.0041), which in turn provides the primary heating source for its geologic activity (see the "
Tidal heating" section for a more detailed explanation of the process).]]
Io plays a significant role in shaping the Jovian magnetic field. The magnetosphere of Jupiter sweeps up gases and dust from Io's thin atmosphere at a rate of 1
tonne per second. This material is mostly composed of
ionized and atomic sulfur, oxygen and chlorine; atomic sodium and potassium; molecular sulfur dioxide and sulfur; and
sodium chloride dust. These materials ultimately have their origin from Io's volcanic activity, but the material that escapes to Jupiter's magnetic field and into interplanetary space comes directly from Io's atmosphere. These materials, depending on their ionized state and composition, ultimately end up in various neutral (non-ionized) clouds and radiation belts in Jupiter's
magnetosphere and, in some cases, are eventually ejected from the Jovian system.
Surrounding Io (up to a distance of 6 Io radii from the moon's surface) is a cloud of neutral sulfur, oxygen, sodium, and potassium atoms. These particles originate in Io's upper atmosphere but are excited from collisions with ions in the
plasma torus (discussed below) and other processes into filling Io's
Hill sphere, which is the region where the moon's gravity is predominant over Jupiter. Some of this material escapes Io's gravitational pull and goes into orbit around Jupiter. Over a 20-hour period, these particles spread out from Io to form a banana-shaped, neutral cloud that can reach as far as 6 Jovian radii from Io, either inside Io's orbit and ahead of the satellite or outside Io's orbit and behind the satellite.
Io orbits within a belt of intense radiation known as the Io plasma torus. The plasma in this donut-shaped ring of ionized sulfur, oxygen, sodium, and chlorine originates when neutral atoms in the "cloud" surrounding Io are ionized and carried along by the Jovian magnetosphere. Particles from Io, detected as variations in magnetospheric plasma, have been detected far into the long magnetotail by
New Horizons. To study similar variations within the plasma torus, researchers measure the
ultraviolet-wavelength light it emits. While such variations have not been definitively linked to variations in Io's volcanic activity (the ultimate source for material in the plasma torus), this link has been established in the neutral sodium cloud.
During an encounter with Jupiter in 1992, the
Ulysses spacecraft detected a stream of dust-sized particles being ejected from the Jupiter system. The dust in these discrete streams travel away from Jupiter at speeds upwards of several hundred kilometers per second, have an average size of 10
μm, and consist primarily of sodium chloride.
Jupiter's
magnetic field lines, which Io crosses, couples Io to Jupiter's polar upper atmosphere through the
generation of an
electric current known as the Io
flux tube. Models based on the
Voyager and
Galileo measurements of the moon's mass, radius and quadrupole gravitational coefficients (numerical values related to how mass is distributed within an object) suggest that its interior is differentiated between an outer, silicate-rich
crust and
mantle and an inner, iron- or
iron sulfide–rich
core. Depending on the amount of sulfur in the core, the core has a radius between 350 and 650 km (220 to 400 mi) if it's composed almost entirely of iron, or between 550 and 900 km (310 to 560 mi) for a core consisting of a mix of iron and sulfur.
Galileo's
magnetometer failed to detect an internal magnetic field at Io, suggesting that the core isn't
convecting.
Modeling of Io's interior composition suggests that the mantle is composed of at least 75% of the magnesium-rich mineral
forsterite, and has a bulk composition similar to that of
L-chondrite and
LL-chondrite meteorites, with higher iron content (compared to
silicon) than the
Moon or
Earth, but lower than
Mars. To support the heat flow observed on Io, 10–20% of Io's mantle may be molten, though regions where high-temperature volcanism has been observed may have higher melt fractions. The
lithosphere of Io, composed of basalt and sulfur deposited by Io's extensive volcanism, is at least 12 km (7 mi) thick, but is likely to be less than 40 km (25 mi) thick. The vertical differences in Io's tidal bulge, between the times Io is at
periapsis and
apoapsis in its orbit, could be as much as 100 m (330 ft). The friction or tidal dissipation produced in Io's interior due to this varying tidal pull, which, without the resonant orbit, would have gone into circularizing Io's orbit instead, creates significant tidal heating within Io's interior, melting a significant amount of the moon's mantle and core. The amount of energy produced is up to 200 times greater than that produced solely from
radioactive decay. This heat is released in the form of volcanic activity, generating its observed high
heat flow (global total: 0.6 to 1.6×10
14 W). The lack of impact craters indicated that Io's surface is geologically young, like the terrestrial surface; volcanic materials continuously bury craters as they're produced. This result was spectacularly confirmed as at least nine active volcanoes were observed by
Voyager 1.
Surface composition
Io's colorful appearance is the result of various materials produced by its extensive volcanism. These materials include
silicates (such as
orthopyroxene),
sulfur, and
sulfur dioxide. Sulfur dioxide frost is ubiquitous across the surface of Io, forming large regions covered in white or grey materials. Sulfur is also seen in many places across the satellite, forming yellow to yellow-green regions. Sulfur deposited in the mid-latitude and polar regions is often radiation damaged, breaking up normally stable 8-chain sulfur. This radiation damage produces Io's red-brown polar regions. A prominent example of a red-ring plume deposit is located at Pele. These red deposits consist primarily of sulfur (generally 3- and 4-chain molecular sulfur), sulfur dioxide, and perhaps Cl
2SO
2. This lack of water is likely due to Jupiter being hot enough early in the
evolution of the solar system to drive off
volatile materials like water in the vicinity of Io, but not hot enough to do so farther out.
Volcanism
The tidal heating produced by Io's forced
orbital eccentricity has led the moon to become one of the most volcanically active worlds in the solar system, with hundreds of volcanic centers and extensive
lava flows. During a major eruption, lava flows tens or even hundreds of kilometers long can be produced, consisting mostly of
basalt silicate lavas with either
mafic or
ultramafic (magnesium-rich) compositions. As a by-product of this activity, sulfur, sulfur dioxide gas and silicate
pyroclastic material (like ash) are blown up to 500 km (310 mi) into space, producing large, umbrella-shaped plumes, painting the surrounding terrain in red, black, and white, and providing material for Io's patchy atmosphere and Jupiter's extensive magnetosphere.
Io's surface is dotted with volcanic depressions known as
paterae. Paterae generally have flat floors bounded by steep walls. These features resemble terrestrial
calderas, but it's unknown if they're produced through collapse over an emptied lava chamber like their terrestrial cousins. One hypothesis suggests that these features are produced through the exhumation of volcanic
sills, and the overlying material is either blasted out or integrated into the sill. Unlike similar features on Earth and Mars, these depressions generally don't lie at the peak of
shield volcanoes and are normally larger, with an average diameter of 41 km (25 mi), the largest being
Loki Patera at 202 km (126 mi).
Lava flows represent another major volcanic terrain on Io. Magma erupts onto the surface from vents on the floor of paterae or on the plains from fissures, producing inflated, compound lava flows similar to those seen at
Kilauea in Hawaii. Images from the
Galileo spacecraft revealed that many of Io's major lava flows, like those at
Prometheus and
Amirani, are produced by the build-up of small breakouts of lava flows on top of older flows. Larger outbreaks of lava have also been observed on Io. For example, the leading edge of the Prometheus flow moved 75 to 95 km (47 to 59 mi) between
Voyager in 1979 and the first
Galileo observations in 1996. A major eruption in 1997 produced more than 3,500 km
2 (1,350 sq mi) of fresh lava and flooded the floor of the adjacent Pillan Patera. Initial estimates suggesting eruption temperatures approaching 2,000 K These plumes appear to be formed in one of two ways. Io's largest plumes are created when sulfur and sulfur dioxide gas dissolve from erupting magma at volcanic vents or lava lakes, often dragging silicate pyroclastic material with them. These plumes form red (from the short-chain sulfur) and black (from the silicate pyroclastics) deposits on the surface. Plumes formed in this manner are among the largest observed at Io, forming red rings more than 1,000 km (620 mi) in diameter. Examples of this plume type include Pele, Tvashtar, and
Dazhbog. Another type of plume is produced when encroaching lava flows vaporize underlying sulfur dioxide frost, sending the sulfur skyward. This type of plume often forms bright circular deposits consisting of sulfur dioxide. These plumes are often less than 100 km (62 mi) tall, and are among the most long-lived plumes on Io. Examples include Prometheus, Amirani, and
Masubi.
Mountains
Io has 100 to 150 mountains. These structures average 6 km (4 mi) in height and reach a maximum of 17.5 ± 1.5 km (10.9 ± 1 mi) at South
Boösaule Montes. Mountains often appear as large (the average mountain is 157 km (98 mi) long), isolated structures with no apparent global tectonic patterns outlined, as is the case on Earth.
Despite the extensive volcanism that gives Io its distinctive appearance, nearly all its mountains are tectonic structures, and are not produced by volcanoes. Instead, most Ionian mountains form as the result of compressive stresses on the base of the lithosphere, which uplift and often tilt chunks of Io's crust through
thrust faulting. The compressive stresses leading to mountain formation are the result of
subsidence from the continuous burial of volcanic materials. This suggests large-scale regions in Io's lithosphere where compression (supportive of mountain formation) and extension (supportive of patera formation) dominate. Locally, however, mountains and paterae often abut one another, suggesting that magma often exploits faults formed during mountain formation to reach the surface. These volcanic mountains are often smaller than the average mountain on Io, averaging only 1 to 2 km (0.6 to 1.2 mi) in height and 40 to 60 km (25 to 37 mi) wide. Other shield volcanoes with much shallower slopes are inferred from the morphology of several of Io's volcanoes, where thin flows radiate out from a central patera, such as at
Ra Patera.
Atmosphere
Io has an extremely thin
atmosphere consisting mainly of sulfur dioxide with a pressure of a billionth of an
atmosphere. The most dramatic source of is volcanism, but the atmosphere is largely sustained by sunlight-driven sublimation of frozen on the surface. The atmosphere is largely confined to the equator, where the surface is warmest and most active volcanic plumes reside. Other variations also exist, with the highest densities near volcanic vents (particularly at sites of volcanic plumes) and on Io's anti-Jovian hemisphere (the side that faces away from Jupiter, where frost is most abundant).
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