Io(Moon Of Jupiter)
(Io:Moon Of Jupiter, Structure, Origin, Discovery, History, Surface)
Io /ˈaɪ.oʊ/ (Jupiter I) is the innermost of the four
Galilean moons of the planet Jupiter. It is the fourth-largest moon, has the
highest density of all the moons, and has the least amount of water of any
known astronomical object in the Solar System. It was discovered in 1610 and
was named after the mythological character Io, a priestess of Hera who became
one of Zeus's lovers.
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 as it is pulled
between Jupiter and the other Galilean satellites—Europa, Ganymede and
Callisto. Several volcanoes produce plumes of sulfur and sulfur dioxide that
climb as high as 500 km (300 mi) above the surface. Io's surface is also dotted
with more than 100 mountains that have been uplifted by extensive compression
at the base of Io's silicate crust. Some of these peaks are taller than Mount
Everest. Unlike most satellites in the outer Solar System, which are mostly
composed of water ice, Io is primarily composed of silicate rock surrounding a
molten iron or iron-sulfide core. Most of Io's surface is composed of extensive
plains coated with sulfur and sulfur-dioxide frost.
Io's volcanism is responsible for many of its unique
features. Its volcanic plumes and lava flows produce large surface changes and
paint the surface in various subtle shades of yellow, red, white, black, and
green, largely due to allotropes and compounds of sulfur. Numerous extensive
lava flows, several more than 500 km (300 mi) in length, also mark the surface.
The materials produced by this volcanism make up Io's thin, patchy atmosphere
and Jupiter's extensive magnetosphere. Io's volcanic ejecta also produce a
large plasma torus around Jupiter.
Io played a significant role in the development of astronomy
in the 17th and 18th centuries. It was discovered in January 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 just 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 Galileospacecraft 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 Io and
Jupiter's magnetosphere and the existence of a belt of high-energy radiation
centered on Io's orbit. Io receives about 3,600 rem (36 Sv) of ionizing
radiation per day.
Further observations have been made by Cassini–Huygens in
2000 and New Horizons in 2007, as well as from Earth-based telescopes and the
Hubble Space Telescope.
From the surface of Io, Jupiter would subtend an arc of
19.5°, making Jupiter appear 39 times the apparent diameter of our Moon.
Observational history
The first reported observation of Io was made by Galileo
Galilei on 7 January 1610 using a 20x-power, refracting telescope at the
University of Padua. However, in that observation, Galileo could not separate
Io and Europa due to the low power of his telescope, so the two were recorded
as a single point of light. Io and Europa were seen for the first time as
separate bodies during Galileo's observations of the Jupiter system the
following day, 8 January 1610 (used as the discovery date for Io by the IAU).
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. Regardless, Marius' first
recorded observation came from 29 December 1609 in the Julian calendar, which
equates to 8 January 1610 in the Gregorian calendar, which Galileo used. 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, including early methods to determine longitude, validating Kepler's
third law of planetary motion, and determining the time required for light to
travel between Jupiter and Earth. Based on ephemerides produced by astronomer
Giovanni Cassini and others, Pierre-Simon Laplace created a mathematical theory
to explain the resonant orbits of Io, Europa, and Ganymede.This resonance was
later found to have a profound effect on the geologies of the three moons.
Improved telescope technology in the late 19th and 20th
centuries allowed astronomers to resolve (that is, see as distinct objects)
large-scale surface features on Io. In the 1890s, Edward E. Barnard was the
first to observe variations in Io's brightness between its equatorial and polar
regions, correctly determining that this was due to differences in color and
albedo between the two regions and not due to Io being egg-shaped, as proposed
at the time by fellow astronomer William Pickering, or two separate objects, as
initially proposed by Barnard. 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.Radiotelescopic 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 3 December 1973 and 2 December 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 because of the
high-radiation environment.
Voyager
Mosaic of Voyager 1 images covering Io's south polar region.
The view includes two of Io's ten highest peaks, the Euboea Montes at upper
extreme left and Haemus Mons at bottom.
When the twin probes Voyager 1 and Voyager 2 passed by Io in
1979, their more advanced imaging system allowed for far more detailed images.
Voyager 1 flew past Io on 5 March 1979 from a distance of 20,600 km (12,800
mi). 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 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 plasmacentered on
Io's orbit (also discovered by Voyager).
Voyager 2 passed Io on 9 July 1979 at a distance of 1,130,000
km (702,000 mi). Though it did not approach nearly as close as Voyager 1,
comparisons between images taken by the two spacecraft showed several surface
changes that had occurred in the four months between the encounters. In
addition, observations of Io as a crescent as Voyager 2 departed the Jovian
system revealed that seven 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. Although no
images were taken during the close flyby on 7 December 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. 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. In December 2000, the Cassini spacecraft had a
distant and brief encounter with the Jupiter system en route to Saturn,
allowing for joint observations with Galileo. These observations revealed a new
plume at Tvashtar Paterae and provided insights into Io's aurorae.
Subsequent observations
Following Galileo's planned destruction 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 28 February 2007. During the
encounter, numerous distant observations of Io were obtained. These included
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.
There are currently two forthcoming missions planned for the
Jupiter system. Juno, launched on 5 August 2011, has limited imaging
capabilities, but it could monitor Io's volcanic activity using its near-infrared
spectrometer, JIRAM. The Jupiter Icy Moon Explorer (JUICE) is a planned
European Space Agency mission to the Jupiter system that is intended to end up
in Ganymede orbit. JUICE has a launch scheduled for 2022, with arrival at
Jupiter planned for January 2030. JUICE will not fly by Io, but it will use its
instruments, such as a narrow-angle camera, to monitor Io's volcanic activity
and measure its surface composition during the two-year Jupiter-tour phase of
the mission prior to Ganymede orbit insertion. The Io Volcano Observer (IVO) is
a proposal for a Discovery-class mission that would launch in 2021. It would
involve multiple flybys of Io while in orbit around Jupiter beginning in 2026.
Interior
Composed primarily of silicate rock and iron, Io is closer
in bulk composition to the terrestrial planets than to other satellites in the
outer Solar System, which are mostly composed of a mix of water ice and
silicates. Io has a density of 3.5275 g/cm3, the highest of any moon in the
Solar System; significantly higher than the other Galilean satellites (Ganymede
and Callisto in particular, whose densities are around 1.9 g/cm3) and slightly
higher than the Moon. Models based on the Voyager and Galileo measurements of
Io'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 a silicate-rich crust and mantle and an iron- or
iron-sulfide-rich core. Io's metallic core makes up approximately 20% of its
mass. Depending on the amount of sulfur in the core, the core has a radius
between 350 and 650 km (220–400 mi) if it is composed almost entirely of iron,
or between 550 and 900 km (340–560 mi) for a core consisting of a mix of iron
and sulfur. Galileo's magnetometerfailed to detect an internal, intrinsic
magnetic field at Io, suggesting that the core is not 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. However, re-analysis of Galileo
magnetometer data in 2009 revealed the presence of an induced magnetic field at
Io, requiring a magma ocean 50 km (31 mi) below the surface. Further analysis
published in 2011 provided direct evidence of such an ocean. This layer is estimated
to be 50 km thick and to make up about 10% of Io's mantle. It is estimated that
the temperature in the magma ocean reaches 1,200 °C. It is not known if the
10–20% partial melting percentage for Io's mantle is consistent with the
requirement for a significant amount of molten silicates in this possible magma
ocean. The lithosphere of Io, composed of basalt and sulfur deposited by Io's
extensive volcanism, is at least 12 km (7 mi) thick, and likely less than 40 km
(25 mi) thick.
Tidal heating
Unlike Earth and the Moon, Io's main source of internal heat
comes from tidal dissipation rather than radioactive isotope decay, the result
of Io's orbital resonance with Europa and Ganymede. Such heating is dependent
on Io's distance from Jupiter, its orbital eccentricity, the composition of its
interior, and its physical state. Its Laplace resonance with Europa and
Ganymede maintains Io's eccentricity and prevents tidal dissipation within Io
from circularizing its orbit. The resonant orbit also helps to maintain Io's
distance from Jupiter; otherwise tides raised on Jupiter would cause Io to
slowly spiral outward from its parent planet. 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
Io'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 1.6×1014 W). Models of its orbit suggest that the amount of tidal
heating within Io changes with time; however, the current amount of tidal
dissipation is consistent with the observed heat flow. Models of tidal heating
and convection have not found consistent planetary viscosity profiles that
simultaneously match tidal energy dissipation and mantle convection of heat to
the surface.
Although there is general agreement that the origin of the
heat as manifested in Io's many volcanoes is tidal heating from the pull of
gravity from Jupiter and its moon Europa, the volcanoes are not in the
positions predicted with tidal heating. They are shifted 30 to 60 degrees to
the east. A study published by Tyler et al. (2015) suggests that this eastern
shift may be caused by an ocean of molten rock under the surface. The movement
of this magma would generate extra heat through friction due to its viscosity.
The study's authors believe that this subsurface ocean is a mixture of molten
and solid rock.
Other moons in the Solar System are also tidally heated, and
they too may generate additional heat through the friction of subsurface magma
or water oceans. This ability to generate heat in a subsurface ocean increases
the chance of life on bodies like Europa and Enceladus.
Surface
Based on their experience with the ancient surfaces of the
Moon, Mars, and Mercury, scientists expected to see numerous impact craters in
Voyager 1's first images of Io. The density of impact craters across Io's
surface would have given clues to Io's age. However, they were surprised to
discover that the surface was almost completely lacking in impact craters, but
was instead covered in smooth plains dotted with tall mountains, pits of
various shapes and sizes, and volcanic lava flows. Compared to most worlds
observed to that point, Io's surface was covered in a variety of colorful
materials (leading Io to be compared to a rotten orange or to pizza) from
various sulfurous compounds. The lack of impact craters indicated that Io's
surface is geologically young, like the terrestrial surface; volcanic materials
continuously bury craters as they are produced. This result was spectacularly
confirmed as at least nine active volcanoes were observed by Voyager 1.
Atmosphere
Auroral glows in Io's upper atmosphere. Different colors
represent emission from different components of the atmosphere (green comes
from emitting sodium, red from emitting oxygen, and blue from emitting volcanic
gases like sulfur dioxide). Image taken while Io was in eclipse.
Io has an extremely thin atmosphere consisting mainly of
sulfur dioxide (SO
Gas in Io's atmosphere is stripped by Jupiter's
magnetosphere, escaping to either the neutral cloud that surrounds Io, or the
Io plasma torus, a ring of ionized particles that shares Io's orbit but
co-rotates with the magnetosphere of Jupiter. Approximately one ton of material
is removed from the atmosphere every second through this process so that it
must be constantly replenished. The most dramatic source of SO
High-resolution images of Io acquired when Io is
experiencing an eclipse reveal an aurora-like glow. As on Earth, this is due to
particle radiation hitting the atmosphere, though in this case the charged
particles come from Jupiter's magnetic field rather than the solar wind. Aurorae
usually occur near the magnetic poles of planets, but Io's are brightest near
its equator. Io lacks an intrinsic magnetic field of its own; therefore,
electrons traveling along Jupiter's magnetic field near Io directly impact Io's
atmosphere. More electrons collide with its atmosphere, producing the brightest
aurora, where the field lines are tangent to Io (i.e. near the equator),
because the column of gas they pass through is longest there. Aurorae
associated with these tangent points on Io are observed to rock with the
changing orientation of Jupiter's tilted magnetic dipole. Fainter aurora from
oxygen atoms along the limb of Io (the red glows in the image at right), and
sodium atoms on Io's night-side (the green glows in the same image) have also
been observed.
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