Exploration of Io

[1] 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.

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.

[1] While the Jovian moons he discovered would later be known as the Galilean satellites, after himself, he proposed the name Medicea Sidera (Medicean Stars) after his new patrons, the de'Medici family of his native Florence.

This showed that previous maps had depicted some shorelines as extending farther than they really did, which caused the apparent area of France to shrink, and led King Louis XIV to comment that "he was losing more territory to his astronomers than to his enemies.

[18] Using Ole Rømer's data and a modern value for the astronomical unit, his measurement that light takes 16.44 minutes to travel the distance of the diameter of Earth's orbit was only 2% greater than the modern-day value, though this was not calculated at the time.

[19] In 1788, Pierre-Simon Laplace used Cassini's ephemerides and those produced by other astronomers in the preceding century to create a mathematical theory explaining the resonant orbits of Io, Europa, and Ganymede.

Improved telescopes and mathematical techniques allowed astronomers in the 19th and 20th centuries to estimate many of Io's physical properties, such as its mass, diameter, and albedo, as well as to resolve large-scale surface features on it.

[21] Beginning in the 1890s, larger telescopes allowed astronomers to directly observe large scale features on the surfaces of the Galilean satellites including Io.

However, there was uncertainty over whether a spacecraft could survive passage through the asteroid belt, where micrometeoroids could cause it physical damage, or the intense Jovian magnetosphere, where charged particles could harm sensitive electronics.

A slight attenuation of the signal before and after the occultation showed that Io had an ionosphere, suggesting the presence of a thin atmosphere with a pressure of 1.0 × 10−7 bar, though the composition was not determined.

The initial hot Jupiter prevented the condensation of water at the orbits of Io and Europa, leading those bodies to have higher densities than the outer two moons.

[41] NASA researchers observed a sharp increase in Io's thermal emission at 5 μm on February 20, 1978, possibly due to an interaction between the satellite and Jupiter's magnetosphere, though volcanism was not ruled out.

[43] A few days before the Voyager 1 encounter, Stan Peale, Patrick Cassen, and R. T. Reynolds published a paper in the journal Science predicting a volcanically modified surface and a differentiated interior, with distinct rock types rather than a homogeneous blend.

[46] On approach to Jupiter in late February and early March 1979, Voyager imaging scientists noticed that Io appeared distinct from the other Galilean satellites.

[48] The data from the Ultraviolet Spectrometer (UVS) revealed a torus of plasma composed of sulfur ions at the orbit of Io, but tilted to match the equator of Jupiter's magnetic field.

[46][48] The close distance of the encounter allowed Voyager to acquire images of the sub-Jovian and south polar regions of Io with a best resolution of less than 0.5 km (0.3 mi) per pixel.

[47] Even in the highest resolution images, no impact craters were observed, suggesting that Io's surface was being regularly renewed by the present-day volcanic activity.

While processing images of Io to enhance the visibility of background stars, navigation engineer Linda Morabito found a 300-kilometer (190 mi) tall cloud along the moon's limb.

[58] Though it did not approach nearly as close to Io 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, including new plume deposits at Aten Patera and Surt.

Changes in the distribution of diffuse plume deposits and additional dark material were observed in the southern portion of Loki Patera, the consequence of a volcanic eruption there.

[60] The blue color of the plumes observed (Amirani, Maui, Masubi, and Loki) suggested that the reflected light from them came from fine grained particles approximately 1 μm in diameter.

[65] En route to Jupiter, the high-gain antenna, folded up like an umbrella to allow the spacecraft to fit in the shuttle cargo bay, failed to open completely.

Analysis of the Doppler shift of Galileo's radio signal showed that Io is differentiated with 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 at Io were made during Galileo's two-year, primary mission.

The focus of this extended mission was joint observation of the Jovian system by both Galileo and Cassini, which performed a distant flyby of Jupiter en route to Saturn on December 30, 2000.

Both encounters in 2001 allowed Galileo to observe Io's polar regions up-close, though imaging from the August 2001 flyby was lost due to a camera malfunction.

[2] In order to prevent potential biological contamination of the possible Europan biosphere, the Galileo mission ended on September 23, 2003 when the spacecraft was intentionally crashed into Jupiter.

[81] The New Horizons spacecraft, en route to Pluto and the Kuiper belt, flew by the Jupiter system on February 28, 2007, approaching Io to a distance of 2,239,000 km (1,391,000 mi).

[83] Like Galileo during its November 1999 flyby of Io and Cassini during encounter in December 2000, New Horizons caught Tvashtar during a major eruption at the same site as the 1999 lava curtain.

The Jupiter Icy Moon Explorer (JUICE) is a planned European Space Agency mission to the Jovian system that is intended to end up in Ganymede orbit.

A painting of a spacecraft with fully extended, umbrella-like radio antenna dish, in front of an orange planetary body at left with several, blue, umbrella-like clouds, with Jupiter in back ground on the right, with its Great Red Spot visible
Painting illustrating a flyby of Io by the Galileo spacecraft
A portrait of the head and upper body of a middle-aged man with a receding hairline and brown beard. He is wearing a black, Italian Renaissance outfit. The text "GAILILEVS GAILILEVS – MATHVS:" is painted to the left of the man's head.
Galileo Galilei, the discoverer of Io
A brass, clock-like mechanical device in a museum display case, with a small card with the number 8 printed on it. The face of the device is split into several rings, with the Roman numerals I through XI (and 0) on one of these rings.
Dutch Orrery of the Jovian system, built c. 1750 , used by Harvard professor John Winthrop
Io, Europa, and Ganymede move counter-clockwise along three concentric circles around Jupiter. Every time Europa reaches the top of its orbit, Io goes around twice in its orbit. Every time Ganymede reaches the top of its orbit, Io goes around four times in its orbit.
Animation showing the Laplace resonance between Io, Europa and Ganymede (conjunctions are highlighted by color changes)
An animation simulating the orbital motion of a small, planetary body as it passes from left to right in front of Jupiter. A dark, circular spot is seen on Jupiter, moving left to right with the same speed, and to the right, of the smaller body.
Simulation of a transit of Jupiter by Io. Io's shadow precedes Io on Jupiter's cloud tops.
A painting of a spacecraft in front of a crescent Jupiter, the distant Sun, and the stars of the Milky Way in the background. The night-side of Jupiter is illuminated.
Artist's rendition of the Pioneer 10 encounter with Jupiter
Two versions of the same image of an orange planetary body; the bottom left half of both is illuminated. The image on the right is darker, so dark features on the surface of the body are more visible.
Only image of Io returned from Pioneer 11
Photo a planetary body covered in numerous dark spots in front of the bright and dark clouds of Jupiter.
Voyager 1 approach image of Io, with Jupiter's clouds in the background
An aerial image of a landscape with numerous flow-like features, irregular shaped, flat-floored pits, tall mountains, and shorter mesas. These features are surrounded by smooth plains, with several areas of bright terrain surrounding some mountains and pits. The boundary between the day-side and night-side cuts across the image from upper right to bottom center. The upper left and lower left corner are black, outside the area of the mosaic.
Mosaic of Voyager 1 images covering Io's south polar region
The thin crescent (open to the right) of the full disk of a planetary body with two bright clouds along the upper left edge of the object and another along the right edge.
Three volcanic plumes seen by Voyager 2 along the limb of Io
A multi-colored image of the full disk of a planetary body, dotted with numerous dark spots. Much of the middle portion of the planetary body is yellow to white/gray, while the polar regions at the top and bottom are generally reddish in color.
Mosaic of images from Galileo acquired in November 1996
Two images, displayed side-by-side, showing a red, diffuse ring with a darker, gray region in the middle. In the image on the right, this red ring is interrupted on its upper right side by a hexagonal dark gray region.
Two Galileo images showing the effects of a major eruption at Pillan Patera in 1997
A portion of a planetary body with a pair of large, mountainous ridges on the left side of the image, a shorter, rugged domical mountain at top center, an elliptical pit near bottom center, and the boundary between the dayside (to the left) and the nightside (to the right) running down the right side of the image. Two small mountain peaks are seen near this boundary at lower right.
Mongibello Mons, as seen by Galileo in February 2000
A colorized image, with a multi-colored region in the middle, elongated left-to-right. The text "I32 Pele" is displayed at top left, and at bottom center, and a color chart of the gradient used. A scale bar shows that the image covers an area 60 kilometers across.
Infrared image showing night-time thermal emission from the lava lake Pele
In the New Horizons image (from 2007), a small area of dark material is present in a bright region near the bottom; this area was not present in the Galileo image (from 1999).
Changes in surface features in the eight years between Galileo and New Horizons observations
Five-image sequence of New Horizons images showing Io's volcano Tvashtar spewing material 330 km above its surface.
Global image of Jupiter's moon Io acquired by Juno's JunoCam camera on 30 December 2023