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Space has lots of different things in it. Planets, nebulas, asteroids, the list goes on. So what would happen if we got rid of a single planet? Specifically, our largest planet Jupiter? How important is this gas giant to our solar system?
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What If is a mini-documentary web series that takes you on an epic journey through hypothetical worlds and possibilities. Join us on an imaginary adventure — grounded in scientific theory — through time, space and chance, as we ask what if some of the most fundamental aspects of our existence were different.
From the discovery of the moon, to what makes it so volcanic, and more! Join us as we explore Io: Jupiter’s Volcanic Moon!
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8. The Discovery Of Io
In many ways, Io is one of the more popular moons of Jupiter. It’s been referenced many a time as we’ll note later. But how did we learn about this very special moon?
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 Jovian system the following day, January 8th, 1610 ( this is 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’s first recorded observation came from 29 December 1609 in the Julian calendar, which equates to January 8th, 1610 in the Gregorian calendar, which Galileo used. Given that Galileo published his work before Marius, Galileo is credited with the discovery.
But the end of the “discovery” did not end there. Because for basically 250 years various astronomers tried to learn more about Io. But because of its place in space all they could usually see was a ball of light. It would take a while for them to start to parse out the details of the moon.
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).
So as you can see, this wasn’t just a discovery of trying to find the moon, but to try and understand what it was and what it was like in regards to its very nature. Which would be further expanded upon in the future via attempts to explore the moon with probes and satellites.
7. The Exploration of Io Part 1
In the late 1960s, a concept known as the Planetary Grand Tour was developed in the United States by NASA and the Jet Propulsion Laboratory (JPL). It would allow a single spacecraft to travel past the asteroid belt and onto each of the outer planets, including Jupiter, if the mission was launched in 1976 or 1977. 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.
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From its discovery, to its importance around Jupiter, and more! Join us as we explore Callisto, Jupiter’s Moon.
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9. Discovery and Naming Of Callisto
Callisto was discovered Jan. 7, 1610, by Italian scientist Galileo Galilei along with Jupiter’s three other largest moons: Ganymede, Europa and Io.
Artemis. Who was also the goddess of the moon for the record. The name was suggested by Simon Marius soon after Callisto’s discovery. Marius attributed the suggestion to Johannes Kepler.
However, the names of the Galilean satellites fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, Callisto is referred to by its Roman numeral designation, a system introduced by Galileo, as Jupiter IV or as “the fourth satellite of Jupiter”.
Now though it’s known as Callisto by most texts, including ones you’ll see in school in hear about when moons like these are discovered. The desire to keep things simple while also rooting much naming in mythology has been desired by astronomers in earlier decades.
8. Orbit and Rotation
Callisto is the outermost of the four Galilean moons of Jupiter. It orbits at a distance of approximately 1,170,000 miles (26.3 times the radius of Jupiter itself). This is significantly larger than the orbital radius of the next-closest Galilean satellite, Ganymede. As a result of this relatively distant orbit, Callisto does not participate in the mean-motion resonance—in which the three inner Galilean satellites are locked—and probably never has.
Like most other regular planetary moons, Callisto’s rotation is locked to be synchronous with its orbit. The length of Callisto’s day, simultaneously its orbital period, is about 16.7 Earth days. Its orbit is very slightly eccentric and inclined to the Jovian equator, with the eccentricity and inclination changing quasi-periodically due to solar and planetary gravitational perturbations on a timescale of centuries. These orbital variations cause the axial tilt (the angle between rotational and orbital axes) to vary between 0.4 and 1.6°.
The dynamical isolation of Callisto means that it has never been appreciably tidally heated, which has important consequences for its internal structure and evolution. Its distance from Jupiter also means that the charged-particle flux from Jupiter’s magnetosphere at its surface is relatively low—about 300 times lower than, for example, that at Europa. Hence, unlike the other Galilean moons, charged-particle irradiation has had a relatively minor effect on Callisto’s surface. The radiation level at Callisto’s surface is equivalent to a dose of aCallisto is named after one of Zeus’s many lovers in Greek mythology. Callisto was a nymph (or, according to some sources, the daughter of Lycaon) who was associated with the goddess of the hunt, bout 0.01 rem per day, which is over ten times higher than Earth’s average background radiation.
6. Surface Of The Moon
Callisto’s rocky, icy surface is the oldest and most heavily cratered in our solar system. The surface is about 4 billion years old and it’s been pummeled, likely by comets and asteroids. Because the impact craters are still visible, scientists think the moon has little geologic activity—there are no active volcanoes or tectonic shifting to erode the craters. Callisto looks like it’s sprinkled with bright white dots that scientists think are the peaks of the craters capped with water ice.
The moons of Jupiter have been something of a fascination for many astronomers and scientists. So when the Earth had the ability to look at the moons via satellites and probes they almost literally jumped at the chance. To the extent that Callisto has been visited many times of the last several decades.
The Pioneer 10 and Pioneer 11 Jupiter encounters in the early 1970s contributed little new information about Callisto in comparison with what was already known from Earth-based observations ironically enough.
The real breakthrough happened later with the Voyager 1 and Voyager 2 flybys in 1979. They imaged more than half of the Callistoan surface with a resolution of 1–2 km, and precisely measured its temperature, mass and shape. A second round of exploration lasted from 1994 to 2003, when the Galileo spacecraft had eight close encounters with Callisto, the last flyby during the C30 orbit in 2001 came as close as 138 km to the surface.
