Rugged mountains, plateaus, and calderas dot the surface of Io. Credit: NASA/JPL/University of Arizonia
Io
Io (pronounced “EYE oh”) is the most volcanically active world in the solar system. Hundreds of volcanoes dot its surface. Lava fountains, dozens of miles high, erupt from some of them. Io has a combination of Jupiter, Europa and Ganymede and their gravitational tug of war to thank for its high activity.
A bit larger than the Moon, Io is Jupiter’s third largest moon. It is tidally locked to Jupiter, meaning that only one side ever faces that planet during its orbit. The Moon is similarly tidally locked to Earth. But unlike the Moon which has a mostly circular orbit, Io’s is elliptical. This causes the moon to experience tremendous tidal forces. As Io goes about its orbit, the gravitational effects on one side are great than that on the other, this causes the Io’s surface to bulge up and down. The same effect can be seen on Earth because of the Moon. But where the Moon pulls Earth’s tides to highs and lows of about 60 feet (18 meters), Io’s surface is fluctuating by as much as 330 feet (100 meters).
But Io causes just as much trouble for Jupiter. At an orbit of about 262,000 miles (422,000 kilometers), Io crosses Jupiter’s magnetic field, turning the moon into a giant electric generator. The current generated by Io then cuts back to Jupiter and creates lightening in the gas giant’s upper atmosphere.
All in all, Io is chaotic and although life has been found even in the most inhospitable locations on Earth, most researchers don’t think this moon is unlikely to host any.
Europa is considered one of the most promising locations in the local search for life. Credit: NASA/JPL-Caltech/SETI Institute
The Voyager spacecrafts flew by Europa in 1979 and scientists got their first hints that the moon’s icy surface might be hiding an ocean of liquid water. Ever since, humanity has imagined countless science fiction scenarios for alien life to be thriving in a Europan ocean.
And sci-fi writers aren’t the only ones intrigued by life on this moon. Europa’s shell is approximately 10 to 15 miles (15 to 25 kilometers) thick and beneath it lies an ocean 40 to 100 miles (60 to 150 kilometers) deep. If true, that would mean Europa holds twice as much water as all of Earth’s oceans combined on a body only a quarter of our planet’s diameter.
Recently the Hubble Space Telescope also spotted signs that some of that water may be leaking out into space, meaning it would be possible for a spacecraft to sample Europa’s ocean without ever touching down.
Of the main requirements astrobiologists think are necessary for life, Europa is seemingly only lacking an energy source. Jupiter is about 5.2 astronomical units — where 1 AU is the average distance between the Sun and Earth — from the Sun, so any sunlight that reaches Europa is about 25 times fainter than at Earth. So, while sunlight isn’t a good energy source, NASA’s Galileo spacecraft saw signs of convection occurring in Europa’s ice layer. Galileo explored the Jupiter system from 1995 to 2003. During that time, it spotted pits and domes on Europa’s surface. Heating from below could be the culprit, causing the cooler, dense ice to sink while the warmer less-dense ice rises.
Though all of Jupiter's moons are of interest, four in particular stand out, the Galilean satellites. These are Jupiter's largest moons, Io
Io
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.
Europa /jʊˈroʊpə/, 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.
Four of Jupiter's moons: Io, Europa, Ganymede and Callisto — known as the Galilean moons — were the first celestial objects to be discovered orbiting an object other than the sun or Earth when Galileo Galilei first observed them in 1610.
Jupiter has 95 moons that have been officially recognized by the International Astronomical Union. But the number doesn't capture the complexity of the Jovian system of moons, rings and asteroids. The giant planet has thousands of small objects in its orbit.
Jupiter: 79 moons, some of which do not yet have proper names. The most famous are the Galilean moons, Io, Europa, Ganymede and Callisto. Saturn: 82 moons, some of which do not yet have proper names. The most famous, by far, is Titan, which is significantly more massive than any other Saturnian moon.
However, a good starting point is to use a magnification of around 50x to 100x to locate Jupiter and get a basic view of its cloud bands and the Galilean moons (Io, Europa, Ganymede, and Callisto). As atmospheric conditions improve, you can increase the magnification to see finer details.
From top to bottom, the moons are Io, Europa, Ganymede and Callisto. Europa is almost the same size as Earth's moon, while Ganymede, the largest moon in the Solar System, is larger than planet Mercury.
Being farther from the Sun, Jupiter is much colder. On average, the temperature on Jupiter's "surface" is -110°C (-160°F). The interior of Jupiter, however, is very hot. The temperature gradually increases as you dive deeper and deeper into the atmosphere.
Even though Jupiter is much larger than Earth, Jupiter rotates much faster on its axis. One day on Jupiter is only 9 hours and 50 minutes! Jupiter takes around 11.86 Earth years to complete one revolution around the sun.
Jupiter has only a couple of well defined rings with faint 'gossamer' rings surrounding them. Saturn on the other hand has dozens of distinct rings of varying thickness, density and composition.
Jupiter is much larger than the stars, so it will appear as a disk to the naked eye. If you are viewing Jupiter from a dark sky location, you may even be able to see some of its bands and moons.
It is at this power that the resolution of a scope most closely matches the resolution of your eye and images are sharpest. This gives you a highest useful power of about 200x to 240x with an 8" scope on an average night, 100x to 120x with a 4" scope, etc.
Although binoculars typically magnify far less than Galileo's telescopes, their sharp views will more than compensate for the lower power. Most binoculars will allow you to see the four largest Jovian moons — Io, Europa, Ganymede, and Callisto (in order of increasing average distance from the planet).
Also, life takes time to develop. We should look for life on worlds where sufficient time has passed for life to get started. Jupiter's icy moon Europa may have these essential ingredients and is as old as Earth.
The Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto) were named by Simon Marius soon after their discovery in 1610. However, these names fell out of favor until the 20th century.
Jupiter's rocky moon Io is the most volcanically active world in the solar system, with hundreds of volcanoes, some erupting lava fountains dozens of miles (or kilometers) high.
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