It’s no accident that Jupiter shares its name with the king of the gods. In addition to being the largest planet in our Solar System – with two and a half times the mass of all the other planets combined – it is also home to some of the largest moons of any Solar planet. Jupiter’s largest moons are known as the Galileans, all of which were discovered by Galileo Galilei and named in his honor.
They include Io, Europa, Ganymede, and Callisto, and are the Solar System’s fourth, sixth, first and third largest satellites, respectively. Together, they contain almost 99.999% of the total mass in orbit around Jupiter, and range from being 400,000 and 2,000,000 km from the planet. Outside of the Sun and eight planets, they are also among the most massive objects in the Solar System, with radii larger than any of the dwarf planets.
New Horizons, you gotta wake up this weekend. There’s so much work ahead of you when you reach Pluto next year! The spacecraft has been sleeping quietly for weeks in its last great hibernation before the dwarf planet close encounter in July. On Saturday (Dec. 6), the NASA craft will open its eyes and begin preparations for that flyby.
How cool will those closeups of Pluto and its moons look? A hint comes from a swing New Horizons took by Jupiter in 2007 en route. It caught a huge volcanic plume erupting off of the moon Io, picked up new details in Jupiter’s atmosphere and gave scientists a close-up of a mysterious “Little Red Spot.” Get a taste of the fun seven years ago in the gallery below.
It takes years of painstaking work to get a spacecraft off the ground. So when you have a spacecraft like JUICE (the Jupiter Icy Moons Explorer) set to launch in 2022, you need to back up about a decade to get things figured out. How will the spacecraft get there? What science instruments will it carry? What will the spacecraft look like and what systems will support its work?
JUICE just hit another milestone in its development a few days ago, when the European Space Agency gave the go-ahead for the “implementation phase” — the part where the spacecraft design begins to take shape. The major goal of the mission will be to better understand those moons around Jupiter that could be host to life.
The spacecraft will reach Jupiter’s system in 2030 and begin with observations of the mighty planet — the biggest in our Solar System — to learn more about the gas giant’s atmosphere, faint rings and magnetic environment. It also will be responsible for teaching us more about Europa (an icy world that could host a global ocean) and Callisto (a moon pockmarked with the most craters of anything in the Solar System.)
Its major departure from past missions, though, will come when JUICE enters orbit around Ganymede. This will the first time any spacecraft has circled an icy moon repeatedly; past views of the moon have only come through flybys by the passing-through spacecraft (such as Pioneer and Voyager) and the Galileo mission, which stuck around Jupiter’s system in the 1990s and early 2000s.
With Ganymede, another moon thought to host a global ocean, JUICE will examine its surface and insides. What makes the moon unique in our neighborhood is its ability to create its own magnetic field, which creates interesting effects when it interacts with Jupiter’s intense magnetic environment.
“Jupiter’s diverse Galilean moons – volcanic Io, icy Europa and rock-ice Ganymede and Callisto – make the Jovian system a miniature Solar System in its own right,” the European Space Agency stated when the mission was selected in 2012.
“With Europa, Ganymede and Callisto all thought to host internal oceans, the mission will study the moons as potential habitats for life, addressing two key themes of cosmic vision: what are the conditions for planet formation and the emergence of life, and how does the Solar System work?”
Talk about a great fall lineup. Three of Jupiter’s four brightest moons plan a rare show for telescopic observers on Friday night – Saturday morning Oct. 11-12. For a span of just over an hour, Io, Europa and Callisto will simultaneously cast shadows on the planet’s cloud tops, an event that hasn’t happened since March 28, 2004.
Who doesn’t remember their first time looking at Jupiter and his entourage of dancing moons in a telescope? Because each moves at a different rate depending on its distance from the planet, they’re constantly on the move like kids in a game of musical chairs. Every night offers a different arrangement.
Some nights all four of the brightest are strung out on one side of the planet, other nights only two or three are visible, the others hidden behind Jupiter’s “plus-sized” globe. Occasionally you’ll be lucky enough to catch the shadow of one of moons as it transits or crosses in front of the planet. We call the event a shadow transit, but to someone watching from Jupiter, the moon glides in front of the sun to create a total solar eclipse.
Since the sun is only 1/5 as large from Jupiter as seen from Earth, all four moons are large enough to completely cover the sun and cast inky shadows. To the eye they look like tiny black dots of varying sizes. Europa, the smallest, mimics a pinprick. The shadows of Io and Callisto are more substantial. Ganymede, the solar system’s largest moon at 3,269 miles (5,262 km), looks positively plump compared to the others. Even a small telescope magnifying around 50x will show it.
The three inner satellites – Io, Europa and Ganymede – have shadow transits every orbit. Distant Callisto only transits when Jupiter’s tilt relative to Earth is very small, i.e. the plane of the planet’s moons is nearly edge-on from our perspective. Callisto transits occur in alternating “seasons” lasting about 3 years apiece. Three years of shadow play are followed by three years of shadowless misses. Single transits are fairly common; you can find tables of them online like this one from Project Pluto or plug in time and date into a free program like Meridian for a picture and list of times.
Seeing two shadows inch across Jupiter’s face is very uncommon, and three are as rare as a good hair day for Donald Trump. Averaged out, triple transits occur once or twice a decade. Friday night Oct. 11 each moon enters like actors in a play. Callisto appears first at 11:12 p.m. EDT followed by Europa and then Io. By 12:32 a.m. all three are in place.
Catch them while you can. Groups like these don’t last long. A little more than an hour later Callisto departs, leaving just two shadows. You’ll find the details below. All times are Eastern Daylight or EDT. Subtract one hour for Central time and add four hours for BST (British Summer Time):
* Callisto’s shadow enters the disk – 11:12 p.m. Oct. 11
* Europa – 11:24 p.m.
* Io – 12:32 a.m. ** TRIPLE TRANSIT from 12:32 – 1:37 a.m.
* Callisto departs – 1:37 a.m.
* Europa departs – 2:01 a.m.
* Io departs – 2:44 a.m.
The triple transit will be seen across the eastern half of the U.S., Europe and western Africa. Those living on the East Coast have the best view in the U.S. with Jupiter some 20-25 degrees high in the northeastern sky around 1 a.m. local time. Things get dicier in the Midwest where Jupiter climbs to only 5-10 degrees. From the mountain states the planet won’t rise until Callisto’s shadow has left the disk, leaving a two-shadow consolation prize. If you live in the Pacific time zone and points farther west, you’ll unfortunately miss the event altogether.
Key to seeing all three shadows clearly, especially if Jupiter is low in the sky, is steady air or what skywatchers call “good seeing”. The sky can be so clear you’d swear there’s a million stars up there, but a look through the telescope will sometimes show dancing, blurry images due to invisible air turbulence. That’s “bad seeing”. Unfortunately, bad seeing is more common near the horizon where we peer through a greater thickness of atmosphere. But don’t let that keep you inside Friday night. With a spell of steady air, all you need is a 4-inch or larger telescope magnifying around 100x to spot all three.
If bad weather blocks the view, there are two more triple transits coming up soon – a 95-minute-long event on June 3, 2014 starring Europa, Ganymede and Callisto (not visible in the Americas) and a 25-minute show on Jan. 24, 2015 featuring Io, Europa and Callisto and visible across Western Europe and the Americas. That’s it until dual triple transits in 2032.
Jupiter’s two moons Ganymede and Callisto could be considered fraternal twins. They have a similar composition and size, but visually, they are different. Also, data from the Galileo and Voyager spacecraft reveal the two moons’ interiors are very dissimilar, as well. The reasons for the differences have eluded scientists for 30 years, but a new study provides an explanation. During the Late Heavy Bombardment, Callisto escaped relatively unscathed, while Ganymede was a battered child; so much so that the later moon melted. “Impacts during this period melted Ganymede so thoroughly and deeply that the heat could not be quickly removed,” said Dr. Amy Barr of the Southwest Research Institute. “All of Ganymede’s rock sank to its center the same way that all the chocolate chips sink to the bottom of a melted carton of ice cream. Callisto received fewer impacts at lower velocities and avoided complete melting.”
Barr and and Dr. Robin Canup created a model showing how Jupiter’s strong gravity focused cometary impactors onto Ganymede and Callisto 3.8 billion years ago, during the LHB period. Each impact onto Ganymede or Callisto’s mixed ice and rock surface creates a pool of liquid water, allowing rock in the melt pool to sink to the moon’s center.
But Ganymede is closer to Jupiter and therefore was hit by twice as many icy impactors as Callisto. Additionally, the impactors hitting Ganymede had a higher average velocity. Modeling by Barr and Canup shows that core formation begun during the late heavy bombardment becomes energetically self-sustaining in Ganymede but not Callisto.
Watch a movie that shows the effect of an outer solar system late heavy bombardment on the interior structure of Callisto (top model in the movie) and Ganymede (bottom).
“Similar to Earth and Venus, Ganymede and Callisto are twins, and understanding how they were born the same and grew up to be so different is of tremendous interest to planetary scientists,” explains Barr. “Our study shows that Ganymede and Callisto record the fingerprints of the early evolution of the solar system, which is very exciting and not at all expected.”
The “Ganymede-Callisto dichotomy,” has been a classical problem in comparative planetology, a field of study that seeks to explain why some solar system objects with similar bulk characteristics have radically different appearances. The study by Barr and Canup also links the evolution of Jupiter’s moons to the orbital migration of the outer planets and the bombardment history of Earth’s moon.