The four Gallilean moons of Jupiter (from innermost) are Io, Europa, Ganymede, and Callisto. Io, Europa, and Ganymede are in a 1:2:4 orbital resonance. Callisto's orbital period is 16.689 days, which is not much more than twice Ganymede's period of 7.1546 days.
If Jupiter's orbit around the Sun was stable, when could we expect Callisto to come into a 1:2:4:8 orbital resonance with the Jupiter's other Gallilean moons?
Related: Does anyone know why three of Jupiter's largest moons orbit in 1:2:4 resonance?
we tried to answer to this question in this article: https://arxiv.org/abs/2001.01106
According to the current estimation of the tidal dissipation in the Jovian system, we expect that Callisto will be captured into resonance in about 1.5 billions of years, forming 1:2:4:8 resonant chain with the other Galilean moons.
Orbits of Jupiter Moons Transformed into Mind-Bending Optical Illusions and Music
Order has risen from the chaos of the creation of our solar system, and that systemization is on display through the gravitational push and pull of the bodies it harbors. A clear example of this is Jupiter and its four largest moons: When their orbital dynamics are converted into rhythms, musical notes and time-lapsed illusions, you'll likely be astounded by what you see and hear.
To understand how it works, you have to first break down the orbits.
After the three inner moons of the Jupiter system were created, they eventually settled into a simple 4:2:1 orbital resonance. In other words, it takes twice as long for Europa, Jupiter's second-closest moon, to orbit Jupiter as it does Io, the closest moon. And, in turn, the third-closest moon, Ganymede, takes four times as long as Io to orbit Jupiter. [Hear the orbits of Trappist-1 system planets turned into music]
The fourth moon, Callisto, completes the quartet of Galilean moons &mdash which were discovered by Italian astronomer Galileo Galilei in 1610 &mdash and it is the farthest of the bunch. It orbits at a 12:5 resonance: 12 orbits of Ganymede are equal to five orbits for Callisto. Using these statistics, Matt Russo, Andrew Santaguida and Dan Tamayo of system-sounds.com have created a musical representation of the orbits. Io, Europa and Ganymede are "playing the same note but in different octaves," when the orbits are converted into musical pitches, according to an explainer on their website &mdash the pitches metaphor is appropriate because octaves follow a similar resonance structure to go an octave higher, you double the frequency of the sound. In the case of Callisto, the pitch occurs at an "interval of an octave plus a minor 3rd," they wrote, since its resonance differs from that of the other moons.
The trio used the ratios of the moons' orbits, combined with an animation of Jupiter and its moons, to generate a rhythmic beat that is based on each moon's resonance, before slowly speeding it up from 30,000 to 250 million times the actual orbital speed (seen at the top of this page &mdash turn on sound). "As [the moons] begin orbiting faster than 20 times per second (20 Hz), the repetitive beat morphs into sustained musical harmony," the explainer goes on to say &mdash based on those octaves mentioned above.
The geometric patterns that form in the animation as it speeds up are obviously not a true representation of what the orbits look like. The patterns occur because the frame rate that the video was created with just can't keep up. The result is a stroboscopic effect, which the System Sounds website compares to a "wagon wheel" optical illusion you see when a "car wheel (is) spinning so fast that it appears to stop and start spinning backwards."
All those factors combined turn into a mind-blowing, 70-second experience that we highly recommend you watch in full screen with a good sound system or headphones. You can even get interactive with it below &mdash just click Play and drag NASA's Juno spacecraft to speed up the orbits you can also turn the moons' orbits on or off by clicking on them.
To take a deeper dive into the musical details and to see some more "hypnotic" orbit animations, visit the System Sounds website.
The Need for a New Theory
Previous attempts to understand the origin of planets and moons were tailored to explain our own solar system. But those theories didn’t work when we tried to apply them to the dizzying array of exoplanetary systems discovered in the last few dozen years.
“The exoplanet revolution over the last two decades has completely rewritten the story of how planets form,” Batygin says. He compares this to becoming a karate master, but then emerging from your dojo and learning about the many other martial arts practiced around the world.
So, based on the most up-to-date exoplanet data, Batygin thinks it might be time to rewrite our understanding of how moons are made.
“The best model up until now of satellite formation is from two decades ago, and things have advanced since then,” he says. “The question of why the icy moons are there is one we know embarrassingly little about.”
The Interior of the Moon Callisto
Most planets and moons have "differentiated" interiors. That is to say, they have clearly defined mantle and core. Callisto, on the other hand does not. Beneath the solid, icy crust, which is some 90 miles thick, there is simply a large volume consisting of a mixture of water ice and rock. The proportion of rock in the mixture becomes greater the nearer to the centre of the moon you get.
It is thought that there may be an ocean of liquid water beneath the crust which could be as much as 125 miles thick, but this is only conjecture.
Another unproven supposition is the presence of a tiny silicate core, which, if it existed, could have a radius of no more than 370 miles.
"most likely Callisto, the outermost of the four" - seems that it's definitely Callisto, since its drawn with little craters - no? 188.8.131.52 (talk) (please sign your comments with
Hmmm. The animation just added agrees with another animation I've seen, in that the three innermost moons never line up all on one side of Jupiter at the same time. So if "Hi" (Io) and "What's your name" (Europa) conjoin on the right side as we're looking, then "What's your name" and "MOOOON!" (Ganymede) should conjoin on the left side. Not that I'm being critical of course.
If the inner moons are tidally locked with Jupiter, can you ostensibly state that they're mooning the outer moons, whenever two such moons line up? lol 184.108.40.206 08:57, 6 December 2013 (UTC)
Even with the resonance, "MOOOOOON!" appears still not to have been able to escape with that effect alone until Cueball's own close approach to Megan brought his own gravity well close enough to hers to give rise to a viable transfer orbit. And appears to be now retrograde, relative to its last orbit. Or possibly on a free-return path, unless Cueball steps back before the return transfer happens or makes an appropriate sideways move to quash the orbital potential. 220.127.116.11 09:34, 6 December 2013 (UTC)
Hang on. "MOOOOOON!" isn't the "Ugh/So annoying/Almost/Yes!" one. Forgot to note the hint of shading. Still, the above applies to the disgusted/elated moon, clearly not liking either of the Valley Girls or the loudmouth Jock. 18.104.22.168 09:38, 6 December 2013 (UTC)
Is it just me or does "MOOOOOON!" have a subtle "MOON MOON" undertone? 22.214.171.124 12:26, 6 December 2013 (UTC)
Am I the only one thinking that the "MOOOOOON!" is a reference to the "SPAAAAACE!" module from portal 2? 126.96.36.199 (talk) (please sign your comments with
No -- Sian (talk) (please sign your comments with
I propose a simpler explanation for Ganymede saying "MOOOOOON!". Europa has asked Ganymede the same question, "What's your name?", every time they go by each other for eons. Ganymede is yelling the answer before it is even completely asked, exasperated at having to repeat it for the umpteeen gazillionth time. --uhillard (talk) 17:05, 12 December 2013 (UTC)
Ganymede interrupts Europa, reminding me of the knock-knock joke, "interrupting cow," "interrupting cow wh. " "MOOOOOO". Ganymede is an interrupting moon.
Maybe moons converse with Cueball, not between them itself? 188.8.131.52 13:14, 6 December 2013 (UTC)
Or with Megan, when opposite to her face? 184.108.40.206 13:39, 6 December 2013 (UTC)
I think the moons are clearly conversing with Cueball. Remember that Io executes a full orbit between every panel. --BlueMoonlet (talk) 17:27, 6 December 2013 (UTC) Animation is incorrect
The current animation has the wrong speed of the outermost moon, which is currently orbiting at a 5:1 ratio to the innermost. They should all line up along a vertical line once every four rotations. In fact the current animation never lines up all three moons at the same time (at least, not on the same side of the planet). -Greg 220.127.116.11 16:06, 6 December 2013 (UTC)
The animation is correct. If you look closely at only Europa and Ganymede, you'll see that they are also in a 2:1 resonance, with conjunctions always taking place at the "6 o'clock" position. Io and Ganymede are in a 4:1 resonance, with conjunctions taking place at 12 o'clock, 4 o'clock, and 8 o'clock. The comic is incorrect in having all three moons on the same side of Jupiter at the same time. That never happens in the actual system, though I don't mind it in the name of artistic license. --BlueMoonlet (talk) 17:27, 6 December 2013 (UTC) The innermost orbit completes 5 rotations for each 1 of the outermost. How is that a 4:1? 18.104.22.168 18:16, 6 December 2013 (UTC) I think you need to count more carefully. Start when both moons are at "12 o'clock". In the time it takes for Ganymede to get back to that position, I see Io go around 4 times. --BlueMoonlet (talk) 18:19, 6 December 2013 (UTC) You're probably counting from 1 to 5, instead of from 0 to 4. I.e. When they're lined up to start, you could call that conjunction #1, but they've done 0 orbits. Wwoods (talk) 19:42, 6 December 2013 (UTC) If it was five to one the planets would line up every other orbit of Ganymede 22.214.171.124
Did someone change the animation? Because when I watch it, they all line up on the right side. 126.96.36.199 22:44, 6 December 2013 (UTC)
Not the animation I've been seeing. Starting at an arbitrary point: [email protected], [email protected], [email protected] (I+E Conjunction) [email protected], [email protected], [email protected] (I+G Opposition) [email protected], [email protected], [email protected] (I+G Conjunction, E Opposed) [email protected], [email protected], [email protected] (I+G Opposition) [email protected],[email protected], [email protected] (I+E Conjunction) [email protected], [email protected], [email protected] (I+G Conjunction) [email protected], [email protected], [email protected] (E+G Conjunction, I Opposed) [email protected], [email protected], [email protected] (I+G Conjunction) . then repeat That's one cycle of Europa and Ganymede, two cycles of Io and Europa (relative to each other, alone) four cycles of Io and Ganymede (likewise). There are two 'in-line' conditions, when Ganymede is in conjunction with one of the other moons, the remaining one in opposition, with Ganemedes other Io conjunctions having Europa off at an angle and the other Io/Europa conjunctions having Ganymede off at a right-angle. If I've managed to note all that down correctly. (Note, this is nothing to do with the following plot regarding the XKCD motions, which I quite admire!) 188.8.131.52 08:09, 7 December 2013 (UTC)
Hard to tell exactly which moon was which, until I plotted their cyclic orbits.
-- DaveC426913 (talk) (please sign your comments with
"You may also notice at the animated picture that, unlike in the fifth and ninth panels of the comic, the three moons are never on the same side of Jupiter at the same time." The animated picture doesn't match this text. In the animated picture it looks like the three moons are on the same side of Jupiter at least twice for each cycle of Ganymede. -- 184.108.40.206 04:26, 7 December 2013 (UTC)
It means they are never lining up on the same side of Jupiter. 220.127.116.11 23:53, 7 December 2013 (UTC) That picture, which is very helpful, confirms that the three moons are all in conjunction with each other in panels 5 and 9 of the comic. In the animation, Io and Europa have their conjunctions at 12 o'clock, so (if the comic were correct) this picture would imply that Ganymede should sometimes also be at 12 o'clock during an I+E conjunction. In fact, the animation shows that Ganymede is always at either 3 o'clock or 9 o'clock during an I+E conjunction. --BlueMoonlet (talk) 18:25, 12 December 2013 (UTC)
Also note that "Io" looks like "Lo" in many fonts, so Io saying Hi is a probably a little Lo-Hi (Low High) pun. And Io passes 10 times, a Io-10 pun. And Europa saying "what's your name" is maybe a pun on Europa sounding like "You Are" a bit? 18.104.22.168 14:42, 10 December 2013 (UTC) Martin.
Randall's Moon Meetings are just a part of the joke, all four Galilean Moons never would meet in that way in reality. Randall does not present science publications here, but just comics, playing with real things. --Dgbrt (talk) 22:28, 12 December 2013 (UTC)
Is it possible that a person having moons is a reference to Ioun Stones? 22.214.171.124 (talk) (please sign your comments with
Jupiter’s equinox and mutual events of its 4 major moons
From left to right – and from innermost to outermost in their orbits – the 4 major moons of Jupiter are Io, Europa, Ganymede and Callisto. Earth’s moon is nearly the same size as Io, but larger than Europa. Ganymede, the largest moon in our solar system, is much bigger than our moon it’s even larger than the planet Mercury. Callisto is the 3rd-largest moon in the solar system – bigger than our moon, but smaller than Ganymede – and also a touch smaller than Saturn’s Titan. Photo via JPL.
The king planet Jupiter will reach an equinox – an event that happens twice in every 12-year orbit of Jupiter – on May 2, 2021. Around Jupiter’s equinoxes, we on Earth always have an opportunity to see a series of mutual events – or you might call them eclipses – of Jupiter’s four major moons. These are the four famous Galilean satellites, Io, Europa, Ganymede and Callisto. The eclipses are already happening. They’ll continue through August 2021. Experienced telescope users will be the ones seeing them if you get an image, please submit it here. As the four large moons revolve around Jupiter, a moon shadow will sometimes eclipse (fall upon) a different moon. Or the body of one Jovian moon will occult (move directly in front) of another moon. For example, on April 4, Io’s shadow will fall upon Europa. The following day – April 5 – Io itself will occult the moon Ganymede.
It hasn’t happened (we haven’t had a Jupiter equinox) since February 4, 2015. The next eclipse season for Jupiter’s moons won’t come again until around Jupiter’s next equinox, on December 16, 2026.
These moons orbit in the plane of Jupiter’s equator. We see the shadows of Jupiter’s moons eclipsing other moons at or near a Jovian equinox, because that’s when the shadows of Jupiter’s moons align with Jupiter’s equatorial plane. At times other than near a Jovian equinox, the moon shadows slant relative to the Jovian equator they angle north or south of the other moons, and no eclipses take place.
Also, from our earthly perspective, we only see Jupiter’s equator edge-on around Jupiter’s equinox. Without this edge-on view, we can no longer see moons occulting other moons.
A moon is said to eclipse another moon when a moon’s shadow falls on the face of another moon. A moon is said to occult another when the body of a moon passes in front of another. Illustration via Dave Dickinson/ Universe Today. This 19-frame animation shows Io (the small, moving object) being partially occulted by Ganymede on January 16, 2003. Hong Kong amateur Yan Chi-keung used a CCD imager attached to his 250-mm, f/20 Maksutov-Cassegrain telescope to capture this 36-minute-long sequence. Image via SkyandTelescope.org.
Jupiter’s four major moons are readily observable through a low-powered telescope anytime Jupiter is visible.
These four moons are called the Galilean moons to honor Galileo Galilei, who famously used one of the first small telescopes to observe the four moons revolving around Jupiter in the early 1600s.
Now and again, you might miss seeing a moon or two they disappear from view when they swing behind Jupiter or in front of Jupiter from our earthly perspective. To find out the positions of Jupiter’s four major moons for right now – or some chosen time – use this handy interactive tool via SkyandTelescope.org.
View at EarthSky Community Photos. | Binoculars will enable you to capture a glimpse of a moon or 2 of Jupiter’s, and a small telescope reveals all 4 of the largest Jovian moons, the Galilean satellites, spread out in line across Jupiter’s midsection. This photo comes from Mohamed Mohamed in Tripoli, Libya. He captured it on July 29, 2020.
With a higher-powered telescope, you can watch the moons’ dark shadows transiting (or crossing) the near side of Jupiter’s disk, and the moons themselves sweeping through Jupiter’s dark shadow on Jupiter’s far side. It is more difficult to view a Jovian moon itself transiting Jupiter, however. That’s because both Jupiter and its transiting moon are basking in sunlight, providing no sharp contrast between the moon and Jupiter.
Hubble portrait of Io, the innermost Galilean moon, in front of Jupiter, and Io’s shadow falling upon Jupiter’s cloud tops. On April 4, 2021, Io’s shadow will actually fall upon the moon Europa, and then, on April 5, 2021, Io itself will occult the moon Ganymede. Read more. Photo via J. Spencer (Lowell Observatory), and NASA/ESA
Callisto, the outermost moon, is the only Galilean moon that doesn’t pass in front and behind Jupiter with each revolution. For a few years – roughly centered on Jupiter’s solstice on January 20, 2024 – Callisto will swing to the north and south of Jupiter at each revolution as seen from Earth. This year, however, it’s a totally different story. Callisto’s shadow not only transits Jupiter at each revolution all year long, but eclipses Io, Europa and Ganymede in the month of April 2021 alone.
With a low-powered telescope – anytime Jupiter is visible – it’s easy to see Jupiter’s four major moons as pinpricks of light lining up upon the same plane.
It takes a practiced observer with a high-powered telescope to tease out Jupiter’s moons eclipsing and occulting other moons upon the great merry-go-round of Jupiter’s equatorial plane.
Either way, it’s a fine year for enjoying the dance of Jupiter’s moons.
The 3 inner moons – Io, Europa and Ganymede – are locked into a 4:2:1 orbital resonance. That is, Io circles Jupiter 4 times for every 2 times that Europa circles Jupiter and every one time that Ganymede does. Image and more explanation via Wikipedia.
Bottom line: Jupiter’s equinox is May 2, 2021, and – around every Jovian equinox – we on Earth can see Jupiter’s moons eclipse each other. These mutual events of Jupiter’s moons are already happening this year they’ll continue through August 2021.
The Truth About Orbital Resonance
The first 1000 people to use this link will get a free trial of Skillshare Premium Membership:
Incredibly, three of the four largest moons of jupiter (Ganymede, Europa and Io) have orbital periods that are whole number ratios with each other (1:2:4). The big gap in Saturn’s rings is caused by a moon much further out that has an orbital period double that of the gap! We’ve even found exoplanet systems with these patterns. They’re all the result of orbital resonance. This video explains how that mechanism works.
CORRECTION: In the video I say that Ganymede, Europa and Io are the largest moons are jupiter. Actually here are the 4 largest moons from largest to smallest:
Here’s my video on resonance:
Here’s my video about bad maths:
This is Dr Becky Smethurst’s channel:
This is Beardyman’s channel:
This is Jay Foreman’s channel:
This is the Veritasium video mentioned at the start:
Here’s the paper I found that explains orbital resonance:
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When will Callisto be in orbital resonance with the rest of Jupiter's big moons? - Astronomy
As befits the colossus of the Solar System, Jupiter has four of the system's largest moons: Ganymede, Callisto, Io and Europa.
Galileo discovered Jupiter's moons in January 1610 and was quick to publish his findings. It was a sensation, because they were the first known moons of another planet. In addition, since they were orbiting Jupiter, it challenged the orthodoxy that everything in the Solar System revolved around the Earth.
Four years later German astronomer Simon Marius published an account claiming that he had discovered the moons first. He didn't get the credit, but the names in use today are those suggested by Marius. They are names of amorous conquests of Zeus (the Greek equivalent of Jupiter). Collectively, we know them as the Galilean moons.
Like our moon, the Galilean moons are tidally locked, which means they keep the same face towards the planet as they orbit. In addition, the three inner moons orbit in what's called a 1:2:4 resonance. This is created by a combination of Jupiter's gravity and the gravity of the moons themselves. Each time Ganymede orbits Jupiter, Europa orbits twice and Io four times.
All of the moons have extremely thin atmospheres. Io's is mostly sulfur dioxide, those of Europa and Ganymede are oxygen, and Callisto's is carbon dioxide. Io, Europa and Ganymede have internal layers (core, mantle and crust), as planets do, but Callisto has a more primitive structure.
My favorite is Io, and not just because it reminds me of a pizza. It's the most volcanically active body in the Solar System, and very interesting.
Io is about the same size as the Moon and orbits at about the same distance from Jupiter as the Moon does from Earth. Yet it has over 400 volcanoes, and a "month" on Io lasts only 42 hours. This is all down to gravity. Caught between Jupiter's strong gravity and that of its companions Callisto and Ganymede, Io is mercilessly squeezed. This raises large land tides in which the surface is pulled up and down as much as 100m (330 ft). The resulting friction releases considerable heat.
The moon's surface is a rugged one with over 100 mountains, some of them taller than Everest. On the other hand, impact craters are absent, as the volcanic activity has renewed the surface.
A complete contrast to fiery, mountainous Io, Europa is covered in ice, and is one of the smoothest objects in the Solar System. The small number of craters shows that the surface is young, possibly as young as 100 million years. (That is young, geologically.) You can see cracks and streaks on Europa's surface. The cracking is caused by tidal heating, but we don't know exactly what the staining is.
The exciting thing about Europa is a deep liquid ocean under the ice. It's about 100 km (60 miles) deep and warmed by the tidal heating. Deep in Earth's oceans there are hydrothermal vents where heat and minerals pour out of the planet's interior. Life has evolved there to use chemical energy rather than the energy of sunlight. Astrobiologists think that if life has evolved elsewhere in the Solar System, then Europa's ocean is one of the likeliest places to find it.
Ganymede is bigger than Mercury and the known dwarf planets. However its lower density means it has only half Mercury's mass. Ganymede has a liquid iron core and is the only moon in the Solar System to have its own magnetic field. Hubble Space Telescope data convinced astronomers in 2015 that there is a very large salty ocean underneath the surface of Ganymede. It could contain more water than all of Earth's surface water combined, but more evidence is needed for that.
The terrain is varied, but broadly, Ganymede has two different types. The dark regions are heavily cratered, evidence of great age. These regions could date back four billion years to the time of heavy bombardment in the early Solar System. The brighter regions show patterns of ridges and grooves which go on for thousands of miles, suggesting later geological activity. Yet the pattern of cratering shows that although they're younger than the dark regions, they're still ancient.
Callisto seems to be the odd one out. It's far enough away that it isn't part of the orbital resonance, and its interior isn't warmed by tidal heating. It's also the least dense of the moons, and shows the least internal layering.
But each of the four moons has a superlative feature and Callisto's distinction is that it's the most heavily cratered satellite in the Solar System. There aren't any notable features related to internal activity - its surface has apparently been primarily shaped by impacts. The largest impact crater is called Valhalla. The crater itself is 360 km (225 miles) across and the rings extend to 1900 km (1190 miles) from its center.
Our Moon's surface is a record of the history of the inner Solar System but Callisto's surface is probably even older, dating back to the beginning of the Solar System.
(1) NASA, “Solar System Exploration” https://solarsystem.nasa.gov/planets/profile.cfm?Object=Jupiter&Display=Sats
(2) Nick Strobel https://www.astronomynotes.com/solarsys/s14.htm
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Early Cometary Bombardment May Explain the Divergent Paths of Jupiter's Biggest Moons
Ganymede and Callisto are the largest of Jupiter's so-called Galilean satellites, the four moons of the giant planet that were discovered 400 years ago, in January 1610, by Italian astronomer Galileo Galilei. Ganymede, the largest moon in the solar system, even bigger than the planet Mercury, boasts its own magnetic field and bears the marks of past tectonic activity. But Callisto, of roughly equal size and with a similar makeup of rock and ice, has neither a magnetic field nor an apparent history of tectonics&mdashthe moons' geologic histories have proceeded very differently.
Ganymede seems more evolved, so to speak&mdashits constituents appear to have differentiated further than those of Callisto. Specifically, most of its rock and metal have migrated to the core, whereas those components are more widely distributed throughout Callisto, which appears to host a smaller core as a result.
The circumstances that could have led Ganymede to differentiation without fully affecting its sibling moon have been debated for years. One suggestion is that Ganymede's orbital history included a phase in which the moon experienced strong gravitational tides that heated the icy body and allowed the rock and metal to coalesce at its center.
In a paper published online Sunday in Nature Geoscience, planetary scientists Amy Barr and Robin Canup of the Southwest Research Institute in Boulder, Colo., propose an alternative scenario: heating by cometary impacts, which should have been plentiful several hundred million years after the moons formed, could have liberated the materials that now constitute Ganymede's core. (Scientific American is part of Nature Publishing Group.) Callisto orbits much farther from Jupiter and so would have endured less bombardment from comets drawn in close to Jupiter by the massive planet's gravitational pull.
Each time a comet strikes an icy satellite, Barr explains, a portion of the moon's surface melts from the heat of the impact the heavier metallic and rocky constituents mixed in sink to the bottom of the melt pool. With enough impacts providing sufficient melting, the sinking rocks' gravitational potential energy is released as heat, producing more melting, and the separation of rock and ice becomes self-sustaining, a process known as "runaway differentiation."
During the solar system's period of intense impacts about 3.8 billion years ago known as the late heavy bombardment, tremendous amounts of cometary material would have been flying around Jupiter and the outer gas-giant planets. Barr and Canup estimate that Ganymede's proximity to Jupiter, the latter of which acts as something of a gravitational sink, led to Ganymede's experiencing double the impacts of Callisto, and at higher velocities, to boot. "Ganymede gets 3.5 times as much energy in the late heavy bombardment as Callisto," Barr says. That energy differential, Barr and Canup realized, could account for Ganymede's much more complete state of differentiation&mdashthe so-called Ganymede&ndashCallisto dichotomy.
By their calculations, a broad range of starting conditions for the source population of comets could produce Ganymede's full differentiation but stop short of runaway differentiation at Callisto. Importantly, the debris disk described by the so-called Nice model, a popular dynamical simulation for the solar system's evolution, would do the job. "There is a huge range of masses of planetesimal disks that lead to the formation of the dichotomy," Barr says, noting that prior hypotheses for the divergent histories of Ganymede and Callisto required fine-tuning of parameters or worked for only a very narrow set of circumstances. "This fits in with what is already known about dynamical sculpting in the outer solar system, and it works for a broad range of parameters," she says.
Planetary scientist William McKinnon of Washington University in Saint Louis notes that work in recent years has complicated a competing explanation for the dichotomy, in which tidal heating during the orbital evolution of the Jovian moons melted Ganymede enough to differentiate it. Some research has in fact shown a strong dynamical preference for Ganymede to have settled quickly into its present orbital resonance with the moons Io and Europa. "And if that's true then there is no later special time for Ganymede to be tidally heated," McKinnon says. The fact that Barr and Canup's model dovetails with a primordial development of the moons' orbits makes it attractive, he adds.
The new hypothesis is "a completely plausible explanation," McKinnon says. "What they've shown is that the effect of a strong late heavy bombardment might be the answer."
Callisto: The Outermost Galilean Moon
Callisto is the stereotypical outer solar system satellite. It is one of the largest and most heavily cratered satellites in the solar system. The surface is very icy and dates back four billion years. Beneath the icy crust is possibly a salty ocean supported by a deeper rocky interior.
Callisto doesn't have any large mountains, show evidence of volcanic or tectonic activity or have any appreciable level of internal heat. Nonetheless, observations of Callisto's magnetic field may cause scientists to add the large moon to the list of possible worlds with subsurface salty oceans.