Jupiter’s 63 Other Moons

This month, we’ve met four of Jupiter’s moons. Four. Which means there are at least sixty-three other moons we haven’t met, and possibly more that have yet to be discovered.

It seems a little unfair to spend so much time on four moons and so little on all the rest, except those remaining sixty-three moons make up less than 1% of the total mass orbiting Jupiter.

Ag13 Galilean MoonsDepending on who you ask (click here or here or here), the four Galilean moons pictured above constitute between 99.997 and 99.999% of the stuff in orbit around Jupiter.  That includes all sixty-seven moons plus Jupiter’s rings.

Jupiter’s moons are divided into three groups. This might be hard to remember, but the four innermost moons are called the “inner moons.” Beyond the inner moons lie the four Galilean moons, and beyond them there’s a cloud of what astronomers call “irregular moons.”

Many of the irregular moons are in eccentric, inclined, or even retrograde orbit. Most if not all of them are either captured asteroids or debris from asteroid collisions. A few may only be temporary residents and might slip loose from Jupiter’s gravity sooner or later.

Compared to the Galilean moons, all these other moons look like pebbles. I don’t feel too bad about skipping them. Granted, the inner moons play an interesting role in shaping and maintaining Jupiter’s rings, but we’re going to be talking a lot about shaping and maintaining planetary rings very soon. I promise.

Aa03 Saturn

Sciency Words: Hot Jupiter

Sciency Words MATH

Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related terminology. Today, we’re looking at the term:


Hot Jupiters are defined as large gas giants, roughly Jupiter-sized, in orbits less than 0.5 AU from their host stars (half the distance between Earth and the Sun). Many hot Jupiters orbit much closer than that.

Ag12 Hot Jupiter

Since the 1990’s, astronomers have catalogued hundreds of hot Jupiters. Current models of planet formation indicate that gas giants cannot form so close to stars, so hot Jupiters must begin life father away and migrate inwards.

These planetary migrations can have dramatic effects on the rest of a star system.

The Creator of Worlds

As star systems coalesce, gas giants like Jupiter are among the first objects to appear. In some cases, a young gas giant might migrate inward while the other planets are still forming. The denser the protoplanetary disk, the more likely it is that a gas giant will migrate.

In computer simulations, researchers found that an inward migrating giant is actually good for a developing star system. Its passage stirs things up, encouraging planet formation.

Terrestrial planets that form in this way would benefit from the mixing of material from different regions of the protoplanetary disk. In the simulations, some ended up with way more water than Earth could ever dream of!

The Destroyer of Worlds

Of course if a giant planet migrates inward after the inner planets form, all bets are off. These smaller planets would either be gobbled up by the giant or hurled out of orbit by the giant’s gravity.

This scenario could happen if a Jupiter-sized planet were nudged by gravitational interactions with other large planets or by interactions with nearby stars. Gas giants in binary star systems would be at especially high risk.

The Destabilizer of Stars

Hot Jupiters are often found in high inclination (tilted) or retrograde (backward) orbits when compared to the orbits of their host stars. For a long time, astronomers wondered what happened to the orbits of these planets. A better question might be what happened to the rotations of these stars?

The presence of such a large object so close to a star could have a destabilizing effect on the star. New research suggests that hot Jupiters cause their stars to tilt sideways or tip upside down. This would explain the highly inclined and retrograde orbits we’ve observed.

Is This Normal?

Astronomers have discovered a whole lot of hot Jupiters, but that doesn’t mean they’re common. It’s just that with our current detection techniques, hot Jupiters are among the easiest planets to spot.

Rare or not, hot Jupiters would be worth closer inspection by futuristic space explorers. What sorts of adventures might these explorers have? Please share in the comments below.


Why Doesn’t Our Solar System Have a Hot Jupiter? from Space Answers.

Build Your Own Orbit (Hot Jupiters) from Artifexian.

“Hot Jupiter” Systems May Harbor Earth-like Planets from Physics.org.

Mystery of “Hot Jupiter” Planets’ Crazy Orbits May Be Solved from Space.com.

What’s Jupiter Hiding?

Jupiter’s hiding something. We can see the cloud tops. We can monitor the planet’s intense winds and observe its enormous cyclonic and anticyclonic storms like the Great Red Spot. But we don’t know what’s happening on the inside.

Maybe all the meteorological activity we see is only skin-deep. Maybe beneath the tumultuous “surface” lies a calm and tranquil atmosphere/ocean of gaseous/liquid hydrogen.

Or perhaps Jupiter’s interior is a violent and chaotic place. Perhaps storms like the Great Red Spot are driven by as yet unknown forces that extend deep into the planet’s innermost layers.

How can we settle the matter?

Ag11 Juno Space Probe

In July of 2016, NASA’s Juno spacecraft will enter a high eccentricity polar orbit around Jupiter. Jupiter’s upper atmosphere includes clouds of water (yes, you read that right… there’s water on Jupiter!). Using a microwave radiometer, Juno will attempt to figure out just how far down the water goes.

Also, as Juno skims near Jupiter, NASA will pay close attention to how Jupiter’s gravity affects the spacecraft. Subtle changes in Juno’s velocity will reveal variations in Jupiter’s gravity, indicating variations in the planet’s density. This technique, called gravity mapping, has been used to study the interiors of other planets, including Earth.

Juno also carries a magnetometer (in the illustration above, it’s that pointy thing connected to one of the solar panels). Since Jupiter’s magnetic field is generated by super pressurized metallic hydrogen and perhaps other metallic elements in the planet’s core, data from the magnetometer should give us a clearer understanding of conditions at the center of Jupiter.

Personally, I like the image of Jupiter’s chaotic surface activity concealing a deep, inner calm. It makes the planet sound really Zen. But we’ll have to wait until 2016 to find out if Jupiter is hiding a violent or tranquil interior.

P.S.: One of Juno’s instruments is named JEDI (short for Jovian Energetic particle Detector Instrument). Because NASA engineers can’t design a spacecraft without making at least one Star Wars reference.

Meet a Moon: Callisto

Our ongoing journey through the Solar System now brings us to Callisto: the least interesting of Jupiter’s Galilean moons. Callisto doesn’t participate in the Laplace resonance. It exhibits no geological activity, past or present. It doesn’t have a magnetic field, and its thin atmosphere is the generic CO2 atmosphere that almost all rocky planetoids in the Solar System have.

Callisto’s surface consists of a mix of rock and water ice, and there may be a small ocean of liquid water deep underground. That’s pretty nifty, I guess, but it’s kind of old news after Europa and Ganymede. And without geological activity to feed nutrients into that ocean, it seems unlikely that life could have developed on Callisto.

Yet NASA has taken a special interest in Callisto. If all goes according to plan, astronauts could set foot on this not-so-small moon as early as 2040.

Ag10 Callisto

No, Callisto may not be as exciting as its neighbors. It lacks Io’s sulfur volcanos, Europa’s potentially habitable oceans, or Ganymede’s protective magnetic field. But according to a 2003 concept study called HOPE (Human Outer Planet Exploration), Callisto may be one of the safest locations in the outer Solar System to build a scientific research base. Why? Precisely because it’s so boring.

  • No geological activity means we don’t have to worry about volcanoes or earthquakes (Callisto-quakes?).
  • Since the odds of Callisto supporting native life are negligible, we don’t have to worry much about contaminating the Callistonian ecosystem… or about having the Callistonian ecosystem contaminate us.
  • Even though Callisto doesn’t have a magnetic field like Ganymede’s to shield us from solar or cosmic rays, Callisto orbits outside Jupiter’s radiation belt. Radiation levels on Callisto are actually lower than on Ganymede.

Once we’ve established an outpost on Callisto, astronauts could use it as a base of operations for further exploration of Jupiter and its other moons. Callisto’s water can also be converted into rocket fuel (liquid hydrogen and liquid oxygen), so the outpost could also serve as a fuel depot for missions beyond Jupiter.

I can’t remember any references to Callisto in science fiction (though the mythical Callisto appeared in a few episodes of Xena). But if this seemingly boring moon has attracted so much attention from NASA, maybe it’s worth exploring as a setting for Sci-Fi stories as well.

P.S.: A mission to Callisto in the 2040’s would follow close on the heels of NASA’s planned mission to Mars in the 2030’s. While the HOPE study made some excellent points about the viability of a Callisto outpost, I won’t comment on how realistic the mission timetable that might be.


Concept for a Human Mission to Callisto in the 2040s from Beyond Earthly Skies.

Is There Life on Callisto? from Mysterious Universe.

Jupiter’s Moons in Fiction: Callisto from Wikipedia.

Sciency Words: Chaos Terrain

Sciency Words PHYS copy

Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related terminology. Today, we’re looking at the term:


Chaos terrain is a weird concept, so I’ve decided to let a master of chaos terrain formation explain.

Ag09 Europa Intro

First, you’ll need an ocean of liquid water with a layer of water ice on top. For best results, I recommend using pure or nearly pure ice and really salty ocean water, so that they’ll have dramatically different melting/freezing points.

Ag09 Chaos Terrain Diagram 1

Next, set up some volcanoes or hydrothermal vents on your ocean floor. A little volcanic activity will cause the sporadic melting and refreezing of your ice, allowing ice water and saltwater to mix. If you do this right, you’ll end up with a salty “lake” trapped between layers of ice.

Ag09 Chaos Terrain Diagram 2

As we all know, liquid water is denser than water ice, so your lake will cause the ice above to sag and eventually cave in.

Ag09 Chaos Terrain Diagram 3

Cracks and fissures will form. Chuncks of ice will break apart, and that salty liquid water will get the chance to seep into the gaps, causing more melting and refreezing mayhem.

Finally, when your lake refreezes, it will expand (remember: ice is less dense than water) pushing all that cracked and broken material upward.

Ag09 Chaos Terrain Diagram 4

The resulting terrain will look truly bizarre—chaotic, you might say!—with huge ice blocks jutting up above an otherwise perfectly smooth landscape.

Ag09 Europa Final Thoughts

So, fellow planets and moons, what else can we do to confuse the humans? Share your ideas in the comments below!


Active Formation of “Chaos Terrain” over Shallow Subsurface Water on Europa from Nature (beware of paywall).

Europa’s Chaos Terrains from NASA Visualization Explorer.

On Jupiter, Cold Air Rises?

In last week’s edition of Sciency Words, we covered the belts and zones of Jupiter’s atmosphere. As a brief summary:

  • Zones are the light colored stripes.
  • Belts are the darker, ruddy orange stripes.

While researching that post, something struck me as odd. The cooler clouds of zones rise above the warmer clouds of belts. That made no sense to me. Cold air rises? Warm air sinks? Isn’t that the opposite of what’s supposed to happen?

But there’s more to these clouds than temperature alone. We also have to consider air pressure.

Just as increasing the pressure of a gas can make it hotter, decreasing the pressure can make it cooler. So rather than picturing cool air masses somehow rising, picture rising air masses cooling off due to decreasing atmospheric pressure. Such a situation can be thermodynamically stable, especially when dealing with the extreme altitudes associated with planetary atmospheres.

Still sound crazy? Well, this phenomenon isn’t unique to Jupiter. Similar changes in air pressure occur here on Earth, which is why mountaintops get so cold while the fields and valleys below stay warm.

In many ways, Jupiter is a mysterious planet. We don’t fully understand what causes its enormous cyclonic and anticyclonic storms, nor do we fully understand what’s going on in the deeper layers of the planet’s interior. We’re not even sure why the Great Red Spot looks red.

But Jupiter isn’t that mysterious. Some things which might seem odd at first glance are actually pretty easy to explain.

Meet a Moon: Ganymede

So far this month, we’ve met two of Jupiter’s famous Galilean moons: Io and Europa. Now it’s time we met the moon named Ganymede.

Ag07 GanymedeEh… no, Ganymede is in fact a moon, not a planet, although it does have some planet-like qualities.

  • Ganymede is oddly large for a moon. It’s much larger than Earth’s moon. In fact, it’s larger (in terms of radius) than the planet Mercury, and it’s almost as large as Mars.
  • Ganymede has it’s own magnetic field, providing some protection from solar and cosmic radiation. No other moon can claim that.
  • Ganymede has a thin oxygen atmosphere. It’s nowhere near an Earth-like atmosphere, so leave your space helmet on. But still… oxygen!

All this, combined with plentiful liquid water beneath Ganymede’s surface, would make you think Ganymede is ripe for human colonization.

And indeed, Ganymede has been portrayed multiple times in science fiction as a major human outpost in the outer Solar System. But before you pack your bags and slap a “Ganymede or bust” sign on your spaceship, a note of caution.

Ganymede orbits within Jupiter’s radiation belts. While Ganymede’s magnetic field would provide some protection, it’s not enough to protect you from the radiation concentrated in those radiation belts.

Of course in a distant Sci-Fi future where humanity has overcome the radiation dangers associated with Lunar or Martian colonization, the colonization of Ganymede will seem much more plausible. In the meantime, NASA has its sights set on a different target for human space exploration.

Next week, we’ll be meeting (and possibly colonizing) a moon named Callisto.

Sciency Words: Belts and Zones

Sciency Words BIO copy

Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related terminology. Today, we’re looking at two closely related terms:


Jupiter: it’s a big ball of hydrogen. Well, mostly hydrogen. The topmost layer of clouds—the part we can see in visible light—is primarily composed of ammonia, hydrogen sulfide, and water. A dash of other as-yet-unidentified chemicals are also mixed in for color.

If you’ve ever looked at Jupiter, you’ve noticed that it has stripes. The stripes are so pronounced that you can see them even with a cheap backyard telescope, and astronomers have been observing these stripes for centuries.

PIA02863 - Jupiter surface motion animation.gifBy a long-standing convention, the two different kinds of stripes seen on Jupiter are called belts and zones.

  • Zones: Zones are characterized by their lightly colored clouds. Winds in zones generally blow west to east, and the cloud tops rise above the clouds in the neighboring belts.
  • Belts: Belts are darker-colored, with winds blowing east to west. You may notice that Jupiter’s famous storms, such as the Great Red Spot, tend to appear where belts and zones border each other. The clouds in belts are known to sink to lower altitudes than the clouds in zones.

Jupiter isn’t the only planet with belts and zones. The Solar System’s other three gas giants show similar, though less visually distinctive, stripiness, and it’s a safe bet gas giants orbiting other stars will too.

This all seems straightforward enough, but while researching zones and belts for today’s post, I learned something that struck me as very odd. Zones, the clouds of which rise upwards, are often described as cold while belts, with their lower altitude clouds, are described as warm. Does this mean that on Jupiter, warm air sinks and cold air rises? Have the laws of thermodynamics been reversed?

Next Wednesday, I will attempt to solve this peculiar riddle.

What Color is the Great Red Spot?

The title of today’s post may seem like a moronic question, but Jupiter’s Great Red Spot is known to change color, ranging from brick red to ruddy orange to salmon pink. At times, it turns almost white. While the Red Spot remains somewhat mysterious, this color variation may have given us a clue as to what the heck is going on on Jupiter.

Ag05 Great Red Spot

Currently, there are two competing explanations for the color of the Great Red Spot (or G.R.S.): the endogenic and photolytic hypotheses.

The Endogenic Hypothesis

As you may already know, the G.R.S. is a gigantic hurricane (technically, it’s an anticyclone). According to the endogenic hypothesis, this huge, vortex-like storm dredges up dark red material from lower layers of Jupiter’s atmosphere. If that’s true, then the G.R.S. is giving us a peek into Jupiter’s interior.

But there’s a problem with this idea. If the G.R.S. is made of layer upon layer of red stuff, it should appear darker, and it certainly shouldn’t change color so much.

Therefore, an alternate hypothesis is currently winning the scientific debate.

The Photolytic Hypothesis

By mixing two chemicals, ammonia and acetylene, in a lab and exposing the mixture to ultraviolet radiation, scientists were able to replicate the distinctive red color associated with the Great Red Spot.

On Jupiter, the G.R.S. billows up above the surrounding clouds, reaching altitudes approximately 8 kilometers (5 miles) above the gas giant’s “surface.” The vortex-like nature of the storm could keep ammonia and acetylene trapped near the top, prolonging their exposure to ultraviolet rays from the Sun.

Beneath the topmost layer, G.R.S. clouds would likely appear light grey or pure white, and the storm’s observed color variations could be explained by influxes of fresh ammonia and acetylene.

Let’s Keep Staring

The scientific debate isn’t over. The photolytic hypothesis, so popular at the moment, will no doubt face challenges. The endogenic hypothesis, which seems to have fallen out of favor lately, might be revived, or perhaps an entirely new explanation will come to light.

Meanwhile, we need to collect more data. Which means humanity will continue to stare at Jupiter’s Red Spot with our telescopes. Occasionally, we might try poking it with our space probes.

Ag05 Grumpy Jupiter


Is Jupiter’s Great Red Spot a Sunburn? from NASA Science News.

Why is Jupiter’s Great Red Spot… Red? from Sky and Telescope.

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Today’s post is part of Jupiter month for the 2015 Mission to the Solar System. Click here to find out more about this ongoing adventure.

Meet a Moon: Europa

Every Monday this month, as part of the 2015 Mission to the Solar System, were meeting a moon of Jupiter. Today, we’re meeting:


Liquid water. Europa has it. In fact, tiny as Europa is, it has more water in its subsurface ocean than in all the seas and oceans on Earth combined. But does this ocean support life? There are a few ways we could try to find out.

Fly Through a Water Plume

In 2012, Hubble observed water vapor spraying into space from Europa’s southern hemisphere. If Europa’s water plumes originate from its subsurface ocean, and if that ocean does indeed support life, then the plumes may include traces of biological material.

On a relatively low budget, a space probe could fly through one of these water plumes, collecting and analyzing samples of the ejected material.

However, Europa’s water plumes are difficult to detect. They seem to be rare and highly unpredictable, so it would require a lot of luck to position our probe in just the right place at just the right time.

Land on the Surface

Among its many claims to fame, Europa has the smoothest, youngest-looking surface of any object in the Solar System.

Ag04 Europa BlushThe lack of any significant cratering suggests that, much like Venus, Europa has been resurfaced recently. Most likely, Europa experiences near continuous resurfacing events as water finds its way to the surface, spreads out, and refreezes.

The lines crisscrossing the moon’s surface may be cracks in the ice that have refrozen, with sea salt and other minerals leaving a reddish-brown residue behind. If so, perhaps biological material has been deposited on Europa’s surface as well, just waiting for some NASA rover to come analyze it.

Dive into the Ocean

But to get a definitive answer about life on Europa, we need to go swimming. A robotic probe could melt its way through the surface ice and begin to explore the watery depths below.

Scientists believe Europa’s ocean has plenty of oxygen. There may also be hydrothermal vents near the bottom. Our underwater probe might discover microbial life clustered around these vents, or perhaps the probe’s cameras will observe schools of alien fish.

In one particularly awesome hypothetical scenario, maybe our intrepid little probe will be attacked by angry Europan sea monsters!

Pending Missions to Europa

NASA has preliminary plans to send a probe, tentatively named Clipper, to Europa. Meanwhile, the European Space Agency (ESA) is working on a mission called JUICE (the JUpiter ICy moon Explorer). While JUICE’s main objective is Ganymede, it will also visit Europa.

Best of all, Clipper and JUICE should reach the Jupiter system at around the same time, so NASA and ESA will have plenty of data to share during these missions.

Budget constraints have ruled out the possibility of landing or swimming on Europa, but mission planners have hinted that they might try to fly through a water plume, if Europa gives them a chance to do it.


Scientists Plan to Hunt for Alien Life on Europa from National Geographic.

Sea Salt in Europa’s Dark Materials from Centauri Dreams.

Europa Clipper Concept Team Aims for Launch in 2022 from Spaceflight Now.