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.

Depending 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.

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?

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.

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.

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.

Links

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.

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.

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.

Eh… 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.