Tag: Ganymede

Our Place in Space: Callisto

~ J.S. Pailly ~ 18 Comments

Hello, friends!  Welcome to Our Place in Space: A to Z!  For this year’s A to Z Challenge, I’ll be taking you on a partly imaginative and highly optimistic tour of humanity’s future in outer space.  If you don’t know what the A to Z Challenge is, click here to learn more.  In today’s post, C is for…

CALLISTO

The major moons of Jupiter are Io, Europa, Ganymede, and Callisto.  In science fiction, Europa and Ganymede seem to get the most attention.  Sci-Fi writers often end up putting human colonists (or at least a handful of plucky human scientists) on the surfaces of one or both of these icy moons.  But today, I’m going to argue that Callisto would be a far more suitable home for future humans.

First off, and most importantly, there’s the issue of radiation.  The space around Jupiter is one of the most dangerous radiation environments in the entire Solar System.  As you can see in the highly technical diagram below, the radiation is most intense in the vicinity of Io.  The radiation levels get better in the vicinity of Europa and continue to taper off when you reach Ganymede.  You’re still soaking up a lot of radiation, though!  Callisto’s radiation levels, however, are fairly low.  You might even describe the radiation levels on Callisto as “survivable.”

Furthermore, planetary protection laws in the future may mean that both Europa and Ganymede are off limits to human settlers.  Scientists today are 99.99% sure that Europa has a vast ocean of liquid water beneath her surface, and (as you know) wherever there’s water, there may also be life.  There’s evidence suggesting Ganymede may have a subsurface ocean, too.  Europa is often said to be the #1 most likely place where we might find alien life here in the Solar System.  While the odds of finding life on Ganymede are considerably lower, the possibility of Ganymedean life shouldn’t be ignored.

There are already international agreements in place regarding extraterrestrial life.  Space agencies like NASA, the E.S.A., and others are legally obligated to do everything they can to protect suspected alien biospheres from our Earth germs (and also to protect Earth’s biosphere from any germs we might find in outer space).  For obvious reasons, these international agreements haven’t exactly been tested in court, and it’s a little unclear how they would be enforced.

But in a future where human civilization is spreading out across the Solar System, I’d imagine bio-contamination laws would become stronger, not weaker.  Europa would almost certainly be declared off-limits to humans, unless it is proven beyond a shadow of a doubt that no aliens currently live there.  Ganymede may end up being off-limits, too, for the same reason.

Meanwhile, we have Callisto.  Scientists who want to study possible biospheres on Europa and Ganymede could set up a research station on Callisto.  From there, they could keep a close eye on the other moons of Jupiter.  They could operate remote-controlled probes to explore Europa and Ganymede without risking contamination, or they could go on brief excursions to Europa and Ganymede themselves (taking proper safety precautions, of course).  While they’re at it, these scientist could also explore Io.  Io is the most volcanically active object in the Solar System.  There is virtually no chance that we’ll find life there, but studying Io’s volcanoes would still be interesting.

I’d be remiss if I didn’t mention this: Callisto might have liquid water beneath her surface, too.  Not as much liquid water as Ganymede, and nowhere near as much as Europa, but still… it’s possible.  Which means there’s a slim possibility that there could be life on Callisto.  But in Callisto’s case, it is a very slim possibility.  Based on what we currently know about Jupiter’s moons, Callisto still seems like the best place for humans to live.  The radiation levels are much lower, the risk of bio-contamination is negligible…  Yeah, if I were a science fiction writer, I’d put my human colonists on Callisto.

Want to Learn More?

In 2003, NASA published a plan to send astronauts to Callisto, with the intention of using Callisto as a base of operations to explore the other Jovian moons.  Click here to read that plan.  Some of the information is out of date, of course, but it’s still got some interesting ideas.  Maybe someday, something like this plan could work!

I’d also recommend this article on Planetary Protection Policy, covering some of the rules that are already in place to protect planets and moons where we might find alien life.

P.S.: If I were a science fiction writer…?  Wait a minute, I am a science fiction writer!  Click here if you want to buy my first book.  It’s not set on Callisto, unfortunately, but it’s still a fun story.

Oops! I Learned Something Wrong About Io

~ J.S. Pailly ~ 10 Comments

Hello, friends!

As you may remember from a previous post, Io is my favorite moon in the Solar System.  He may not be the prettiest moon, and he certainly isn’t the most habitable.  I, for one, would never, ever, ever want to live there.  You see, Io is the most volcanically active object in the Solar System.  He is constantly—and I do mean constantly!—spewing up this mixture of molten hot sulfur compounds.  It gets everywhere, and it is totally gross.

But it’s also super fascinating—fascinating enough that Io ended up becoming my #1 favorite moon in the whole Solar System.  I’ve read a lot about Io over the years.  I thought I understood Io pretty well.  But I was wrong.  One of the facts in my personal collection of Io-related facts was based on a fundamental misunderstanding of how Io’s volcanism works.  Let me explain:

Io is caught in this gravitational tug of war between his planet (Jupiter) and his fellow Galilean moons (Europa, Ganymede, and Callisto).  Jupiter’s gravity pulls one way; the moons pull another; Io is caught in the middle, feeling understandably queasy.  I always thought this gravitational tug-of-war was directly responsible for Io’s volcanic activity.  But it’s not.  Recently, while reading a book called Alien Oceans: The Search for Life in the Depths of Space, I realized that I had some unlearning to do.

The gravitational tug-of-war has forced Io into a highly elliptical (non-circular) orbit.  This means there are times when Io gets very close to Jupiter, and times when Io is much farther away.  When Io’s orbit brings him close to Jupiter, Jupiter’s gravity compresses Io’s crust.  And when Io moves father away, his crust gets a chance to relax.  This cycle of compressing and relaxing—of squeezing and unsqueezing—causes Io’s interior to get hot, which, in turn, keeps Io’s volcanoes erupting.

This squeezing and unsqueezing action wouldn’t happen if not for Io’s highly elliptical orbit, so the gravitational tug-of-war with Jupiter’s other moons is still partially responsible for Io’s volcanism.  But the tug-of-war is not the direct cause of Io’s volcanism, as I always assumed it to be.

I wanted to share all this with you today because some of you may have had the same misunderstanding about Io that I did.  Hopefully I’ve cleared that up for you!  But also, I think this is a good example of how the process of lifelong learning works.  If you’re a lifelong learner (as I am), you may have favorite topics that you think you know an awful lot about.  But there’s always more to learn, and sometimes learning more means unlearning a few things that you thought you already knew.

WANT TO LEARN MORE?

If you’re an Io fanatic like me, I highly recommend Alien Oceans: The Search for Life in the Depths of Space by Kevin Peter Hand.  The book is mainly about Europa and the other icy/watery moons of the outer Solar System, but there’s a surprising amount of information in there about Io, too.  Apparently, if it turns out that Europa really is home to alien life (as many suspect her to be), then Io may have played a crucial role in making that alien life possible.

Sciency Words: Stagnant Lid

~ J.S. Pailly ~ 6 Comments

Sciency Words: (proper noun) a special series here on Planet Pailly focusing on the definitions and etymologies of science or science-related terms.  Today’s Sciency Word is:

STAGNANT LID

Here on Earth, we have earthquakes.  Lots and lots of earthquakes.  And that’s very odd.

Maybe we should be thankful for all those earthquakes.  Our planet’s system of plate tectonics is unique in the Solar System.  Frequent earthquakes are a sign that Earth’s tectonic plates are still moving, that our planet is still geologically healthy.  The alternative would be stagnant lid tectonics, and that’s something we Earthlings probably don’t want.

In this 1996 paper, planetary scientists V.S. Solomatov and L.N. Moresi coined the term “stagnant lid” to describe what was happening on Venus—or rather what was not happening.  Venus doesn’t have active plate tectonics.  Maybe she did once, long ago.  If so, Venus’s plates somehow got stuck together, forming a rigid, inflexible shell.

The term stagnant lid has since been applied to almost every other planetary body in the Solar System, with the obvious exceptions of the four gas giants, and the possible exceptions of two of Jupiter’s moons: Europa and Ganymede.

According to this paper from Geoscience Frontiers, neither Europa nor Ganymede have truly Earth-like plate tectonics, but something similar may be happening.  The authors of that paper refer to the situation on Europa and Ganymede as “fragmented lid tectonics” or “ice floe tectonics.”  The upcoming Europa Clipper and JUICE missions should tell us more about how similar or different this is to Earth’s plate tectonics.

A stagnant lid does not necessarily mean that a planet or moon is geologically dead.  Venus and Io both have active volcanoes, for example, and it was recently confirmed that Mars has marsquakes.  But none of these stagnant lid worlds seem to be as lively as Earth—and I mean that in more ways than one.

If you buy into the Rare Earth Hypothesis, plate tectonics is one of those features that makes Earth so rare. Plate tectonics is something Earth has that other planets don’t, and thus it may be an important factor in why Earth can support life when so many other worlds can’t.

Sciency Words A to Z: JUICE

~ J.S. Pailly ~ 25 Comments

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, J is for:

JUICE

Speaking as a space enthusiast and a citizen of the United States, I have to confess I’m a bit disappointed with the status of the American space program.  While there have been some success stories—New Horizons, Curiosity, Scott Kelly’s year in space—I can’t help but feel like NASA has spent the last decade or so floundering.

However, it’s encouraging to see that so many other space agencies around the world are starting to pick up the slack.  My favorite example of this is the JUICE mission, a project of the European Space Agency (E.S.A.).

Astrobiologists have taken a keen interest in the icy moons of Jupiter.  There’s compelling evidence that one of those moons (Europa) has an ocean of liquid water beneath its surface.  There’s also a growing suspicion that two more of those moons (Ganymede and Callisto) may have subsurface oceans as well.

The original plan was for NASA and the E.S.A. to pool their resources for one big, giant mission to the Jupiter system.  But then the 2008 financial crisis hit.  The U.S. Congress was loath to spend money on anything—especially space stuff.  “Due to the unavailability of the proposed international partnerships […]”—that’s how this E.S.A. report describes the matter.

So the E.S.A. decided to go it alone. Personally, I think this was a very brave move.  E.S.A. has never done a mission to the outer Solar System before, not without NASA’s help.  But there has to be a first time for everything, right?  And so JUICE—the JUpiter ICy moons Explorer—began.  It’s not my favorite acronym, but it works.

According to E.S.A.’s website, JUICE will conduct multiple flybys of Europa and Callisto before settling into orbit around Ganymede.  You may be wondering why JUICE won’t be orbiting Europa.  This is in large part because of the radiation environment around Jupiter.  Europa may be more exciting to astrobiologists, but Ganymede is a safer place to park your spacecraft.

Meanwhile, NASA has recovered much of the funding it lost after the 2008 financial crisis, and they’re once again planning to send their own mission to the Jupiter system.  So maybe NASA and E.S.A. will get to explore those icy moons together after all!  Or maybe not.  According to this article from the Planetary Society, NASA’s budget is under threat once again.

I guess we’ll have to wait and see, but no matter what happens to NASA’s budget, E.S.A. seems fully committed to JUICE.  So speaking as a space enthusiast, at least I have that to look forward to.

Next time on Sciency Words A to Z, how do you measure the size of an alien civilization?

Sciency Words: Frost Line

~ J.S. Pailly ~ 10 Comments

Welcome to a very special holiday edition of Sciency Words! Today’s science or science-related term is:

FROST LINE

When a new star is forming, it’s typically surrounded by a swirling cloud of dust and gas called an accretion disk. Heat radiating from the baby star plus heat trapped in the disk itself vaporizes water and other volatile chemicals, which are then swept off into space by the solar wind.

But as you move farther away from the star, the temperature of the accretion disk tends to drop. Eventually, you reach a point where it’s cold enough for water to remain in its solid ice form. This is known as the frost line (or snow line, or ice line, or frost boundary).

Of course not all volatiles freeze or vaporize at the same temperature. When necessary, science writers will specify which frost line (or lines) they’re talking about. For example, a distinction might be made between the water frost line versus the nitrogen frost line versus the methane frost line, etc. But in general, if you see the term frost line by itself without any specifiers, I think you can safely assume it’s the water frost line.

Even though our Sun’s accretion disk is long gone, the frost line still loosely marks the boundary between the warmth of the inner Solar System and the coldness of the outer Solar System. The line is smack-dab in the middle of the asteroid belt, and it’s been observed that main belt asteroids tend to be rockier or icier depending on which side of the line they’re on.

It was easier for giant planets like Jupiter and Saturn to form beyond the frost line, since they had so much more solid matter to work with. And icy objects like Europa, Titan, and Pluto—places so cold that water is basically a kind of rock—only exist as they do because they formed beyond the frost line. This has led to the old saying:

dc23-outer-solar-system-christmas-party

Okay, maybe that’s not an old saying, but I really wanted this to be a holiday-themed post.

Sciency Words: Facula

~ J.S. Pailly ~ 2 Comments

Sciency Words BIO copy

When Voyager 1 trained its camera on the moons of Jupiter, scientists back on Earth had no idea what to expect. Turned out they were right. Voyager was snapping photos of geological features unlike anything anyone had ever seen before. Which meant it was time to make up some new sciency words!

FACULA

Last week, we learned about the word macula (plural, maculae): a special term for dark spots on the surface of a moon or other planetary body. Now if you’re going to invent a special term for dark spots, you really ought to have a term for bright spots too. And that term is facula (plural, faculae).

To an ancient Roman, facula meant “little torch.” To a modern planetary scientist, it refers to a surface feature that looks brighter than the surrounding terrain. The term was first used this way to describe bright, circular features seen on Ganymede.

sp16-faculae-on-ganymede

If you think Ganymede’s faculae look a little like craters, you’d be on the right track. Like most moons in the outer Solar System, Ganymede is composed of a mixture of rock and ice, and it may have a layer of liquid water beneath its surface.

So the craters left by asteroid impacts on Ganymede sometimes get filled in with icy slush. The slush freezes, and the crater is virtually erased. Only the crater rim remains, and you can see a color difference between old and new surface ice.

The term facula can be used to describe almost any bright spot on a planet-like surface, not just resurfaced craters. For example, there are faculae on the dwarf planet Ceres. Ceres’s faculae are still being investigated by the Dawn spacecraft, but the current best guess is that they’re salt deposits—perhaps salt left behind after very briny water boiled into space.

For next week’s edition of Sciency Words, we’ll conclude our visit to the moons of Jupiter with a quick trip to Io.

Bonus Sciency Word: An impact crater that gets filled in and smoothed over, like the craters on Ganymede, is also called a palimpsest.