Sciency Words: Moon

September 29, 2017

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


There are three things I want to cover with today’s post. Firstly, for anyone who may not already know, Earth’s moon is officially called the Moon (with a capital M). Unless you don’t speak English, in which case it’s called whatever it’s called in your language, provided that you treat the word as a proper noun. This according to the International Astronomy Union (I.A.U.), the one and only organization with the authority to name and classify astronomical objects.

Phases of the Moon.

Of course the Moon is not the only moon out there, so I also want to talk a little about the official I.A.U. sanctioned definition of the word moon. Unfortunately there isn’t one, which seems odd given how the I.A.U. are such stickers about their official definition of the word planet.

A common unofficial definition is that a moon is any naturally occurring object orbiting a planet, dwarf planet, or other kind of minor planet (such as an asteroid or comet). Except this definition creates some problems:

Saturn has like a bazillion moons!

Since there’s no lower limit on size or mass, you could consider each and every fleck of ice in Saturn’s rings to be a moon.

The Moon isn’t a moon!

In a very technical sense, the Moon does not orbit the Earth. The Earth and Moon both orbit their combined center of mass, a point called a barycenter. In the case of the Earth-Moon system, the barycenter happens to lie deep inside the Earth, so this distinction may not seem important, but…

Pluto is Charon’s moon, and Charon is Pluto’s!

The barycenter of the Pluto-Charon system is a point in empty space between the two objects. Pluto is the larger of the pair, so we generally consider Charon to be Pluto’s moon; however, you could argue that Pluto and Charon are moons of each other. You could even write a love song about their relationship.

Of course I’m not seriously arguing that Saturn has billions upon billions of moons, nor am I arguing that our own Moon is not really a moon. There does seem to be some ambiguity about Charon’s status (is Charon a moon, or are Pluto and Charon binary dwarf planets?), but I’m not sure if this ambiguity has caused any real confusion in scientific discourse.

Still, as we learn more about moons in our own Solar System and also moons in other star systems, I think the I.A.U. will eventually have to come up with an official definition. And that brings me to the third and final thing I wanted to cover today: exomoons.

An exomoon would be defined as a moon (whatever that is) orbiting a planet or other planetary body outside our Solar System. Finding exoplanets is hard enough, so as you can imagine, searching for exomoons really stretches the limits of current telescope technology. But astronomers are trying, and next month (October, 2017) the Hubble Space Telescope will be making special observations of a planet named Kepler-1625b in an attempt to confirm a possible exomoon detection.

In Memory of Cassini

September 20, 2017

Last week, NASA’s Cassini Mission came to an end when the spacecraft crashed into the planet Saturn. This was, of course, a planned event: a way for the mission to end in a blaze of glory, collect a little extra data about Saturn’s atmosphere, and also protect Saturn’s potentially habitable moons (Titan, Enceladus, and possibly also Dione) from microorganisms that may have hitched a ride from Earth aboard the spacecraft.

Cassini’s last few days were an oddly emotional time, at least for me. Somehow knowing that the end was coming, that everything was proceeding according to schedule, made it a little harder to bear. When the words “data downlink ended” started appearing in my Twitter feed, I got a little misty eyes and had to walk away from the computer for a while.

This despite the fact that I never got to know Cassini all that well. I never really followed the Cassini Mission closely (especially compared the way I follow Juno). Looking back through my old posts, it seems Cassini only ever appeared on this blog twice. Once for that time it spotted sunlight glinting off the surface of Titan’s methane lakes…

… and once more for the time it used precise measurements of Enceladus’s librations to determine that Enceladus does indeed have an ocean of water beneath its crust.

So today I thought I’d turn the floor over to several of the moons of Saturn and also Saturn herself. They’re the ones who got to know Cassini well. Not me. It’s right that they get the chance to give Cassini’s eulogy.

The Titan Mission That Could’ve Been

July 31, 2017

This is a follow-up to my recent post about NASA’s next flagship-class mission. There seemed to be a lot of interest in the comments about a possible mission to Titan and/or Enceladus, Saturn’s most famous moons.

The competition for flagship mission funding can get pretty intense. The Titan Saturn System Mission (or T.S.S.M.) was a strong contender last time around, as was a proposed mission to Europa, the most watery moon of Jupiter.

According to Titan Unveiled by Ralph Lorenz and Jacqueline Mitton, things got a little nasty when the Europa team started calling Titan “Callisto with weather,” the implication being that Titan was geologically boring.

Callisto, by the way, is a large by often overlooked moon of Jupiter.

Ultimately Team Europa won. NASA deemed their proposal to be closer to launch-readiness. Now after a few years delay due to a certain global financial meltdown, the Europa Clipper Mission appears to be on track for a 2022 launch date (fingers crossed).

As excited as I am for Europa Clipper, the mission to Titan would’ve been really cool too. It actually would have included three—possibly four—spacecraft.

  • A lake-lander to explore Titan’s liquid methane lakes.
  • A hot air balloon to explore the organic chemical fog surrounding Titan.
  • A Titan orbiter to observe Titan from space and also relay data from the lander and balloon back to Earth.
  • And a possible Enceladus orbiter, built by the European Space Agency, which would have tagged along for the ride to Saturn.

It’s a shame T.S.S.M. didn’t get the green light from NASA. Just think: we would’ve had so many cool things going on at once in the Saturn System, enough to almost rival the activity we’ve got going on on Mars!

But now once Europa Clipper is safely on its way (again, fingers crossed), Team Titan will have another shot at getting their mission off the ground.

Sciency Words: Frost Line

December 23, 2016

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


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:


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

Can You See Saturn from Titan?

August 10, 2016

As I continue my exploration of Titan, there’s something I was really hoping to see.

Ag10 Saturn in the Sky

Like Earth’s moon, Titan is tidally locked. That means as Titan orbits Saturn, the same side of the moon is always oriented toward the planet.

So in theory, all I have to do is make my way to the Saturn-facing hemisphere, look up in the sky, and behold the majesty of the Ringed Planet.

I’m sorry to report that today science has crushed my dreams. Titan is shrouded in a haze of aerosol particles called tholins. The tholin haze is not as dense as you might assume (which is why I thought I might be able to see Saturn).

But this diffuse haze extends from the surface all the way up to an altitude of approximately 300 km. For the sake of comparison, typical Earth clouds form at altitudes between 3 and 12 km, and the unofficial boundary between Earth’s atmosphere and space is about 100 km up. So you could say that Titan’s haze is 200 km taller than Earth’s entire atmosphere (and Titan still has a few more atmospheric layers above the haze too).

Dense or not, there’s more than enough tholin haze overhead to block my view of Saturn. In fact, it’s enough that I can’t tell which way the sun is.

Ag10 Saturn Not in the Sky

Of course, Titan does experience seasonal changes which can affect the tholin haze. Maybe if I came back at a different time of year (Titan’s year equals almost 30 Earth years), I might be able to see something. But I doubt it.

Saturn’s Story: Rings, Moons, and Alien Life

April 20, 2016

Where did Saturn’s rings come from? It is possible that the rings were always there, that they formed 4.5 billion years ago along with the rest of the Solar System. However, it seems more likely—a heck of a lot more likely—that the rings formed recently.

About 100 million years ago, Saturn would have had a different collection of moons than it does today. Then catastrophe struck. Moons started ramming into each other, or perhaps they strayed too close to Saturn (crossing the Roche limit) and were ripped apart by Saturn’s gravity.

Sp03 Poor Unfortunate Moon

The rings we see today are basically the icy debris left by that previous generation of moons. It’s also starting to look like many of Saturn’s current moons also formed around that time, accreting from the rubble.

Enceladus may be one of those newly formed moons. Enceladus is of particular interest to astrobiologists. Its subsurface ocean would be an ideal environment for life, but as I said last week, that’s only if life has had sufficient time to evolve. 100 million years doesn’t give evolution a much time to do its magic.

However, astrobiologists have taken a keep interest in another of Saturn’s moons: Titan. So I want to mention something important. Titan is not a young moon. It did not coalesce from lunar debris 100 million years ago. Titan is probably 4.5 billion years old, making it as old as Saturn, as old as the Solar System itself.

In fact, Titan would have been there when that previous generation of moons was destroyed. Titan would have watched it happen.

Ap09 Titan and Saturn's Rings

So while I’m less confident about the prospects of Enceladian life than I used to be, the odds of finding life on Titan are as good as they ever were.