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.