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:

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.


Meet a Moon: Dione

October 24, 2016

Regarding Greek mythology, it seems no one’s really sure who Dione was. Ancient sources contradict each other, and modern scholars think there may have actually been more than one mythical woman who went by that name. But I was able to find out this much: according to Wikipedia, at least one of these Diones was “sometimes associated with water or the sea.”

What that in mind, I’d like to introduce you to Dione, one of Saturn’s moons.

oc24-meet-dione

You sure are, Dione. In fact, I can’t think of a better description for you.

The Waters of Enceladus

Over the last decade or so, one of Saturn’s other moons has become famous for having an ocean of liquid water beneath its surface. That moon is called Enceladus. We know about Enceladus’s water for two reasons:

  • Geysers: Enceladus has a series of cracks (called tiger stripes) in its south polar region, and saltwater shoots out of these cracks at regular intervals.
  • Libration: Enceladus wobbles in place (librates) more than it should. This is best explained by the presence of a layer of liquid separating the moon’s crust from its core.

It’s still a mystery how Enceladus generates enough heat to keep its liquid water from freezing, but at this point, it’s pretty clear the water is there.

The Waters of Dione

Dione doesn’t librate the way Enceladus does, and we haven’t noticed any saltwater geysers, but a recent paper in Geophysical Research Letters says Dione might have a subsurface ocean too.

The authors of the paper created a new theoretical model for icy moons, a model which fits precisely with observations of Enceladus. Then they applied this new model to Dione and concluded that Dione should have a subsurface ocean.

This raises two questions that are fairly easily answered.

  • Where are Dione’s geysers?: Dione may not spew saltwater (anymore), but it does have cracks and fissures in its surface, suggesting that it may have had active “tiger stripe” geysers in the past.
  • What about Dione’s libration?: The new model suggests that Dione should librate, but not as much as Enceladus does. The Cassini spacecraft (currently orbiting Saturn) does not have instruments sensitive enough to detect the predicted libration.

So there you have it. According to at least one theoretical model, Dione should have a subsurface ocean, but we cannot yet confirm that it does. And it’ll probably be awhile before we can send a new spacecraft to Saturn to find out one way or another.

But hey, how appropriate is it that we named this moon, which might have a subsurface ocean, depending on your theoretical model, after a mythical figure that might sometimes have been associated with water, depending on which ancient sources your reading!


All These Worlds Are Yours: A Book Review

October 11, 2016

In his book All These Worlds Are Yours: The Scientific Search for Alien Life, author Jon Willis gives you $4 billion. How many authors do that? Okay, it’s imaginary money, and you’re only allowed to spend it on astrobiological research. But still… $4 billion, just for reading a book!

If you’re new to the subject of astrobiology, All These Worlds is an excellent introduction. It covers all the astrobiological hotspots of the Solar System and beyond, and unlike most books on this subject, it doesn’t gloss over the issue of money.

There are so many exciting possibilities, so many opportunities to try to find alien life. But realistically, you can only afford one or maybe two missions on your $4 billion budget. So you’ll have to pick and choose. You’ll have to make some educated guesses about where to look.

Do you want to gamble everything on Mars, or would you rather spend your money on Titan or Europa? Or do you want to build a space telescope and go hunting for exoplanets? Or donate all your money to SETI? Willis lays out the pros and cons of all your best options.

My only complaint about this book is that Enceladus (a moon of Saturn) didn’t get its own chapter. Instead, there’s a chapter on Europa and Enceladus, which was really a chapter about Europa with a few pages on Enceladus at the end.

oc11-enceladus-is-angry

I agree, Enceladus. On the other hand, Enceladus is sort of like Europa’s mini-me. So while I disagree with the decision to lump the two together, I do understand it.

In summary, I’d highly recommend this book to anyone interested in space exploration, and especially to those who are new or relatively knew to the subject of astrobiology. Minimal prior scientific knowledge is required, although some basic familiarity with the planets of the Solar System would help.

P.S.: How would you spend your $4 billion? I’d spend mine on a mission to Europa, paying special attention to the weird reddish-brown material found in Europa’s lineae and maculae.


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.


Enceladus: Too Young for Life?

April 11, 2016

Does life exist on Enceladus? Maybe. I don’t know. It depends.

Ap05 Enceladian Jellyfish 1

Calm down, Enceladian jellyfish thing. We’re still trying to figure this out.

Astronomers recently determined that Enceladus has a decoupled crust, meaning there is a global ocean of liquid water hidden beneath this tiny moon’s surface. Just the kind of environment that could support alien life. But there’s a problem.

A recent scientific paper, entitled “Dynamical Evidence for a Late Formation of Saturn’s Moons,” suggests that Enceladus may be too young for life to have evolved there. (Please note: I typically try to read these sorts of papers in their entirety, but this one is 50 pages long. I think I got the gist of it, but I cannot honestly say I read the whole thing).

After examining tidal forces, orbital resonances, and orbital inclinations within the Saturnian system, the paper reaches two possible conclusions:

  • Either the Saturnian system changes at an oddly slow rate…
  • Or Saturn’s rings and many of its moons—including Enceladus—formed very, very recently.

If you were a dinosaur, specifically a dinosaur who knew how to use a telescope, you may have been able to watch as a previous generation of Saturn’s moons were destroyed. Perhaps these moons collided with each other. Perhaps they were torn asunder by Saturn’s gravity. Perhaps you would have wondered, with your walnut-sized dinosaur brain, if anything like that could ever happen to Earth.

Anyway, the destruction of Saturn’s old moons left a whole lot of debris, forming a disk of icy and rocky material around the planet. Much of that debris is still there in the rings, but some of it accreted together, forming new moons like Enceladus.

All of this happened, according to this “Dynamical Evidence” paper, about 100 million years ago. That doesn’t give life a whole lot of time to evolve, which is bad news for out Enceladian jellyfish friend.

Ap05 Enceladian Jellyfish 2

Sorry, buddy.


Sciency Words: Decouple

April 8, 2016

Sciency Words BIO copy

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 all expand our scientific vocabularies together. Today’s term is:

DECOUPLE

After years of speculation, last September we finally learned the truth. Enceladus, one of Saturn’s moons, has a decoupled crust.

Ap04 Enceladus Decoupled

Sorry, Enceladus. I meant no offense. In fact, having a decoupled crust might be a good thing.

We’ve known for some time now that there is liquid water somewhere beneath Enceladus’s surface. Some of that water periodically spurts out of geysers located near the moon’s south pole. You can see this happening in the totally legit Hubble image of Enceladus above.

The big question: how much liquid water is there? Are we talking about a modest subsurface lake near the south pole…

Ap04 Enceladus Subsurface Lake

… or does Enceladus have a vast global ocean hidden beneath its surface? You know, the kind of environment that might support life.

Ap04 Enceladus Subsurface Ocean

By carefully observing Enceladus as it orbits Saturn, astronomers noticed that the moon appears to wobble more than it should. The best explanation for this: the moon’s crust is not attached to anything solid. It floats freely atop a layer of liquid. Enceladus’s crust is—to use the technical term—decoupled from the moon’s rocky interior.

That’s pretty compelling evidence for a global subsurface ocean, and while it doesn’t prove that Enceladus supports life, it does make the possibility a whole lot more likely.

Or does it? Let me check on that for Monday’s post.

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Alternative definition of decouple: to remove any couples from a party or similar social event, leaving all the single gents and ladies free to mingle.