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.

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.


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.

Sciency Words: Macula

September 9, 2016

Sciency Words PHYS 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!


Last week, we talked about Europa’s lineae: the reddish-brown cracks and fissures crisscrossing this small moon’s surface. But those weren’t the only surprises Voyager 1 observed. Let’s zoom in for a closer look.

Sp09 Macula on Europa

Europa has these peculiar dark splotches on its surface, similar in coloration to the lineae. Scientists came up with the term macula (plural maculae) to describe them. It comes from the Latin word for “spot” or “blemish.” It’s related indirectly to the word immaculate, which literally means “without blemish.”

Although Europa’s maculae were discovered in 1979, it wasn’t until 2011 that anyone could adequately explain them. It seems that Europa’s thick ice shell has a complex relationship with the ocean of liquid water deep beneath the surface, resulting in frequent patterns of melting and refreezing.

Sometimes “lakes” of liquid water become embedded between layers of ice. This causes surface ice to sag and cave in, breaking up into chunky, tightly packed icebergs. Some sort of material (possibly organic material) seeps up with the meltwater, causing the dark discoloration.

Eventually, the lake beneath a macula will freeze. Since ice is less dense than water, this forces the now cracked and broken surface ice to rise above the surrounding landscape. In the process, the already strange-looking maculae transform into even stranger-looking chaos terrain.

The term macula can be used to describe almost any dark, spotty or splotchy feature on a planetary body. That doesn’t mean they have anything in common beyond superficial appearances. For example, while maculae on Europa seem to be caused by melting and refreezing ice, maculae on Titan may be related to some sort of volcanic activity.

For next week’s edition of Sciency Words, we’ll move on to Ganymede. Europa wasn’t the only Jovian moon showing off strange, never-before-seen geological features when Voyager arrived.

Sciency Words: Linea

September 2, 2016

Sciency Words MATH

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!


Europa has the youngest, smoothest-looking surface of any object in the Solar System.

Ag04 Europa Blush

But as you can see in the totally legit Voyager 1 image above, Europa’s icy blue surface is not without blemish.

It’s crisscrossed with cracks and fissures that appear to be filled with some sort of reddish-brown substance. Astronomers adopted the term linea (plural lineae) to describe these features. This was not astronomers at their most creative or imaginative; linea is just the Latin word for line.

Europa has a subsurface ocean of liquid water. There might be alien sea creatures swimming around in that subsurface ocean. Or there might not. If we want to find out, Europa’s lineae may be a good place to start looking.

The reddish-brown substance is believed to seep up through the cracking, fissuring ice. Does it include organic material? Amino acids? Maybe some sort of alien DNA? Hopefully NASA’s Europa Clipper mission will be able to find out (pending Congressional approval and so forth).

Lineae are most commonly associated with Europa, but the term has also been used to describe line-like features on a handful of other worlds, including Mars (although Mars’s recurring slope lineae are very different from lineae on Europa).

In next week’s edition of Sciency Words, we’ll continue exploring the moons of Jupiter. There are plenty of other terms that had to be invented following Voyager 1’s visit.

P.S.: I once ate at a seafood restaurant named Nova Europa. I know it was supposed to be Mediterranean-themed, but that is not what I was thinking about when they served my calamari.

Europa: My Favorite Moon

March 16, 2016

Fun fact about me: Europa is my favorite moon.

Mr07 Moon 1

Oh, sorry Moon. You’re cool too. It’s just… you don’t have an ocean. Or chaos terrain. Or possible alien life. It’s nothing personal.

Mr07 Moon 2

Anyway, on Monday I told you that Congress wants NASA to put a robotic lander on the surface of Europa. But the really interesting bit is deep beneath the surface, where the ice turns to liquid water. Is anything alive down there? Any microbes? Maybe fish? What about alien mermaids?

A lander can’t investigate that sort of stuff. At least not directly. But if you’ve ever seen a picture of Europa…

Mr07 Europa

… you’ll notice the surface is covered in dark reddish-colored lines. These lines appear to be cracks. It’s believed that warm water sometimes forces its way to the surface, carrying with it a mix of minerals and possibly other materials from the oceans below. It’s these minerals which cause the reddish discoloration.

So while a lander can’t sample the ocean water directly, it could examine the materials that have been deposited on the surface. Now, if you’ll allow me to switch my science blogger hat for my science fiction writer hat, I’ll tell you exactly what the Europa lander will find.

Salt. Lots of salt. That won’t surprise anyone. It’s been long assumed that Europa’s ocean is much saltier than the oceans here on Earth. It must be; otherwise the ocean would freeze.

The lander will also detect other minerals as well. And amino acids. That’ll raise some eyebrows, but amino acids aren’t that uncommon. We’ve found them on other planets and we’ve found spectrographic evidence of them all across space. As I reported in last week’s Molecular Monday post, there are literally hundreds or perhaps thousands of different kinds of amino acids in our universe.

Mr07 Surface of Europa

That will make headlines. No, it’s not the same 21 amino acids coded for by human DNA, but this cannot be a coincidence. What natural phenomenon, other than life, could produce such a select few amino acids in such large quantities?

But wait, there’s more. These 21 amino acids have something in common. They have the same chirality. And that’s the part where the entire scientific community freaks out.

Tune in for Friday’s edition of Sciency Words to find out what the heck chirality is and why it’s so important in the search for alien life.

P.S.: My second favorite moon is Titan, followed by Io, Miranda, and Triton. Oh, and Naiad! I love Naiad. But Earth’s Moon totally makes in my top ten. Probably.

Europa: To Land or Not to Land?

March 14, 2016

NASA has big plans for Europa. It is, after all, Jupiter’s most interesting moon.

Ag04 Europa Blush

But the details of these big plans have been in a state of flux for a while. The mission would undoubtedly include an orbiter, but should it orbit Europa or Jupiter? What about also sending a lander or rover? Or maybe a submarine? Europa does have an ocean somewhere beneath its icy shell.

Okay, there’s no way Congress would pay for all that, so NASA decided to scale down its ambitions. In other words, the mission was descoped. No landers, no rovers, and definitely no submarines. Also, the orbiter would orbit Jupiter. Entering orbit of Jupiter requires less delta-v, and therefore less fuel, than trying to enter orbit of any specific Jovian moon.

But even after scaling everything down, this Europa mission would still come with a hefty price tag. Congress held hearings. This couldn’t go well. So what happened?

Congress told NASA to put the lander back in the mission plan and put up money to pay for it.

Mr06 Europa 1

That’s right. Congress suggested—no, commanded—that NASA include a lander as part of its Europa Clipper mission and provided money to pay for it. I guess you could say this mission was de-descoped. Or maybe it was rescoped.

Now I’m not naïve enough to think that Congress has suddenly developed a deep appreciation for planetary science. It’s more likely this lander will be built by some company that donated generously to someone’s campaign, or maybe it will be built within some influential congress-person’s district.

I’m not a political blogger, so I don’t want to get into that. What I do want to say is this: we’re going to Europa, baby!

Mr06 Europa 2


We’re Going to Europa from SciShow Space.

A Lander for NASA’s Europa Mission from The Planetary Society.

Congress: NASA Must Not Only Go to Europa, It Must Land from Ars Technica. This last link is particular interesting because it suggests that NASA doesn’t really want to go to Europa at all, but Congress is forcing them to do it.