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.

Who’s Eating Titan’s Acetylene?

October 3, 2016

The first Monday of the month is Molecular Monday, the day I write about my least favorite subject from school: chemistry.

Molecular Mondays Header

I’d planned to write something about ammonia today. Ammonia might (might!) serve as a good substitute for water in some alien biochemistry.

But then I was reminded of something. Something important. Something I’m kicking myself for not covering before. So once again, let’s turn our attention to Saturn’s largest moon: Titan.


Making Acetylene on Titan

As we’ve discussed previously, methane gas and other chemicals break apart in Titan’s upper atmosphere. This allows carbon, hydrogen, nitrogen, and possibly other elements to recombine in new ways. The result is a mishmash of organic chemicals collectively refered to as tholins.

Tholins tend to be sticky, yucky, and orange. They slowly fall to Titan’s surface, covering the moon with sticky, yucky, orange sludge. One chemical in the tholin mix should be acetylene (C2H2). In fact, acetylene is a fairly simple molecule compared to the rest of the tholin gunk on Titan, so we should find lots of it.

But we don’t. We’ve detected little to no acetylene accumulation on Titan’s surface. Maybe this means there’s something wrong with our detection techniques. Or maybe some as-yet-unidentified chemical process breaks up acetylene molecules as they fall through Titan’s atmosphere.

Or maybe (maybe!) something eats the acetylene as soon as it touches the ground.

Eating Titan’s Acetylene

I first read about this a few years ago in Astrobiology: A Very Short Introduction. It came up again, in greater detail, in the book I’m currently reading: All These Worlds Are Yours. The case of Titan’s missing acetylene is a hot topic for astrobiologists.

There’s a rather simple chemical reaction that might (might!) explain what’s going on.

C2H2 + 3H2 –> 2CH4 + energy

That’s one acetylene molecule reacting with three hydrogen molecules to produce two methane molecules and some energy. The kind of energy that weird Titanian microorganisms could use to survive (maybe).

In my opinion, it still seems unlikely that life could have evolved on the surface of Titan, if only because liquid methane (Titan’s “water”) is not a good solvent for amino acids. But unlikely is not the same as impossible.

It’s worth noting at this point that a few other weird things are happening on Titan. Hydrogen gas seems to mysteriously disappear near Titan’s surface, and no one has adequately explained how Titan replenishes its atmospheric methane (all the methane should have turned into tholins by now).

If Titan does have an acetylene-eating, hydrogen-breathing microbe that expels methane as a waste product, that would conveniently solve three mysteries at once. I can’t help but think, though, that this might be a little too convenient to be true.

Molecular Monday: Liquid Water vs. Liquid Methane

September 5, 2016

Molecular Mondays Header

Welcome to Molecular Monday! On the first Monday of the month, we take a closer look at the atoms and molecules that make up our physical universe. Today, we’re comparing some of the properties of:


So you’re a moon or other planetary body, and you want to get some biochemical action going on. First, you need some organic substances. Titan has set a great example with the tholin haze that forms spontaneously in its atmosphere.

Next, you need a liquid to dissolve that organic material in, in the hopes that the organic material will recombine as amino acids, peptide chains, and ultimately DNA. But which liquid should you choose? Liquid water (as seen on Earth) or liquid methane (as seen on Titan)?

Pick Water!

Water (H2O) makes an excellent solvent for our purposes because it’s a polar molecule. There are two big reasons for water’s polarity.

  • First, oxygen has an extremely high electronegativity, meaning oxygen atoms like to yank electrons away from other atoms. Within a water molecule, oxygen’s electron-hogging tendencies cause it to become negatively charged, while the two hydrogen atoms become positive.
  • Second, you know how water molecules have that Mickey Mouse shape? Because of that shape, with the two hydrogen atoms bent toward each other, the positive charges accumulate on one side of the molecule and the negative charge accumulates on the other.

Thus, water is a polar molecule, and it’ll go around interacting with other polar molecules, like tholins or amino acids.

Don’t Pick Methane

Unlike water, methane (CH4) is a nonpolar molecule. Why?

  • Carbon is slightly more electronegative than hydrogen, but not by much, so the atoms in a methane molecule share electrons almost equally. This minimizes the electric charges that might build up inside the molecule.
  • Methane molecules are symmetrical, with the carbon atom in the center and the four hydrogens evenly spaced around in, like the four corners of an equilateral pyramid.

Sp05 Methane vs Water

Any electrical charges in a methane molecule balance out, due to the molecule’s symmetry. And those charges are fairly weak anyway, due to the similar electronegativities of carbon and hydrogen.

I won’t be so bold as to say life can’t develop in a liquid methane environment, but the idea does seem a bit farfetched in light of the chemistry. Polar molecules like tholins just aren’t likely to dissolve in a methane lake, like the lakes found on Titan.

On the other hand, the universe keeps surprising us, and the giant lake monster I recently met on Titan might dispute my assessment of Titan’s biochemical potential.

P.S.: Titan’s lakes also contain liquid ethane, but that doesn’t really change anything. Ethane is also nonpolar.

How to Fly on Titan

August 31, 2016

My trip to Titan is almost over. Soon, I’ll have to find some other planet or moon to blog from. But before I leave, there’s one last thing I want to do: fly.

Titan’s atmosphere is about 50% denser than the atmosphere on Earth. Combine that with the low surface gravity (a mere 14% of Earth normal) and it should be possible, theoretically, for me to put on some wings and flap around in the sky like a bird.

Taking my cue from the myth of Icarus, my artificial wings will not be made of wax, although it’s cold enough here on Titan that there’d be no danger of wax wings melting.

So with my non-wax wings strapped to my arms, I leap into the air, and….

Ag31 Flight on Titan 1

Okay, that didn’t go according to plan, so I turn to the Internet for help (the wifi on Titan is surprisingly good, by the way). I soon find this helpful article from the Journal of Physics Special Topics.

One option is that I try to get a running start. I’m really going to have to sprint here; average human running speed (6 m/s) won’t cut it. I need to reach a minimum of 11 m/s. And…

Ag31 Flight on Titan 2

… nope. I’m a nerd, not an athlete. Sprinting isn’t my thing.

So my next option is to build bigger wings. According to the paper from Physics Special Topics, the total area of my wings needs to be at least 4.7 m2. I’ll go for 5 m2, just to be safe.

The good news is that once I’m off the ground, I won’t need to use much energy to stay aloft. Flying on Titan should be “effortless and relatively easy […] without any sort of propulsion device.” Sounds like just a little light flapping should do the trick.

Okay, so here we go.

Ag31 Flight on Titan 3

P.S.: The Journal of Physics Special Topics may be my new favorite scientific periodical, with articles covering topics like cows jumping over the Moon, the effects of general relativity on Santa Claus, and the atmospheric loss caused by opening a portal between Earth and the Moon (as depicted in the video game Portal 2).

Do Not Go Swimming on Titan

August 24, 2016

I’ve made a friend on Titan: a giant, multi-tentacled monster that swims around in Titan’s lakes of liquid methane. Today, my new friend invited me to go swimming with him. While that does sound like fun, there are a few problems with that idea:

  • I can’t exactly change into my swimming trunks. There’s no oxygen in Titan’s atmosphere, so I need my bulky spacesuit. But even if that weren’t a problem….
  • Liquid methane is a cryogenic fluid. It’s not quite as cold as liquid nitrogen, but still… If I stick my toe in the methane, my toe will probably flash freeze and shatter. But even if that weren’t a problem….
  • I would sink straight to the bottom of the lake. Liquid methane is significantly less dense than water and significantly less dense than the human body. I wouldn’t be able to float, and I certainly wouldn’t be able to swim.

However, I didn’t want to disappoint my new friend, and I did come prepared for a possible excursion over a methane lake. So I hurried back to my spaceship and grabbed my boat.

Ag24 Lake Monster of Titan

Life on Titan: Infrared Eyes

August 22, 2016

I’ve been exploring the surface of Titan for several weeks now. During my time here, I have not discovered alien life, but alien life sure has discovered me. Fortunately, the Titanian lake monster I met on Friday is friendly, and he was super excited about meeting someone “from the stars.”

“Wait,” I said, “you know about the stars?”

“Oh yes,” the lake monster said. “I look up at them, twinkling in the night, and also the great orb with the rings around it.”

This really left me flummoxed. I can’t see Saturn at all from the surface of Titan (and I was pretty upset about it too). I certainly can’t see the stars. I can’t even see the Sun through all the tholin haze layered up in Titan’s atmosphere.

However, the tholin haze does allow certain wavelengths of light to pass through, mostly in the infrared part of the spectrum. The haze is almost completely transparent at a wavelength of 2000 nanometers (nm), which is how the infrared camera on the Cassini spacecraft has been able to photograph Titan’s surface.

The human eye can only detect light between roughly 400 and 700 nm. That’s because humans evolved on a planet where the 400 to 700 nm range is dominant, while life on Titan evolved in an environment where infrared light shines the most clearly.

So my new lake monster friend sees in infrared, possibly right around the 2000 nm range, and when he looks up into the sky he can see the Sun and stars and even Saturn, while all I see is gloomy orange haze.