October Is Europa Month Here on Planet Pailly!

Hello, friends!  Let’s talk about aliens!

If we want to find alien life, where should we look?  Well, if money were no object, I’d say we should look anywhere and everywhere we can.  Phosphorous on Venus?  Could be aliens.  Let’s check it out.  Melty zones beneath the surface of Pluto?  Let’s check that out too.  Ariel?  Dione?  Ceres?  Let’s check them all for signs of alien life!

But money is an object.  We simply don’t have the resources to explore all of these places.  Space exploration is expensive.  Space exploration will always be expensive so long as we’re stuck using rocket-based propulsion.  The Tsiolkovsky rocket equation makes it so.

Whenever you’re working within a restrictive budget, you need to think strategically.  With that in mind, astrobiologists (scientists who specialize in the search for alien organisms) have focused their efforts on four worlds within our Solar System.  Their names are Mars, Europa (moon of Jupiter), Enceladus (moon of Saturn), and Titan (another moon of Saturn).

This month, I’m going to take you on a deep dive (no pun intended) into Europa.  In my opinion, of the four worlds I just listed, Europa is the #1 most likely place for alien life to be found.  I don’t mean to denigrate Mars, Enceladus, or Titan.  There are good reasons to think we might find life in those places, too.  But there are also good reasons to think we might not.

  • Mars: Life may have existed on Mars once, long ago.  But then the Martian oceans dried up.  We’re unlikely to find anything there now except, perhaps, fossils.
  • Enceladus: Enceladus’s age is disputed.  She may be only a few hundred million years old, in which case she may be too young to have developed life.
  • Titan: If you want to believe in life on Titan, you have to get a little imaginative about how Titanian biochemistry would work.

Europa doesn’t have those issues.  Unlike Mars, Europa has an ocean of liquid water right now, in modern times.  Unlike Enceladus, Europa’s age is not disputed; she’s definitely old enough for life.  And unlike Titan, Europa doesn’t require us to get imaginative about biochemistry.  The same carbon-based/water-based biochemistry we use here on Earth would work just as well for the Europans.

There are still good reasons to search for aliens on Mars, Enceladus, and Titan.  Finding fossils on Mars would be super exciting!  Enceladus’s age is, as I said, in dispute, with some estimates suggesting she’s very young, but others telling us she’s plenty old.  And while life on Titan would be very different than life on Earth, scientists don’t have to imagine too hard to find plausible ways for Titanian biochemistry to work.

But if I were a gambler, I’d put my money on Europa.  And if I were in charge of NASA’s budget, I’d invest heavily in Europa research and Europa missions.  Europa just seems like the safest bet to me, if we want to find alien life. And in the coming month, I plan to go into more detail about why I feel that way.

WANT TO LEARN MORE?

If you’re interested in learning more about the Tsiolkovsky Rocket Equation, you may enjoy this article from NASA called “The Tyranny of the Rocket Equation” (because NASA is the American space agency, and anything Americans don’t like is tyranny).

As for astrobiology, I highly recommend All These Worlds Are Yours: The Scientific Search for Alien Life, by Jon Willis.  Willis frames the search for alien life just as I did in this post: alien life could be anywhere, but you only have a limited budget to use to find it.  So how would you spend that money?

Sciency Words: Ice

Sciency Words: (proper noun) a special series here on Planet Pailly focusing on the definitions and etymologies of science or science-related terms.  Today’s Sciency Word is:

ICE

I have a friend who teases me whenever I use the word ice. This is because, depending on what we’re talking about, I can’t just say “ice.”  As soon as the conversation turns to space stuff (as it often does when I’m around, for some reason), I feel the need to say “water ice.” I feel the need—no, the compulsion to specify that I mean the frozen form of water, as opposed to the frozen form of something else.

In more normal, down-to-earth sorts of conversation, I don’t feel that same compulsion.  Water ice is the only kind of ice we’re likely to encounter here on Earth. On rare occasions, if you’re at a science fair, or maybe a Halloween party, you might encounter carbon dioxide ice (a.k.a. dry ice).  But that’s a very rare special case.

However, as soon as we start talking about other planets and moons, or comets and asteroids, the word ice takes on a much broader meaning. In these more cosmic conversations, you really do need to be specific about which ice you’re talking about. To quote from a recent issue of The Planetary Report:

In the strictest definition, ice is the solid form of water.  However, planetary astronomers often use “ice” to refer to the solid form of any condensable molecule.

Beyond Earth, and especially in the outer Solar System, we find all sorts of crazy ices, like ammonia ice, methane ice, or nitrogen ice.  Along with the water ice and CO2 ice we Earthlings are more familiar with, these ices make up the hard crusts of many planetary bodies, like Titan or Pluto.

We also find ice crystals (of various types) forming in the clouds of planets like Uranus and Neptune.  In fact, Uranus and Neptune are often called “ice giants” in large part because of all those weird ices found in their atmospheres.

Starting next week, I’m planning to take a much closer look at those ice giant planets.  I expect my research to turn up plenty of questions, but very few answers.  Uranus and Neptune are, at this point, the least well explored planets in the Solar System.

So stay tuned!

P.S.: I want to start referring to all forms of igneous rock as “magma ice.”  After all, what is igneous rock but frozen magma?  I can’t think of any good reason why the term “magma ice” shouldn’t apply.

Sciency Words A to Z: Tholin

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, T is for:

THOLIN

Have you ever been stuck trying to say something, but you just don’t have the right words to say it?  In the 1970’s, planetary scientists Carl Sagan and Bishun Khare had that problem.

They’d conducted a series of experiments using gaseous chemicals that were known to be common in outer space, chemcials like ammonia, methane, water, hydrogen sulfide… they mixed all these chemicals together and zapped them with either an electric spark or ultraviolet light.  Then they studied the orangey-brown gunk that formed as a result.

Initially, this gooey gunk was thought to be a polymer, but as reported in this 1979 paper, Sagan and Khare soon determined that wasn’t what it was.

It is clearly not a polymer—a repetition of the same monomeric unit—and some other term is needed.

Sagan and Khare propose the word “tholin,” which is sort of a pun.  It’s taken from two Greek words that are spelled the same, except for an accent mark that’s shifted from one vowel to another.  One word means “muddy,” the other means “dome” or “vault,” as in the great dome or vault of the sky.  Sagan and Khare go on to mention that they were “tempted by the phrase ‘star-tar.’”

Tholin may be present on some asteroids and comets, and tholin or tholin-like material has been observed on several moons in the outer Solar System, most notably Titan.  We may have even found tholin on Pluto, and several other red-hued dwarf planets could have it too.

So what specifically is this stuff?  Well, I can’t really say.  Tholin is not a specific substance but rather a general category of organic matter.  As planetary scientist Sarah Hörst explains in this article:

The best analogy I have been able to come up with is “salad.”  Salad, like tholin, is a mixture of a number of different compounds and spans a fairly broad range of materials.  Most of us would agree on a case by case basis whether or not something is a salad, but the definition is not at all specific and the material itself depends on the starting materials, temperature, etc.

So there are many different tholins out there.  The tholin we might find inside a comet is probably different from the tholin we find on Pluto, which is different from the tholin we find on Titan.  What all these tholins have in common is that they’re the kind of yucky gunk you’d expect life to make, except life didn’t make it.

However, while life doesn’t make tholin, tholin could, in theory, be used to make life.  Or at least, once life gets started, tholin can serve as a source of food for primitive microorganisms.

Titan has long been the poster child for tholin chemistry, simply because Titan has so much of this stuff.  More than enough, you’d think, for some sort of biological activity to get started—assuming it hasn’t already!  However, with all that tholin lying around, sending astronauts to explore Titan properly may prove to be a sticky proposition.

Next time on Sciency Words A to Z, there’s no way we’ll find life on Venus… right?

Sciency Words A to Z: Rare Earth Hypothesis

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, R is for:

THE RARE EARTH HYPOTHESIS

Once upon a time, it was believed that the Sun, Moon, planets, and all the stars revolved around the Earth.  This was known as the geocentric theory.

Copernicus, Galileo, Kepler, and others set us straight about our planet’s physical location in space.  However, it is still sometimes asserted that Earth is special or unique in other ways.  Such assertions are often referred to in a derogatory sense as “geocentrisms.”

It’s tempting to dismiss the Rare Earth Hypothesis as just another geocentrism.  The idea was first presented in 2000 in a book called Rare Earth: Why Complex Life is Uncommon in the Universe by Peter Ward and Donald Brownlee.  In that book, Ward and Brownlee go through all the conditions they say were necessary for complex life to develop on this planet.  Crucially, they point out all the ways things could have gone wrong, all the ways complex life on Earth could have been prematurely snuffed out.

In other words, we are very, very, very lucky to be here, according to Ward and Brownlee, and the odds of finding another planet that was as lucky as Earth must be astronomically low.  Sure, there might be lots of planets where biology got started. Simple microorganisms may be quite common.  But complex, multicellular life like we have here on Earth—that’s rare.  And intelligent life forms like us are rarer still.  Perhaps intelligent life is so rare that we’re the only ones.

My favorite response to the Rare Earth Hypothesis comes from NASA astronomer Chris McKay.  In All These Worlds Are Yours, McKay’s argument is described as the Rare Titan Hypothsis.

Imagine intelligent life has developed on Titan (such a thing seems unlikely, I know, but there may be something living on Titan).  Titanian scientists look through their telescopes and soon realize that no other world in the Solar System is quite like their own.  Earth, for example, if too hot for life as the Titanians know it, and there’s far too much of that poisonous oxygen in the atmosphere anyway.  Furthermore, water would wreak havoc on what the Titanians would consider a biomolecule.

Perhaps a pair of Titanian scientists then decide to publish a book.  They list all the conditions required for complex life to develop on Titan, point out all the ways Titanian life could have been snuffed out prematurely, and argue that the odds of finding another Titan-like world must be astronomically low.

Personally, I think there’s some validity to the Rare Earth Hypothesis, but McKay’s point is worth bearing in mind.  There could be many different ways for life to develop in our universe.  Earth is but one example.  Planets that are just like Earth may indeed be rare—extremely rare—but there’s no reason to conclude that Earth-like life is the only kind of complex life out there.

Next time on Sciency Words A to Z… oh my gosh, we’ve finally made it to S!  It’s finally time to talk about SETI!

In Memory of Cassini

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

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.

NASA’s Next Flagship Mission

Let’s imagine you’re NASA. You have two big flagship-class missions coming up: one to search for life on Mars (launcing in 2020) and another to search for life on Europa (launching in 2022). These flagship missions are big, expensive projects, so Congress only lets you do one or two per decade.

After 2022, the next flagship mission probably won’t launch until the late 2020’s or early 2030’s, but still… now is the time for you to start thinking about it. So after Mars and Europa, where do you want to go next? Here are a few ideas currently floating around:

  • Orbiting Enceladus: If you want to keep looking for life in the Solar System, Enceladus (a moon of Saturn) is a good pick. It’s got an ocean of liquid water beneath it surface, and thanks to the geysers in the southern hemisphere, Enceladus is rather conveniently spraying samples into space for your orbiter to collect.
  • Splash Down on Titan: If there’s life on Titan (another moon of Saturn), it’ll be very different from life we’re familiar with here on Earth. But the organic chemicals are there in abundance, and it would be interesting to splash down in one of Titan’s lakes of liquid methane. If we built a submersible probe, we could even go see if anything’s swimming around in the methane-y depths.
  • Another Mars Rover: Yes, we have multiple orbiters and rovers exploring Mars already, but some of that equipment is getting pretty old and will need to be replaced soon. If we’re serious about sending humans to Mars, it’s important to keep the current Mars program going so we know what we’re getting ourselves into.
  • Landing on Venus: Given the high temperature and pressure on Venus, this is a mission that won’t last long—a few days tops—but Venus is surprisingly similar to Earth in many ways. Comparing and contrasting the two planets taught us how important Earth’s ozone layer is and just what can happen if a global greenhouse effect get’s out of control. Who knows what else Venus might teach us about our home?
  • Orbiting Uranus: This was high on NASA’s list of priorities at the beginning of the 2010’s, and it’s expected to rank highly again in the 2020’s. We know next to nothing about Uranus or Neptune, the ice giants of our Solar System. Given how many ice giants we’ve discovered orbiting other stars, it would be nice if we could learn more about the ones in our backyard.
  • Orbiting Neptune: Uranus is significantly closer to Earth than Neptune, but there’s an upcoming planetary alignment in the 2030’s that could make Neptune a less expensive, more fuel-efficient choice. As an added bonus, we’d also get to visit Triton, a Pluto-like object that Neptune sort of kidnapped and made into a moon.

If it were up to me, I know which one of these missions I’d pick. But today we’re imagining that you are NASA. Realistically Congress will only agree to pay for one or two of these planetary science missions in the coming decade. So what would be your first and second choices?

Sciency Words: Frost Line

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.

All These Worlds Are Yours: A Book Review

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.

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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?

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.

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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.