New Horizons workshop, day 1: Chemistry & climate on Pluto & other cold places
Posted by Emily Lakdawalla
30-08-2011 23:27 CDT
Today and tomorrow I'm attending the New Horizons Workshop on Icy Surface Processes. New Horizons is on its way to Pluto, and still has about three years until it starts gathering science data there; but now is the time when the science team has to complete their plan of observations for the short few months of the flyby. To help them predict what might be happening on Pluto and Charon, so that they can take the right kinds of observations of the right things at the right time, they've invited lots of people who are not on the science team to present their research on topics relevant to the surfaces of Pluto and Charon.
I am, of course, neither a New Horizons team member nor a scientist, so I'm pleased they let me come. This created some amount of nervousness that I'd be Tweeting every stray remark. I had to reassure people that I can't type that much, while not mentioning that really my readers probably aren't interested in every stray remark! I am Tweeting some stuff, though, so check that out.
The first day was all about the composition of the surface and atmosphere of Pluto, Charon, Triton, and other distant places. Triton is widely regarded as being a good proxy for Pluto because it's similar in size, composition, and solar system location, though it doesn't get quite as far from the Sun as Pluto does.
The day began with Alan Stern calling me out for my T-shirt, which I had worn just to make trouble. He talked about something that Hal Weaver also discussed at SBAG, namely the implications of the discovery of a fourth Pluto moon for the planning of New Horizons' close approach. I think I will have to devote a separate blog entry to the details of this issue, but the upshot is that there is some concern of gravel-sized particles being in New Horizons' path -- something that would end the mission before it had a chance to return any data from the encounter -- so they are planning a second encounter trajectory to have in their back pocket while they study the problem further. This second encounter trajectory is called the "Safe Haven Bailout Trajectory," or (in the inevitable acronym) SHBT, pronounced "Shabbat." This got a chuckle from the audience.
Alan gave a run-down of the instruments' capabilities (highly redundant with each other in case of any one part failing) and the current health of the spacecraft (excellent, with no backup systems currently in use). Then he gave an overview of the encounter geometry, which he compared to Voyager 2's Uranus encounter. Basically, the Pluto system is tilted such that the orbits of the moons look like a bull's eye, which is advantageous for some science but which presents problems near close approach, because all the close approaches to all the bodies happen at nearly the same moment, which makes for some tough choices in prioritizing observations. Alan said they've managed to plan in very good observations of Pluto, Charon, Nix, and Hydra. The new moon -- and any other moons they may find between now and the flyby -- may be a bit harder. If more moons are found, they'll probably just have to pick one to perform high-res observations of, and leave the others forever as pinpoints of light.
There was a lot of chemistry in the rest of the talks today. I find it easier to follow chemistry than physics but after a couple of hours I was definitely feeling brain fatigue. But I soldiered on with note-taking; I'll try to hit some high points.
Jeff Kargel talked about what we've recently learned from other outer solar system bodies that might be relevant to Pluto and Charon. He opened with the point that materials present on these bodies in only trace amounts can be very important for understanding the surface. He drew an analogy to Earth: water makes up only 0.1% of Earth's mass but "we get oceans." On Pluto and Charon we've detected ices of methane, ethane, carbon monoxide, and nitrogen. Are other species not present, he asked? Or are they sequestered below the surface? or covered over by kilometers of other ices?
One chemical that outer planets scientists really like is ammonia, because its presence in an icy crust depresses the melting temperature of water, making liquid water possible at much lower temperatures, and facilitating ice volcanism. There is certainly some ammonia in Enceladus. But Jeff said that wherever you get ammonia with carbon dioxide they combine chemically to form ammonium carbonate or ammonium bicarbonate salts or even urea, and you run out of ammonia pretty quickly. "Ammonia may be an endangered species in the outer solar system." At this point I spaced out a bit but I snapped back to attention when he claimed that active nitrogen cryovolcanism is possible on Pluto, comparable to Earth in volcanic mass transfer per unit area per unit time. Eruptions, if they happen, would be "mildly explosive." Also, Jeff wants us to know he is veryexcited about New Horizons! I think everyone in the room agrees but it's funny how rarely people beyond mission leadership actually say stuff like that out loud.
Reggie Hudson talked about radiolysis on Pluto. This is where energetic ions -- protons or alpha particles or other ions of various compositions -- smash into the icy crust. An awful lot of energy is delivered to Pluto this way. He offered a primer on radiation chemistry to the audience: "You send in one ion (say a proton) and ion creates a track or train of thousands of ionizations and excitations. Some of the [excitations] cause extra chains of events. So one incident radioactive entity can give rise to thousands of events."
Then Hudson went on to show results of experiments where they bombarded various pure ices and combinations of ices with protons to see what all this radiation chemistry produces. I found this part of the talk interesting because of all the crazy chemical species he mentioned, many of which I'd never heard of. Carbon suboxide? Diazomethane? Diazomethane has the chemical formula CH2N2. It's a CH2 group (a carbon with single bonds to two hydrogens) in a chain with two nitrogen atoms. Chemists love this stuff, he said, because it's a great place to store a CH2 group, which can react with "pretty much anything else that contains a carbon-hydrogen bond" to add a CH2 to them to make carbon chains longer, producing "big long organics."
In the lab, he said, they predict that they can form diazomethane (CH2N2), hydrogen cyanide (HCN), hydrogen isocyanide (HNC), isocyanic acid (HNCO), hydrogen peroxide (H2O2), and hydrazine (N2H4). He pointed to each one in turn and rattled off: "Toxic, explosive and toxic, can't buy it, reacts, explosive, rocket fuel." Since I know an organic chemist (Mike Malaska) I sent an email to him asking him to elaborate on the awfulness of these species and he happily explained:
Some are doubly nasty
Diazomethane is explosive AND toxic (when working with this in a chemistry lab, you need to be careful not to use glassware with any tiny nicks or cracks in it or containing ground glass, the surface imperfections can cause crystallization and then spontaneous detonation.)
Hydrogen cyanide - toxic AND explosive (needs to be kept acidic or boom!)
Hydrogen isocyanide - not a happy gas, wants to convert to hydrogen cyanide (see above), but if kept really cold (i.e. interstellar medium) it won't convert to its buddy hydrogen cyanide. (can't buy it)
Isocyanic acid (H-C=N=O) toxic and reactive in high concentrations.
Hydrogen peroxide - explosive in high concentrations (but can be used to make brunettes into blondes in lower concentrations) (also can be used as a rocket fuel component)
Hydrazine - explosive (rocket fuel)
Not mentioned but also maybe possible is HCNO, fulminic acid, which is HIGHLY EXPLOSIVE and toxic. It is used to make detonators.
The punch line is that it's a bit difficult to do laboratory experiments involving this stuff. So it'd be nice if astronomers could tell the experimenters whether or not any of this stuff actually exists on surfaces in the outer solar system.
Robert Hodyss gave a similar talk but on photolysis instead of radiolysis, mentioning several species that would "make good targets in visible and near-ultraviolet spectroscopy" for New Horizons' instruments, namely hydrogen cyanide (HCN) and other things I don't even know the names of like CN and CNN and C2-.
Dale Cruikshank introduced his talk by saying, "The discussion so far has been small molecules. I'm fond of small molecules because you can find them." (This earned a chuckle from the audience.) "But I'm also interested in macromolecular chemistry." He explained that while small molecules can be detected uniquely using spectroscopy -- this absorption feature for water, that one for carbon monoxide, and so on -- with larger chemicals, it's a bit more ambiguous; you can see spectral signs of functional groups that might be found in many different chemicals (like, for example, carbon triple bonded to nitrogen) rather than identifiable specific molecules. He talked a lot about tholins, the gunky materials made in Carl Sagan and Bhishun Khare's famous experiments. He mentioned that they were "very potent coloring agents" and showed that it's possible to produce models of outer solar system spectra using tholins for color that model observed spectra very well. But, he said, "You can model almost anything you want with a modest spectral library." This earned another guffaw from the audience.
At this point I mentioned in my notes that I've heard about four different pronunciations for Charon. I asked about this at dinner this evening and was told that the classically correct pronunciation is like "Care-on" but that those "in the know" use the "Sharon" pronunciation, which is what Charon's discoverer preferred. He preferred it because Sharon was his wife's name. But it's not really kosher to name solar system bodies (except teeny asteroids) after living people. People seem to be evenly split on the pronunciation, with minor variations.
John Spencer gave a talk on thermal segregation, a topic I've covered before in the blog to explain Iapetus' dark-bright dichotomy. Today, he talked about the process more generally across the outer solar system. Thermal segregation happens when you have an atmosphere that is in "vapor pressure equilibrium." What that means: no matter how cold a substance is, there is always some amount of the substance in gaseous form above the solid stuff. That happens because the atoms in it don't all have the same speed (temperature), and there will always be a few that move fast enough to escape their bond to the solid material and float upward, forming a vapor. Vapor pressure gets higher when things are warmer. It's lower if the material is particularly good at bonding to itself (the big example here is water, with its polar molecules that form strong hydrogen bonding). There are actually quite a few objects in the solar system with atmospheres that are in vapor pressure equilibrium, where atoms are leaving the surface and sticking back to the surface at the same rate overall. Earth isn't one of them, because its gaseous nitrogen atmosphere is not in equilibrium with a solid nitrogen ice. Things that are in vapor pressure equilibrium include Mars, Io, Ganymede, and Pluto. Pluto is much colder than Mars, but Pluto's ices are much more volatile than Mars' dominant carbon dioxide.
Okay. Now here's where it gets a bit harder, but this stuff showed up again later in the day, so I'll try to explain it. If the mean free path of a molecule in the atmosphere is greater than the scale height of the atmosphere -- put another way, if a molecule is not likely to strike another molecule as it moves through the atmosphere -- then what you really have is an exosphere, like on Mercury, where molecules move on ballistic trajectories. But if the molecule can't move through the atmosphere without hitting another molecule, then you have a "collisional" atmosphere whose particles move together -- that is, they make wind. If the winds blow fast enough to move the particles around globally before they refreeze, then the temperatures of the atmosphere and surface frosts are globally uniform. Mars and Pluto both lie in that regime. He spent a few minutes on an interesting aside about Io, a special case, which has a collisional atmosphere but not the global circulation needed to make it isothermal. But getting back to the heart of the matter, he showed that thermal segregation really is an important process not just on the best known cases of Iapetus and Callisto but also on Europa and Ganymede (but not Dione). Major hallmarks of thermal segregation are bright pole-facing slopes and topographic control, where bright stuff tends to be at higher elevations and dark stuff at low elevations. "We may well see similar things on Pluto," he predicted. Pluto certainly has a wide variety of albedo across its surface.
At this point I noted how the discussion after papers involved lots of different people in the room; some would refer to papers from the 1960s or 80s, some to unpublished work that others might not be familiar with. These are exactly the sorts of interchanges that the workshop format was intended to facilitate, so that's good.
Tim Titus gave a talk on bright and dark fans in Mars' "cryptic region." Yes, Mars. He made a pretty good case, actually, for Mars' seasonal carbon dioxide caps as analogs for the seasonal ice formation and sublimation processes that are likely to be important at Pluto (not to mention Triton). Sublimation of carbon dioxide in the spring drives plumes of carbon dioxide with entrained dirt that spout above the frosty surface; at least that's the theory.
There were only two talks after this but my brain felt really, really full. The penultimate talk was John Stansberry on global circulation models for Pluto and Triton. The interesting fact that got through my stupor to reach my notes was that Coriolis forces are likely to be important in Pluto's atmosphere. I think one of the major points I came away from the day with was the idea that Pluto's atmosphere really is a complicated thing worthy of the name, not at all like the exospheres of small icy moons and Mercury. Which is not to say that exospheres aren't interesting, because as has been pointed out for Mercury they provide an amazing opportunity for an orbiting spacecraft to directly sample the composition of the planet's surface. But Pluto's isn't an exosphere, it's a real atmosphere with wind and Coriolis effects and a tropopause and whatnot. Triton is the same. Pluto may not have this "real" an atmosphere for all of its year. Triton always does. Quaoar might also have it all year. Makemake is like Pluto. Eris may be a unique case that is sometimes Triton-like, sometimes Io-like, and sometimes exospheric, depending on where it is in its orbit.
The last talk was by Leslie Young, on modeling of the atmosphere's interaction with frosts on the surface over the course of the seasons. Models can sometimes be deadly but she had great animations to show. Pluto's seasons are pretty dramatic because of its highly elliptical orbit, which causes solar insolation to vary by a factor of nearly 3. On top of that, its solstices are nearly coincident with its aphelion and perihelion, making its seasons more extreme than they would otherwise be. In one of her models, she showed how bright seasonal frost caps form on the winter pole and continue to exist well after equinox before suddenly disappearing completely.
There was a poster session but, like I said, my brain was full, so rather than attempt much with the posters I just sat down to write, and had a good Thai meal, and some Freddy's frozen custard. Now it's 9:15 at night -- and there's another full day of this tomorrow! Tomorrow should be easier for me, though, because it's geology, not composition and chemistry. That's more my thing.