Tuesday, March 10, 2009

Planetary Astronomy, Clouds, and the Space Station

Jen writes:
I have a couple of questions, driven by my 5 year old's insatiable curiosity. First, what exactly is a planetary astronomer? We are not sure which questions fall within one's domain and which do not?
Well, planetary astronomy is usually described as just the study of solar system bodies. The specific fields of study fall under a few categories:

  • Planetary atmospheres - winds, storms, clouds, and the like (this is what I personally research)
  • Planetary geology - surface features, volcanism, earthquakes, tectonics, tidal processes, etc.
  • Planetary interiors - core processes, mantle processes, convection - anything under the crust.
  • Planetary magnetism - interaction of a planet's magnetic field with the space environment
  • Small bodies - Comets, meteors, and such
  • Rings - gravitational interactions of planetary rings with moons, gravitational wakes, etc.
I'm probably forgetting a few aspects here, so forgive me if my list isn't entirely exhaustive...but hopefully you get the idea.

That said, I'm happy to answer any astronomy questions I can get my hands on. The previous post was all about the much, much larger scale of galaxies and galaxy clusters. Thankfully, I did some galactic cluster research earlier in my grad school career, and have taken classes in most aspects of astronomy.

Jen continues:
Second, my son has not been happy with any of my explanations of how clouds are formed (this is one reason for the first question, since I was pretty sure it doesn't qualify as planetary astronomy). I have told him, so far, that they are made when water evaporates, that the water freezes, etc. but I think the concept of freezing bits of water floating around in the sky just sounds like mom most have something wrong :).
Ah, this question is right up my alley. You're actually mostly correct here.

I think the difficult part to conceptualize is the idea of "water vapor" always in the air. We're not talking about rain or mist particles here, but water as an actual gas. Simply by virtue of having liquid water on our surface, there is vapor pressure - some of that liquid will evaporate and become a gas floating in the air.

Depending on which climate you live in, it's usually ubiquitous (though here in the desert we have fairly little). It's mostly invisible, though noticeable as a white haze when you look at really distant objects like mountains or skyscrapers on the horizon. It's definitely noticeable on a muggy day when you step outside and can just "feel" the humidity, though...you'll notice on those days that distant objects appear even hazier.

It's also painfully apparent when collecting spectra from a telescope. A major difficulty is removing the signal of Earth's water vapor...particularly if you're looking for water vapor on other planets. In fact, whole sections of infrared wavelengths are simply unusable because they're so saturated with the signature of water vapor.

I think the best way to conceive of water vapor is if you have an ice-cold beverage on a hot, humid day. Within seconds, the outside of the cold glass or can becomes wet. The water wasn't just spontaneously created, and didn't leak from inside the glass...though technically it did "materialize from thin air".

The issue it this: just like hot tea is better at dissolving sugar than iced tea, so a warm atmosphere can hold more water vapor than a cold atmosphere. This is also why your weather forecast will talk about "relative humidity" - 50% humidity at 90° F has quite a bit more water vapor dissolved in it than 50% humidity at 40° F. The percentage symbol in there is just the percent of how much total water vapor the atmosphere at that temperature *could* hold. We refer to a humidity of 100% as being saturated. Anything above this, and the atmosphere won't be able to easily hold on to all that water vapor.

Now, imagine a parcel of warm atmosphere at the surface, holding quite a bit of this invisible water vapor. If that atmosphere begins to rise - usually due to convection - it will encounter a colder environment as it increases in altitude. As it's also at a lower pressure higher up, the parcel will begin to expand and cool down in the process - so what happens to the water vapor?

Well, let's say the parcel started out at 90% relative humidity. As it cools, that colder air reaches a point of going beyond its 100% saturation value. Even though the total amount of water vapor doesn't change, the amount of water vapor the parcel could potentially hold is decreased as it cools, until it's holding more than its limit.

That "super-saturated" water vapor has to go somewhere. Usually water will start condensing out onto the surface of tiny particles suspended in the air, known as nucleation sites. There's lots of natural sources of nucleation sites - suspended dust, for example - though smog particles work equally well, if not better.

So, as this water condenses out, it goes from a gas phase to either liquid water or ice...it all depends on what the ambient temperature is. The low fluffy cumulus clouds we see are usually liquid water, since temperatures aren't *that* cold as they form fairly low. The high, wispy cirrus clouds are formed in a much higher, colder environment, so they're usually made of ice.

The same thing happens to your ice-cold beverage on a hot muggy day. The air right around the glass is a bit colder than the surrounding air, so water vapor begins condensing out onto the outside surface of the glass.

It's also extremely similar to the process for making rock candy. Loads of sugar are dissolved into hot water, until it reaches its saturation point. As one then begins to cools the water, it becomes super-saturated and the sugar tries to condense out into crystals onto whatever nucleation site it can find...usually a stick is placed in the solution to provide a nucleation point, resulting in a tasty, diabetic-coma-inducing snack.

Now, there's one other interesting bit here...I mentioned in a previous post about water on the moon that 32° F ice has much less energy than 32° F liquid water. Similarly, liquid water at a certain temperature has much less energy than water vapor at the same temperature. As the water vapor condenses out into liquid water, then, that extra energy has to go somewhere - usually into heating up the surrounding parcel of air. As a result, the parcel is even more buoyant and will continue rising, possibly condensing out even more water vapor depending on ambient conditions aloft. So, there's a weird feedback cycle which causes a parcel to lose most of its water vapor as it continues to ascend.

Jen also asks:
His last question is: What is the space station for? Is it like a train station in that rockets stop there and then travel onwards? When do space ships visit it, or not visit it?
Well, that's actually a question a lot of us have been wondering.

The original concept was to provide exactly this sort of train station, but so far that hasn't really panned out. At this point, the only space ships visiting it (the Space Shuttle and Russian Soyuz capsules) have been to resupply, swap crew, and boost its slowly degrading orbit.

I think part of it was established with the idea that manned space exploration never made such leaps and bounds as when we were in a space race with the Russians. Now that the cold war has ended, manned space exploration has been proceeding at a lethargic pace. Some of the reasoning may have been that we could jump start the program again by introducing a spirit of cooperation this time instead of competition. The whole point was to have multiple nations contributing to its construction to foster a renewed level of innovation.

Unfortunately, this hasn't panned out terribly well. Several of the International Space Station (ISS) mission modules have not been up to spec - I believe the Russian-built Zvezda module had terrible noise problems. (Remember, in space there's nowhere for sound to go, so once you start ringing a bell, it keeps ringing.) To some extent there's a feeling the ISS has become a white elephant, funneling manned exploration resources that could be used for something more constructive.

That said, it's still a bit of a thrill to watch it pass overhead. If you want to see it for yourself, check out the Heavens-Above website. In the configuration section, enter your location using the map or database feature, and submit. You'll be given fly-over times for the ISS as well as star map so you know where to look for it at which times. In fact, it will do this for all visible satellites for any given night - the ISS just happens to be the brightest. Cool stuff.


  1. Thanks for all your answers ... I have one follow up question: Why don't the water and ice droplets fall (or maybe when don't they fall and when do they?)

  2. Please expand on sound in space. A ringing bell that keeps ringing blows my mind.

  3. @jmankoff:

    Actually, water and ice droplets do fall - it's what we call "rain and snow". :)

    The issue here is that the particle has to grow large enough that gravitational forces on it are stronger than inertial surface forces keeping it suspended in air. It's a bit like how a single speck of dust can float in a dust mote, while a lump of dirt will just fall to the ground.

    This again has to do with the surface-area-to-volume ratio I mentioned in my explanation of the Poynting-Robertson effect:

    "Terminal velocity" is the maximum velocity a falling body can reach before air resistance keeps it falling at constant speed. It's directly proportional to the surface-area-to-volume ratio, and for very small, microscopic droplets (or dust specks) that terminal velocity is so small that the droplet essentially just moves with the surrounding parcel of air holding it. It's only when it grows large enough - and thus when the surface-area-to-volume ratio decreases enough - that it can overcome that atmospheric drag force and fall to Earth.


    Sound is just compression waves transmitted though a medium, most commonly air. The whole reason you hear, say, someone ringing a bell is because a resonating bell also resonates the air around it...those waves travel through the air, and start your eardrum resonating, which sends a signal to your brain. At the same time, the bell eventually stops ringing because the air around it is stealing energy of the bell's resonating motion.

    In space, there is no air, so there's no sound to transmit. If someone bangs against the side of the space shuttle, the shuttle begins resonating...but there's no surrounding medium to steal energy from it, so it just continues resonating.

    Eventually, though it's damped by internal inhomogeneities of the resonating object. If you imagine you fill a bell with foam or some other damping material, it would be pretty hard to ring it.

    In practice, they introduce these internal inhomogeneities into spacecraft to do just that. There's a lot of sound-absorbing foam installed inside spacecraft to take on that energy-stealing role - and is exactly what they had to do for the Zvezda module when they realized it didn't have enough.

    - Planetary Astronomer "In Space, No One Can Hear You Scream" Mike.

  4. Is this alo visible on google sky?

  5. For our school sciece fair we are focusing on the question why do the stars twinkle? I was wondreing if you could maybe share some info with us that would help with our project!