Hoppin’ water molecules

chladny-2Before you get any further in this post, follow this link to Steve Mould’s demonstration of  Chladni figures.  (I’ll wait here.)  It’s a neat demo and the effect plays into some recent discoveries in planetary science.

Steve’s couscous grains dance to the vibrations of the iron plate they’re sitting on.  The patterns happen because he controls where those vibrations happen.  Or more importantly, don’t happen (see his fingers pinching the plate?).

The study of vibration goes back to Pythagoras, the ancient Greek geek who determined that a plucked stretched string invariably exhibits a whole number of peaks and nodes.  (A node is a point on the string that doesn’t move, like those dots on the chart).  I’m so tempted to yammer about the relationship between nodes and quantum mechanics, but I’ve already posted on that topic.sines

The important point for this post is that Steve’s demonstration shows individual particles, each moving under the influence of random impacts, nonetheless winding up at a common destination.  They’re repeatedly kicked away from points where the iron plate is fluctuating strongly.  If a particle suddenly finds itself on a non-fluctuating nodal point (or nodal line, which is just a collection of nodal points), it stays there because why not?

The basic principle applies to numerous phenomena in Physics, Chemistry and other Sciences.  The particles in Chladni’s experiment were grains of sand.  Steve used coucous grains, which work better in video.  But they could also be molecules.  On the Moon.

Back in the 2000s there was intense debate in the lunar astronomy community.  One argument went, “The Solar Wind teems with hydrogen ions (H+).  The Moon’s surface rocks are mostly silicon oxides.  Those H+ ions will yank oxygen O2- ions off exposed rocks to make H2O molecules.  There has to be water on the Moon!”

The other side of the argument (in real Science there’s always at least one other side) went, “Maybe so, but Solar radiation also contains high-energy electrons and photons that’ll rip those molecules apart.  Water can’t survive up there!”

If/when we plant a Moon colony, the colonists will need water.  Either it gets shipped up from Earth — EXPENSIVE — or we find and mine water up there.  NASA did the only thing that could be done — they sent up a spacecraft for a close look.   When the Lunar Reconnaissance Orbiter (the LRO) launched in 2009 it carried half-a-dozen instruments.  One of them was the Lyman Alpha Mapping Project (LAMP) camera.

LAMP was the embodiment of a sly trick.  Buried in starlight’s ultraviolet spectrum are photons (a.k.a. Lyman-α  light) with a wavelength of 121.6 nanometers.  They’re generated by excited hydrogen atoms and they’re (mostly) absorbed by hydrogen atoms but reflected by rock that doesn’t contain hydrogen.

LAMP’s camera was designed to be sensitive to just those Lyman-α photons.  As LRO circled the Moon, the LAMP camera recorded what fraction of those special photons was bouncing off the Moon.  By subtraction, it told us  what fraction was being absorbed by surface hydrogen.

LAMP did find water.  The fun facts are its form and location — it was frost, buried in “fluffy soils” in the walls of craters.
water-moonThis photo, part of the LAMP exhibit at the Denver Museum of Nature and Science, shows why.  It’s a model of a cratered Moon lit by sunlight.

An H2O molecule may develop anywhere on the Moon’s surface.  Then it experiences life’s usual slings and arrows (well, electrons and photons) that might blast it apart or might merely give it a kinetic kick to somewhere else.  That process continues until the molecule or a descendant drops into a nice shady crater.

The best craters would be the ones in the polar regions, where sunlight arrives at a low angle and the crater walls are permanently shadowed like the one at the top in the model.  That’s exactly were LAMP found the most dark spots.  HAH —  Chladni in space!

But there’s more.  In 2012, NASA’s MESSENGER spacecraft produced evidence for water on Mercury, the hottest planet in the Solar System.  Once again, those molecules were hiding in polar craters along with a few other surprising molecular species.  That knocked my socks off when I read the scientific report.

~~ Rich Olcott

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A Brief History of Atmospheres

miller-ureyLong ago in a far-away career, I taught a short-course about then-current theories on the origin of life.  The lab portion of the course centered on the 1952 Miller-Urey experiment the first demonstration that amino acids could be produced abiotically.

Imagine my surprise when I learned that Miller’s original lab apparatus is on exhibit at the Denver Museum of Nature and Science, where I volunteer now.

The diagram’s notes describe the basic experiment.  Load up the system with whatever gases you think might have been in the primeval atmosphere.  Start cooling water running through the condenser (double-wall tubing below the upper sphere) and gently heat the water sample you’ve put in the bottom sphere.  Vapors travel up the tube into the top sphere where there’s a spark arcing between two electrodes (the black lines at 45º).  Water vapor passing by sweeps any gas-spark reaction products back down to the bottom sphere.

Let the whole thing stew for a while (Miller ran his for a week, we let ours go for two), then draw off and analyze a sample of the solution in the bottom sphere.  In Stanley’s day his analytical techniques found 5 amino acids.  In 1971 (I think) we found (I think) 8 or 9.  More recent work-ups of Miller’s sealed original samples found 25, including all 20 considered essential to life.  So yeah, if you supply enough energy to a methane-ammonia-water system (Miller added hydrogen to that; we didn’t, for safety reasons) you can make the building blocks for proteins.

The experiment has been repeated probably thousands of times by different researchers in the last half-century.  Some replications were duplicates of Miller’s, some started with recipes derived from other theories about what Earth’s early atmosphere looked like.

And there’s the problem.  In Miller’s day we thought that Earth’s atmosphere was basically comet-tail concentrate.  That’d be mostly water vapor along with a couple of volatiles like methane and ammonia.  Later on we realized that much of our atmosphere is volcano belch — a hodgepodge of carbon dioxide, carbon monoxide, methane, hydrogen, sulfur gases, nitrogen, argon, helium, various acids, multiple kinds of rock dust….  Some of that is left-overs from Earth’s initial stages; some has been generated by subsequent geological processes like serpentinization, which can generate both methane and hydrogen.

If you’re running a Miller experiment you’re free to load the apparatus with whatever mixture you think other scientists might think is reasonable for an Earth on the verge of Biology.  No oxygen, though — the chemistry of ancient rocks rules out significant atmospheric O2 before about 3½ billion years ago.

Now the researchers are playing variations on the theme, asking whether conditions on (or within) Titan could also generate complex compounds that given a billion years could self-organize into anything we’d recognize as life.  So what did Titan’s atmosphere look like a few billion years ago?

That’s a toughie, because we don’t have on-the-ground (or out-of-the-ground) data like we have for Earth.  We’ll have to make do with theory, which starts with this chart.

solar_system_escape_velocity_vs_surface_temperature-svg
Molecular escape velocities

At any given temperature you can calculate the average energy per gas molecule (any kind of gas).  Combine that with the known mass of a specific kind of molecule and you can compute its speed.

On any given world you can calculate the minimum speed (the escape velocity) that an object (rocket, rock or gas molecule) needs to have in order to overcome the world’s gravitational pull.

The chart combines both calculations for some important molecules for worlds in the Solar System that have atmospheres.  For instance, Earth’s average temperature (give or take a few dozen degrees) is  300ºK=27ºC≈80ºF.  From the chart, hydrogen and helium should be able to (and do) leave our atmosphere quickly.  However, Earth’s gravity is sufficient to hold onto its original dowry of the heavier species.  By contrast, the four massive planets would have to warm up by hundreds of degrees before they lose even the light gases.

Sure enough, Titan’s atmosphere is mostly nitrogen.  The astronomers measure its methane and hydrogen content in parts per thousand but those concentrations aren’t the same going from top to bottom of Titan’s atmosphere.  Therein lies an intriguing tale, but it’ll have to wait for the next post.

~~ Rich Olcott