Differential elemental ablation of micrometeoroids

Bad Astronomy
The entire universe in blog form
April 25 2009 9:37 AM

Differential elemental ablation of micrometeoroids

Sometimes, I love the jargony science speak!

The title refers to something I find totally cool.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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Space is not empty. This close to a star like the Sun, even after billions of years, space is filled with junk. Tiny bits of rock, ice, and metal are everywhere, the leftover shrapnel from asteroid collisions, or detritus sloughed off of comets. Every day, the Earth plows through many tons of such material, which mostly burns up in our atmosphere.

Stardust reentering Earth’s air
Scientists have now detected for the first time (PDF journal paper here) that as a particle enters our atmosphere, the different materials in it burn off at different times. They found that sodium and potassium burn off first, when temperatures are still low in the meteoroid. As the little chunk of cosmic fluff penetrates deeper into our atmosphere, the air thickens and the meteoroid heats up. When it hits about 1800 K (1500 C or 2800 F) materials like silicon, iron, and magnesium that have a higher vaporization energy -- that, is, need to get hotter to vaporize -- start to burn off. At 2500 K (2200 C or 4000 F) the calcium, titanium, and aluminum finally boil away.

What's amazing to me is that were able to determine this at all. The micrometeoroids we're talking about here are very tiny, maybe 10-11 to 10-4 grams -- in some cases, too small to see. Even at the bigger end that's a tiny little piece of debris. Normally, the chemicals in an object like this would be measured using spectra -- breaking the light up into colors and examining them; different elements emit different colors of light.

But these meteors are too small to create enough light to measure. So scientists got clever: they used radar! Radar reflects off of ionized air, and the amount of ionization -- the amount of free electrons in the air -- changes the strength of that reflection. By carefully measuring just how strongly a meteor reflects radar as it burns up, scientists were able to figure out just when various elements burned off the hot little visitor.

This is the first time measurements like this have been done, and show that this appears to be the main method that micron-sized particles of metal get into the mesosphere and lower thermosphere, the region of the atmosphere around 100 km (60 miles) high. That may not seem terribly important, but this is one more component that makes up the vastly complex tapestry of the Earth's atmosphere. Every time a new puzzle piece falls into place is a time when we understand a little bit more about the ocean of air above our heads. And since we need it to, say, breathe, I'm glad there are folks out there working so hard to piece it all together.

Plus, I'm fascinated by meteors. These little guys come screaming in from outer space, booking along at dozens of kilometers per second relative to us. They slam into our air, heat it up, vaporize, and leave that lovely and ephemeral glowing tail stretched out behind them. There's always a thrill, a shock, when you see one, and it's hard not to gasp and say "Oooooohhhhhhhh!" when one flashes into your field of view.

And, like everything else in the Universe, there are multiple facets to meteors; we can appreciate them for their sudden flare of beauty and surprise... and we can also study them to see what they can teach us. Science and beauty are two sides of the same coin, and don't let anyone ever tell you differently.

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