From the post in Discover's Bad Astronomy Blog (Phil Plait)...
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.
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.
April 25th, 2009 9:37 AM by Phil Plait
From the abstract...
"First observation of micrometeoroid differential ablation in the atmosphere"
D. Janches, Colorado Research Associates, NorthWest Research Associates, Boulder, Colorado, USA, L. P. Dyrud, Center for Remote Sensing, Inc., Fairfax, Virginia, USA, S. L. Broadley, School of Chemistry, University of Leeds, Leeds, UK, J. M. C. Plane, School of Chemistry, University of Leeds, Leeds, UK
Abstract:
Every day, billions of microgram-sized-extraterrestrial particles enter and ablate in the upper layers of the Earth's atmosphere, depositing their mass in the mesosphere and lower thermosphere (MLT). This evaporated meteoric mass is the source of global layers of neutral metal atoms, sporadic E layers of metal ions, and meteoric smoke particles. Because their kinetic energy is insufficient to produce detectable optical emissions, these particles can only be observed using sensitive radars, which detect the plasma (i.e., electrons) either immediately surrounding the meteoroid (head-echo), or left behind along its path (trail-echo). Here we show that observed short-scale temporal features in the radar returned signal from the meteor head-echo are explained by differential ablation of the chemical constituents. These results represent the first observation of this mass-loss process, indicating that this is the main mechanism through which the meteoric mass of micron-sized particles is deposited in the MLT.
Link: Discover Magazine Blog
Link: Abstract
Link: Journal paper [PDF]
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