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LEONID DAILY NEWS: November 9, 2000

Graph of spectrum of a Leonid in red and near-infrared

Figure left: the spectrum of a Leonid meteor in the red and near-infrared. Intensity is plotted versus wavelength.


The bright light of a meteor originates in a warm wake, not in the meteor's hot head as was thought before, according to a new spectroscopic study published in an upcoming issue of Earth, Moon and Planets.

Spectroscopy is a powerful technique to identify chemical processes in the meteor path and learn about physical conditions such as temperature. Spectroscopy turns white light into its array of colors and measures the intensity of specific colors that are emitted by atoms and molecules. Those colors are expressed in terms of the wavelength of the light.

During the 1998 Leonid MAC, the emission of molecules created in the meteor plasma could be measured for the first time in enough detail to derive the temperature and internal excitation of the air plasma. Peter Jenniskens and Mike Wilson used a cooled un-intensified CCD camera with an objective grating to measure the meteor's visible light intensity. The result is shown as a blue line in the figure to the right. The wavelength scale corresponds to colors ranging from a red-orange left to a deep red on the right.

The spectrum betrays the presence of oxygen atoms and a broad band caused by the molecule nitrogen (N2) shortly after it is created by the recombination of nitrogen atoms (N).

The spectrum is so detailed that the measurement can be compared directly with models of air plasmas. Christophe Laux and Denis Packan of Stanford University calculated the expected emission spectra for two very similar temperatures. A temperature of 4,300 Kelvin (about 4,000 degrees Celsius), seems to match the data best (red line). Some bumps near the point of the arrow are actually a residual signature of the recombination process, not included in the calculations. A slightly higher temperature of 4,670 K (black line) matches one of the oxygen lines better, a surprisingly small difference for what should be highly non-equilibrium conditions.

Meteor model by Boyd

Figure left shows the new meteor model by Iain Boyd. Figure right is a video animation showing the growth of the wake.

4,300 K is a very low temperature, given that air molecules collide with the meteoroid at an astonishing 160,000 miles per hour. To understand where this rather balmy temperature originates, Iain Boyd of the University of Michigan calculated this model of the meteor. For the first time, he used Monte-Carlo type calculations, taking into account that the meteoroids are very small and shocks are not typically formed.

Meteor movie

The result paints a fascinating picture of a meteor. The light we are seeing is from a warm wake, not from the meteoroid head. The cause: air is warmed by interaction with an ablation vapor cloud in front of the meteoroid. Without evaporation of meteoric matter (situation "a"), there is no wake and all light comes from the head of the meteor. When ablation is included in the calculations (situation "b"), a wake results with just about the right temperature to explain the observed spectrum.

The resulting picture of a meteor opens new pathways for organic molecules to survive the plunge in the atmosphere. Moreover, the temperature of the wake turns out to be just right for breaking up the C-O band in carbon monoxide, an important step in creating potentially interesting atmospheric chemistry in Earth's early atmosphere (Full paper -PDF).

Previous news items:
Nov. 09 - New meteor picture
Nov. 08 - Spin city
Nov. 07 - Meteors affect atmospheric chemistry
Nov. 06 - Listen to this!
Nov. 04 - Fear of heights?
Nov. 03 - The pale (infra-red) dot
Nov. 02 - Twin showers
Nov. 01 - Leonids approaching Earth
Oct. 31 - Prospects for Moon Impact Studies
Oct. 30 - Comet dust crumbled less fine

These and other results of Leonid storm research will appear in a special issue of the peer-reviewed journal "Earth, Moon and Planets", published by Kluwer Academic Publishers, the Netherlands.

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