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LEONID DAILY NEWS: November 15, 2000

Variation of HCN radio emission during the day

The record of HCN emission at different times between 5 and 11 hours Hawaiian Standard Time.


Hydrogen cyanide, HCN is a molecule that is often associated with death, rather than with life, but it is one of those compounds that lead to interesting chemistry for life's precursor molecules when in contact with water. In a recent issue of Earth, Moon and Planets, Didier Despois and colleagues of the Observatory of Bordeaux, France, report on the mysterious disappearance of HCN in the upper atmosphere in the night after the Leonid shower.

HCN is a natural occuring compound in the Earth's lower and middle atmosphere below 50 km altitude and thought to result from living (and dying) organisms. It is also found in rather high abundance in the mesopause, at the 70-90 km altitude where meteoroids ablate their matter. It is possible that this HCN has a similar low altitude source. An extraterrestrial source of HCN from meteors is possible as well, but remains to be proven.

In an effort to measure the HCN abundance in the upper atmosphere to see if meteoroids contribute to the high abundance of this molecule at 70-90 km altitude, Didier Despois and co-workers measured the atmospheric abundance from radio measurements at several astronomical observatories in the world during the 1998 and 1999 Leonid showers. Unfortunately, bad weather spoiled all but a few nights during this period.

Graph of HCN abundance variation with altitude

A compilation of all measurements and models of the vertical distribution of the HCN molecule in the atmosphere.

In the few clear nights prior to the shower, the HCN band (observed in frequency switched mode) would be a composite of a broad feature due to lower atmospheric HCN molecules at high pressure, on top of which was a narrow peak from low pressure high altitude HCN above 70 km.

The night of the peak was clouded at all observing sites. The night after the peak (the second "1866" peak was over Hawaii), the observers noticed something very unusual. The high altitude HCN signature had all but disappeared. Only at sunrise did the feature return.

Instead of an increase of HCN, a decrease was observed. This suggests that the meteors had a significant effect on the upper atmospheric chemistry, changing the delicate chemical balance responsible for the HCN abundance.


This observation needs further confirmation during future Leonid storms. Submillimeter spectroscopy is uniquely suited for airborne application, whereby the technique can be combined with airglow measurements at optical and near-IR wavelengths. Many other molecules than HCN can potentially be measured. Preparations are being made to incorporate this technique in future Leonid MAC missions for November 2001 and 2002 (Full paper - PDF).

Previous news items:
Nov. 15 - HCN disappears mysteriously
Nov. 14 - Meteor shower from space
Nov. 13 - Organic fingerprint
Nov. 12 - Train airglow chemistry
Nov. 11 - Hard bits and persisting glows
Nov. 10 - Meteoroid debris detected
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
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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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