The record of HCN emission at different times between 5 and 11 hours
Hawaiian Standard Time.
MYSTERIOUS DISAPPEARANCE OF HCN
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.
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.
FUTURE LEONID MAC MISSIONS
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).