This band is difficult to observe from the ground because UV radiation is absorbed by ozone in the Earth’s atmosphere (a good thing, since UV light is harmful).
It has been even more difficult in the case of 67P/Churyumov-Gerasimenko as the comet could only be observed low in the sky in twilight.
This means that the line of sight to the comet passes through even more of the atmosphere – in astronomical terms, the observations were taken at high airmass – effectively doubling the
absorption of UV light. These challenges meant that detecting OH was not possible elsewhere, but was theoretically possible with the WHT and ISIS.
Thus the WHT was used to observe 67P/Churyumov-Gerasimenko for an international team led by Colin Snodgrass (Open University) and Alan Fitzsimmons (Queen's University Belfast) in the early morning twilight of the 20 August and again on 2 September.
Even though only 3% of the UV light from the OH molecules made it through the atmosphere, there was enough water being
released by the comet that ISIS made a clear detection of this gas. The spectrum above has been corrected for the absorption
caused by the atmosphere and instrumental effects, to show the true amounts of light reaching the Earth.
This can be compared with direct measurements from Rosetta: The MIRO instrument onboard Rosetta measured the comet to be
releasing up to 300 kg of water per second around perihelion, whereas the Earth-based observations gave 90 kg per second.
Given the variability in the amount of gas being released from the comet and uncertainties in the physical models applied to the data,
these results can be considered consistent. The numbers will probably move closer as the scientists refine
their models by combining both sets of data.
And what about that CN (cyanide) gas? Through a quirk of quantum physics, it is much better at scattering light from the Sun than OH.
So even though that emission was almost as bright as the OH, it was due to only 0.13 kg of HCN being released per second from the
nucleus, before the HCN molecules break apart (dissociated) to form the CN we see from Earth. Along with all the other gases in the
spectrum such as C2, C3 and CH, the ground based data give a good view of some of the constituents of the coma of 67P/Churyumov-Gerasimenko at distances beyond the unique vantage point of Rosetta.
More information: ESA Rosetta blog