A comet’s water ice cycle follows a daily routine, according to data collected by European space probe Rosetta. At the time, the comet was about 500 million km (300 million miles) from the Sun and the neck was one of the most active areas.As the comet spins, taking just over 12 hours to turn around once completely, the various regions receive different lighting.”We saw the tell-tale signature of water ice in the spectra [color make-up] of the study region but only when certain portions were cast in shadow”, said de Sanctis. The surface ice is likely to be replenished as water escapes from the comet’s interior via cracks caused by the contrast in day and night temperatures.

The ice was spotted in data gathered last August using Rosetta’s VIRTIS instrument – a spectrometer that was created to map the comet’s chemical composition.

Their findings are published today in the journal Nature.

“Water fuel undertaking is adjusted from the diurnal sequence, and that we notice that it is clear that company of glaciers on the outside is adjusted likewise”, said Maria Cristina De Sanctis, who might be in charge of analysis and inventor with the Institute for Astrophysics and Space Planetology in Rome.

The scientists also calculated how much water vapour was being emitted by the patch that they analysed with VIRTIS, and showed that this accounted for about 3 per cent of the total amount of water vapour coming out from the whole comet at the same time, as measured by Rosetta’s MIRO microwave sensor.

Scientists were originally bewildered by how the comet’s surface began to rapidly change after a period of relative stability. Left over from the formation the solar system billions of years ago, they zip around the Sun along eccentric orbits.

“How and where exactly the sources of cometary activity arise has been a largely unsolved mystery in comet research”, said co-author Dr Gabriele Arnold of the German Aerospace Center. The water signal was stronger when the neck was in shadow and weaker during the comet’s day. The ice then transitions, or sublimates, to gaseous water vapor when that region shifts into sunlight. Thus, this cycle could be common on comets.

A group of Italian scientists headed by Maria Cristina De Sanctis measures the concentration of ice in the “neck” area of the comet, the narrower zone connecting two larger areas of the comet that looks something like two enormous potatoes pressed together.

Sierks says comets like 67P are sublimating away all the time through cycles like this, using up their water. As the comet continues to rotate, it plunges the region into darkness.

The data suggest that water ice on and a few centimeters below the surface “sublimates” when illuminated by sunlight, turning it into gas that then flows away from the comet. But, because the surface of the comet is cold, this gas effectively freezes again thereby covering the patch of comet’s surface with a fresh layer of ice. This process keeps the comet’s surface rich in water ice.