Icy messages from a dead spacecraft

Researchers have found dry ice on a comet for the first time, a discovery that hints at the extreme temperature shifts on the surface as its seasons change. 

The finding is part of a series of cometary secrets revealed by two separate papers published this week in the journal Science, focusing on images collected by the Rosetta space probe. 

Rosetta completed its mission earlier this year but continues to feed new information about its target – Comet 67P/Churyumov-Gerasimenko – which it observed closely for around two years.

In the first paper, a research team led by Gianrico Filacchione at the National Institute for Astrophysics in Rome studied infrared spectral images of Comet 67P’s solid core, known as the nucleus.

The team’s modelling zeroed in on part of the surface area dubbed the Anhur region, approximately 60 by 80 metres. About 0.1% of the zone consisted of solid carbon dioxide – enough dry ice to keep a Broadway season of Into The Woods well stocked for weeks.

{%recommended 4051%}

“Carbon dioxide is one of the most abundant species in cometary nuclei, but due to its high volatility [carbon dioxide] ice is generally only found beneath the surface,” the researchers report.

This observation adds to a growing awareness of the extreme seasonal changes that can occur on a comet’s surface.

Comet 67P’s climate isn’t only affected by its orbit and distance from the sun, but also by the irregular surface of the nucleus, which can hide parts of its topography from light for extended periods.

The region bearing dry ice had experienced a four-year winter season before being thrust into sunlight in mid-January 2015. The images analysed by the research team were taken in March 2015, and by mid-April, just three weeks later, the ice had disappeared.  

Ice was also a central theme of the second paper, published by a research team led by Sonia Fornasier of the LESIA Observatory at Paris Diderot University in France.

In it, the scientists traced the evolution of colours on Comet 67P’s surface between day and night, and through changes of season. 

The researchers tracked images of the comet as it moved from its coldest period in early 2015, into its perihelion – the season of highest temperatures, when it is closest to the sun – which it reached around August 2015.

They observed the emergence of a blue hue, as parts of the comet moved from shadow into sunlight, indicating the presence of ice.

“The whole nucleus became relatively bluer near perihelion, as increasing activity removed the surface dust, implying that water ice is widespread underneath the surface,” the researchers explain.

The imaging also revealed icy patches of roughly 1,500 square metres, which appeared during colder seasons and vanished within 10 days. 

The findings exemplify just how diverse cometary surface environments can be, and add to our growing understanding of how they are formed.