A study published by an international team reveals the comet is as old as the solar system itself, as is the ice buried inside it.
This ice has now settled a longstanding debate over the nature of ice in comets, and has helped uncover secrets of when comets were first formed.
The data used to identify the ice was gathered by Rosetta’s Rosina instrument – a mass spectrometer that measured the amounts of nitrogen, carbon monoxide and argon in the comet’s ice.
The results were compared with data from labs that looked for amorphous ice, and models describing the composition of ice that can trap molecules of gas.
The ratios of molecular nitrogen and argon found in Churi correspond to those in the gas hydrate model, while the amount of argon detected in Churi is a hundred times smaller than the quantity that can be trapped in amorphous ice.
Computer-generated image shows Rosetta, the billion-dollar comet-chasing-spacecraft, that was launched by Esa in 2004 and arrived at the comet Churi in 2014. Results from its Rosina instrument have shown ice on the comet is as old as the solar system, answering questions about when comets first formed
Nitrogen to carbon monoxide, and argon to carbon monoxide ratios compared to laboratory data and models. Green and blue areas represent the variations the ratios measured by the Rosina instrument. By studying these levels, it shows the ice in the comet definitely has a crystalline structure
The ice in the comet, therefore, definitely has a crystalline structure.
Until now, there were two opposing hypotheses. One was that the ice is crystalline and the water molecules are arranged in a regular pattern, and the other that the ice is amorphous, with disordered molecules.
This question is important because of its implications for the origin and formation of comets and the solar system.
Gas hydrates are made of crystalline ice that formed in the protosolar nebula, which gave rise to the early solar system, from the crystallisation of grains of water ice and the adsorption of gas molecules onto their surfaces as the nebula slowly cooled.
The finding means scientists can now determine the age of comets.
GOODBYE TO PHILAE LANDER
Artist’s impression of the Rosetta’s lander, Philae, on the surface of the comet
Rosetta’s accompanying Philae lander, a robot the size of a washing machine, dropped onto the comet in November 2014.
But after a troubled landing and 60 hours of operation, there has largely been radio silence from Philae.
In February this year, ground control sadly bid farewell to Philae and said the chances of re-establishing contact with the robot is basically zero.
The probe’s historic landing famously happened several times in succession.
Its first bounce looping nearly 0.6 miles (1km) back from the comet’s surface and lasting a remarkable 110 minutes.
When it finally settled, its precise location was unknown but images and other data suggested the solar powered robot was sitting in the shade at an awkward angle.
In particular, the crystalline structure means it was formed in the potosolar nebula, a cloud of dust that gathered before the solar system formed.
This makes the ice as old as our solar system, around 4.6 billion years old.
The gas hydrates agglomerated by Churi must have formed between -228 °C and -223 °C to produce the observed abundances.
The discovery was made by an international team led by researchers at the centre national de la recherche scientifique (CNRS) and Marseille University.
The work has been published in The Astrophysical Journal Letters.
If comets are made of crystalline ice, this means that they must have formed at the same time as the solar system, rather than earlier in the interstellar medium.
The crystalline structure of comets also shows that the protosolar nebula was hot and dense enough to turn ice from the interstellar medium into gas.
This work also supports currently believed scenarios for the formation of the gas giant planets, as well as their moons, which require the agglomeration of crystalline ice.